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SUSE Linux Enterprise Server 15 SP1

Security and Hardening Guide

Introduces basic concepts of system security, covering both local and network security aspects. Shows how to use the product inherent security software like AppArmor, SELinux, or the auditing system that reliably collects information about any security-relevant events. Supports the administrator with security-related choices and decisions in installing and setting up a secure SUSE Linux Enterprise Server and additional processes to further secure and harden that installation.

Publication Date: October 15, 2021
About This Guide
Available Documentation
Giving Feedback
Documentation Conventions
Product Life Cycle and Support
1 Security and Confidentiality
1.1 Overview
1.2 Passwords
1.3 Backups
1.4 System Integrity
1.5 File Access
1.6 Networking
1.7 Software Vulnerabilities
1.8 Malware
1.9 Important Security Tips
1.10 Reporting Security Issues
2 Common Criteria
2.1 Introduction
2.2 Evaluation Assurance Level (EAL)
2.3 Generic Guiding Principles
2.4 For More Information
I Authentication
3 Authentication with PAM
3.1 What is PAM?
3.2 Structure of a PAM Configuration File
3.3 The PAM Configuration of sshd
3.4 Configuration of PAM Modules
3.5 Configuring PAM Using pam-config
3.6 Manually Configuring PAM
3.7 For More Information
4 Using NIS
4.1 Configuring NIS Servers
4.2 Configuring NIS Clients
5 Setting Up Authentication Clients Using YaST
5.1 Configuring an Authentication Client with YaST
5.2 SSSD
6 LDAP—A Directory Service
6.1 Structure of an LDAP Directory Tree
6.2 Installing the Software for 389 Directory Server
6.3 Manually Configuring a 389 Directory Server
6.4 Setting Up a 389 Directory Server with YaST
6.5 Manually Administering LDAP Data
6.6 For More Information
7 Network Authentication with Kerberos
7.1 Conceptual Overview
7.2 Kerberos Terminology
7.3 How Kerberos Works
7.4 User View of Kerberos
7.5 Installing and Administering Kerberos
7.6 Setting up Kerberos using LDAP and Kerberos Client
7.7 Kerberos and NFS
7.8 For More Information
8 Active Directory Support
8.1 Integrating Linux and Active Directory Environments
8.2 Background Information for Linux Active Directory Support
8.3 Configuring a Linux Client for Active Directory
8.4 Logging In to an Active Directory Domain
8.5 Changing Passwords
9 Setting Up a FreeRADIUS Server
9.1 Installation and Testing on SUSE Linux Enterprise
II Local Security
10 Physical Security
10.1 System Locks
10.2 Locking Down the BIOS
10.3 Security via the Boot Loaders
10.4 Retiring Linux Servers with Sensitive Data
10.5 Restricting Access to Removable Media
11 Automatic Security Checks with seccheck
11.1 Seccheck Timers
11.2 Enabling Seccheck Timers
11.3 Daily, Weekly, and Monthly Checks
11.4 Automatic Logout
12 Software Management
12.1 Removing Unnecessary Software Packages (RPMs)
12.2 Patching Linux Systems
13 File Management
13.1 Disk Partitions
13.2 Checking File Permissions and Ownership
13.3 Default umask
13.4 SUID/SGID Files
13.5 World-Writable Files
13.6 Orphaned or Unowned Files
14 Encrypting Partitions and Files
14.1 Setting Up an Encrypted File System with YaST
14.2 Encrypting Files with GPG
15 User Management
15.1 Various Account Checks
15.2 Enabling Password Aging
15.3 Stronger Password Enforcement
15.4 Password and Login Management with PAM
15.5 Restricting root Logins
15.6 Setting an Inactivity Timeout for Interactive Shell Sessions
15.7 Preventing Accidental Denial of Service
15.8 Displaying Login Banners
15.9 Connection Accounting Utilities
16 Spectre/Meltdown Checker
16.1 Using spectre-meltdown-checker
16.2 Additional Information about Spectre/Meltdown
17 Configuring Security Settings with YaST
17.1 Security Overview
17.2 Predefined Security Configurations
17.3 Password Settings
17.4 Boot Settings
17.5 Login Settings
17.6 User Addition
17.7 Miscellaneous Settings
18 Authorization with PolKit
18.1 Conceptual Overview
18.2 Authorization Types
18.3 Querying Privileges
18.4 Modifying Configuration Files
18.5 Restoring the Default Privileges
19 Access Control Lists in Linux
19.1 Traditional File Permissions
19.2 Advantages of ACLs
19.3 Definitions
19.4 Handling ACLs
19.5 ACL Support in Applications
19.6 For More Information
20 Certificate Store
20.1 Activating Certificate Store
20.2 Importing Certificates
21 Intrusion Detection with AIDE
21.1 Why Use AIDE?
21.2 Setting Up an AIDE Database
21.3 Local AIDE Checks
21.4 System Independent Checking
21.5 For More Information
III Network Security
22 X Window System and X Authentication
23 SSH: Secure Network Operations
23.1 ssh—Secure Shell
23.2 scp—Secure Copy
23.3 sftp—Secure File Transfer
23.4 The SSH Daemon (sshd)
23.5 SSH Authentication Mechanisms
23.6 Port Forwarding
23.7 Adding and Removing Public Keys on an Installed System
23.8 For More Information
24 Masquerading and Firewalls
24.1 Packet Filtering with iptables
24.2 Masquerading Basics
24.3 Firewalling Basics
24.4 firewalld
24.5 Migrating From SuSEfirewall2
24.6 For More Information
25 Configuring a VPN Server
25.1 Conceptual Overview
25.2 Setting Up a Simple Test Scenario
25.3 Setting Up Your VPN Server Using a Certificate Authority
25.4 Setting Up a VPN Server or Client Using YaST
25.5 For More Information
26 Enabling FIPS 140-2
26.1 Enabling FIPS
IV Confining Privileges with AppArmor
27 Introducing AppArmor
27.1 AppArmor Components
27.2 Background Information on AppArmor Profiling
28 Getting Started
28.1 Installing AppArmor
28.2 Enabling and Disabling AppArmor
28.3 Choosing Applications to Profile
28.4 Building and Modifying Profiles
28.5 Updating Your Profiles
29 Immunizing Programs
29.1 Introducing the AppArmor Framework
29.2 Determining Programs to Immunize
29.3 Immunizing cron Jobs
29.4 Immunizing Network Applications
30 Profile Components and Syntax
30.1 Breaking an AppArmor Profile into Its Parts
30.2 Profile Types
30.3 Include Statements
30.4 Capability Entries (POSIX.1e)
30.5 Network Access Control
30.6 Profile Names, Flags, Paths, and Globbing
30.7 File Permission Access Modes
30.8 Mount Rules
30.9 Pivot Root Rules
30.10 PTrace Rules
30.11 Signal Rules
30.12 Execute Modes
30.13 Resource Limit Control
30.14 Auditing Rules
31 AppArmor Profile Repositories
32 Building and Managing Profiles with YaST
32.1 Manually Adding a Profile
32.2 Editing Profiles
32.3 Deleting a Profile
32.4 Managing AppArmor
33 Building Profiles from the Command Line
33.1 Checking the AppArmor Status
33.2 Building AppArmor Profiles
33.3 Adding or Creating an AppArmor Profile
33.4 Editing an AppArmor Profile
33.5 Unloading Unknown AppArmor Profiles
33.6 Deleting an AppArmor Profile
33.7 Two Methods of Profiling
33.8 Important File Names and Directories
34 Profiling Your Web Applications Using ChangeHat
34.1 Configuring Apache for mod_apparmor
34.2 Managing ChangeHat-Aware Applications
35 Confining Users with pam_apparmor
36 Managing Profiled Applications
36.1 Reacting to Security Event Rejections
36.2 Maintaining Your Security Profiles
37 Support
37.1 Updating AppArmor Online
37.2 Using the Man Pages
37.3 For More Information
37.4 Troubleshooting
37.5 Reporting Bugs for AppArmor
38 AppArmor Glossary
V SELinux
39 Configuring SELinux
39.1 Why use SELinux?
39.2 SELinux policy overview
39.3 Installing SELinux packages
39.4 Installing an SELinux policy
39.5 Modifying the GRUB 2 bootloader
39.6 Configuring SELinux
39.7 Managing SELinux
39.8 Troubleshooting
VI The Linux Audit Framework
40 Understanding Linux Audit
40.1 Introducing the Components of Linux Audit
40.2 Configuring the Audit Daemon
40.3 Controlling the Audit System Using auditctl
40.4 Passing Parameters to the Audit System
40.5 Understanding the Audit Logs and Generating Reports
40.6 Querying the Audit Daemon Logs with ausearch
40.7 Analyzing Processes with autrace
40.8 Visualizing Audit Data
40.9 Relaying Audit Event Notifications
41 Setting Up the Linux Audit Framework
41.1 Determining the Components to Audit
41.2 Configuring the Audit Daemon
41.3 Enabling Audit for System Calls
41.4 Setting Up Audit Rules
41.5 Configuring Audit Reports
41.6 Configuring Log Visualization
42 Introducing an Audit Rule Set
42.1 Adding Basic Audit Configuration Parameters
42.2 Adding Watches on Audit Log Files and Configuration Files
42.3 Monitoring File System Objects
42.4 Monitoring Security Configuration Files and Databases
42.5 Monitoring Miscellaneous System Calls
42.6 Filtering System Call Arguments
42.7 Managing Audit Event Records Using Keys
43 Useful Resources
A Achieving PCI DSS Compliance
A.1 What Is the PCI DSS?
A.2 Focus of This Document: Areas Relevant to the Operating System
A.3 Requirements in Detail
B GNU Licenses
B.1 GNU Free Documentation License
List of Examples
3.1 PAM Configuration for sshd (/etc/pam.d/sshd)
3.2 Default Configuration for the auth Section (common-auth)
3.3 Default Configuration for the account Section (common-account)
3.4 Default Configuration for the password Section (common-password)
3.5 Default Configuration for the session Section (common-session)
3.6 pam_env.conf
6.1 Excerpt from CN=schema
6.2 Basic Instance Configuration File
6.3 A .dsrc File for Remote Administration
6.4 A .dsrc File for Local Administration
7.1 Example KDC Configuration, /etc/krb5.conf
24.1 Callback Port Configuration for the nfs Kernel Module in /etc/modprobe.d/60-nfs.conf
24.2 Commands to Define a new firewalld RPC Service for NFS
25.1 VPN Server Configuration File
25.2 VPN Client Configuration File
28.1 Output of aa-unconfined
33.1 Learning Mode Exception: Controlling Access to Specific Resources
33.2 Learning Mode Exception: Defining Permissions for an Entry
39.1 Security context settings using ls -Z
39.2 Verifying that SELinux is functional
39.3 Getting a list of booleans and verifying policy access
39.4 Getting file context information
39.5 The default context for directories in the root directory
39.6 Showing SELinux settings for processes with ps Zaux
39.7 Viewing default file contexts
39.8 Example lines from /etc/audit/audit.log
39.9 Analyzing audit messages
39.10 Viewing which lines deny access
39.11 Creating a policy module allowing an action previously denied
40.1 Example output of auditctl -s
40.2 Example Audit Rules—Audit System Parameters
40.3 Example Audit Rules—File System Auditing
40.4 Example Audit Rules—System Call Auditing
40.5 Deleting Audit Rules and Events
40.6 Listing Rules with auditctl -l
40.7 A simple audit event—viewing the audit log
40.8 An Advanced Audit Event—Login via SSH
40.9 Example /etc/audisp/audispd.conf
40.10 Example /etc/audisp/plugins.d/syslog.conf

Copyright © 2006– 2021 SUSE LLC and contributors. All rights reserved.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or (at your option) version 1.3; with the Invariant Section being this copyright notice and license. A copy of the license version 1.2 is included in the section entitled GNU Free Documentation License.

For SUSE trademarks, see https://www.suse.com/company/legal/. All other third-party trademarks are the property of their respective owners. Trademark symbols (®, ™ etc.) denote trademarks of SUSE and its affiliates. Asterisks (*) denote third-party trademarks.

All information found in this book has been compiled with utmost attention to detail. However, this does not guarantee complete accuracy. Neither SUSE LLC, its affiliates, the authors nor the translators shall be held liable for possible errors or the consequences thereof.

About This Guide Edit source

This manual introduces the basic concepts of system security on SUSE Linux Enterprise Server. It covers extensive documentation about the authentication mechanisms available on Linux, such as NIS or LDAP. It deals with aspects of local security like access control lists, encryption and intrusion detection. In the network security part you learn how to secure computers with firewalls and masquerading, and how to set up virtual private networks (VPN). This manual shows how to use security software like AppArmor (which lets you specify per program which files the program may read, write, and execute) or the auditing system that collects information about security-relevant events.

1 Available Documentation Edit source

Note
Note: Online Documentation and Latest Updates

Documentation for our products is available at https://documentation.suse.com/, where you can also find the latest updates, and browse or download the documentation in various formats. The latest documentation updates are usually available in the English version of the documentation.

Note
Note: Manual Pages

Many commands are described in detail in their manual pages. You can view manual pages by running the man command followed by a specific command name. If the man command is not installed on your system, install it by running zypper install man.

The following documentation is available for this product:

Article “Installation Quick Start

This Quick Start guides you step-by-step through the installation of SUSE® Linux Enterprise Server 15 SP1.

Book “Deployment Guide

This guide details how to install single or multiple systems, and how to exploit the product-inherent capabilities for a deployment infrastructure. Choose from various approaches: local installation from physical installation media, customizing the standard installation images, network installation server, mass deployment using a remote-controlled, highly-customized, automated installation process, and initial system configuration.

Book “Administration Guide

Covers system administration tasks like maintaining, monitoring and customizing an initially installed system.

Book “Virtualization Guide

Describes virtualization technology in general, and introduces libvirt—the unified interface to virtualization—and detailed information on specific hypervisors.

Book “Storage Administration Guide

Provides information about how to manage storage devices on a SUSE Linux Enterprise Server.

Book “AutoYaST Guide

AutoYaST is a system for unattended mass deployment of SUSE Linux Enterprise Server systems using an AutoYaST profile containing installation and configuration data. The manual guides you through the basic steps of auto-installation: preparation, installation, and configuration.

Book “Security and Hardening Guide

Introduces basic concepts of system security, covering both local and network security aspects. Shows how to use the product inherent security software like AppArmor, SELinux, or the auditing system that reliably collects information about any security-relevant events. Supports the administrator with security-related choices and decisions in installing and setting up a secure SUSE Linux Enterprise Server and additional processes to further secure and harden that installation.

Book “System Analysis and Tuning Guide

An administrator's guide for problem detection, resolution and optimization. Find how to inspect and optimize your system by means of monitoring tools and how to efficiently manage resources. Also contains an overview of common problems and solutions and of additional help and documentation resources.

Book “Repository Mirroring Tool Guide”

An administrator's guide to Subscription Management Tool—a proxy system for SUSE Customer Center with repository and registration targets. Learn how to install and configure a local SMT server, mirror and manage repositories, manage client machines, and configure clients to use SMT.

Book “GNOME User Guide

Introduces the GNOME desktop of SUSE Linux Enterprise Server. It guides you through using and configuring the desktop and helps you perform key tasks. It is intended mainly for end users who want to make efficient use of GNOME as their default desktop.

The release notes for this product are available at https://www.suse.com/releasenotes/.

2 Giving Feedback Edit source

Your feedback and contribution to this documentation is welcome! Several channels are available:

Service Requests and Support

For services and support options available for your product, refer to https://www.suse.com/support/.

To open a service request, you need a subscription at SUSE Customer Center. Go to https://scc.suse.com/support/requests, log in, and click Create New.

Bug Reports

Report issues with the documentation at https://bugzilla.suse.com/. To simplify this process, you can use the Report Documentation Bug links next to headlines in the HTML version of this document. These preselect the right product and category in Bugzilla and add a link to the current section. You can start typing your bug report right away. A Bugzilla account is required.

Contributions

To contribute to this documentation, use the Edit Source links next to headlines in the HTML version of this document. They take you to the source code on GitHub, where you can open a pull request. A GitHub account is required.

For more information about the documentation environment used for this documentation, see the repository's README.

Mail

Alternatively, you can report errors and send feedback concerning the documentation to <>. Make sure to include the document title, the product version and the publication date of the documentation. Refer to the relevant section number and title (or include the URL) and provide a concise description of the problem.

3 Documentation Conventions Edit source

The following notices and typographical conventions are used in this documentation:

  • /etc/passwd: directory names and file names

  • PLACEHOLDER: replace PLACEHOLDER with the actual value

  • PATH: the environment variable PATH

  • ls, --help: commands, options, and parameters

  • user: users or groups

  • package name : name of a package

  • Alt, AltF1: a key to press or a key combination; keys are shown in uppercase as on a keyboard

  • File, File › Save As: menu items, buttons

  • AMD/Intel This paragraph is only relevant for the AMD64/Intel 64 architecture. The arrows mark the beginning and the end of the text block.

    IBM Z, POWER This paragraph is only relevant for the architectures IBM Z and POWER. The arrows mark the beginning and the end of the text block.

  • Dancing Penguins (Chapter Penguins, ↑Another Manual): This is a reference to a chapter in another manual.

  • Commands that must be run with root privileges. Often you can also prefix these commands with the sudo command to run them as non-privileged user.

    root # command
    tux > sudo command
  • Commands that can be run by non-privileged users.

    tux > command
  • Notices

    Warning
    Warning: Warning Notice

    Vital information you must be aware of before proceeding. Warns you about security issues, potential loss of data, damage to hardware, or physical hazards.

    Important
    Important: Important Notice

    Important information you should be aware of before proceeding.

    Note
    Note: Note Notice

    Additional information, for example about differences in software versions.

    Tip
    Tip: Tip Notice

    Helpful information, like a guideline or a piece of practical advice.

4 Product Life Cycle and Support Edit source

SUSE products are supported for up to 13 years. To check the life cycle dates for your product, see https://www.suse.com/lifecycle/.

For SUSE Linux Enterprise, the following life cycles and release cycles apply:

  • SUSE Linux Enterprise Server has a 13-year life cycle: 10 years of general support and three years of extended support.

  • SUSE Linux Enterprise Desktop has a 10-year life cycle: seven years of general support and three years of extended support.

  • Major releases are published every four years. Service packs are published every 12-14 months.

  • SUSE supports previous SUSE Linux Enterprise service packs for six months after the release of a new service pack.

For some products, Long Term Service Pack Support (LTSS) is available. Find information about our support policy and options at https://www.suse.com/support/policy.html and https://www.suse.com/support/programs/long-term-service-pack-support.html.

Modules have a different life cycle, update policy, and update timeline than their base products. Modules contain software packages and are fully supported parts of SUSE Linux Enterprise Server. For more information, see the Article “Modules and Extensions Quick Start.

4.1 Support Statement for SUSE Linux Enterprise Server Edit source

To receive support, you need an appropriate subscription with SUSE. To view the specific support offerings available to you, go to https://www.suse.com/support/ and select your product.

The support levels are defined as follows:

L1

Problem determination, which means technical support designed to provide compatibility information, usage support, ongoing maintenance, information gathering and basic troubleshooting using available documentation.

L2

Problem isolation, which means technical support designed to analyze data, reproduce customer problems, isolate problem area and provide a resolution for problems not resolved by Level 1 or prepare for Level 3.

L3

Problem resolution, which means technical support designed to resolve problems by engaging engineering to resolve product defects which have been identified by Level 2 Support.

For contracted customers and partners, SUSE Linux Enterprise Server is delivered with L3 support for all packages, except for the following:

  • Technology Previews

  • Sound, graphics, fonts and artwork.

  • Packages that require an additional customer contract.

  • Some packages shipped as part of the module Workstation Extension are L2-supported only.

  • Packages with names ending in -devel (containing header files and similar developer resources) will only be supported together with their main packages.

SUSE will only support the usage of original packages. That is, packages that are unchanged and not recompiled.

4.2 Technology Previews Edit source

Technology previews are packages, stacks, or features delivered by SUSE to provide glimpses into upcoming innovations. The previews are included for your convenience to give you the chance to test new technologies within your environment. We would appreciate your feedback! If you test a technology preview, please contact your SUSE representative and let them know about your experience and use cases. Your input is helpful for future development.

However, technology previews come with the following limitations:

  • Technology previews are still in development. Therefore, they may be functionally incomplete, unstable, or in other ways not suitable for production use.

  • Technology previews are not supported.

  • Technology previews may only be available for specific hardware architectures.

  • Details and functionality of technology previews are subject to change. As a result, upgrading to subsequent releases of a technology preview may be impossible and require a fresh installation.

  • Technology previews can be dropped at any time. For example, if SUSE discovers that a preview does not meet the customer or market needs, or does not prove to comply with enterprise standards. SUSE does not commit to providing a supported version of such technologies in the future.

For an overview of technology previews shipped with your product, see the release notes at https://www.suse.com/releasenotes/.

1 Security and Confidentiality Edit source

This chapter introduces basic concepts of computer security. Threats and basic mitigation techniques are described. The chapter also provides references to other chapters, guides and websites with further information.

1.1 Overview Edit source

One main characteristic of Linux is its ability to handle multiple users at the same time (multiuser) and to allow these users to simultaneously perform tasks (multitasking) on the same computer. To users, there is no difference between working with data stored locally and data stored in the network.

Due to the multiuser capability, data from different users has to be stored separately to guarantee security and privacy. Also important is the ability to keep data available in spite of a lost or damaged data medium, for example a hard disk.

This chapter is primarily focused on confidentiality and privacy. But a comprehensive security concept includes a regularly updated, workable, and tested backup. Without a backup, restoring data after it has been tampered with or after a hardware failure is very hard.

Use a defense-in-depth approach to security: Assume that no single threat mitigation can fully protect your systems and data, but multiple layers of defense will make an attack much harder. Components of a defense-in-depth strategy can be the following:

  • Hashing passwords (for example with PBKDF2, bcrypt, or scrypt) and salting them

  • Encrypting data (for example with AES)

  • Logging, monitoring, and intrusion detection

  • Firewall

  • Antivirus scanner

  • Defined and documented emergency procedures

  • Backups

  • Physical security

  • Audits, security scans, and intrusion tests

SUSE Linux Enterprise Server includes software that addresses the requirements of the list above. The following sections provide starting points for securing your system.

1.2 Passwords Edit source

On a Linux system, only hashes of passwords are stored. Hashes are one-way algorithms that make it easy to encrypt data. At the same time, hash algorithms make it very hard to compute the original secret from the hash.

The hashes are stored in the file /etc/shadow, which cannot be read by normal users. Because restoring passwords is possible with powerful computers, hashed passwords should not be visible to regular users.

The National Institute of Standards and Technology (NIST) publishes a guideline for passwords, which is available at https://pages.nist.gov/800-63-3/sp800-63b.html#sec5

For details about how to set a password policy, see Section 17.3, “Password Settings. For general information about authentication on Linux, see Part I, “Authentication”.

1.3 Backups Edit source

If your system is compromised, backups can be used to restore a prior system state. When bugs or accidents occur, backups can also be used to compare the current system against an older version. For production systems, it is very important to take some backups off-site for cases like disasters (for example off-site storage of tapes/recordable media, or off-site initiated).

For legal reasons, some firms and organizations must be careful about backing up too much information and holding it too long. If your environment has a policy regarding the destruction of old paper files, you might need to extend this policy to Linux backup tapes as well.

The rules about physical security of servers apply to backups as well. Additionally it is advisable to encrypt backup data. This can be done either per individual backup archive or for the complete backup file system, if applicable. Should a backup medium ever be lost, for example during transportation, the data will be protected against unauthorized access. The same applies if a backup system itself is compromised. To some extent encryption also ensures the integrity of the backups. Keep in mind, however, that the appropriate people need to be able to decrypt backups in emergency situations. Also, the case that an encryption key itself is compromised and needs to be replaced should be considered.

If a system is known to be compromised or suspected to be compromised, then it is vital to determine the integrity status of backups. If a system compromise has not been detected for a longer time, then it is possible that backups already include manipulated configuration files or malicious programs. Keeping a long enough history of backups allows to inspect for possible unwarranted differences.

Even in the absence of any known security breach, a regular inspection of differences among important configuration files in backups can help with finding security issues (maybe even accidental misconfigurations). This approach is best suited for files and environments where the content doesn't change too frequently.

1.4 System Integrity Edit source

If it is possible to physically access a computer, the firmware and boot process can be manipulated to gain access as soon as an authorized person boots the machine. While not all computers can be locked into inaccessible rooms, your first step should be physically locking the server room.

Also remember that disposing of old equipment must be handles in a secure manner. Securing the boot loader and restricting removable media also provide useful physical security, See Chapter 10, Physical Security for more information.

Consider taking the following additional measures:

  • Configure your system so it cannot be booted from a removable device.

  • Protect the boot process with a UEFI password, Secure Boot, and a GRUB2 password.

  • Linux systems are started by a boot loader that usually allows passing additional options to the booted kernel. You can prevent others from using such parameters during boot by setting an additional password for the boot loader. This is crucial to system security. Not only does the kernel itself run with root permissions, but it is also the first authority to grant root permissions at system start-up.

    For more information about setting a password in the boot loader, see Book “Administration Guide”, Chapter 14 “The Boot Loader GRUB 2”, Section 14.2.6 “Setting a Boot Password”.

  • Enable hard disk encryption. For more information, see Chapter 14, Encrypting Partitions and Files.

  • Use AIDE to detect any changes in your system configuration. For more information, see Chapter 21, Intrusion Detection with AIDE.

1.5 File Access Edit source

Because of the everything is a file approach in Linux, file permissions are important for controlling access to most resources. This means that by using file permissions, you can define access to regular files and directories as well as hardware devices. By default, most hardware devices are only accessible for root. However, some devices, for example serial ports, can be accessible for normal users.

As a general rule, always work with the most restrictive privileges possible for a given task. For example, it is definitely not necessary to be root to read or write e-mail. If the mail program has a bug, this bug could be exploited for an attack that acts with exactly the permissions of the program at the time of the attack. By following the above rule, minimize the possible damage.

For details, see Section 19.1, “Traditional File Permissions” and Section 19.2, “Advantages of ACLs”.

AppArmor and SELinux allow you to set constraints for applications and users. For details, see Part IV, “Confining Privileges with AppArmor and Part V, “SELinux”.

If there is a chance that hard disks could be accessed outside of the installed operating system, for example by booting a live system or removing the hardware, encrypt the data. SUSE Linux Enterprise Server allows you to encrypt partitions containing data and the operating system. For details, see Chapter 14, Encrypting Partitions and Files.

1.6 Networking Edit source

Securing network services is a crucial task. Aim to secure as many layers of the OSI model as possible.

All communication should be authenticated and encrypted with up-to-date cryptographic algorithms on the transport or application layer. Use a Virtual Private Network (VPN) as an additional secure layer on physical networks.

SUSE Linux Enterprise Server provides many options for securing your network:

  • Use openssl to create X509 certificates. These certificates can be used for encryption and authentication of many services. You can set up your own certificate authority (CA) and use it as a source of trust in your network. For details, see man openssl.

  • Usually, at least parts of networks are exposed to the public Internet. Reduce attack surfaces by closing ports with firewall rules and by uninstalling or at least disabling unrequired services. For details, see Chapter 24, Masquerading and Firewalls.

  • Use OpenVPN to secure communication channels over insecure physical networks. For details, see Chapter 25, Configuring a VPN Server.

  • Use strong authentication for network services. For details, see Part I, “Authentication”.

1.7 Software Vulnerabilities Edit source

Software vulnerabilities are issues in software that can be exploited to obtain unauthorized access or misuse systems. Vulnerabilities are especially critical if they affect remote services, such as HTTP servers. Computer systems are very complex, therefore they always include certain vulnerabilities.

When such issues become known, they must usually be fixed in the software by software developers. The resulting update must then be installed by system administrators in a timely and safe manner on affected systems.

Vulnerabilities are usually announced on centralized databases, for example the National Vulnerability Database, which is maintained by the US government. You can subscribe to feeds to stay informed about newly discovered vulnerabilities. In some cases the problems induced by the bugs can be mitigated until a software update is provided. Vulnerabilities are assigned a Common Vulnerabilities and Exposures (CVE) number and a Common Vulnerability Scoring System (CVSS) score. The score helps identify the severity of vulnerabilities.

SUSE provides a feed of security advisories. It is available at https://www.suse.com/en-us/support/update/. There is also a list of security updates by CVE number available at https://www.suse.com/en-us/security/cve/.

Note
Note: Backports and Version Numbers

SUSE employs the practice of applying the important source code fixes onto older stable versions of software (backporting). Therefore, even if the version number of a software in SUSE Linux Enterprise Server is lower than that of the latest version number from the upstream project, the software version in SUSE Linux Enterprise Server may already contain the latest fixes for vulnerabilities.

For more information, see Book “Upgrade Guide”, Chapter 6 “Backports of Source Code”.

In general, administrators should be prepared for severe vulnerabilities in their systems. This includes hardening all computers as far as possible. Also, we recommend to have predefined procedures in place for quickly installing updates for severe vulnerabilities.

To reduce the damage of possible attacks, use restrictive file permissions. See Section 19.1, “Traditional File Permissions”.

Other useful links:

1.8 Malware Edit source

Malware is software that is intended to interrupt the normal functioning of a computer or steal data. This includes viruses, worms, ransomware, or rootkits. Sometimes malware uses software vulnerabilities to attack a computer. However, in many cases it is accidentally executed by a user, especially when installing third-party software from unknown sources. SUSE Linux Enterprise Server provides an extensive list of programs (packages) in its download repositories. This reduces the need to download third-party software. All packages provided by SUSE are signed. The package manager of SUSE Linux Enterprise Server checks the signatures of packages after the download to verify their integrity.

The command rpm --checksig RPM_FILE shows whether the checksum and the signature of a package are correct. You can find the signing key on the first DVD of SUSE Linux Enterprise Server and on most key servers worldwide.

You can use the ClamAV antivirus software to detect malware on your system. ClamAV can be integrated into several services, for example mail servers and HTTP proxies. This can be used to filter malware before it reaches the user.

Restrictive user privileges can reduce the risk of accidental code execution.

1.9 Important Security Tips Edit source

The following tips are a quick summary of the sections above:

  • Stay informed about the latest security issues. Get and install the updated packages recommended by security announcements as quickly as possible.

  • Avoid using root privileges whenever possible. Set restrictive file permissions.

  • Only use encrypted protocols for network communication.

  • Disable any network services you do not absolutely require.

  • Conduct regular security audits. For example, scan your network for open ports.

  • Monitor the integrity of files on your systems with AIDE (Advanced Intrusion Detection Environment).

  • Take proper care when installing any third-party software.

  • Check all your backups regularly.

  • Check your log files, for example with logwatch.

  • Configure the firewall to block all ports that are not explicitly whitelisted.

  • Design your security measures to be redundant.

  • Use encryption where possible, for example for hard disks of mobile computers.

1.10 Reporting Security Issues Edit source

If you discover a security-related problem, first check the available update packages. If no update is available, write an e-mail to <>. Include a detailed description of the problem and the version number of the package concerned. We encourage you to encrypt e-mails with GPG.

You can find a current version of the SUSE GPG key at https://www.suse.com/support/security/contact/.

2 Common Criteria Edit source

Common Criteria is the best known and most widely used methodology to evaluate and measure the security value of an IT product. The methodology aims to be independent, as an independent laboratory conducts the evaluation, which a certification body will certify afterward. Security Functional Requirements (SFR) are summarized in so-called Protection Profiles (PP). If the definition of a Security Target (ST) and the Evaluation Assurance Levels (EAL) are comparable, this allows the comparison of security functions of different products. (The definition of a Security Target typically references the PP—if one exists that fits the purpose of the product.)

2.1 Introduction Edit source

A clear definition of security in IT products is challenging. Security should be considered a process that never ends, not a static condition that can be met or not. A Common Criteria certificate (below EAL7) does not make a clear statement about the error-proneness of the system, but it adds an important value to the product that cannot be described with the presence of technology alone: That someone has independently inspected the design of the system in such way that it corresponds to the claims that are made, and that explicit care has been taken in producing and maintaining the product.

The certificate states a degree of maturity of both the product with its security functions and the processes of the company that has designed, built and engineered the product, and that will maintain the product across its lifecycle. As such, Common Criteria aims to be fairly holistic with its approach to take everything into account that is relevant for the security of an IT product.

2.2 Evaluation Assurance Level (EAL) Edit source

The Evaluation Assurance Level denotes the degree of confidence that the product fulfills the described claims. The levels are from 1 through 7:

  • EAL1: Functionally tested

  • EAL2: Structurally tested

  • EAL3: Methodically tested and checked

  • EAL4: Methodically designed, tested and reviewed

  • EAL5: Semi-formally designed and tested

  • EAL6: Semi-formally verified design and tested

  • EAL7: Formally verified design and tested

While EAL1 only provides basic assurance for products to meet security requirements, EAL2 to 4 are medium assurance levels. EAL5-EAL7 describe medium-to-high and high assurance. EAL4 is expected to be the highest level of assurance that a product can have if it has not been designed from the start to achieve a higher level of assurance.

2.3 Generic Guiding Principles Edit source

Much of the advice in this guide is based on the following guidelines. Consider them when defining your own security processes or deciding about configurations that are not explicitly covered here.

Use Data Encryption Whenever Possible

Be aware that cryptography is certainly useful, but only for the specific purposes that it is good for. Using cryptography is not a generic recipe for better security in a system, its use may even impose additional risk on the system. Make informed decisions about the use of cryptography, and feel obliged to have a reason for your decisions. A false sense of security can be more harmful than the weakness itself.

SUSE Linux Enterprise Server supports encryption for:

  • Network connections (the openssl command, stunnel), for remote login (openssh, man ssh(1))

  • Files (gpg)

  • Entire file systems at block layer (dm-crypt, cryptsetup)

  • VPN (ipsec, openvpn)

Minimal Package Installation

It is useful to restrict the installed packages in your system to a minimum. Binaries not installed cannot be executed.

During installation of the system, you can limit the set of packages that is installed. For example, you can deselect all packages and select only those that you want to use. For example, the selection of the apache2-mod_perl package in YaST would automatically cause all packages to be selected for installation that are needed for the Apache package to operate. Dependencies have often been artificially cut down to handle the system's dependency tree more flexibly. You can chose the minimal system, and build the dependency tree from there with your (leaf) package selection.

Service Isolation—Run Different Services on Separate Systems

Whenever possible, a server should be dedicated to serving exactly one service or application. This limits the number of other services that could be compromised if an attacker can successfully exploit a software flaw in one service (assuming that flaw allows access to others).

The use of AppArmor for services that are provided on a system is an effective means of containment. For more information, see Part IV, “Confining Privileges with AppArmor and the man page of apparmor.

The use of virtualization technology is supported with SUSE Linux Enterprise Server. While virtualization is generally designed for server consolidation purposes, it is also usefulness for service isolation. However, virtualization technology cannot match or substitute the separation strength that is given by running services on different physical machines! Be aware that the capability of the hypervisor to separate virtual machines is not higher or stronger than the Linux kernel's capability to separate processes and their address spaces.

System Fingerprinting and Backups

Doing regular backups and having a fingerprint of your system is vital, especially in the case of a successful attack against your system. Make it an integral part of your security routine to verify that your backups work.

A fast and directly accessible backup adds confidence about the integrity of your system. However, it is important that the backup mechanism/solution has adequate versioning support so that you can trace changes in the system. As an example: The installation times of packages (rpm -q --queryformat='%{INSTALLTIME} %{NAME}\n' PACKAGE NAME) must correspond to the changed files in the backup log files.

Several tools exist on SUSE Linux Enterprise Server 15 SP1 which can be used for the detection of unknown, yet successful attacks. It does not take much effort to configure them.

In particular, we recommend using the file and directory integrity checker AIDE (Advanced Intrusion Detection Environment). When run for initialization, it creates a hash database of all files in the system that are listed in its configuration file. This allows verifying the integrity of all cataloged files at a later time.

Warning
Warning: Backdoors

If you use AIDE, copy the hash database to a place that is inaccessible for potential attackers. Otherwise, the attacker may modify the integrity database after planting a backdoor, thereby defeating the purpose of the integrity measurement.

An attacker may also have planted a backdoor in the kernel. Apart from being very hard to detect, the kernel-based backdoor can effectively remove all traces of the system compromise, so system alterations become almost invisible. Consequently, an integrity check needs to be done from a rescue system (or any other independent system with the target system's file systems mounted manually).

Be aware that the application of security updates invalidates the integrity database. rpm -qlv packagename lists the files that are contained in a package. The RPM subsystem is very powerful with the data that it maintains. It is accessible with the --queryformat command line option. A differential update of integrity database with the changed files becomes more manageable with some fine-grained usage of RPM.

2.4 For More Information Edit source

The Common Criteria evaluations inspect a specific configuration of the product in an evaluated setup. How to install and configure the reference system that was used as baseline in the Common Criteria evaluation is documented in the Administrator's Guide part of the Common Criteria evaluation documentation.

However, it would be incorrect to understand the evaluated configuration as a hardened configuration. The removal of setuid bits and the prescription of administrative procedures after installation help to reach a specific configuration that is sane. But this is not sufficient for a hardening claim.

  • For more information about SUSE Linux Enterprise Server security certifications and features, see https://www.suse.com/support/security/certifications/.

  • Find a list of SUSE security resources at https://www.suse.com/support/security/.

  • Apart from the documentation that comes with the Common Criteria effort, see also the following manual pages:

    pam(8), pam(5)
    apparmor(7) and referred man pages
    rsyslogd(8), syslog(8), syslogd(8)
    fstab(5), mount(8), losetup(8), cryptsetup(8)
    haveged(8), random(4)
    ssh(1), sshd(8), ssh_config(5), sshd_config(5), ssh-agent(1), ssh-add(1), ssh-keygen(1)
    cron(1), crontab(5), at(1), atd(8)
    systemctl(1), daemon(7), systemd.unit(5), systemd.special(5), kernel-command-line(7), bootup(7), systemd.directives

Part I Authentication Edit source

3 Authentication with PAM

Linux uses PAM (pluggable authentication modules) in the authentication process as a layer that mediates between user and application. PAM modules are available on a system-wide basis, so they can be requested by any application. This chapter describes how the modular authentication mechanism works and how it is configured.

4 Using NIS

When multiple Unix systems in a network access common resources, it becomes imperative that all user and group identities are the same for all machines in that network. The network should be transparent to users: their environments should not vary, regardless of which machine they are actually using. This can be done by means of NIS and NFS services. NFS distributes file systems over a network and is discussed in Book “Administration Guide”, Chapter 34 “Sharing File Systems with NFS”.

NIS (Network Information Service) can be described as a database-like service that provides access to the contents of /etc/passwd, /etc/shadow, and /etc/group across networks. NIS can also be used for other purposes (making the contents of files like /etc/hosts or /etc/services available, for example), but this is beyond the scope of this introduction. People often refer to NIS as YP, because it works like the network's yellow pages.

5 Setting Up Authentication Clients Using YaST

Whereas Kerberos is used for authentication, LDAP is used for authorization and identification. Both can work together. For more information about LDAP, see Chapter 6, LDAP—A Directory Service, and about Kerberos, see Chapter 7, Network Authentication with Kerberos.

6 LDAP—A Directory Service

The Lightweight Directory Access Protocol (LDAP) is a protocol designed to access and maintain information directories. LDAP can be used for user and group management, system configuration management, address management, and more. This chapter provides a basic understanding of how LDAP works.

7 Network Authentication with Kerberos

Kerberos is a network authentication protocol which also provides encryption. This chapter describes how to set up Kerberos and integrate services like LDAP and NFS.

8 Active Directory Support

Active Directory* (AD) is a directory-service based on LDAP, Kerberos, and other services. It is used by Microsoft* Windows* to manage resources, services, and people. In a Microsoft Windows network, Active Directory provides information about these objects, restricts access to them, and enforces po…

9 Setting Up a FreeRADIUS Server

The RADIUS (Remote Authentication Dial-In User Service) protocol has long been a standard service for manage network access. It performs authentication, authorization, and accounting (AAA) protocol for very large businesses such as Internet service providers and cellular network providers, and is al…

3 Authentication with PAM Edit source

Linux uses PAM (pluggable authentication modules) in the authentication process as a layer that mediates between user and application. PAM modules are available on a system-wide basis, so they can be requested by any application. This chapter describes how the modular authentication mechanism works and how it is configured.

3.1 What is PAM? Edit source

System administrators and programmers often want to restrict access to certain parts of the system or to limit the use of certain functions of an application. Without PAM, applications must be adapted every time a new authentication mechanism, such as LDAP, Samba, or Kerberos, is introduced. However, this process is time-consuming and error-prone. One way to avoid these drawbacks is to separate applications from the authentication mechanism and delegate authentication to centrally managed modules. Whenever a newly required authentication scheme is needed, it is sufficient to adapt or write a suitable PAM module for use by the program in question.

The PAM concept consists of:

  • PAM modules, which are a set of shared libraries for a specific authentication mechanism.

  • A module stack with of one or more PAM modules.

  • A PAM-aware service which needs authentication by using a module stack or PAM modules. Usually a service is a familiar name of the corresponding application, like login or su. The service name other is a reserved word for default rules.

  • Module arguments, with which the execution of a single PAM module can be influenced.

  • A mechanism evaluating each result of a single PAM module execution. A positive value executes the next PAM module. The way a negative value is dealt with depends on the configuration: no influence, proceed up to terminate immediately and anything in between are valid options.

3.2 Structure of a PAM Configuration File Edit source

PAM can be configured in two ways:

File based configuration (/etc/pam.conf)

The configuration of each service is stored in /etc/pam.conf. However, for maintenance and usability reasons, this configuration scheme is not used in SUSE Linux Enterprise Server.

Directory based configuration (/etc/pam.d/)

Every service (or program) that relies on the PAM mechanism has its own configuration file in the /etc/pam.d/ directory. For example, the service for sshd can be found in the /etc/pam.d/sshd file.

The files under /etc/pam.d/ define the PAM modules used for authentication. Each file consists of lines, which define a service, and each line consists of a maximum of four components:

TYPE  CONTROL
 MODULE_PATH  MODULE_ARGS

The components have the following meaning:

TYPE

Declares the type of the service. PAM modules are processed as stacks. Different types of modules have different purposes. For example, one module checks the password, another verifies the location from which the system is accessed, and yet another reads user-specific settings. PAM knows about four different types of modules:

auth

Check the user's authenticity, traditionally by querying a password. However, this can also be achieved with a chip card or through biometrics (for example, fingerprints or iris scan).

account

Modules of this type check if the user has general permission to use the requested service. As an example, such a check should be performed to ensure that no one can log in with the user name of an expired account.

password

The purpose of this type of module is to enable the change of an authentication token. Usually this is a password.

session

Modules of this type are responsible for managing and configuring user sessions. They are started before and after authentication to log login attempts and configure the user's specific environment (mail accounts, home directory, system limits, etc.).

CONTROL

Indicates the behavior of a PAM module. Each module can have the following control flags:

required

A module with this flag must be successfully processed before the authentication may proceed. After the failure of a module with the required flag, all other modules with the same flag are processed before the user receives a message about the failure of the authentication attempt.

requisite

Modules having this flag must also be processed successfully, in much the same way as a module with the required flag. However, in case of failure a module with this flag gives immediate feedback to the user and no further modules are processed. In case of success, other modules are subsequently processed, like any modules with the required flag. The requisite flag can be used as a basic filter checking for the existence of certain conditions that are essential for a correct authentication.

sufficient

After a module with this flag has been successfully processed, the requesting application receives an immediate message about the success and no further modules are processed, provided there was no preceding failure of a module with the required flag. The failure of a module with the sufficient flag has no direct consequences, in the sense that any subsequent modules are processed in their respective order.

optional

The failure or success of a module with this flag does not have any direct consequences. This can be useful for modules that are only intended to display a message (for example, to tell the user that mail has arrived) without taking any further action.

include

If this flag is given, the file specified as argument is inserted at this place.

MODULE_PATH

Contains a full file name of a PAM module. It does not need to be specified explicitly, as long as the module is located in the default directory /lib/security (for all 64-bit platforms supported by SUSE® Linux Enterprise Server, the directory is /lib64/security).

MODULE_ARGS

Contains a space-separated list of options to influence the behavior of a PAM module, such as debug (enables debugging) or nullok (allows the use of empty passwords).

In addition, there are global configuration files for PAM modules under /etc/security, which define the exact behavior of these modules (examples include pam_env.conf and time.conf). Every application that uses a PAM module actually calls a set of PAM functions, which then process the information in the various configuration files and return the result to the requesting application.

To simplify the creation and maintenance of PAM modules, common default configuration files for the types auth, account, password, and session modules have been introduced. These are retrieved from every application's PAM configuration. Updates to the global PAM configuration modules in common-* are thus propagated across all PAM configuration files without requiring the administrator to update every single PAM configuration file.

The global PAM configuration files are maintained using the pam-config tool. This tool automatically adds new modules to the configuration, changes the configuration of existing ones or deletes modules (or options) from the configurations. Manual intervention in maintaining PAM configurations is minimized or no longer required.

Note
Note: 64-Bit and 32-Bit Mixed Installations

When using a 64-bit operating system, it is possible to also include a runtime environment for 32-bit applications. In this case, make sure that you also install the 32-bit version of the PAM modules.

3.3 The PAM Configuration of sshd Edit source

Consider the PAM configuration of sshd as an example:

Example 3.1: PAM Configuration for sshd (/etc/pam.d/sshd)
#%PAM-1.0 1
auth     requisite      pam_nologin.so                              2
auth     include        common-auth                                 3
account  requisite      pam_nologin.so                              2
account  include        common-account                              3
password include        common-password                             3
session  required       pam_loginuid.so                             4
session  include        common-session                              3
session  optional       pam_lastlog.so   silent noupdate showfailed 5

1

Declares the version of this configuration file for PAM 1.0. This is merely a convention, but could be used in the future to check the version.

2

Checks, if /etc/nologin exists. If it does, no user other than root may log in.

3

Refers to the configuration files of four module types: common-auth, common-account, common-password, and common-session. These four files hold the default configuration for each module type.

4

Sets the login UID process attribute for the process that was authenticated.

5

Displays information about the last login of a user.

By including the configuration files instead of adding each module separately to the respective PAM configuration, you automatically get an updated PAM configuration when an administrator changes the defaults. Formerly, you needed to adjust all configuration files manually for all applications when changes to PAM occurred or a new application was installed. Now the PAM configuration is made with central configuration files and all changes are automatically inherited by the PAM configuration of each service.

The first include file (common-auth) calls three modules of the auth type: pam_env.so, pam_gnome_keyring.so and pam_unix.so. See Example 3.2, “Default Configuration for the auth Section (common-auth)”.

Example 3.2: Default Configuration for the auth Section (common-auth)
auth  required  pam_env.so                   1
auth  optional  pam_gnome_keyring.so         2
auth  required  pam_unix.so  try_first_pass 3

1

pam_env.so loads /etc/security/pam_env.conf to set the environment variables as specified in this file. It can be used to set the DISPLAY variable to the correct value, because the pam_env module knows about the location from which the login is taking place.

2

pam_gnome_keyring.so checks the user's login and password against the GNOME key ring

3

pam_unix checks the user's login and password against /etc/passwd and /etc/shadow.

The whole stack of auth modules is processed before sshd gets any feedback about whether the login has succeeded. All modules of the stack having the required control flag must be processed successfully before sshd receives a message about the positive result. If one of the modules is not successful, the entire module stack is still processed and only then is sshd notified about the negative result.

When all modules of the auth type have been successfully processed, another include statement is processed, in this case, that in Example 3.3, “Default Configuration for the account Section (common-account)”. common-account contains only one module, pam_unix. If pam_unix returns the result that the user exists, sshd receives a message announcing this success and the next stack of modules (password) is processed, shown in Example 3.4, “Default Configuration for the password Section (common-password)”.

Example 3.3: Default Configuration for the account Section (common-account)
account  required  pam_unix.so  try_first_pass
Example 3.4: Default Configuration for the password Section (common-password)
password  requisite  pam_cracklib.so
password  optional   pam_gnome_keyring.so  use_authtok
password  required   pam_unix.so  use_authtok nullok shadow try_first_pass

Again, the PAM configuration of sshd involves only an include statement referring to the default configuration for password modules located in common-password. These modules must successfully be completed (control flags requisite and required) whenever the application requests the change of an authentication token.

Changing a password or another authentication token requires a security check. This is achieved with the pam_cracklib module. The pam_unix module used afterward carries over any old and new passwords from pam_cracklib, so the user does not need to authenticate again after changing the password. This procedure makes it impossible to circumvent the checks carried out by pam_cracklib. Whenever the account or the auth type are configured to complain about expired passwords, the password modules should also be used.

Example 3.5: Default Configuration for the session Section (common-session)
session  required  pam_limits.so
session  required  pam_unix.so  try_first_pass
session  optional  pam_umask.so
session  optional  pam_systemd.so
session  optional  pam_gnome_keyring.so auto_start only_if=gdm,gdm-password,lxdm,lightdm
session  optional  pam_env.so

As the final step, the modules of the session type (bundled in the common-session file) are called to configure the session according to the settings for the user in question. The pam_limits module loads the file /etc/security/limits.conf, which may define limits on the use of certain system resources. The pam_unix module is processed again. The pam_umask module can be used to set the file mode creation mask. Since this module carries the optional flag, a failure of this module would not affect the successful completion of the entire session module stack. The session modules are called a second time when the user logs out.

3.4 Configuration of PAM Modules Edit source

Some PAM modules are configurable. The configuration files are located in /etc/security. This section briefly describes the configuration files relevant to the sshd example—pam_env.conf and limits.conf.

3.4.1 pam_env.conf Edit source

pam_env.conf can be used to define a standardized environment for users that is set whenever the pam_env module is called. With it, preset environment variables using the following syntax:

VARIABLE  [DEFAULT=VALUE]  [OVERRIDE=VALUE]
VARIABLE

Name of the environment variable to set.

[DEFAULT=<value>]

Default VALUE the administrator wants to set.

[OVERRIDE=<value>]

Values that may be queried and set by pam_env, overriding the default value.

A typical example of how pam_env can be used is the adaptation of the DISPLAY variable, which is changed whenever a remote login takes place. This is shown in Example 3.6, “pam_env.conf”.

Example 3.6: pam_env.conf
REMOTEHOST  DEFAULT=localhost          OVERRIDE=@{PAM_RHOST}
DISPLAY     DEFAULT=${REMOTEHOST}:0.0  OVERRIDE=${DISPLAY}

The first line sets the value of the REMOTEHOST variable to localhost, which is used whenever pam_env cannot determine any other value. The DISPLAY variable in turn contains the value of REMOTEHOST. Find more information in the comments in /etc/security/pam_env.conf.

3.4.2 pam_mount.conf.xml Edit source

The purpose of pam_mount is to mount user home directories during the login process, and to unmount them during logout in an environment where a central file server keeps all the home directories of users. With this method, it is not necessary to mount a complete /home directory where all the user home directories would be accessible. Instead, only the home directory of the user who is about to log in, is mounted.

After installing pam_mount, a template for pam_mount.conf.xml is available in /etc/security. The description of the various elements can be found in the manual page man 5 pam_mount.conf.

A basic configuration of this feature can be done with YaST. Select Network Settings › Windows Domain Membership › Expert Settings to add the file server; see Book “Administration Guide”, Chapter 35 “Samba”, Section 35.5 “Configuring Clients”.

Note
Note: LUKS2 Support

LUKS2 support was added to cryptsetup 2.0, and SUSE Linux Enterprise Server has included support for LUKS2 in pam_mount since SUSE Linux Enterprise Server 12 SP3.

3.4.3 limits.conf Edit source

System limits can be set on a user or group basis in limits.conf, which is read by the pam_limits module. The file allows you to set hard limits, which may not be exceeded, and soft limits, which may be exceeded temporarily. For more information about the syntax and the options, see the comments in /etc/security/limits.conf.

3.5 Configuring PAM Using pam-config Edit source

The pam-config tool helps you configure the global PAM configuration files (/etc/pam.d/common-*) and several selected application configurations. For a list of supported modules, use the pam-config --list-modules command. Use the pam-config command to maintain your PAM configuration files. Add new modules to your PAM configurations, delete other modules or modify options to these modules. When changing global PAM configuration files, no manual tweaking of the PAM setup for individual applications is required.

A simple use case for pam-config involves the following:

  1. Auto-generate a fresh Unix-style PAM configuration.  Let pam-config create the simplest possible setup which you can extend later on. The pam-config --create command creates a simple Unix authentication configuration. Pre-existing configuration files not maintained by pam-config are overwritten, but backup copies are kept as *.pam-config-backup.

  2. Add a new authentication method.  Adding a new authentication method (for example, LDAP) to your stack of PAM modules comes down to a simple pam-config --add --ldap command. LDAP is added wherever appropriate across all common-*-pc PAM configuration files.

  3. Add debugging for test purposes.  To make sure the new authentication procedure works as planned, turn on debugging for all PAM-related operations. The pam-config --add --ldap-debug turns on debugging for LDAP-related PAM operations. Find the debugging output in the systemd journal (see Book “Administration Guide”, Chapter 17 “journalctl: Query the systemd Journal”).

  4. Query your setup.  Before you finally apply your new PAM setup, check if it contains all the options you wanted to add. The pam-config --query --MODULE command lists both the type and the options for the queried PAM module.

  5. Remove the debug options.  Finally, remove the debug option from your setup when you are entirely satisfied with the performance of it. The pam-config --delete --ldap-debug command turns off debugging for LDAP authentication. In case you had debugging options added for other modules, use similar commands to turn these off.

For more information on the pam-config command and the options available, refer to the manual page of pam-config(8).

3.6 Manually Configuring PAM Edit source

If you prefer to manually create or maintain your PAM configuration files, make sure to disable pam-config for these files.

When you create your PAM configuration files from scratch using the pam-config --create command, it creates symbolic links from the common-* to the common-*-pc files. pam-config only modifies the common-*-pc configuration files. Removing these symbolic links effectively disables pam-config, because pam-config only operates on the common-*-pc files and these files are not put into effect without the symbolic links.

Warning
Warning: Include pam_systemd.so in Configuration

If you are creating your own PAM configuration, make sure to include pam_systemd.so configured as session optional. Not including the pam_systemd.so can cause problems with systemd task limits. For details, refer to the man page of pam_systemd.so.

3.7 For More Information Edit source

In the /usr/share/doc/packages/pam directory after installing the pam-doc package, find the following additional documentation:

READMEs

In the top level of this directory, there is the modules subdirectory holding README files about the available PAM modules.

The Linux-PAM System Administrators' Guide

This document comprises everything that the system administrator should know about PAM. It discusses a range of topics, from the syntax of configuration files to the security aspects of PAM.

The Linux-PAM Module Writers' Manual

This document summarizes the topic from the developer's point of view, with information about how to write standard-compliant PAM modules.

The Linux-PAM Application Developers' Guide

This document comprises everything needed by an application developer who wants to use the PAM libraries.

The PAM Manual Pages

PAM in general and the individual modules come with manual pages that provide a good overview of the functionality of all the components.

4 Using NIS Edit source

When multiple Unix systems in a network access common resources, it becomes imperative that all user and group identities are the same for all machines in that network. The network should be transparent to users: their environments should not vary, regardless of which machine they are actually using. This can be done by means of NIS and NFS services. NFS distributes file systems over a network and is discussed in Book “Administration Guide”, Chapter 34 “Sharing File Systems with NFS”.

NIS (Network Information Service) can be described as a database-like service that provides access to the contents of /etc/passwd, /etc/shadow, and /etc/group across networks. NIS can also be used for other purposes (making the contents of files like /etc/hosts or /etc/services available, for example), but this is beyond the scope of this introduction. People often refer to NIS as YP, because it works like the network's yellow pages.

4.1 Configuring NIS Servers Edit source

To distribute NIS information across networks, either install one single server (a master) that serves all clients, or NIS slave servers requesting this information from the master and relaying it to their respective clients.

4.1.1 Configuring a NIS Master Server Edit source

To manage the NIS Server functionality with YaST, install the yast2-nis-server package by running the zypper in yast2-nis-server command as root. To configure a NIS master server for your network, proceed as follows:

  1. Start YaST › Network Services › NIS Server.

  2. If you need just one NIS server in your network or if this server is to act as the master for further NIS slave servers, select Install and Set Up NIS Master Server. YaST installs the required packages.

    Tip
    Tip: Already Installed NIS Server Software

    If NIS server software is already installed on your machine, initiate the creation of a NIS master server by clicking Create NIS Master Server.

    NIS Server Setup
    Figure 4.1: NIS Server Setup
  3. Determine basic NIS setup options:

    1. Enter the NIS domain name.

    2. Define whether the host should also be a NIS client (enabling users to log in and access data from the NIS server) by selecting This Host is also a NIS Client.

    3. If your NIS server needs to act as a master server to NIS slave servers in other subnets, select Active Slave NIS Server Exists.

      The option Fast Map Distribution is only useful with Active Slave NIS Servers Exist. It speeds up the transfer of maps to the slaves.

    4. Select Allow Changes to Passwords to allow users in your network (both local users and those managed through the NIS server) to change their passwords on the NIS server (with the command yppasswd). This makes the options Allow Changes to GECOS Field and Allow Changes to Login Shell available. GECOS means that the users can also change their names and address settings with the command ypchfn. Shell allows users to change their default shell with the command ypchsh (for example, to switch from Bash to sh). The new shell must be one of the predefined entries in /etc/shells.

    5. Select Open Port in Firewall to have YaST adapt the firewall settings for the NIS server.

      Master Server Setup
      Figure 4.2: Master Server Setup
    6. Leave this dialog with Next or click Other Global Settings to make additional settings.

      Other Global Settings include changing the source directory of the NIS server (/etc by default). In addition, passwords can be merged here. The setting should be Yes to create the user database from the system authentication files /etc/passwd, /etc/shadow, and /etc/group. Also, determine the smallest user and group ID that should be offered by NIS. Click OK to confirm your settings and return to the previous screen.

      Changing the Directory and Synchronizing Files for a NIS Server
      Figure 4.3: Changing the Directory and Synchronizing Files for a NIS Server
  4. If you previously enabled Active Slave NIS Server Exists, enter the host names used as slaves and click Next. If no slave servers exist, this configuration step is skipped.

  5. Continue to the dialog for the database configuration. Specify the NIS Server Maps, the partial databases to transfer from the NIS server to the client. The default settings are usually adequate. Leave this dialog with Next.

  6. Check which maps should be available and click Next to continue.

    NIS Server Maps Setup
    Figure 4.4: NIS Server Maps Setup
  7. Determine which hosts are allowed to query the NIS server. You can add, edit, or delete hosts by clicking the appropriate button. Specify from which networks requests can be sent to the NIS server. Normally, this is your internal network. In this case, there should be the following two entries:

    255.0.0.0     127.0.0.0
    0.0.0.0       0.0.0.0

    The first entry enables connections from your own host, which is the NIS server. The second one allows all hosts to send requests to the server.

    Setting Request Permissions for a NIS Server
    Figure 4.5: Setting Request Permissions for a NIS Server
  8. Click Finish to save your changes and exit the setup.

4.1.2 Configuring a NIS Slave Server Edit source

To configure additional NIS slave servers in your network, proceed as follows:

  1. Start YaST › Network Services › NIS Server.

  2. Select Install and Set Up NIS Slave Server and click Next.

    Tip
    Tip

    If NIS server software is already installed on your machine, initiate the creation of a NIS slave server by clicking Create NIS Slave Server.

  3. Complete the basic setup of your NIS slave server:

    1. Enter the NIS domain.

    2. Enter host name or IP address of the master server.

    3. Set This Host is also a NIS Client if you want to enable user logins on this server.

    4. Adapt the firewall settings with Open Ports in Firewall.

    5. Click Next.

  4. Enter the hosts that are allowed to query the NIS server. You can add, edit, or delete hosts by clicking the appropriate button. Specify all networks from which requests can be sent to the NIS server. If it applies to all networks, use the following configuration:

    255.0.0.0     127.0.0.0
    0.0.0.0       0.0.0.0

    The first entry enables connections from your own host, which is the NIS server. The second one allows all hosts with access to the same network to send requests to the server.

  5. Click Finish to save changes and exit the setup.

4.2 Configuring NIS Clients Edit source

To use NIS on a workstation, do the following:

  1. Start YaST › Network Services › NIS Client.

  2. Activate the Use NIS button.

  3. Enter the NIS domain. This is usually a domain name given by your administrator or a static IP address received by DHCP. For information about DHCP, see Book “Administration Guide”, Chapter 33 “DHCP”.

    Setting Domain and Address of a NIS Server
    Figure 4.6: Setting Domain and Address of a NIS Server
  4. Enter your NIS servers and separate their addresses by spaces. If you do not know your NIS server, click Find to let YaST search for any NIS servers in your domain. Depending on the size of your local network, this may be a time-consuming process. Broadcast asks for a NIS server in the local network after the specified servers fail to respond.

  5. Depending on your local installation, you may also want to activate the automounter. This option also installs additional software if required.

  6. If you do not want other hosts to be able to query which server your client is using, go to the Expert settings and disable Answer Remote Hosts. By checking Broken Server, the client is enabled to receive replies from a server communicating through an unprivileged port. For further information, see man ypbind.

  7. Click Finish to save them and return to the YaST control center. Your client is now configured with NIS.

5 Setting Up Authentication Clients Using YaST Edit source

Whereas Kerberos is used for authentication, LDAP is used for authorization and identification. Both can work together. For more information about LDAP, see Chapter 6, LDAP—A Directory Service, and about Kerberos, see Chapter 7, Network Authentication with Kerberos.

5.1 Configuring an Authentication Client with YaST Edit source

YaST allows setting up authentication to clients using different modules:

5.2 SSSD Edit source

Two of the YaST modules are based on SSSD: User Logon Management and LDAP and Kerberos Authentication.

SSSD stands for System Security Services Daemon. SSSD talks to remote directory services that provide user data and provides various authentication methods, such as LDAP, Kerberos, or Active Directory (AD). It also provides an NSS (Name Service Switch) and PAM (Pluggable Authentication Module) interface.

SSSD can locally cache user data and then allow users to use the data, even if the real directory service is (temporarily) unreachable.

5.2.1 Checking the Status Edit source

After running one of the YaST authentication modules, you can check whether SSSD is running with:

root # systemctl status sssd
sssd.service - System Security Services Daemon
   Loaded: loaded (/usr/lib/systemd/system/sssd.service; enabled)
   Active: active (running) since Thu 2015-10-23 11:03:43 CEST; 5s ago
   [...]

5.2.2 Caching Edit source

To allow logging in when the authentication back-end is unavailable, SSSD will use its cache even if it was invalidated. This happens until the back-end is available again.

To invalidate the cache, run sss_cache -E (the command sss_cache is part of the package sssd-tools).

To completely remove the SSSD cache, run:

tux > sudo systemctl stop sssd
tux > sudo rm -f /var/lib/sss/db/*
tux > sudo systemctl start sssd

6 LDAP—A Directory Service Edit source

The Lightweight Directory Access Protocol (LDAP) is a protocol designed to access and maintain information directories. LDAP can be used for user and group management, system configuration management, address management, and more. This chapter provides a basic understanding of how LDAP works.

Ideally, a central server stores the data in a directory and distributes it to all clients using a well-defined protocol. The structured data allow a wide range of applications to access them. A central repository reduces the necessary administrative effort. The use of an open and standardized protocol such as LDAP ensures that as many client applications as possible can access such information.

A directory in this context is a type of database optimized for quick and effective reading and searching. The type of data stored in a directory tends to be long lived and changes infrequently. This allows the LDAP service to be optimized for high performance concurrent reads, whereas conventional databases are optimized for accepting a large number of writes to data in a short time.

6.1 Structure of an LDAP Directory Tree Edit source

This section introduces the layout of an LDAP directory tree, and provides the basic terminology used with regard to LDAP. If you are familiar with LDAP, read on at Section 6.3, “Manually Configuring a 389 Directory Server”.

An LDAP directory has a tree structure. All entries (called objects) of the directory have a defined position within this hierarchy. This hierarchy is called the directory information tree (DIT). The complete path to the desired entry, which unambiguously identifies it, is called the distinguished name or DN. An object in the tree is identified by its relative distinguished name (RDN). The distinguished name is built from the RDNs of all entries on the path to the entry.

The relations within an LDAP directory tree become more evident in the following example, shown in Figure 6.1, “Structure of an LDAP Directory”.

Structure of an LDAP Directory
Figure 6.1: Structure of an LDAP Directory

The complete diagram is a fictional directory information tree. The entries on three levels are depicted. Each entry corresponds to one box in the image. The complete, valid distinguished name for the fictional employee Geeko Linux, in this case, is cn=Geeko Linux,ou=doc,dc=example,dc=com. It is composed by adding the RDN cn=Geeko Linux to the DN of the preceding entry ou=doc,dc=example,dc=com.

The types of objects that can be stored in the DIT are globally determined following a Schema. The type of an object is determined by the object class. The object class determines what attributes the relevant object must or may be assigned. The Schema contains all object classes and attributes which can be used by the LDAP server. Attributes are a structured data type. Their syntax, ordering and other behavior is defined by the Schema. LDAP servers supply a core set of Schemas which can work in a broad variety of environments. If a custom Schema is required, you can upload it to an LDAP server.

Table 6.1, “Commonly Used Object Classes and Attributes” offers a small overview of the object classes from 00core.ldif and 06inetorgperson.ldif used in the example, including required attributes (Req. Attr.) and valid attribute values. After installing 389-ds, these can be found in usr/share/dirsrv/schema.

Table 6.1: Commonly Used Object Classes and Attributes

Object Class

Meaning

Example Entry

Req. Attr.

domain

name components of the domain

example

displayName

organizationalUnit

organizational unit

doc

ou

nsPerson

person-related data for the intranet or Internet

Geeko Linux

cn

Example 6.1, “Excerpt from CN=schema” shows an excerpt from a Schema directive with explanations.

Example 6.1: Excerpt from CN=schema
attributetype (1.2.840.113556.1.2.102 NAME 'memberOf' 1
       DESC 'Group that the entry belongs to' 2
       SYNTAX 1.3.6.1.4.1.1466.115.121.1.12 3
       X-ORIGIN 'Netscape Delegated Administrator') 4

objectclass (2.16.840.1.113730.3.2.333 NAME 'nsPerson' 5
       DESC 'A representation of a person in a directory server' 6
       SUP top STRUCTURAL 7
       MUST ( displayName $ cn ) 8
       MAY ( userPassword $ seeAlso $ description $ legalName $ mail \
             $ preferredLanguage ) 9
       X-ORIGIN '389 Directory Server Project’
  ...

1

The name of the attribute, its unique object identifier (OID, numerical), and the abbreviation of the attribute.

2

A brief description of the attribute with DESC. The corresponding RFC, on which the definition is based, may also mentioned here.

3

The type of data that can be held in the attribute. In this case, it is a case-insensitive directory string.

4

The source of the schema element (for example, the name of the project).

5

The definition of the object class nsPerson begins with an OID and the name of the object class (like the definition of the attribute).

6

A brief description of the object class.

7

The SUP top entry indicates that this object class is not subordinate to another object class.

8

With MUST list all attribute types that must be used with an object of the type nsPerson.

9

With MAY list all attribute types that are optionally permitted with this object class.

6.2 Installing the Software for 389 Directory Server Edit source

The 389-ds package contains the 389 Directory Server and the administration tools. If the package is not installed yet, install it with the following command:

tux > sudo zypper install 389-ds

After installation, you can set up the server either manually (as described in Section 6.3) or create a very basic setup with YaST (as described in Section 6.4).

6.3 Manually Configuring a 389 Directory Server Edit source

Setting up the 389 Directory Server takes the following basic steps:

The 389 Directory Server is controlled by three primary commands:

dsctl

Manages a local instance and requires root permissions. Requires you to be connected to a terminal which is running the directory server instance. Used for starting, stopping, backing up the database, and more.

dsconf

The primary tool used for administration and configuration of the server. Manages an instance's configuration via its external interfaces. This allows you to make configuration changes remotely on the instance.

dsidm

Used for identity management (managing users, groups, passwords etc.). The permissions are granted by access controls, so, for example, users can reset their own password or change details of their own account.

6.3.1 Creating the 389 Directory Server Instance Edit source

You create the instance with the dscreate command. It can take a configuration file (*.inf) which defines the instance configuration settings. Alternatively, the command can be run interactively.

Note
Note: Instance Name

If not specified otherwise, the default instance name is localhost. The instance name cannot be changed after the instance has been created.

To check the name of an instance you have not created yourself, use the dsctl -l command.

Example 6.2 shows an example configuration file that you can use as a starting point. Alternatively, use dscreate create-template to create a template *.inf file. The template is commented and pre-filled, so you can adjust its variables to your needs. For more details, see the man page of dscreate.

  1. If you want to set up a trial instance, start an editor and save the following as /tmp/instance.inf:

    Example 6.2: Basic Instance Configuration File
    # /tmp/instance.inf
    [general]
    config_version = 2
    
    [slapd]
    root_password = YOUR_PASSWORD_FOR_CN=DIRECTORY_MANAGER1
    
    [backend-userroot]
    sample_entries = yes
    suffix = dc=example,dc=com

    1

    Set the root_password to the password for the directory server root user. The password is used for LDAP server administration only.

  2. To create the 389 Directory Server instance from Example 6.2, run:

    tux > sudo dscreate from-file /tmp/instance.inf

    This creates a working LDAP server.

  3. If dscreate should fail, the messages will tell you why. For more details, repeat the command with the -v option:

    tux > sudo dscreate -v from-file /tmp/instance.inf
  4. Check the status of the server with:

    tux > sudo dsctl localhost status
    instance 'Localhost' is running
  5. In case you want to delete the instance later on:

    tux > sudo dsctl localhost remove --do-it

    With this command, you can also remove partially installed or corrupted instances.

6.3.2 Using CA Certificates for TLS Edit source

You can manage the CA certificates for 389 Directory Server with the following command line tools: certutil, openssl, and pk12util.

For testing purposes, you can create a self-signed certificate with dscreate. Find the certificate at /etc/dirsrv/slapd-localhost/ca.crt. For remote administration, copy the certificate to a readable location. For production environments, contact a CA authority of your organization's choice and request a server certificate, a client certificate, and a root certificate.

Make sure to meet the following requirements before executing the procedure below:

  • You have a server certificate and a private key to use for the TLS connection.

  • You have set up an NSS (Network Security Services) database (for example, with the certutil command).

Before you can import an existing private key and certificate into the NSS (Network Security Services) database, you need to create a bundle of the private key and the server certificate. This results in a *.p12 file.

Important
Important: *.p12 File and Friendly Name

When creating the PKCS12 bundle, you must encode a friendly name in the *.p12 file.

Make sure to use Server-Cert as the friendly name. Otherwise the TLS connection will fail, because the 389 Directory Server searches for this exact string.

Keep in mind that the friendly name cannot be changed after you import the *.p12 file into the NSS database.

  1. Use the following command to create the PKCS12 bundle with the required friendly name:

    root # openssl pkcs12 -export -in SERVER.crt \
      -inkey SERVER.key -out SERVER.p12 \
      -name Server-Cert

    Replace SERVER.crt with the server certificate and SERVER.key with the private key to be bundled. With -out, specify the name of the *.p12 file. Use -name to set the friendly name to use, Server-Cert.

  2. Before you can import the file into the NSS database, you need to obtain its password. To do this, use the following command:

    pk12util -i PATH_TO_SERVER.p12 -d sql:PATH_TO_NSS_DB -n Server-cert -W SERVER.p12_PASSWORD

    You can then find the password in the pwdfile.txt file in the PATH_TO_NSS_DB directory.

  3. Now import the SERVER.p12 file, into your NSS database:

    pk12util -i SERVER.p12 -d PATH_TO_NSS_DB

6.3.3 Configuring Admin Credentials for Remote/Local Access Edit source

For remote or local administration of the 389 Directory Server, you can create a .dsrc configuration file in your home directory. This saves you typing your user name and connection details with every command. Example 6.3 shows an example configuration file for remote administration, whereas Example 6.4 shows one for local administration.

Example 6.3: A .dsrc File for Remote Administration
# cat ~/.dsrc
[localhost]
uri = ldaps://REMOTE_URI 1
basedn = dc=example,dc=com
binddn = cn=Directory Manager
tls_cacertdir = PATH_TO_CERTDIR 2

1

Needs to point to the LDAP server instance. If not specified otherwise, the default instance name is localhost.

2

Path to the certificate, at a readable location or on the client machine, from which you use the ds* commands.

If you want to administer the instance on the same host where the 389 Directory Server runs, use the configuration file in Example 6.4.

Example 6.4: A .dsrc File for Local Administration
# cat ~/.dsrc
[localhost]
# Note that '/' is replaced with '%%2f'.
uri = ldapi://%%2fvar%%2frun%%2fslapd-localhost.socket 1
basedn = dc=example,dc=com
binddn = cn=Directory Manager

1

When using ldapi on the server where the 389 Directory Server instance is running, your UID/GID will be detected. If it is 0/0 (which means you are logged in as root user), the ldapi binds the local root as the directory server root dn (cn=Directory Manager) of the instance. This allows local administration of the server, but also allows you to set a machine-generated password for cn=Directory Manager that no human knows. Whoever has administrator rights on the server hosting the 389 Directory Server instance can access the instance as cn=Directory Manager.

6.3.4 Configuring LDAP Users and Groups Edit source

Users and groups can be created and managed with the dsidm command. It either runs interactively or you can use it with arguments from the command line.

In the following example, we add two users, wilber and geeko, by specifying their data via command-line arguments.

Procedure 6.1: Creating LDAP Users
  1. Create the user wilber:

    tux > sudo dsidm localhost user create --uid wilber \
      --cn wilber --displayName 'Wilber Fox' --uidNumber 1000 --gidNumber 1000 \
      --homeDirectory /home/wilber
  2. To look up a user's distinguished name (fully qualified name to the directory object, which is guaranteed unique):

    tux > sudo dsidm localhost user get wilber
    dn: uid=wilber,ou=people,dc=example,dc=com
    [...]

    The system prompts you for the directory server root user password (unless you configured remote or local access as described in Section 6.3.3, “Configuring Admin Credentials for Remote/Local Access”).

    You need the distinguished name for actions such as changing the password for a user.

  3. To set or change the password for wilber:

    1. tux > sudo dsidm localhost account reset_password \
        uid=wilber,ou=people,dc=example,dc=com

      The system prompts you for the directory server root user password (unless you configured remote or local access as described in Section 6.3.3, “Configuring Admin Credentials for Remote/Local Access”).

    2. Enter the new password for wilber twice.

      If the action was successful, you get the following message:

      reset password for uid=wilber,ou=people,dc=example,dc=com
  4. Create the user geeko:

    tux > sudo dsidm localhost user create --uid \
      --cn geeko --displayName 'Suzanne Geeko' \
      --uidNumber 1001 --gidNumber 1001 --homeDirectory /home/geeko
Procedure 6.2: Creating LDAP Groups and Assigning Users to Them

In the following, we create a group, server_admins, and assign the user wilber to this group.

  1. Create the group:

    tux > sudo dsidm localhost group create

    You will be prompted for a group name:

    Enter value for cn :
  2. Enter the name for the group, for example: server_admins.

  3. Add the user wilber to the group:

    tux > sudo dsidm localhost group add_member server_admins uid=wilber,ou=people,dc=example,dc=com
    added member: uid=wilber,ou=people,dc=example,dc=com
  4. Verify if authentication works:

    tux > sudo ldapwhoami -H ldaps://localhost -D \
      uid=wilber,ou=people,dc=example,dc=com -W -x

    If you are prompted for the LDAP password of wilber, authentication works.

    If the command fails with the following error, you are probably using a self-signed certificate:

    ldap_sasl_bind(SIMPLE): Can't contact LDAP server (-1)

    In that case, edit /etc/openldap/ldap.conf and add the path to the certificate. For example:

    TLS_CACERT /etc/dirsrv/slapd-localhost/ca.crt

    Alternatively, include the path to the certificate in the whoami command:

    tux > sudo LDAPTLS_CACERT=/etc/dirsrv/slapd-localhost/ca.crt \
      ldapwhoami -H ldaps://localhost -D \
      uid=wilber,ou=people,dc=example,dc=com -W -x

6.3.5 Setting Up SSSD Edit source

SSSD (System Security Services Daemon) is a daemon that communicates with remote identity providers and allows pam and nsswitch to consume that data. SSSD can have multiple back-ends, cache users and groups and provides features like SSH key distributions.

  1. On a separate server, install the sssd package:

    tux > sudo zypper in sssd
  2. Disable and stop the nscd daemon because it conflicts with sssd:

    tux > sudo systemctl disable nscd && systemctl stop nscd
  3. Create the SSSD configuration and restrict the login to the members of the group server_admins that we created in Procedure 6.2:

    Tip
    Tip

    The memberOf plugin needs to be enabled, so that clients can log in and authorise.

    tux > sudo dsidm localhost client_config sssd.conf server_admins
  4. Review the output and paste (or redirect) it to /etc/sssd/sssd.conf. If required, edit the configuration file according to your needs.

  5. To configure the certificates on your client, copy ca.crt from the LDAP server to your client:

    tux > sudo mkdir -p /etc/openldap/certs
    cp [...]>/ca.crt /etc/openldap/certs/
    /usr/bin/c_rehash /etc/openldap/certs
  6. Enable and start SSSD:

    tux > sudo systemctl enable sssd
    systemctl start sssd
  7. To make sure SSSD is part of PAM and NSS, follow the instructions in sections Configure PAM (SUSE) and Configure NSS (SUSE) at https://www.port389.org/docs/389ds/howto/howto-sssd.html.

  8. Verify if the client can provide the details for user wilber:

    tux > sudo id wilber
        uid=1000(wilber) gid=100(users) groups=100(users)

    If everything is set up correctly, wilber can access the 389 Directory Server instance via SSH to the machine where you have installed and configured SSSD. However, geeko will fail to do so, because geeko does not belong to the group server_admins that we have configured in Procedure 6.2.

6.4 Setting Up a 389 Directory Server with YaST Edit source

You can use YaST to quickly create a very basic setup of the 389 Directory Server.

6.4.1 Creating an 389 Directory Server Instance with YaST Edit source

  1. In YaST, click Network Services › Create New Directory Server. Alternatively, start the module from command line with yast2 ldap-server.

    In the window that opens, you need to fill in all mandatory text fields.

  2. Enter the Fully qualified domain name of the 389 Directory Server. It must be resolvable from the host.

  3. In Directory server instance name, enter a local name for the LDAP server instance.

    Note
    Note: Instance Name

    The instance name cannot be changed after the instance has been created. If you plan for only one LDAP server, use the default instance name localhost. However, if you plan to host multiple LDAP servers, use meaningful names for the individual instances.

  4. In Directory suffix, enter the base domain name of the LDAP tree. It is your domain name split by component. For example, example.com becomes dc=example,dc=com.

  5. In the mandatory security options, enter the password for the directory manager (LDAP's root/admin account) and repeat the password in the next step. The password must be at least 8 characters long.

  6. To run 389 Directory Server with a CA certificate, specify both of the following options:

    1. Enter the path to the Server TLS certificate authority in PEM format, with which the server certificates have been signed.

    2. Enter the path to the Server TLS certificate and key in PKCS12 format with friendly name "Server-Cert". The *.p12 file contains the server's private key and certificate. These must have been signed by the CA in PEM format that you have specified above. The friendly name must be Server-Cert, see Section 6.3.2, “Using CA Certificates for TLS” for details.

      If you do not specify a CA certificate here, a self-signed certificate will be created automatically. After the instance has been created, find the related files in /etc/dirsrv/slapd-INSTANCENAME.

  7. If you are ready to create the instance, click OK.

    YaST displays a message whether the creation was successful and where to find the log files.

The setup with YaST provides only a very basic configuration of the 389 Directory Server. To fine-tune more settings, see Section 6.3, “Manually Configuring a 389 Directory Server” or the documentation mentioned in Section 6.6, “For More Information”.

6.4.2 Configuring an LDAP Client with YaST Edit source

YaST includes the module LDAP and Kerberos Client that helps define authentication scenarios involving either LDAP or Kerberos.

It can also be used to join Kerberos and LDAP separately. However, in many such cases, using this module may not be the first choice, such as for joining Active Directory (which uses a combination of LDAP and Kerberos). For more information, see Section 5.1, “Configuring an Authentication Client with YaST”.

Start the module by selecting Network Services › LDAP and Kerberos Client.

LDAP and Kerberos Client Window
Figure 6.2: LDAP and Kerberos Client Window

To configure an LDAP client, follow the procedure below:

  1. In the window LDAP and Kerberos Client, click Change Settings.

    Make sure that the tab Use a Directory as Identity Provider (LDAP) is chosen.

  2. Specify one or more LDAP server URLs or host names under Enter LDAP server locations. For security reasons, we recommend to use LDAPS:// URLs only. When specifying multiple addresses, separate them with spaces.

  3. Specify the appropriate LDAP distinguished name (DN) under DN of Search Base. For example, a valid entry could be dc=example,dc=com.

  4. If your LDAP server supports TLS encryption, choose the appropriate security option under Secure LDAP Connection.

    To first ask the server whether it supports TLS encryption and be able to downgrade to an unencrypted connection if it does not, use Secure Communication via StartTLS.

  5. Activate other options as necessary:

    • You can Allow users to authenticate via LDAP and Automatically Create Home Directories on the local computer for them.

    • If you want to cache LDAP entries locally, use Cache LDAP Entries For Faster Response.

      Warning
      Warning: Potential Security Risk with Caching

      Using the cache incurs security risks, depending on the mechanism used.

      nscd

      If you define an authorization rule (for example, members of group admin can log in), and you remove a user from that group, the client cache will not see that change until the cache expires or refreshes. So a user whose account has been revoked can still log in for a period of time later.

      sssd

      This caching mechanism constantly checks if group memberships are still valid. Thus the cache risk only exists if the sssd daemon is disconnected from the LDAP server for any reason.

    • Specify the types of data that should be used from the LDAP source, such as Users and Groups, Super-User Commands, and Network Disk Locations (network-shared drives that can be automatically mounted on request).

    • Specify the distinguished name (DN) and password of the user under whose name you want to bind to the LDAP directory in DN of Bind User and Password of the Bind User.

      Otherwise, if the server supports it, you can also leave both text boxes empty to bind anonymously to the server.

      Warning
      Warning: Authentication Without Encryption

      When using authentication without enabling transport encryption using TLS or StartTLS, the password will be transmitted in the clear.

    Under Extended Options, you can additionally configure timeouts for BIND operations.

  6. To check whether the LDAP connection works, click Test Connection.

  7. To leave the dialog, click OK. Then wait for the setup to complete.

    Finally, click Finish.

6.5 Manually Administering LDAP Data Edit source

The command line tools provided by the openldap2-client package (like ldapsearch or ldapmodify) can be used for administration of data in the LDAP directory. However, they are low-level tools and hard to use. For details about their use, refer to the respective man pages and documentation.

6.6 For More Information Edit source

For more information about 389 Directory Server, see the upstream documentation, available at https://www.port389.org/docs/389ds/documentation.html.

7 Network Authentication with Kerberos Edit source

Kerberos is a network authentication protocol which also provides encryption. This chapter describes how to set up Kerberos and integrate services like LDAP and NFS.

7.1 Conceptual Overview Edit source

An open network provides no means of ensuring that a workstation can identify its users properly, except through the usual password mechanisms. In common installations, the user must enter the password each time a service inside the network is accessed. Kerberos provides an authentication method with which a user registers only once and is trusted in the complete network for the rest of the session. To have a secure network, the following requirements must be met:

  • Have all users prove their identity for each desired service and make sure that no one can take the identity of someone else.

  • Make sure that each network server also proves its identity. Otherwise an attacker might be able to impersonate the server and obtain sensitive information transmitted to the server. This concept is called mutual authentication, because the client authenticates to the server and vice versa.

Kerberos helps you meet these requirements by providing strongly encrypted authentication. Only the basic principles of Kerberos are discussed here. For detailed technical instruction, refer to the Kerberos documentation.

7.2 Kerberos Terminology Edit source

The following glossary defines some Kerberos terminology.

credential

Users or clients need to present some kind of credentials that authorize them to request services. Kerberos knows two kinds of credentials—tickets and authenticators.

ticket

A ticket is a per-server credential used by a client to authenticate at a server from which it is requesting a service. It contains the name of the server, the client's name, the client's Internet address, a time stamp, a lifetime, and a random session key. All this data is encrypted using the server's key.

authenticator

Combined with the ticket, an authenticator is used to prove that the client presenting a ticket is really the one it claims to be. An authenticator is built using the client's name, the workstation's IP address, and the current workstation's time, all encrypted with the session key known only to the client and the relevant server. An authenticator can only be used once, unlike a ticket. A client can build an authenticator itself.

principal

A Kerberos principal is a unique entity (a user or service) to which it can assign a ticket. A principal consists of the following components:

USER/INSTANCE@REALM
  • primary:  The first part of the principal. In the case of users, this is usually the same as the user name.

  • instance (optional) Additional information characterizing the primary. This string is separated from the primary by a /.

    tux@example.org and tux/admin@example.org can both exist on the same Kerberos system and are treated as different principals.

  • realm:  Specifies the Kerberos realm. Normally, your realm is your domain name in uppercase letters.

mutual authentication

Kerberos ensures that both client and server can be sure of each other's identity. They share a session key, which they can use to communicate securely.

session key

Session keys are temporary private keys generated by Kerberos. They are known to the client and used to encrypt the communication between the client and the server for which it requested and received a ticket.

replay

Almost all messages sent in a network can be eavesdropped, stolen, and resent. In the Kerberos context, this would be most dangerous if an attacker manages to obtain your request for a service containing your ticket and authenticator. The attacker could then try to resend it (replay) to impersonate you. However, Kerberos implements several mechanisms to deal with this problem.

server or service

Service is used to refer to a specific action to perform. The process behind this action is called a server.

7.3 How Kerberos Works Edit source

Kerberos is often called a third-party trusted authentication service, which means all its clients trust Kerberos's judgment of another client's identity. Kerberos keeps a database of all its users and their private keys.

To ensure Kerberos is working correctly, run both the authentication and ticket-granting server on a dedicated machine. Make sure that only the administrator can access this machine physically and over the network. Reduce the (networking) services running on it to the absolute minimum—do not even run sshd.

7.3.1 First Contact Edit source

Your first contact with Kerberos is quite similar to any login procedure at a normal networking system. Enter your user name. This piece of information and the name of the ticket-granting service are sent to the authentication server (Kerberos). If the authentication server knows you, it generates a random session key for further use between your client and the ticket-granting server. Now the authentication server prepares a ticket for the ticket-granting server. The ticket contains the following information—all encrypted with a session key only the authentication server and the ticket-granting server know:

  • The names of both, the client and the ticket-granting server

  • The current time

  • A lifetime assigned to this ticket

  • The client's IP address

  • The newly-generated session key

This ticket is then sent back to the client together with the session key, again in encrypted form, but this time the private key of the client is used. This private key is only known to Kerberos and the client, because it is derived from your user password. Now that the client has received this response, you are prompted for your password. This password is converted into the key that can decrypt the package sent by the authentication server. The package is unwrapped and password and key are erased from the workstation's memory. As long as the lifetime given to the ticket used to obtain other tickets does not expire, your workstation can prove your identity.

7.3.2 Requesting a Service Edit source

To request a service from any server in the network, the client application needs to prove its identity to the server. Therefore, the application generates an authenticator. An authenticator consists of the following components:

  • The client's principal

  • The client's IP address

  • The current time

  • A checksum (chosen by the client)

All this information is encrypted using the session key that the client has already received for this special server. The authenticator and the ticket for the server are sent to the server. The server uses its copy of the session key to decrypt the authenticator, which gives it all the information needed about the client requesting its service, to compare it to that contained in the ticket. The server checks if the ticket and the authenticator originate from the same client.

Without any security measures implemented on the server side, this stage of the process would be an ideal target for replay attacks. Someone could try to resend a request stolen off the net some time before. To prevent this, the server does not accept any request with a time stamp and ticket received previously. In addition to that, a request with a time stamp differing too much from the time the request is received is ignored.

7.3.3 Mutual Authentication Edit source

Kerberos authentication can be used in both directions. It is not only a question of the client being the one it claims to be. The server should also be able to authenticate itself to the client requesting its service. Therefore, it sends an authenticator itself. It adds one to the checksum it received in the client's authenticator and encrypts it with the session key, which is shared between it and the client. The client takes this response as a proof of the server's authenticity and they both start cooperating.

7.3.4 Ticket Granting—Contacting All Servers Edit source

Tickets are designed to be used for one server at a time. Therefore, you need to get a new ticket each time you request another service. Kerberos implements a mechanism to obtain tickets for individual servers. This service is called the ticket-granting service. The ticket-granting service is a service (like any other service mentioned before) and uses the same access protocols that have already been outlined. Any time an application needs a ticket that has not already been requested, it contacts the ticket-granting server. This request consists of the following components:

  • The requested principal

  • The ticket-granting ticket

  • An authenticator

Like any other server, the ticket-granting server now checks the ticket-granting ticket and the authenticator. If they are considered valid, the ticket-granting server builds a new session key to be used between the original client and the new server. Then the ticket for the new server is built, containing the following information:

  • The client's principal

  • The server's principal

  • The current time

  • The client's IP address

  • The newly-generated session key

The new ticket has a lifetime, which is either the remaining lifetime of the ticket-granting ticket or the default for the service. The lesser of both values is assigned. The client receives this ticket and the session key, which are sent by the ticket-granting service. But this time the answer is encrypted with the session key that came with the original ticket-granting ticket. The client can decrypt the response without requiring the user's password when a new service is contacted. Kerberos can thus acquire ticket after ticket for the client without bothering the user.

7.4 User View of Kerberos Edit source

Ideally, a user only contact with Kerberos happens during login at the workstation. The login process includes obtaining a ticket-granting ticket. At logout, a user's Kerberos tickets are automatically destroyed, which makes it difficult for anyone else to impersonate this user.

The automatic expiration of tickets can lead to a situation when a user's login session lasts longer than the maximum lifespan given to the ticket-granting ticket (a reasonable setting is 10 hours). However, the user can get a new ticket-granting ticket by running kinit. Enter the password again and Kerberos obtains access to desired services without additional authentication. To get a list of all the tickets silently acquired for you by Kerberos, run klist.

Here is a short list of applications that use Kerberos authentication. These applications can be found under /usr/lib/mit/bin or /usr/lib/mit/sbin after installing the package krb5-apps-clients. They all have the full functionality of their common Unix and Linux brothers plus the additional bonus of transparent authentication managed by Kerberos:

  • telnet, telnetd

  • rlogin

  • rsh, rcp, rshd

  • ftp, ftpd

  • ksu

You no longer need to enter your password for using these applications because Kerberos has already proven your identity. ssh, if compiled with Kerberos support, can even forward all the tickets acquired for one workstation to another one. If you use ssh to log in to another workstation, ssh makes sure that the encrypted contents of the tickets are adjusted to the new situation. Simply copying tickets between workstations is not sufficient because the ticket contains workstation-specific information (the IP address). XDM and GDM offer Kerberos support, too. Read more about the Kerberos network applications in Kerberos V5 UNIX User's Guide at https://web.mit.edu/kerberos.

7.5 Installing and Administering Kerberos Edit source

A Kerberos environment consists of several components. A key distribution center (KDC) holds the central database with all Kerberos-relevant data. All clients rely on the KDC for proper authentication across the network. Both the KDC and the clients need to be configured to match your setup:

General Preparations

Check your network setup and make sure it meets the minimum requirements outlined in Section 7.5.1, “Kerberos Network Topology”. Choose an appropriate realm for your Kerberos setup, see Section 7.5.2, “Choosing the Kerberos Realms”. Carefully set up the machine that is to serve as the KDC and apply tight security, see Section 7.5.3, “Setting Up the KDC Hardware”. Set up a reliable time source in your network to make sure all tickets contain valid time stamps, see Section 7.5.4, “Configuring Time Synchronization”.

Basic Configuration

Configure the KDC and the clients, see Section 7.5.5, “Configuring the KDC” and Section 7.5.6, “Configuring Kerberos Clients”. Enable remote administration for your Kerberos service, so you do not need physical access to your KDC machine, see Section 7.5.7, “Configuring Remote Kerberos Administration”. Create service principals for every service in your realm, see Section 7.5.8, “Creating Kerberos Service Principals”.

Enabling Kerberos Authentication

Various services in your network can use Kerberos. To add Kerberos password-checking to applications using PAM, proceed as outlined in Section 7.5.9, “Enabling PAM Support for Kerberos”. To configure SSH or LDAP with Kerberos authentication, proceed as outlined in Section 7.5.10, “Configuring SSH for Kerberos Authentication” and Section 7.5.11, “Using LDAP and Kerberos”.

7.5.1 Kerberos Network Topology Edit source

Any Kerberos environment must meet the following requirements to be fully functional:

  • Provide a DNS server for name resolution across your network, so clients and servers can locate each other. Refer to Book “Administration Guide”, Chapter 32 “The Domain Name System” for information on DNS setup.

  • Provide a time server in your network. Using exact time stamps is crucial to a Kerberos setup, because valid Kerberos tickets must contain correct time stamps. Refer to Book “Administration Guide”, Chapter 31 “Time Synchronization with NTP” for information on NTP setup.

  • Provide a key distribution center (KDC) as the center piece of the Kerberos architecture. It holds the Kerberos database. Use the tightest possible security policy on this machine to prevent any attacks on this machine compromising your entire infrastructure.

  • Configure the client machines to use Kerberos authentication.

The following figure depicts a simple example network with only the minimum components needed to build a Kerberos infrastructure. Depending on the size and topology of your deployment, your setup may vary.

Kerberos Network Topology
Figure 7.1: Kerberos Network Topology
Tip
Tip: Configuring Subnet Routing

For a setup similar to the one in Figure 7.1, “Kerberos Network Topology”, configure routing between the two subnets (192.168.1.0/24 and 192.168.2.0/24). Refer to Book “Administration Guide”, Chapter 19 “Basic Networking”, Section 19.4.1.5 “Configuring Routing” for more information on configuring routing with YaST.

7.5.2 Choosing the Kerberos Realms Edit source

The domain of a Kerberos installation is called a realm and is identified by a name, such as EXAMPLE.COM or simply ACCOUNTING. Kerberos is case-sensitive, so example.com is actually a different realm than EXAMPLE.COM. Use the case you prefer. It is common practice, however, to use uppercase realm names.

It is also a good idea to use your DNS domain name (or a subdomain, such as ACCOUNTING.EXAMPLE.COM). As shown below, your life as an administrator can be much easier if you configure your Kerberos clients to locate the KDC and other Kerberos services via DNS. To do so, it is helpful if your realm name is a subdomain of your DNS domain name.

Unlike the DNS name space, Kerberos is not hierarchical. So if you have a realm named EXAMPLE.COM with two subrealms named DEVELOPMENT and ACCOUNTING, these subordinate realms do not inherit principals from EXAMPLE.COM. Instead, you would have three separate realms, and you would need to configure cross-realm authentication for each realm, so that users from one realm can interact with servers or other users from another realm.

For the sake of simplicity, let us assume you are setting up only one realm for your entire organization. For the remainder of this section, the realm name EXAMPLE.COM is used in all examples.

7.5.3 Setting Up the KDC Hardware Edit source

The first thing required to use Kerberos is a machine that acts as the key distribution center, or KDC for short. This machine holds the entire Kerberos user database with passwords and all information.

The KDC is the most important part of your security infrastructure—if someone breaks into it, all user accounts and all of your infrastructure protected by Kerberos is compromised. An attacker with access to the Kerberos database can impersonate any principal in the database. Tighten security for this machine as much as possible:

  1. Put the server machine into a physically secured location, such as a locked server room to which only a very few people have access.

  2. Do not run any network applications on it except the KDC. This includes servers and clients—for example, the KDC should not import any file systems via NFS or use DHCP to retrieve its network configuration.

  3. Install a minimal system first then check the list of installed packages and remove any unneeded packages. This includes servers, such as inetd, portmap, and CUPS, plus anything X-based. Even installing an SSH server should be considered a potential security risk.

  4. No graphical login is provided on this machine as an X server is a potential security risk. Kerberos provides its own administration interface.

  5. Configure /etc/nsswitch.conf to use only local files for user and group lookup. Change the lines for passwd and group to look like this:

    passwd:         files
    group:          files

    Edit the passwd, group, and shadow files in /etc and remove the lines that start with a + character (these are for NIS lookups).

  6. Disable all user accounts except root's account by editing /etc/shadow and replacing the hashed passwords with * or ! characters.

7.5.4 Configuring Time Synchronization Edit source

To use Kerberos successfully, make sure that all system clocks within your organization are synchronized within a certain range. This is important because Kerberos protects against replayed credentials. An attacker might be able to observe Kerberos credentials on the network and reuse them to attack the server. Kerberos employs several defenses to prevent this. One of them is that it puts time stamps into its tickets. A server receiving a ticket with a time stamp that differs from the current time rejects the ticket.

Kerberos allows a certain leeway when comparing time stamps. However, computer clocks can be very inaccurate in keeping time—it is not unheard of for PC clocks to lose or gain half an hour during a week. For this reason, configure all hosts on the network to synchronize their clocks with a central time source.

A simple way to do so is by installing an NTP time server on one machine and having all clients synchronize their clocks with this server. Do this by running an NTP daemon chronyd as a client on all these machines. The KDC itself needs to be synchronized to the common time source as well. Because running an NTP daemon on this machine would be a security risk, it is probably a good idea to do this by running chronyd -q via a cron job. To configure your machine as an NTP client, proceed as outlined in Book “Administration Guide”, Chapter 31 “Time Synchronization with NTP”, Section 31.1 “Configuring an NTP Client with YaST”.

A different way to secure the time service and still use the NTP daemon is to attach a hardware reference clock to a dedicated NTP server and an additional hardware reference clock to the KDC.

It is also possible to adjust the maximum deviation Kerberos allows when checking time stamps. This value (called clock skew) can be set in the krb5.conf file as described in Section 7.5.6.3, “Adjusting the Clock Skew”.

7.5.5 Configuring the KDC Edit source

This section covers the initial configuration and installation of the KDC, including the creation of an administrative principal. This procedure consists of several steps:

  1. Install the RPMs.  On a machine designated as the KDC, install the following software packages: krb5, krb5-server and krb5-client packages.

  2. Adjust the Configuration Files.  The /etc/krb5.conf and /var/lib/kerberos/krb5kdc/kdc.conf configuration files must be adjusted for your scenario. These files contain all information on the KDC. See Section 7.5.5.1, “Configuring the Server”.

  3. Create the Kerberos Database.  Kerberos keeps a database of all principal identifiers and the secret keys of all principals that need to be authenticated. Refer to Section 7.5.5.2, “Setting Up the Database” for details.

  4. Adjust the ACL Files: Add Administrators.  The Kerberos database on the KDC can be managed remotely. To prevent unauthorized principals from tampering with the database, Kerberos uses access control lists. You must explicitly enable remote access for the administrator principal to enable them to manage the database. The Kerberos ACL file is located under /var/lib/kerberos/krb5kdc/kadm5.acl. Refer to Section 7.5.7, “Configuring Remote Kerberos Administration” for details.

  5. Adjust the Kerberos Database: Add Administrators.  You need at least one administrative principal to run and administer Kerberos. This principal must be added before starting the KDC. Refer to Section 7.5.5.3, “Creating a Principal” for details.

  6. Start the Kerberos Daemon.  After the KDC software is installed and properly configured, start the Kerberos daemon to provide Kerberos service for your realm. Refer to Section 7.5.5.4, “Starting the KDC” for details.

  7. Create a Principal for Yourself.  You need a principal for yourself. Refer to Section 7.5.5.3, “Creating a Principal” for details.

7.5.5.1 Configuring the Server Edit source

Configuring a Kerberos server is highly variable, dependent on your network architecture, DNS and DHCP configuration, realms, and many other considerations. You must have a default realm, and domain- to-realm mappings. The following example demonstrates a minimal configuration. This is not a copy-and-paste example; see https://web.mit.edu/kerberos/krb5-latest/doc/admin/conf_files/index.html for detailed information on Kerberos configuration.

Example 7.1: Example KDC Configuration, /etc/krb5.conf
[libdefaults]
 dns_canonicalize_hostname = false
 rdns = false
 default_realm = example.com
 ticket_lifetime = 24h
 renew_lifetime = 7d

[realms]
  example.com = {
  kdc = kdc.example.com.:88
  admin_server = kdc.example.com
  default_domain = example.com
 }
 
 [logging]
 kdc = FILE:/var/log/krb5kdc.log
 admin_server = FILE:/var/log/kadmind.log
 default = SYSLOG:NOTICE:DAEMON

[domain_realm]
 .example.com = example.com
 example.com = example.com

7.5.5.2 Setting Up the Database Edit source

Your next step is to initialize the database where Kerberos keeps all information about principals. Set up the database master key, which is used to protect the database from accidental disclosure (in particular if it is backed up to tape). The master key is derived from a pass phrase and is stored in a file called the stash file. This is so you do not need to enter the password every time the KDC is restarted. Make sure that you choose a good pass phrase, such as a sentence from a book opened to a random page.

When you make tape backups of the Kerberos database (/var/lib/kerberos/krb5kdc/principal), do not back up the stash file (which is in /var/lib/kerberos/krb5kdc/.k5.EXAMPLE.COM). Otherwise, everyone able to read the tape could also decrypt the database. Therefore, keep a copy of the pass phrase in a safe or some other secure location, because you will need it to restore your database from backup tape after a crash.

To create the stash file and the database, run:

tux > sudo kdb5_util create -r EXAMPLE.COM -s

You will see the following output:

Initializing database '/var/lib/kerberos/krb5kdc/principal' for realm 'EXAMPLE.COM',
master key name 'K/M@EXAMPLE.COM'
You will be prompted for the database Master Password.
It is important that you NOT FORGET this password.
Enter KDC database master key:  1
Re-enter KDC database master key to verify:  2

1

Type the master password.

2

Type the password again.

To verify, use the list command:

tux > kadmin.local

kadmin> listprincs

You will see several principals in the database, which are for internal use by Kerberos:

K/M@EXAMPLE.COM
kadmin/admin@EXAMPLE.COM
kadmin/changepw@EXAMPLE.COM
krbtgt/EXAMPLE.COM@EXAMPLE.COM

7.5.5.3 Creating a Principal Edit source

Create two Kerberos principals for yourself: one normal principal for everyday work and one for administrative tasks relating to Kerberos. Assuming your login name is geeko, proceed as follows:

tux > kadmin.local

kadmin> ank geeko

You will see the following output:

geeko@EXAMPLE.COM's Password: 1
Verifying password: 2

1

Type geeko's password.

1

Type geeko's password again.

Next, create another principal named geeko/admin by typing ank geeko/admin at the kadmin prompt. The admin suffixed to your user name is a role. Later, use this role when administering the Kerberos database. A user can have several roles for different purposes. Roles act like completely different accounts that have similar names.

7.5.5.4 Starting the KDC Edit source

Start the KDC daemon and the kadmin daemon. To start the daemons manually, enter:

tux > sudo systemctl start krb5kdc
sudo systemctl start kadmind

Also make sure that the services KDC (krb5kdc) and kadmind (kadmind) are started by default when the server machine is rebooted. Enable them by entering:

tux > sudo systemctl enable krb5kdc kadmind

or by using the YaST Services Manager.

7.5.6 Configuring Kerberos Clients Edit source

When the supporting infrastructure is in place (DNS, NTP) and the KDC has been properly configured and started, configure the client machines. To configure a Kerberos client, use one of the two manual approaches described below.

When configuring Kerberos, there are two approaches you can take—static configuration in the /etc/krb5.conf file or dynamic configuration with DNS. With DNS configuration, Kerberos applications try to locate the KDC services using DNS records. With static configuration, add the host names of your KDC server to krb5.conf (and update the file whenever you move the KDC or reconfigure your realm in other ways).

DNS-based configuration is generally a lot more flexible and the amount of configuration work per machine is a lot less. However, it requires that your realm name is either the same as your DNS domain or a subdomain of it. Configuring Kerberos via DNS also creates a security issue: An attacker can seriously disrupt your infrastructure through your DNS (by shooting down the name server, spoofing DNS records, etc.). However, this amounts to a denial of service at worst. A similar scenario applies to the static configuration case unless you enter IP addresses in krb5.conf instead of host names.

7.5.6.1 Static Configuration Edit source

One way to configure Kerberos is to edit /etc/krb5.conf. The file installed by default contains various sample entries. Erase all of these entries before starting. krb5.conf is made up of several sections (stanzas), each introduced by the section name in brackets like [this].

To configure your Kerberos clients, add the following stanza to krb5.conf (where kdc.example.com is the host name of the KDC):

[libdefaults]
        default_realm = EXAMPLE.COM

[realms]
        EXAMPLE.COM = {
                kdc = kdc.example.com
                admin_server = kdc.example.com
        }

The default_realm line sets the default realm for Kerberos applications. If you have several realms, add additional statements to the [realms] section.

Also add a statement to this file that tells applications how to map host names to a realm. For example, when connecting to a remote host, the Kerberos library needs to know in which realm this host is located. This must be configured in the [domain_realms] section:

[domain_realm]
.example.com = EXAMPLE.COM
www.example.org = EXAMPLE.COM

This tells the library that all hosts in the example.com DNS domains are in the EXAMPLE.COM Kerberos realm. In addition, one external host named www.example.org should also be considered a member of the EXAMPLE.COM realm.

7.5.6.2 DNS-Based Configuration Edit source

DNS-based Kerberos configuration makes heavy use of SRV records. See (RFC2052) A DNS RR for specifying the location of services at https://datatracker.ietf.org/doc/html/rfc2052.

The name of an SRV record, as far as Kerberos is concerned, is always in the format _service._proto.realm, where realm is the Kerberos realm. Domain names in DNS are case-insensitive, so case-sensitive Kerberos realms would break when using this configuration method. _service is a service name (different names are used when trying to contact the KDC or the password service, for example). _proto can be either _udp or _tcp, but not all services support both protocols.

The data portion of SRV resource records consists of a priority value, a weight, a port number, and a host name. The priority defines the order in which hosts should be tried (lower values indicate a higher priority). The weight value is there to support some sort of load balancing among servers of equal priority. You probably do not need any of this, so it is okay to set these to zero.

MIT Kerberos currently looks up the following names when looking for services:

_kerberos

This defines the location of the KDC daemon (the authentication and ticket granting server). Typical records look like this:

_kerberos._udp.EXAMPLE.COM.  IN  SRV    0 0 88 kdc.example.com.
_kerberos._tcp.EXAMPLE.COM.  IN  SRV    0 0 88 kdc.example.com.
_kerberos-adm

This describes the location of the remote administration service. Typical records look like this:

_kerberos-adm._tcp.EXAMPLE.COM. IN  SRV    0 0 749 kdc.example.com.

Because kadmind does not support UDP, there should be no _udp record.

As with the static configuration file, there is a mechanism to inform clients that a specific host is in the EXAMPLE.COM realm, even if it is not part of the example.com DNS domain. This can be done by attaching a TXT record to _kerberos.host_name, as shown here:

_kerberos.www.example.org.  IN TXT "EXAMPLE.COM"

7.5.6.3 Adjusting the Clock Skew Edit source

The clock skew is the tolerance for accepting tickets with time stamps that do not exactly match the host's system clock. Usually, the clock skew is set to 300 seconds (five minutes). This means a ticket can have a time stamp somewhere between five minutes behind and five minutes ahead of the server's clock.

When using NTP to synchronize all hosts, you can reduce this value to about one minute. The clock skew value can be set in /etc/krb5.conf like this:

[libdefaults]
        clockskew = 60

7.5.7 Configuring Remote Kerberos Administration Edit source

To be able to add and remove principals from the Kerberos database without accessing the KDC's console directly, tell the Kerberos administration server which principals are allowed to do what by editing /var/lib/kerberos/krb5kdc/kadm5.acl. The ACL (access control list) file allows you to specify privileges with a precise degree of control. For details, refer to the manual page with man 8 kadmind.

For now, grant yourself the privilege to administer the database by putting the following line into the file:

geeko/admin              *

Replace the user name geeko with your own. Restart kadmind for the change to take effect.

You should now be able to perform Kerberos administration tasks remotely using the kadmin tool. First, obtain a ticket for your admin role and use that ticket when connecting to the kadmin server:

tux > kadmin -p geeko/admin
Authenticating as principal geeko/admin@EXAMPLE.COM with password.
Password for geeko/admin@EXAMPLE.COM:
kadmin:  getprivs
current privileges: GET ADD MODIFY DELETE
kadmin:

Using the getprivs command, verify which privileges you have. The list shown above is the full set of privileges.

As an example, modify the principal geeko:

tux > kadmin -p geeko/admin
Authenticating as principal geeko/admin@EXAMPLE.COM with password.
Password for geeko/admin@EXAMPLE.COM:

kadmin:  getprinc geeko
Principal: geeko@EXAMPLE.COM
Expiration date: [never]
Last password change: Wed Jan 12 17:28:46 CET 2005
Password expiration date: [none]
Maximum ticket life: 0 days 10:00:00
Maximum renewable life: 7 days 00:00:00
Last modified: Wed Jan 12 17:47:17 CET 2005 (admin/admin@EXAMPLE.COM)
Last successful authentication: [never]
Last failed authentication: [never]
Failed password attempts: 0
Number of keys: 2
Key: vno 1, Triple DES cbc mode with HMAC/sha1, no salt
Key: vno 1, DES cbc mode with CRC-32, no salt
Attributes:
Policy: [none]

kadmin:  modify_principal -maxlife "8 hours" geeko
Principal "geeko@EXAMPLE.COM" modified.
kadmin:  getprinc geeko
Principal: geeko@EXAMPLE.COM
Expiration date: [never]
Last password change: Wed Jan 12 17:28:46 CET 2005
Password expiration date: [none]
Maximum ticket life: 0 days 08:00:00
Maximum renewable life: 7 days 00:00:00
Last modified: Wed Jan 12 17:59:49 CET 2005 (geeko/admin@EXAMPLE.COM)
Last successful authentication: [never]
Last failed authentication: [never]
Failed password attempts: 0
Number of keys: 2
Key: vno 1, Triple DES cbc mode with HMAC/sha1, no salt
Key: vno 1, DES cbc mode with CRC-32, no salt
Attributes:
Policy: [none]
kadmin:

This changes the maximum ticket life time to eight hours. For more information about the kadmin command and the options available, see the krb5-doc package or refer to the man 8 kadmin manual page.

7.5.8 Creating Kerberos Service Principals Edit source

So far, only user credentials have been discussed. However, Kerberos-compatible services usually need to authenticate themselves to the client user, too. Therefore, special service principals must be in the Kerberos database for each service offered in the realm. For example, if ldap.example.com offers an LDAP service, you need a service principal, ldap/ldap.example.com@EXAMPLE.COM, to authenticate this service to all clients.

The naming convention for service principals is SERVICE/HOSTNAME@REALM, where HOSTNAME is the host's fully qualified host name.

Valid service descriptors are:

Service Descriptor

Service

host

Telnet, RSH, SSH

nfs

NFSv4 (with Kerberos support)

HTTP

HTTP (with Kerberos authentication)

imap

IMAP

pop

POP3

ldap

LDAP

Service principals are similar to user principals, but have significant differences. The main difference between a user principal and a service principal is that the key of the former is protected by a password. When a user obtains a ticket-granting ticket from the KDC, they needs to type their password, so Kerberos can decrypt the ticket. It would be inconvenient for system administrators to obtain new tickets for the SSH daemon every eight hours or so.

Instead, the key required to decrypt the initial ticket for the service principal is extracted by the administrator from the KDC only once and stored in a local file called the keytab. Services such as the SSH daemon read this key and use it to obtain new tickets automatically, when needed. The default keytab file resides in /etc/krb5.keytab.

To create a host service principal for jupiter.example.com enter the following commands during your kadmin session:

tux > kadmin -p geeko/admin
Authenticating as principal geeko/admin@EXAMPLE.COM with password.
Password for geeko/admin@EXAMPLE.COM:
kadmin:  addprinc -randkey host/jupiter.example.com
WARNING: no policy specified for host/jupiter.example.com@EXAMPLE.COM;
defaulting to no policy
Principal "host/jupiter.example.com@EXAMPLE.COM" created.

Instead of setting a password for the new principal, the -randkey flag tells kadmin to generate a random key. This is used here because no user interaction is wanted for this principal. It is a server account for the machine.

Finally, extract the key and store it in the local keytab file /etc/krb5.keytab. This file is owned by the superuser, so you must be root to execute the next command in the kadmin shell:

kadmin:  ktadd host/jupiter.example.com
Entry for principal host/jupiter.example.com with kvno 3, encryption type Triple
DES cbc mode with HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
Entry for principal host/jupiter.example.com with kvno 3, encryption type DES
cbc mode with CRC-32 added to keytab WRFILE:/etc/krb5.keytab.
kadmin:

When completed, make sure that you destroy the admin ticket obtained with kinit above with kdestroy.

7.5.9 Enabling PAM Support for Kerberos Edit source

Warning
Warning: Incomplete Configuration Locks Users Out

An incomplete Kerberos configuration may completely lock you out of your system, including the root user. To prevent this, add the ignore_unknown_principals directive to the pam_krb5 module after you have added the pam_krb5 module to the existing PAM configuration files as described below.

tux > sudo pam-config --add --krb5-ignore_unknown_principals

This will direct the pam_krb5 module to ignore some errors that would otherwise cause the account phase to fail.

SUSE® Linux Enterprise Server comes with a PAM module named pam_krb5, which supports Kerberos login and password update. This module can be used by applications such as console login, su, and graphical login applications like GDM. That is, it can be used in all cases where the user enters a password and expects the authenticating application to obtain an initial Kerberos ticket on their behalf. To configure PAM support for Kerberos, use the following command:

tux > sudo pam-config --add --krb5

The above command adds the pam_krb5 module to the existing PAM configuration files and makes sure it is called in the right order. To make precise adjustments to the way in which pam_krb5 is used, edit the file /etc/krb5.conf and add default applications to PAM. For details, refer to the manual page with man 5 pam_krb5.

The pam_krb5 module was specifically not designed for network services that accept Kerberos tickets as part of user authentication. This is an entirely different matter, and is discussed below.

7.5.10 Configuring SSH for Kerberos Authentication Edit source

OpenSSH supports Kerberos authentication in both protocol version 1 and 2. In version 1, there are special protocol messages to transmit Kerberos tickets. Version 2 does not use Kerberos directly anymore, but relies on GSSAPI, the General Security Services API. This is a programming interface that is not specific to Kerberos—it was designed to hide the peculiarities of the underlying authentication system, be it Kerberos, a public-key authentication system like SPKM, or others. However, the included GSSAPI library only supports Kerberos.

To use sshd with Kerberos authentication, edit /etc/ssh/sshd_config and set the following options:

# These are for protocol version 1
#
# KerberosAuthentication yes
# KerberosTicketCleanup yes

# These are for version 2 - better to use this
GSSAPIAuthentication yes
GSSAPICleanupCredentials yes

Then restart your SSH daemon using sudo systemctl restart sshd.

To use Kerberos authentication with protocol version 2, enable it on the client side as well. Do this either in the system-wide configuration file /etc/ssh/ssh_config or on a per-user level by editing ~/.ssh/config. In both cases, add the option GSSAPIAuthentication yes.

You should now be able to connect using Kerberos authentication. Use klist to verify that you have a valid ticket, then connect to the SSH server. To force SSH protocol version 1, specify the -1 option on the command line.

Tip
Tip: Additional Information

The file /usr/share/doc/packages/openssh/README.kerberos discusses the interaction of OpenSSH and Kerberos in more detail.

Tip
Tip: Additional Directives for Protocol Version 2

The GSSAPIKeyExchange mechanism (RFC 4462) is supported. This directive specifies how host keys are exchanged. For more information, see the sshd_config manual page (man sshd_config).

7.5.11 Using LDAP and Kerberos Edit source

While Kerberos provides authentication, LDAP is used for authorization and identification. Both services can work together.

For secure connections,389 Directory Server supports different ways of encrypting data: SSL/TLS connections, Start TLS connections, and SASL authentication. Simple Authentication and Security Layer (SASL) is a network protocol designed for authentication. The SASL implementation used on SUSE Linux Enterprise Server is cyrus-sasl. Kerberos authentication is performed through GSS-API (General Security Services API), provided by the cyrus-sasl-gssapi package. Using GSS-API, 389 Directory Server uses Kerberos tickets to authenticate sessions and encrypt data.

With the SASL framework you can use different mechanisms to authenticate a user to the server. In Kerberos, authentication is always mutual. This means that not only have you authenticated yourself to the 389 Directory Server, but also the 389 Directory Server has authenticated itself to you. In particular, this means communication is with the desired server, rather than with a random service set up by an attacker.

To enable Kerberos to bind to the 389 Directory Server, create a principal ldap/ldap.example.com and add that to the keytab. The credentials used by the 389 Directory Server to authenticate are given to other servers by the keytab. 389 Directory Server assigns a keytab through the KRB5_KTNAME environment variable.

To set the variable, proceed as follows:

  1. tux > sudo systemctl edit dirsrv@INSTANCE

    If you used the default name for the 389 Directory Server instance, replace INSTANCE with localhost.

  2. Add the following:

    [Service]
      Environment=KRB5_KTNAME=/etc/dirsrv/slapd-INSTANCE/krb5.keytab
  3. The keytab file needs to be readable by the account under which the 389 Directory Server runs (for example, dirserv):

    tux > sudo chown dirsrv:dirsrv /etc/dirsrv/slapd-INSTANCE/krb5.keytab
    tux > sudo chmod 600 /etc/dirsrv/slapd-INSTANCE/krb5.keytab

7.5.11.1 Using Kerberos Authentication with LDAP Edit source

To obtain and cache an initial ticket-granting ticket, use the principal that has been created in Section 7.5.5.3, “Creating a Principal”:

tux > kinit geeko@EXAMPLE.COM

To check if GSSAPI authentication works, run:

tux > ldapwhoami -Y GSSAPI -H ldap://ldapkdc.example.com
dn: uid=testuser,ou=People,dc=example,dc=com
    

GSSAPI uses the ccache to authenticate the user to the 389 Directory Server server without the user's password.

7.5.11.2 Configuring SASL Identity Mapping Edit source

When processing a SASL bind request, the 389 Directory Server maps the SASL authentication ID (used to authenticate to the Directory Server) with an LDAP entry stored within the server. When using Kerberos, the SASL user ID usually has the following format: userid@REALM, such as tux@example.com. This ID must be converted into the DN of the user's Directory Server entry, such as uid=tux,ou=people,dc=example,dc=com. The 389 Directory Server comes with some default maps for most common configurations. However, you can create customized maps. Procedure 7.1, “Managing Maps” shows how to list and display a map, how to delete a map and how to create a custom map.

Procedure 7.1: Managing Maps
  1. To list the existing SASL maps:

    tux > dsconf INSTANCE sasl list
    Kerberos uid mapping
    rfc 2829 dn syntax
    rfc 2829u syntax
    uid mapping
  2. To display a map:

    tux > sudo dsconf INSTANCE sasl get "Kerberos uid mapping"
    dn: cn=Kerberos uid mapping,cn=mapping,cn=sasl,cn=config
    cn: Kerberos uid mapping
    nsSaslMapBaseDNTemplate: dc=\2,dc=\3
    nsSaslMapFilterTemplate: (uid=\1)
    nsSaslMapRegexString: \(.*\)@\(.*\)\.\(.*\)
    objectClass: top
    objectClass: nsSaslMapping
  3. The default map only works if your dc has two components. To delete the map (if it does not work for you):

    tux > sudo dsconf INSTANCE sasl delete "Kerberos uid mapping"
    Deleting SaslMapping cn=Kerberos uid mapping,cn=mapping,cn=sasl,cn=config :
    Successfully deleted cn=Kerberos uid mapping,cn=mapping,cn=sasl,cn=config
  4. To create a new map:

    tux > sudo dsconf localhost sasl create --cn=bhgssapi --nsSaslMapRegexString "\
    (.*\)@EXAMPLE.NET.DE" --nsSaslMapBaseDNTemplate="dc=example,dc=net,dc=de" --nsSaslMapFilterTemplate="(uid=\1)"
    tux > sudo Enter value for nsSaslMapPriority :
    Successfully created bhgssapi
  5. Display the newly created map with:

    tux > sudo dsconf localhost sasl get "bhgssapi"
    dn: cn=bhgssapi,cn=mapping,cn=sasl,cn=config
    cn: bhgssapi
    nsSaslMapBaseDNTemplate: dc=example,dc=net,dc=de
    nsSaslMapFilterTemplate: (uid=\1)
    nsSaslMapPriority: 100
    nsSaslMapRegexString: \(.*\)@EXAMPLE.NET.DE
    objectClass: top
    objectClass: nsSaslMapping

    With this, you can check only the users of a specific realm and remap them to a different dc base. As you can see, the new map has 3 dc components, so the default maps would not have worked for this realm (EXAMPLE.NET.DE), only for a realm like EXAMPLE.NET.

7.6 Setting up Kerberos using LDAP and Kerberos Client Edit source

YaST includes the module LDAP and Kerberos Client that helps define authentication scenarios involving either LDAP or Kerberos.

It can also be used to join Kerberos and LDAP separately. However, in many such cases, using this module may not be the first choice, such as for joining Active Directory (which uses a combination of LDAP and Kerberos). For more information, see Section 5.1, “Configuring an Authentication Client with YaST”.

Start the module by selecting Network Services › LDAP and Kerberos Client.

LDAP and Kerberos Client Window
Figure 7.2: LDAP and Kerberos Client Window

To configure a Kerberos client, follow the procedure below:

  1. In the window LDAP and Kerberos Client, click Change Settings.

    Choose the tab Authentication via Kerberos.

    Tab Authentication via Kerberos
  2. Click Add Realm.

  3. In the appearing dialog, specify the correct Realm name. Usually, the realm name is an uppercase version of the domain name. Additionally, you can specify the following:

    • To apply mappings from the realm name to the domain name, activate Map Domain Name to the Realm and/or Map Wildcard Domain Name to the Realm.

    • You can specify the Host Name of Administration Server, the Host Name of Master Key Distribution Server and additional Key Distribution Centers.

      All of these items are optional if they can be automatically discovered via the SRV and TXT records in DNS.

    • To manually map Principals to local user names, use Custom Mappings of Principal Names to User Names.

      You can also use auth_to_local rules to supply such mappings using Custom Rules for Mapping Principal Names to User Names. For more information about using such rules, see the official documentation at https://web.mit.edu/kerberos/krb5-current/doc/admin/conf_files/krb5_conf.html#realms and an article at https://community.hortonworks.com/articles/14463/auth-to-local-rules-syntax.html.

    Continue with OK.

  4. To add more realms, repeat from Step 2.

  5. Enable Kerberos users logging in and creation of home directories by activating Allow Kerberos Users to Authenticate and Automatically Create Home Directory.

  6. If you left empty the optional text boxes in Step 3, make sure to enable automatic discovery of realms and key distribution centers by activating Use DNS TXT Record to Discover Realms and Use DNS SRV Record to Discover KDC Servers.

  7. You can additionally activate the following:

    • Allow Insecure Encryption (for Windows NT) allows the encryption types listed as weak at https://web.mit.edu/kerberos/krb5-current/doc/admin/conf_files/kdc_conf.html#encryption-types.

    • Allow KDC on Other Networks to Issue Authentication Tickets allows forwarding of tickets.

    • Allow Kerberos-Enabled Services to Take on The Identity Of a User allows the use of proxies between the computer of the user and the key distribution center.

    • Issue Address-Less Tickets for Computers Behind NAT allows granting tickets to users behind networks using network address translation.

  8. To set up allowed encryption types and define the name of the keytab file which lists the names of principals and their encrypted keys, use the Extended Options.

  9. Finish with OK and Finish.

    YaST may now install extra packages.

To check if the setup of the Kerberos back-end inside of LDAP was successful, proceed as follows:

  1. Directly access the KDC database on the host of the 389 Directory Server:

    tux > sudo kadmin.local
  2. List the principals:

    kadmin.local > listprincs
  3. Create a principal:

    kadmin.local > ank admin@EXAMPLE.COM

    It is written to the 389 Directory Server database.

  4. tux > sudo ldapsearch -D 'cn=Directory Manager' -w password -b 'cn=EXAMPLE.COM,cn=kdc,dc=example,dc=com' -H ldaps://localhost
  5. Check if the principle data from Kerberos is stored in LDAP. If yes, you get an output similar to the following:

    tux > sudo admin@EXAMPLE.COM, EXAMPLE.COM, kdc, example.com
    dn: krbprincipalname=admin@EXAMPLE.COM,cn=EXAMPLE.COM,cn=kdc,dc=example,dc=com
    krbLoginFailedCount: 0
    krbPrincipalName: admin@EXAMPLE.COM
    krbPrincipalKey:: MIG2oAMCAQGhAwIBAaIDAgEBowMCAQGkgZ8wgZwwVKAHMAWgAwIBAKFJMEeg
     AwIBEqFABD4gAKXAsMf7oV5vITzV5OpclhdomR+SdIRCkouS2GeNF9lVgxjT29RpnipNlCjgGOkpr
     93d0nh82WhrrAF6bzBEoAcwBaADAgEAoTkwN6ADAgERoTAELhAAFiGRiI0yUjBteGHhTB6ESJYsYJ
     WxFa4UslUNZD1GEQGlZ/0nltLsyD2ytGc=
    krbLastPwdChange: 20190702032802Z
    krbExtraData:: AAJCzxpdcm9vdC9hZG1pbkBFWEFNUExFLkNPTQA=
    krbExtraData:: AAgBAA==
    objectClass: krbprincipal
    objectClass: krbprincipalaux
    objectClass: krbTicketPolicyAux
    objectClass: top
  6. Obtain and cache an initial ticket-granting ticket:

    tux > sudo kinit admin@EXAMPLE.COM
  7. Display a list of currently cashed Kerberos tickets:

    tux > sudo klist
    Ticket cache: DIR::/run/user/0/krb5cc/tkt
    Default principal: admin@EXAMPLE.COM
    
    Valid starting     Expires            Service principal
    07/02/19 13:29:04  07/03/19 13:29:04  krbtgt/EXAMPLE.COM@EXAMPLE.COM

7.7 Kerberos and NFS Edit source

Most NFS servers can export file systems using any combination of the default trust the network form of security, known as sec=sys, and three different levels of Kerberos-based security, sec=krb5, sec=krb5i, and sec=krb5p. The sec option is set as a mount option on the client. It is often the case that the NFS service will first be configured and used with sec=sys, and then Kerberos can be imposed afterwards. In this case it is likely that the server will be configured to support both sec=sys and one of the Kerberos levels, and then after all clients have transitioned, the sec=sys support will be removed, thus achieving true security. The transition to Kerberos should be fairly transparent if done in an orderly manner. However there is one subtle detail of NFS behavior that works differently when Kerberos is used, and the implications of this need to be understood and possibly addressed. See Section 7.7.1, “Group Membership”.

The three Kerberos levels indicate different levels of security. With more security comes a need for more processor power to encrypt and decrypt messages. Choosing the right balance is an important consideration when planning a roll-out of Kerberos for NFS.

krb5 provides only authentication. The server can know who sent a request, and the client can know that the server sent a reply. No security is provided for the content of the request or reply, so an attacker with physical network access could transform the request or reply, or both, in various ways to deceive either server or client. They cannot directly read or change any file that the authenticated user could not read or change, but almost anything is theoretically possible.

krb5i adds integrity checks to all messages. With krb5i, an attacker cannot modify any request or reply, but they can view all the data exchanged, and so could discover the content of any file that is read.

krb5p adds privacy to the protocol. As well as reliable authentication and integrity checking, messages are fully encrypted so an attacker can only know that messages were exchanged between client and server, and cannot extract other information directly from the message. Whether information can be extracted from message timing is a separate question that Kerberos does not address.

7.7.1 Group Membership Edit source

The one behavioral difference between sec=sys and the various Kerberos security levels that might be visible is related to group membership. In Unix and Linux, each file system access comes from a process that is owned by a particular user and has a particular group owner and a number of supplemental groups. Access rights to files can vary based on the owner and the various groups.

With sec=sys, the user-id, group-id, and a list of up to 16 supplementary groups are sent to the server in each request.

If a user is a member of more than 16 supplementary groups, the extra groups are lost and some files may not be accessible over NFS that the user would normally expect to have access to. For this reason, most sites that use NFS find a way to limit all users to at most 16 supplementary groups.

If the user runs the newgrp command or runs a set-group-id program, either of which can change the list of groups they are a member of, these changes take effect immediately and provide different accesses over NFS.

With Kerberos, group information is not sent in requests at all. Only the user is identified (using a Kerberos principal), and the server performs a lookup to determine the user ID and group list for that principal. This means that if the user is a member of more than 16 groups, all of these group memberships will be used in determining file access permissions. However it also means that if the user changes a group-id on the client in some way, the server will not notice the change and will not take it into account in determining access rights.

In most cases, the improvement of having access to more groups brings a real benefit, and the loss of not being able to change groups is not noticed as it is not widely used. A site administrator considering the use of Kerberos should be aware of the difference though and ensure that it will not actually cause problems.

7.7.2 Performance and Scalability Edit source

Using Kerberos for security requires extra CPU power for encrypting and decrypting messages. How much extra CPU power is required and whether the difference is noticeable will vary with different hardware and different applications. If the server or client are already saturating the available CPU power, it is likely that a performance drop will be measurable when switching from sec=sys to Kerberos. If there is spare CPU capacity available, it is quite possible that the transition will not result in any throughput change. The only way to be sure how much impact the use of Kerberos will have is to test your load on your hardware.

The only configuration options that might reduce the load will also reduce the quality of the protection offered. sec=krb5 should produce noticeably less load than sec=krb5p but, as discussed above, it doesn't produce very strong security. Similarly it is possible to adjust the list of ciphers that Kerberos can choose from, and this might change the CPU requirement. However the defaults are carefully chosen and should not be changed without similar careful consideration.

The other possible performance issue when configuring NFS to use Kerberos involves availability of the Kerberos authentication servers, known as the KDC or Key Distribution Center.

The use of NFS adds load to such servers to the same degree that adding the use of Kerberos for any other services adds some load. Every time a given user (Kerberos principal) establishes a session with a service, for example by accessing files exported by a particular NFS server, the client needs to negotiate with the KDC. Once a session key has been negotiated, the client server can communicate without further help for many hours, depending on details of the Kerberos configuration, particularly the ticket_lifetime setting.

The concerns most likely to affect the provisioning of Kerberos KDC servers are availability and peak usage.

As with other core services such as DNS, LDAP or similar name-lookup services, having two servers that are reasonably "close" to every client provides good availability for modest resources. Kerberos allows for multiple KDC servers with flexible models for database propagation, so distributing servers as needed around campuses, buildings, and even cabinets is fairly straightforward. The best mechanism to ensure each client finds a nearby Kerberos server is to use split-horizon DNS with each building (or similar) getting different details from the DNS server. If this is not possible, then managing the /etc/krb5.conf file to be different at different locations is a suitable alternative.

As access to the Kerberos KDC is infrequent, load is only likely to be a problem at peak times. If thousands of people all log in between 9:00 and 9:05, then the servers will receive many more requests-per-minute than they might in the middle of the night. The load on the Kerberos server is likely to be more than that on an LDAP server, but not orders of magnitude more. A sensible guideline is to provision Kerberos replicas in the same manner that you provision LDAP replicas, and then monitor performance to determine if demand ever exceeds capacity.

7.7.3 Master KDC, Multiple Domains, and Trust Relationships Edit source

One service of the Kerberos KDC that is not easily distributed is the handling of updates, such as password changes and new user creation. These must happen at a single master KDC.

These updates are not likely to happen with such frequency that any significant load will be generated, but availability could be an issue. It can be annoying if you want to create a new user or change a password, and the master KDC on the other side of the world is temporarily unavailable.

When an organization is geographically distributed and has a policy of handling administration tasks locally at each site, it can be beneficial to create multiple Kerberos domains, one for each administrative center. Each domain would then have its own master KDC which would be geographically local. Users in one domain can still get access to resources in another domain by setting up trust relationships between domains.

The easiest arrangement for multiple domains is to have a global domain (for example, EXAMPLE.COM) and various local domains (for example, ASIA.EXAMPLE.COM, EUROPE.EXAMPLE.COM). If the global domain is configured to trust each local domain, and each local domain is configured to trust the global domain, then fully transitive trust is available between any pair of domains, and any principal can establish a secure connection with any service. Ensuring appropriate access rights to resources, for example files provided by that service, will be dependent on the user name lookup service used, and the functionality of the NFS file server, and is beyond the scope of this document.

7.8 For More Information Edit source

The official site of MIT Kerberos is https://web.mit.edu/kerberos. There, find links to any other relevant resource concerning Kerberos, including Kerberos installation, user, and administration guides.

The book Kerberos—A Network Authentication System by Brian Tung (ISBN 0-201-37924-4) offers extensive information.

8 Active Directory Support Edit source

Active Directory* (AD) is a directory-service based on LDAP, Kerberos, and other services. It is used by Microsoft* Windows* to manage resources, services, and people. In a Microsoft Windows network, Active Directory provides information about these objects, restricts access to them, and enforces policies. SUSE® Linux Enterprise Server lets you join existing Active Directory domains and integrate your Linux machine into a Windows environment.

8.1 Integrating Linux and Active Directory Environments Edit source

With a Linux client (configured as an Active Directory client) that is joined to an existing Active Directory domain, benefit from various features not available on a pure SUSE Linux Enterprise Server Linux client:

Browsing Shared Files and Directories with SMB

GNOME Files (previously called Nautilus) supports browsing shared resources through SMB.

Sharing Files and Directories with SMB

GNOME Files supports sharing directories and files as in Windows.

Accessing and Manipulating User Data on the Windows Server

Through GNOME Files, users can access their Windows user data and can edit, create, and delete files and directories on the Windows server. Users can access their data without having to enter their password multiple times.

Offline Authentication

Users can log in and access their local data on the Linux machine even if they are offline or the Active Directory server is unavailable for other reasons.

Windows Password Change

This port of Active Directory support in Linux enforces corporate password policies stored in Active Directory. The display managers and console support password change messages and accept your input. You can even use the Linux passwd command to set Windows passwords.

Single-Sign-On through Kerberized Applications

Many desktop applications are Kerberos-enabled (kerberized), which means they can transparently handle authentication for the user without the need for password reentry at Web servers, proxies, groupware applications, or other locations.

Note
Note: Managing Unix Attributes from Windows Server* 2016 and Later

In Windows Server 2016 and later, Microsoft has removed the role IDMU/NIS Server and along with it the Unix Attributes plug-in for the Active Directory Users and Computers MMC snap-in.

However, Unix attributes can still be managed manually when Advanced Options are enabled in the Active Directory Users and Computers MMC snap-in. For more information, see Clarification regarding the status of Identity Management for Unix (IDMU) & NIS Server Role in Windows Server 2016 Technical Preview and beyond.

Alternatively, use the method described in Procedure 8.1, “Joining an Active Directory Domain Using User Logon Management to complete attributes on the client side (in particular, see Step 6.c).

The following section contains technical background for most of the previously named features.

8.2 Background Information for Linux Active Directory Support Edit source

Many system components need to interact flawlessly to integrate a Linux client into an existing Windows Active Directory domain. The following sections focus on the underlying processes of the key events in Active Directory server and client interaction.

To communicate with the directory service, the client needs to share at least two protocols with the server:

LDAP

LDAP is a protocol optimized for managing directory information. A Windows domain controller with Active Directory can use the LDAP protocol to exchange directory information with the clients. To learn more about LDAP in general and about the open source port of it, refer to Chapter 6, LDAP—A Directory Service.

Kerberos

Kerberos is a third-party trusted authentication service. All its clients trust Kerberos's authorization of another client's identity, enabling kerberized single-sign-on (SSO) solutions. Windows supports a Kerberos implementation, making Kerberos SSO possible even with Linux clients. To learn more about Kerberos in Linux, refer to Chapter 7, Network Authentication with Kerberos.

Depending on which YaST module you use to set up Kerberos authentication, different client components process account and authentication data:

Solutions Based on SSSD
  • The sssd daemon is the central part of this solution. It handles all communication with the Active Directory server.

  • To gather name service information, sssd_nss is used.

  • To authenticate users, the pam_sss module for PAM is used. The creation of user homes for the Active Directory users on the Linux client is handled by pam_mkhomedir.

    For more information about PAM, see Chapter 3, Authentication with PAM.

Solution Based On Winbind (Samba)
  • The winbindd daemon is the central part of this solution. It handles all communication with the Active Directory server.

  • To gather name service information, nss_winbind is used.

  • To authenticate users, the pam_winbind module for PAM is used. The creation of user homes for the Active Directory users on the Linux client is handled by pam_mkhomedir.

    For more information about PAM, see Chapter 3, Authentication with PAM.

Figure 8.1, “Schema of Winbind-based Active Directory Authentication” highlights the most prominent components of Winbind-based Active Directory authentication.

Schema of Winbind-based Active Directory Authentication
Figure 8.1: Schema of Winbind-based Active Directory Authentication

Applications that are PAM-aware, like the login routines and the GNOME display manager, interact with the PAM and NSS layer to authenticate against the Windows server. Applications supporting Kerberos authentication (such as file managers, Web browsers, or e-mail clients) use the Kerberos credential cache to access user's Kerberos tickets, making them part of the SSO framework.

8.2.1 Domain Join Edit source

During domain join, the server and the client establish a secure relation. On the client, the following tasks need to be performed to join the existing LDAP and Kerberos SSO environment provided by the Windows domain controller. The entire join process is handled by the YaST Domain Membership module, which can be run during installation or in the installed system:

  1. The Windows domain controller providing both LDAP and KDC (Key Distribution Center) services is located.

  2. A machine account for the joining client is created in the directory service.

  3. An initial ticket granting ticket (TGT) is obtained for the client and stored in its local Kerberos credential cache. The client needs this TGT to get further tickets allowing it to contact other services, like contacting the directory server for LDAP queries.

  4. NSS and PAM configurations are adjusted to enable the client to authenticate against the domain controller.

During client boot, the winbind daemon is started and retrieves the initial Kerberos ticket for the machine account. winbindd automatically refreshes the machine's ticket to keep it valid. To keep track of the current account policies, winbindd periodically queries the domain controller.

8.2.2 Domain Login and User Homes Edit source

The login manager of GNOME (GDM) has been extended to allow the handling of Active Directory domain login. Users can choose to log in to the primary domain the machine has joined or to one of the trusted domains with which the domain controller of the primary domain has established a trust relationship.

User authentication is mediated by several PAM modules as described in Section 8.2, “Background Information for Linux Active Directory Support”. If there are errors, the error codes are translated into user-readable error messages that PAM gives at login through any of the supported methods (GDM, console, and SSH):

Password has expired

The user sees a message stating that the password has expired and needs to be changed. The system prompts for a new password and informs the user if the new password does not comply with corporate password policies (for example the password is too short, too simple, or already in the history). If a user's password change fails, the reason is shown and a new password prompt is given.

Account disabled

The user sees an error message stating that the account has been disabled and to contact the system administrator.

Account locked out

The user sees an error message stating that the account has been locked and to contact the system administrator.

Password has to be changed

The user can log in but receives a warning that the password needs to be changed soon. This warning is sent three days before that password expires. After expiration, the user cannot log in.

Invalid workstation

When a user is restricted to specific workstations and the current SUSE Linux Enterprise Server machine is not among them, a message appears that this user cannot log in from this workstation.

Invalid logon hours

When a user is only allowed to log in during working hours and tries to log in outside working hours, a message informs the user that logging in is not possible at that time.

Account expired

An administrator can set an expiration time for a specific user account. If that user tries to log in after expiration, the user gets a message that the account has expired and cannot be used to log in.

During a successful authentication, the client acquires a ticket granting ticket (TGT) from the Kerberos server of Active Directory and stores it in the user's credential cache. It also renews the TGT in the background, requiring no user interaction.

SUSE Linux Enterprise Server supports local home directories for Active Directory users. If configured through YaST as described in Section 8.3, “Configuring a Linux Client for Active Directory”, user home directories are created when a Windows/Active Directory user first logs in to the Linux client. These home directories look and feel identical to standard Linux user home directories and work independently of the Active Directory Domain Controller.

Using a local user home, it is possible to access a user's data on this machine (even when the Active Directory server is disconnected) as long as the Linux client has been configured to perform offline authentication.

8.2.3 Offline Service and Policy Support Edit source

Users in a corporate environment must have the ability to become roaming users (for example, to switch networks or even work disconnected for some time). To enable users to log in to a disconnected machine, extensive caching was integrated into the winbind daemon. The winbind daemon enforces password policies even in the offline state. It tracks the number of failed login attempts and reacts according to the policies configured in Active Directory. Offline support is disabled by default and must be explicitly enabled in the YaST Domain Membership module.

When the domain controller has become unavailable, the user can still access network resources (other than the Active Directory server itself) with valid Kerberos tickets that have been acquired before losing the connection (as in Windows). Password changes cannot be processed unless the domain controller is online. While disconnected from the Active Directory server, a user cannot access any data stored on this server. When a workstation has become disconnected from the network entirely and connects to the corporate network again later, SUSE Linux Enterprise Server acquires a new Kerberos ticket when the user has locked and unlocked the desktop (for example, using a desktop screen saver).

8.3 Configuring a Linux Client for Active Directory Edit source

Before your client can join an Active Directory domain, some adjustments must be made to your network setup to ensure the flawless interaction of client and server.

DNS

Configure your client machine to use a DNS server that can forward DNS requests to the Active Directory DNS server. Alternatively, configure your machine to use the Active Directory DNS server as the name service data source.

NTP

To succeed with Kerberos authentication, the client must have its time set accurately. It is highly recommended to use a central NTP time server for this purpose (this can be also the NTP server running on your Active Directory domain controller). If the clock skew between your Linux host and the domain controller exceeds a certain limit, Kerberos authentication fails and the client is logged in using the weaker NTLM (NT LAN Manager) authentication. For more details about using Active Directory for time synchronization, see Procedure 8.2, “Joining an Active Directory Domain Using Windows Domain Membership.

Firewall

To browse your network neighborhood, either disable the firewall entirely or mark the interface used for browsing as part of the internal zone.

To change the firewall settings on your client, log in as root and start the YaST firewall module. Select Interfaces. Select your network interface from the list of interfaces and click Change. Select Internal Zone and apply your settings with OK. Leave the firewall settings with Next › Finish. To disable the firewall, check the Disable Firewall Automatic Starting option, and leave the firewall module with Next › Finish.

Active Directory Account

You cannot log in to an Active Directory domain unless the Active Directory administrator has provided you with a valid user account for that domain. Use the Active Directory user name and password to log in to the Active Directory domain from your Linux client.

8.3.1 Choosing Which YaST Module to Use for Connecting to Active Directory Edit source

YaST contains multiple modules that allow connecting to an Active Directory:

8.3.2 Joining Active Directory Using User Logon Management Edit source

The YaST module User Logon Management supports authentication at an Active Directory. Additionally, it also supports the following related authentication and identification providers:

Identification Providers
  • Delegate to third-party software library Support for legacy NSS providers via a proxy.

  • FreeIPA FreeIPA and Red Hat Enterprise Identity Management provider.

  • Generic directory service (LDAP) An LDAP provider. For more information about configuring LDAP, see man 5 sssd-ldap.

  • Local SSSD file database An SSSD-internal provider for local users.

Authentication Providers
  • Delegate to third-party software library Relay authentication to another PAM target via a proxy.

  • FreeIPA FreeIPA and Red Hat Enterprise Identity Management provider.

  • Generic Kerberos service An LDAP provider.

  • Generic directory service (LDAP) Kerberos authentication.

  • Local SSSD file database An SSSD-internal provider for local users.

  • This domain does not provide authentication service Disables authentication explicitly.

To join an Active Directory domain using SSSD and the User Logon Management module of YaST, proceed as follows:

Procedure 8.1: Joining an Active Directory Domain Using User Logon Management
  1. Open YaST.

  2. To be able to use DNS auto-discovery later, set up the Active Directory Domain Controller (the Active Directory server) as the name server for your client.

    1. In YaST, click Network Settings.

    2. Select Hostname/DNS, then enter the IP address of the Active Directory Domain Controller into the text box Name Server 1.

      Save the setting with OK.

  3. From the YaST main window, start the module User Logon Management.

    The module opens with an overview showing different network properties of your computer and the authentication method currently in use.

    Overview window showing the computer name, IP address, and its authentication setting.
    Figure 8.2: Main Window of User Logon Management
  4. To start editing, click Change Settings.

  5. Now join the domain.

    1. Click Add Domain.

    2. In the appearing dialog, specify the correct Domain name. Then specify the services to use for identity data and authentication: Select Microsoft Active Directory for both.

      Ensure that Enable the domain is activated.

      Click OK.

    3. (Optional) Usually, you can keep the default settings in the following dialog. However, there are reasons to make changes:

      • If the Local Host Name Does Not Match the Host Name Set on the Domain Controller.  Find out if the host name of your computer matches what the name your computer is known as to the Active Directory Domain Controller. In a terminal, run the command hostname, then compare its output to the configuration of the Active Directory Domain Controller.

        If the values differ, specify the host name from the Active Directory configuration under AD hostname. Otherwise, leave the appropriate text box empty.

      • If You Do Not Want to Use DNS Auto-Discovery.  Specify the Host names of Active Directory servers that you want to use. If there are multiple Domain Controllers, separate their host names with commas.

    4. To continue, click OK.

      If not all software is installed already, the computer will now install missing software. It will then check whether the configured Active Directory Domain Controller is available.

    5. If everything is correct, the following dialog should now show that it has discovered an Active Directory Server but that you are Not yet enrolled.

      In the dialog, specify the Username and Password of the Active Directory administrator account (usually Administrator).

      To make sure that the current domain is enabled for Samba, activate Overwrite Samba configuration to work with this AD.

      To enroll, click OK.

      Enrolling into a Domain
      Figure 8.3: Enrolling into a Domain
    6. You should now see a message confirming that you have enrolled successfully. Finish with OK.

  6. After enrolling, configure the client using the window Manage Domain User Logon.

    Configuration Window of User Logon Management
    Figure 8.4: Configuration Window of User Logon Management
    1. To allow logging in to the computer using login data provided by Active Directory, activate Allow Domain User Logon.

    2. (Optional) Optionally, under Enable domain data source, activate additional data sources such as information on which users are allowed to use sudo or which network drives are available.

    3. To allow Active Directory users to have home directories, activate Create Home Directories. The path for home directories can be set in multiple ways—on the client, on the server, or both ways:

      • To configure the home directory paths on the Domain Controller, set an appropriate value for the attribute UnixHomeDirectory for each user. Additionally, make sure that this attribute replicated to the global catalog. For information on achieving that under Windows, see https://support.microsoft.com/en-us/kb/248717.

      • To configure home directory paths on the client in such a way that precedence will be given to the path set on the domain controller, use the option fallback_homedir.

      • To configure home directory paths on the client in such a way that the client setting will override the server setting, use override_homedir.

      As settings on the Domain Controller are outside of the scope of this documentation, only the configuration of the client-side options will be described in the following.

      From the side bar, select Service Options › Name switch, then click Extended Options. From that window, select either fallback_homedir or override_homedir, then click Add.

      Specify a value. To have home directories follow the format /home/USER_NAME, use /home/%u. For more information about possible variables, see the man page sssd.conf (man 5 sssd.conf), section override_homedir.

      Click OK.

  7. Save the changes by clicking OK. Then make sure that the values displayed now are correct. To leave the dialog, click Cancel.

8.3.3 Joining Active Directory Using Windows Domain Membership Edit source

To join an Active Directory domain using winbind and the Windows Domain Membership module of YaST, proceed as follows:

Procedure 8.2: Joining an Active Directory Domain Using Windows Domain Membership
  1. Log in as root and start YaST.

  2. Start Network Services › Windows Domain Membership.

  3. Enter the domain to join at Domain or Workgroup in the Windows Domain Membership screen (see Figure 8.5, “Determining Windows Domain Membership”). If the DNS settings on your host are properly integrated with the Windows DNS server, enter the Active Directory domain name in its DNS format (mydomain.mycompany.com). If you enter the short name of your domain (also known as the pre–Windows 2000 domain name), YaST must rely on NetBIOS name resolution instead of DNS to find the correct domain controller.

    Determining Windows Domain Membership
    Figure 8.5: Determining Windows Domain Membership
  4. To use the SMB source for Linux authentication, activate Also Use SMB Information for Linux Authentication.

  5. To automatically create a local home directory for Active Directory users on the Linux machine, activate Create Home Directory on Login.

  6. Check Offline Authentication to allow your domain users to log in even if the Active Directory server is temporarily unavailable, or if you do not have a network connection.

  7. To change the UID and GID ranges for the Samba users and groups, select Expert Settings. Let DHCP retrieve the WINS server only if you need it. This is the case when some machines are resolved only by the WINS system.

  8. Configure NTP time synchronization for your Active Directory environment by selecting NTP Configuration and entering an appropriate server name or IP address. This step is obsolete if you have already entered the appropriate settings in the stand-alone YaST NTP configuration module.

  9. Click OK and confirm the domain join when prompted for it.

  10. Provide the password for the Windows administrator on the Active Directory server and click OK (see Figure 8.6, “Providing Administrator Credentials”).

    Providing Administrator Credentials
    Figure 8.6: Providing Administrator Credentials

After you have joined the Active Directory domain, you can log in to it from your workstation using the display manager of your desktop or the console.

Important
Important: Domain Name

Joining a domain may not succeed if the domain name ends with .local. Names ending in .local cause conflicts with Multicast DNS (MDNS) where .local is reserved for link-local host names.

Note
Note: Only Administrators Can Enroll a Computer

Only a domain administrator account, such as Administrator, can join SUSE Linux Enterprise Server into Active Directory.

8.3.4 Checking Active Directory Connection Status Edit source

To check whether you are successfully enrolled in an Active Directory domain, use the following commands:

  • klist shows whether the current user has a valid Kerberos ticket.

  • getent passwd shows published LDAP data for all users.

8.4 Logging In to an Active Directory Domain Edit source

Provided your machine has been configured to authenticate against Active Directory and you have a valid Windows user identity, you can log in to your machine using the Active Directory credentials. Login is supported for GNOME, the console, SSH, and any other PAM-aware application.

Important
Important: Offline Authentication

SUSE Linux Enterprise Server supports offline authentication, allowing you to log in to your client machine even when it is offline. See Section 8.2.3, “Offline Service and Policy Support” for details.

8.4.1 GDM Edit source

To authenticate a GNOME client machine against an Active Directory server, proceed as follows:

  1. Click Not listed.

  2. In the text box Username, enter the domain name and the Windows user name in this form: DOMAIN_NAME\USER_NAME.

  3. Enter your Windows password.

If configured to do so, SUSE Linux Enterprise Server creates a user home directory on the local machine on the first login of each user authenticated via Active Directory. This allows you to benefit from the Active Directory support of SUSE Linux Enterprise Server while still having a fully functional Linux machine at your disposal.

8.4.2 Console Login Edit source

Besides logging in to the Active Directory client machine using a graphical front-end, you can log in using the text-based console or even remotely using SSH.

To log in to your Active Directory client from a console, enter DOMAIN_NAME\USER_NAME at the login: prompt and provide the password.

To remotely log in to your Active Directory client machine using SSH, proceed as follows:

  1. At the login prompt, enter:

    tux > ssh DOMAIN_NAME\\USER_NAME@HOST_NAME

    The \ domain and login delimiter is escaped with another \ sign.

  2. Provide the user's password.

8.5 Changing Passwords Edit source

SUSE Linux Enterprise Server helps the user choose a suitable new password that meets the corporate security policy. The underlying PAM module retrieves the current password policy settings from the domain controller, informing the user about the specific password quality requirements a user account typically has by means of a message on login. Like its Windows counterpart, SUSE Linux Enterprise Server presents a message describing:

  • Password history settings

  • Minimum password length requirements

  • Minimum password age

  • Password complexity

The password change process cannot succeed unless all requirements have been successfully met. Feedback about the password status is given both through the display managers and the console.

GDM provides feedback about password expiration and the prompt for new passwords in an interactive mode. To change passwords in the display managers, provide the password information when prompted.

To change your Windows password, you can use the standard Linux utility, passwd, instead of having to manipulate this data on the server. To change your Windows password, proceed as follows:

  1. Log in at the console.

  2. Enter passwd.

  3. Enter your current password when prompted.

  4. Enter the new password.

  5. Reenter the new password for confirmation. If your new password does not comply with the policies on the Windows server, this feedback is given to you and you are prompted for another password.

To change your Windows password from the GNOME desktop, proceed as follows:

  1. Click the Computer icon on the left edge of the panel.

  2. Select Control Center.

  3. From the Personal section, select About Me › Change Password.

  4. Enter your old password.

  5. Enter and confirm the new password.

  6. Leave the dialog with Close to apply your settings.

9 Setting Up a FreeRADIUS Server Edit source

The RADIUS (Remote Authentication Dial-In User Service) protocol has long been a standard service for manage network access. It performs authentication, authorization, and accounting (AAA) protocol for very large businesses such as Internet service providers and cellular network providers, and is also popular for small networks. It authenticates users and devices, authorizes those users and devices for certain network services, and tracks use of services for billing and auditing. You don't have to use all three of the AAA protocols, but only the ones you need. For example, you may not need accounting but only client authentication, or perhaps all you want is accounting, and client authorization is managed by something else.

It is extremely efficient and manages thousands of requests on modest hardware. Of course it works for all network protocols and not just dialup, but the name remains the same.

RADIUS operates in a distributed architecture, sitting separately from the Network Access Server (NAS). User access data is stored on a central RADIUS server that is available to multiple NAS. The NAS provide the physical access to the network, such as a managed Ethernet switch, or wireless access point.

FreeRADIUS is the open source RADIUS implementation, and is the most widely-used RADIUS server. In this chapter you will learn how to install and test a FreeRADIUS server. Because of the numerous possible use cases, after your initial setup is working correctly your next stop is the official documentation, which is detailed and thorough (see https://freeradius.org/documentation/).

9.1 Installation and Testing on SUSE Linux Enterprise Edit source

The following steps set up a simple test system. When you have verified that the server is operating correctly and you are ready to create a production configuration, you will have several undo steps to perform before starting your production configuration.

First install the freeradius-server and freeradius-server-utils packages. Then enter /etc/raddb/certs and run the bootstrap script to create a set of test certificates:

root # zypper in freeradius-server
root # cd /etc/raddb/certs
root # ./bootstrap

The README in the certs directory contains a great deal of useful information. When the bootstrap script has completed, start the server in debugging mode:

root # radiusd -X
[...]
Listening on auth address * port 1812 bound to server default
Listening on acct address * port 1813 bound to server default
Listening on auth address :: port 1812 bound to server default
Listening on acct address :: port 1813 bound to server default
Listening on auth address 127.0.0.1 port 18120 bound to server inner-tunnel
Listening on proxy address * port 54435
Listening on proxy address :: port 58415
Ready to process requests

When you see the "Listening" and "Ready to process requests" lines, your server has started correctly. If it does not start, read the output carefully because it tells you what went wrong. You may direct the output to a text file with tee:

tux > radiusd -X | tee radiusd.text

The next step is to test authentication with a test client and user. The client is a client of the RADIUS server, such as a wireless access point or switch. Clients are configured in /etc/raddb/client.conf. Human users are configured in /etc/raddb/mods-config/files/authorize.

Open /etc/raddb/mods-config/files/authorize and uncomment the following lines:

bob   Cleartext-Password := "hello"
Reply-Message := "Hello, %{User-Name}"

A test client, client localhost, is provided in /etc/raddb/client.conf, with a secret of testing123. Open a second terminal, and as an unprivileged user use the radtest command to log in as bob:

tux > radtest bob hello 127.0.0.1 0 testing123
Sent Access-Request Id 241 from 0.0.0.0:35234 to 127.0.0.1:1812 length 73
        User-Name = "bob"
        User-Password = "hello"
        NAS-IP-Address = 127.0.0.1
        NAS-Port = 0
        Message-Authenticator = 0x00
        Cleartext-Password = "hello"
Received Access-Accept Id 241 from 127.0.0.1:1812 to 0.0.0.0:0 length 20

In your radius -X terminal, a successful login looks like this:

(3) pap: Login attempt with password
(3) pap: Comparing with "known good" Cleartext-Password
(3) pap: User authenticated successfully
(3)     [pap] = ok
[...]
(3) Sent Access-Accept Id 241 from 127.0.0.1:1812 to 127.0.0.1:35234 length 0
(3) Finished request
Waking up in 4.9 seconds.
(3) Cleaning up request packet ID 241 with timestamp +889

Now run one more login test from a different computer on your network. Create a client configuration on your server by uncommenting and modifying the following entry in clients.conf:

client private-network-1 }
  ipaddr          = 192.0.2.0/24
  secret          = testing123-1 
  {

Enter the IP address of your test client machine. On the client machine, install freeradius-server-utils, which provides a number of useful test commands. Try logging in from the client as bob, using the radtest command. It is better to use the IP address of the RADIUS server rather than the hostname because it is faster:

tux > radtest bob hello 192.168.2.100 0 testing123-1

If your test logins fail, review all the output to learn what went wrong. There are several test users and test clients provided. The configuration files are full of useful information, and we recommend studying them. When you are satisfied with your testing and ready to create a production configuration, remove all the test certificates in /etc/raddb/certs and replace them with your own certificates, comment out all the test users and clients, and stop radiusd by pressing Ctrlc. Manage the radiusd.service with systemctl, just like any other service.

To learn how to fit a FreeRADIUS server in your network, see https://freeradius.org/documentation/ and https://networkradius.com/freeradius-documentation/ for in-depth references and howtos.

Part II Local Security Edit source

10 Physical Security

Physical security should be one of the utmost concerns. Linux production servers should be in locked data centers where only people have access that have passed security checks. Depending on the environment and circumstances, you can also consider boot loader passwords.

11 Automatic Security Checks with seccheck

The seccheck SUSE Security Checker is a set of shell scripts designed to automatically check the local security of a system on a regular schedule, and emails reports to the root user, or any user as configured by the administrator.

12 Software Management

A very important step in securing a Linux system is to determine the primary function(s) or role(s) of the Linux server. Otherwise, it can be difficult to understand what needs to be secured and securing these Linux systems can prove ineffective. Therefore, it is critical to look at the default list…

13 File Management

Servers should have separate file systems for at least /, /boot, /usr, /var, /tmp, and /home. This prevents, for example, that logging space and temporary space under /var and /tmp fill up the root partition. Third-party applications should be on separate file systems as well, for example under /opt…

14 Encrypting Partitions and Files

Encrypting files, partitions, and entire disks prevents unauthorized access to your data and protects your confidential files and documents.

15 User Management

It is important that all system and vendor accounts that are not used for logins are locked. To get a list of unlocked accounts on your system, you can check for accounts that do not have an encrypted password string starting with ! or * in the /etc/shadow file. If you lock an account using passwd -…

16 Spectre/Meltdown Checker

spectre-meltdown-checker is a shell script to test if your system is vulnerable to the several speculative execution vulnerabilities that are present in nearly all CPUs manufactured in the past 20 years. This is a hardware flaw that potentially allows an attacker to read all data on the system. On cloud computing services, where multiple virtual machines are on a single physical host, an attacker can gain access to all virtual machines. Fixing these vulnerabilities requires re-designing and replacing CPUs. Until this happens, there are several software patches that mitigate these vulnerabilities. If you have kept your SUSE systems updated, all of these patches should already be installed.

spectre-meltdown-checker generates a detailed report. It is impossible to guarantee that your system is secure, but it shows you which mitigations are in place, and potential vulnerabilities

17 Configuring Security Settings with YaST

The YaST module Security Center and Hardening offers a central clearinghouse to configure security-related settings for SUSE Linux Enterprise Server. Use it to configure security aspects such as settings for the login procedure and for password creation, for boot permissions, user creation or for default file permissions. Launch it from the YaST control center by Security and Users › Security Center and Hardening. The Security Center dialog always starts with the Security Overview, and other configuration dialogs are available from the right pane.

18 Authorization with PolKit

PolKit (formerly known as PolicyKit) is an application framework that acts as a negotiator between the unprivileged user session and the privileged system context. Whenever a process from the user session tries to carry out an action in the system context, PolKit is queried. Based on its configuration—specified in a so-called policy—the answer could be yes, no, or needs authentication. Unlike classical privilege authorization programs such as sudo, PolKit does not grant root permissions to an entire session, but only to the action in question.

19 Access Control Lists in Linux

POSIX ACLs (access control lists) can be used as an expansion of the traditional permission concept for file system objects. With ACLs, permissions can be defined more flexibly than with the traditional permission concept.

20 Certificate Store

Certificates play an important role in the authentication of companies and individuals. Usually certificates are administered by the application itself. In some cases, it makes sense to share certificates between applications. The certificate store is a common ground for Firefox, Evolution, and NetworkManager. This chapter explains some details.

21 Intrusion Detection with AIDE

Securing your systems is a mandatory task for any mission-critical system administrator. Because it is impossible to always guarantee that the system is not compromised, it is very important to do extra checks regularly (for example with cron) to ensure that the system is still under your control. This is where AIDE, the Advanced Intrusion Detection Environment, comes into play.

10 Physical Security Edit source

Physical security should be one of the utmost concerns. Linux production servers should be in locked data centers where only people have access that have passed security checks. Depending on the environment and circumstances, you can also consider boot loader passwords.

Additionally, consider questions like:

  • Who has direct physical access to the host?

  • Of those that do, should they?

  • Can the host be protected from tampering and should it be?

The amount of physical security needed on a particular system depends on the situation, and can also vary widely by available funds.

10.1 System Locks Edit source

Most server racks in data centers include a locking feature. Usually this will be a hasp/cylinder lock on the front of the rack that allows you to turn an included key to a locked or unlocked position – granting or denying entry. Cage locks can help prevent someone from tampering or stealing devices/media from the servers, or opening the cases and directly manipulating/sabotaging the hardware. Preventing system reboots or the booting from alternate devices is also important (for example CD/DVDs/USB drives/etc.).

Some servers also have case locks. These locks can do different things according to the designs of the system vendor and construction. Many systems are designed to self-disable if attempts are made to open the system without unlocking. Others have device covers that will not let you plug in or unplug keyboards or mice. While locks are sometimes a useful feature, they are usually lower quality and easily defeated by attackers with ill intent.

10.2 Locking Down the BIOS Edit source

Tip
Tip: Secure Boot

This section describes only basic methods to secure the boot process. To find out about more advanced boot protection using UEFI and the secure boot feature, see Book “Administration Guide”, Chapter 13 “UEFI (Unified Extensible Firmware Interface)”, Section 13.1 “Secure Boot”.

The BIOS (Basic Input/Output System) or its successor UEFI (Unified Extensible Firmware Interface) is the lowest level of software/firmware on PC class systems. Other hardware types (POWER, IBM Z) that run Linux also have low-level firmware that performs similar functions as the PC BIOS. When this document references the BIOS, it usually means BIOS and/or UEFI. The BIOS dictates system configuration, puts the system into a well defined state and provides routines for accessing low-level hardware. The BIOS executes the configured Linux boot loader (like GRUB 2) to boot the host.

Most BIOS implementations can be configured to prevent unauthorized users from manipulating system and boot settings. This is typically done by setting a BIOS admin or boot password. The admin password only needs to be entered for changing the system configuration but the boot password will be required during every normal boot. For most use cases it is enough to set an admin password and restrict booting to the built-in hard disk. This way an attacker will not be able to simply boot a Linux live CD or USB thumb drive, for example. Although this does not provide a high level of security (a BIOS can be reset, removed or modified – assuming case access), it can be another deterrent.

Many BIOS firmwares have various other security related settings. Check with the system vendor, the system documentation or examine the BIOS during a system boot to find out more.

Important
Important: Booting when a BIOS Boot Password Is Set

If a system has been set up with a boot password, the host will not boot up unattended (for example in case of a system reboot or power failure). This is a trade-off.

Important
Important: Losing the BIOS Admin Password

Once a system is set up for the first time, the BIOS admin password will not be required often. Don't forget the password or you will need to clear the BIOS memory via hardware manipulation to get access again.

10.3 Security via the Boot Loaders Edit source

The Linux boot loader GRUB 2, which is used by default in SUSE Linux Enterprise Server, can have a boot passwords set. It also provides a password feature, so that only administrators can start the interactive operations (for example editing menu entries and entering the command line interface). If a password is specified, GRUB 2 will disallow any interactive control until you press the key C and E and enter a correct password.

You can refer to the GRUB 2 man page for examples.

It is very important to keep in mind that when setting these passwords they will need to be remembered! Also, enabling these passwords might merely slow an intrusion, not necessarily prevent it. Again, someone could boot from a removable device, and mount your root partition. If you are using BIOS-level security and a boot loader, it is a good practice to disable the ability to boot from removable devices in your computer's BIOS, and then also password-protecting the BIOS itself.

Also keep in mind that the boot loader configuration files will need to be protected by changing their mode to 600 (read/write for root only), or others will be able to read your passwords or hashes!

10.4 Retiring Linux Servers with Sensitive Data Edit source

Security policies usually contain some procedures for the treatment of storage media that is going to be retired or disposed of. Disk and media wipe procedures are frequently prescribed as is complete destruction of the media. You can find several free tools on the Internet. A search of dod disk wipe utility will yield several variants. To retire servers with sensitive data, it is important to ensure that data cannot be recovered from the hard disks. To ensure that all traces of data are removed, a wipe utility—such as scrub—can be used. Many wipe utilities overwrite the data several times. This assures that even sophisticated methods are not able to retrieve any parts of the wiped data. Some tools can even be operated from a bootable removable device and remove data according to the U.S. Department of Defense (DoD) standards. Note that many government agencies specify their own standards for data security. Some standards are stronger than others, yet may require more time to implement.

Important
Important: Wiping Wear Leveling Devices

Some devices, like SSDs, use wear leveling and do not necessarily write new data in the same physical locations. Such devices usually provide their own erasing functionality.

10.4.1 scrub: Disk Overwrite Utility Edit source

scrub overwrites hard disks, files, and other devices with repeating patterns intended to make recovering data from these devices more difficult. It operates in three basic modes: on a character or block device, on a file, or on a directory specified. For more information, set the manual page man 1 scrub.

Supported Scrub Methods
nnsa

4-pass NNSA Policy Letter NAP-14.1-C (XVI-8) for sanitizing removable and non-removable hard disks, which requires overwriting all locations with a pseudo random pattern twice and then with a known pattern: ran- dom(x2), 0x00, verify.

dod

4-pass DoD 5220.22-M section 8-306 procedure (d) for sanitizing removable and non-removable rigid disks which requires overwriting all addressable locations with a character, its complement, a random character, then verify. Note: scrub performs the random pass first to make verification easier: random, 0x00, 0xff, verify.

bsi

9-pass method recommended by the German Center of Security in Information Technologies (http://www.bsi.bund.de): 0xff, 0xfe, 0xfd, 0xfb, 0xf7, 0xef, 0xdf, 0xbf, 0x7f.

gutmann

The canonical 35-pass sequence described in Gutmann's paper cited below.

schneier

7-pass method described by Bruce Schneier in "Applied Cryptography" (1996): 0x00, 0xff, random(x5)

pfitzner7

Roy Pfitzner's 7-random-pass method: random(x7).

pfitzner33

Roy Pfitzner's 33-random-pass method: random(x33).

usarmy

US Army AR380-19 method: 0x00, 0xff, random. (Note: identical to DoD 522.22-M section 8-306 procedure (e) for sanitizing magnetic core memory).

fillzero

1-pass pattern: 0x00.

fillff

1-pass pattern: 0xff.

random

1-pass pattern: random(x1).

random2

2-pass pattern: random(x2).

old

6-pass pre-version 1.7 scrub method: 0x00, 0xff, 0xaa, 0x00, 0x55, verify.

fastold

5-pass pattern: 0x00, 0xff, 0xaa, 0x55, verify.

custom=string

1-pass custom pattern. String may contain C-style numerical escapes: \nnn (octal) or \xnn (hex).

10.5 Restricting Access to Removable Media Edit source

In some environments it is required to restrict access to removable media such as USB storage or optical devices. The tools coming with the udisks2 package help with such a configuration.

  1. Create a user group whose users will be allowed to mount and eject removable devices, for example mmedia_all:

    tux > sudo groupadd mmedia_all
  2. Add a specific user tux to the new group:

    tux > sudo usermod -a -G mmedia_all tux
  3. Create the /etc/polkit-1/rules.d/10-mount.rules file with the following content:

    tux > cat /etc/polkit-1/rules.d/10-mount.rules
    polkit.addRule(function(action, subject) {
     if (action.id =="org.freedesktop.udisks2.eject-media"
      && subject.isInGroup("mmedia_all")) {
       return polkit.Result.YES;
      }
    });
    
    polkit.addRule(function(action, subject) {
     if (action.id =="org.freedesktop.udisks2.filesystem-mount"
      && subject.isInGroup("mmedia_all")) {
       return polkit.Result.YES;
      }
    });
    Important
    Important: Naming of the Rules File

    The name of a rules file must start with a digit, otherwise it will be ignored.

    Rules files are processed in alphabetical order. Functions are called in the order they were added until one of the functions returns a value. Therefore, to add an authorization rule that is processed before other rules, put it in a file in /etc/polkit-1/rules.d with a name that sorts before other rules files, for example /etc/polkit-1/rules.d/10-mount.rules. Each function should return a value from polkit.Result.

  4. Restart udisks2:

    root # systemctl restart udisks2
  5. Restart polkit

    root # systemctl restart polkit

11 Automatic Security Checks with seccheck Edit source

The seccheck SUSE Security Checker is a set of shell scripts designed to automatically check the local security of a system on a regular schedule, and emails reports to the root user, or any user as configured by the administrator.

If seccheck is not installed on your system, install it with sudo zypper in seccheck. These scripts are controlled by systemd timers, which are not enabled by default, but must be enabled by the administrator.

11.1 Seccheck Timers Edit source

There are four seccheck timers:

  • /usr/lib/systemd/system/seccheck-daily.timer

  • /usr/lib/systemd/system/seccheck-monthly.timer

  • /usr/lib/systemd/system/seccheck-weekly.timer

  • /usr/lib/systemd/system/seccheck-autologout.timer

seccheck-daily.timer, seccheck-monthly.timer, and seccheck-weekly.timer run multiple checks as described in Section 11.3, “Daily, Weekly, and Monthly Checks”. seccheck-autologout.timer logs out inactive users, see Section 11.4, “Automatic Logout”.

You can change the recipient of the seccheck mails from root to any user in /etc/sysconfig/seccheck. The following example changes it to an admin user named firewall:

SECCHK_USER="firewall"

11.2 Enabling Seccheck Timers Edit source

Manage your timers with systemctl, just like any other systemd timer. The following example enables and starts seccheck-daily.timer:

tux > sudo systemctl enable --now seccheck-daily.timer

List all active timers:

tux > sudo systemctl list-timers

List all enabled timers, active and inactive:

tux > sudo systemctl list-timers --all

11.3 Daily, Weekly, and Monthly Checks Edit source

seccheck performs the following daily checks:

/etc/passwd check

length/number/contents of fields, accounts with same UID accounts with UID/GID of 0 or 1 beside root and bin

/etc/shadow check

length/number/contents of fields, accounts with no password

/etc/group check

length/number/contents of fields

user root checks

secure umask and PATH

/etc/ftpusers

checks if important system users are put there

/etc/aliases

checks for mail aliases which execute programs

.rhosts check

checks if users' .rhosts file contain + signs

home directory

checks if home directories are writable or owned by someone else

dot-files check

checks many dot-files in the home directories if they are writable or owned by someone else

mailbox check

checks if user mailboxes are owned by user and are readable

NFS export check

exports should not be exported globally

NFS import check

NFS mounts should have the nosuid option set

promisc check

checks if network cards are in promiscuous mode

list modules

lists loaded modules

list sockets

lists open ports

The following table lists the weekly checks:

password check

runs john to crack the password file, user will receive an e-mail notice to change their password

RPM md5 check

checks for changed files via RPM's MD5 checksum feature

suid/sgid check

lists all suid and sgid files

exec group write

lists all executables which are group/world-writable

writable check

lists all files which are world-writable (including executables)

device check

lists all devices

Important
Important: Auditing Passwords with john

To enable password auditing, it is necessary to first install the package john, the John the Ripper fast password cracker. The package is available on the openSUSE Build Service at https://build.opensuse.org/package/show/security/john.

The monthly check prints a complete report, and the daily and weekly checks print diffs.

11.4 Automatic Logout Edit source

The seccheck-autologout.timer timer runs every 10 minutes, checks both remote and local terminal sessions for inactivity, and terminates them if an idle time is exceeded.

Configure your desired timeouts in /etc/security/autologout.conf file. Parameters include default idle and logout delay times, and the configuration for limiting maximum idle times specific to users, groups, TTY devices and SSH sessions. /etc/security/autologout.conf includes several configuration examples.

12 Software Management Edit source

12.1 Removing Unnecessary Software Packages (RPMs) Edit source

A very important step in securing a Linux system is to determine the primary function(s) or role(s) of the Linux server. Otherwise, it can be difficult to understand what needs to be secured and securing these Linux systems can prove ineffective. Therefore, it is critical to look at the default list of software packages and remove any unnecessary packages or packages that do not comply with your defined security policies.

Generally, an RPM software package consists of the following:

  • The package's meta data that is written to the RPM database upon installation.

  • The package's files and directories.

  • Scripts that are being executed before and after installation and removal.

Packages generally do not impose any security risk to the system unless they contain:

  1. setuid or setgid bits on any of the installed files

  2. group- or world-writable files or directories

  3. a service that is activated upon installation/activated by default.

Assuming that neither of the three conditions above apply, a package is merely a collection of files. Neither installation nor deinstallation of such packages has influence on the security value of the system.

Nevertheless, it is useful to restrict the installed packages in your system to a minimum. Doing this will result in fewer packages that require updates and will simplify maintenance efforts when security alerts and patches are released. It is a best practice not to install, among others, development packages or desktop software packages (for example, an X Server) on production servers. If you do not need them, you should also not install, for example, the Apache Web server or Samba file sharing server.

Important
Important: Requirements of Third-party Installers

Many third-party vendors like Oracle and IBM require a desktop environment and development libraries to run installers. To avoid this from having an impact on the security of their production servers, many organizations work around this by creating a silent installation (response file) in a development lab.

Also, other packages like FTP and Telnet daemons should not be installed as well unless there is a justified business reason for it. ssh, scp or sftp should be used as replacements, see .

One of the first action items should be to create a Linux image that only contains RPMs needed by the system and applications, and those needed for maintenance and troubleshooting purposes. A good approach is to start with a minimum list of RPMs and then add packages as needed. This process is time-consuming but usually worth the effort.

Tip
Tip: Just Enough Operating System (JeOS)

The SUSE Appliance Program includes a component called JeOS (Just Enough Operating System). JeOS has a very small footprint and can be customized to fit the specific needs of a system developer. Main uses of JeOS are for hardware/software appliance or virtual machine development. Key benefits of JeOS are efficiency, higher performance, increased security and simplified management.

If JeOS is not an option for you, a good choice is the minimal installation pattern.

To generate a list of all installed packages, use the following command:

root # zypper packages -i

To retrieve details about a particular package, run:

root # zypper info PACKAGE_NAME

To check for and report potential conflicts and dependencies when deleting a package, run:

root # zypper rm -D PACKAGE_NAME

This can be very useful, as running the removal command without a test can often yield a lot of complaints and require manual recursive dependency hunting.

Important
Important: Removal of Essential System Packages

When removing packages, be careful not to remove any essential system packages. This could put your system into a broken state in which it can no longer be booted or repaired. If you are uncertain about this, then it is best to do a complete backup of your system before you start to remove any packages.

For the final removal of one or more packages use the following zypper command with the added -u switch, which causes any dependencies that are becoming unused by removing the named packages, to be removed as well:

root # zypper rm -u PACKAGE_NAME

12.2 Patching Linux Systems Edit source

Building an infrastructure for patch management is another very important part of a proactive and secure production Linux environment.

It is recommended to have a written security policy and procedure to handle Linux security updates and issues. For example, a security policy should detail the time frame for assessment, testing, and roll out of patches. Network related security vulnerabilities should get the highest priority and should be addressed immediately within a short time frame. The assessment phase should occur within a testing lab, and initial roll out should occur on development systems first

A separate security log file should contain details on which Linux security announcements have been received, which patches have been researched and assessed, when patches have been applied, etc.

SUSE releases their patches in three categories, security, recommended and optional. There are a few options that can be used to keep systems patched, up to date and secure. Each system can register and then retrieve updates via the SUSE Update Web site using the included YaST tool—YaST Online Update. SUSE has also created the Repository Mirroring Tool (RMT), an efficient way to maintain a local repository of available/released patches/updates/fixes that systems can then pull from (reducing Internet traffic). SUSE also offers SUSE Manager for the maintenance, patching, reporting and centralized management of Linux systems, not only SUSE, but other distributions as well.

12.2.1 YaST Online Update Edit source

On a per-server basis, installation of important updates and improvements is possible using the YaST Online Update tool. Current updates for the SUSE Linux Enterprise family are available from the product specific update catalogs containing patches. Installation of updates and improvements is accomplished using YaST and selecting Online Update in the Software Group. All new patches (except the optional ones) that are currently available for your system will already be marked for installation. Clicking Accept will then automatically install these patches.

12.2.2 Automatic Online Update Edit source

YaST also offers the possibility to set up an automatic update. Select Software ›  Automatic Online Update. Configure a Daily or a Weekly update. Some patches, such as kernel updates, require user interaction, which would cause the automatic update procedure to stop. Check Skip Interactive Patches for the update procedure to proceed automatically.

In this case, run a manual Online Update from time to install patches that require interaction.

When Only Download Patches is checked, the patches are downloaded at the specified time but not installed. They must be installed manually using rpmor zypper.

12.2.3 Repository Mirroring Tool—RMT Edit source

The Repository Mirroring Tool for SUSE Linux Enterprise goes one step further than the Online Update process by establishing a proxy system with repository and registration targets. This helps customers centrally manage software updates within the firewall on a per-system basis, while maintaining their corporate security policies and regulatory compliance.

The downloadable RMT (http://download.suse.com/) is integrated with SUSE Customer Center (https://scc.suse.com/) and provides a repository and registration target that is synchronized with it. This can be very helpful in tracking entitlements in large deployments. The RMT maintains all the capabilities of SUSE Customer Center, while allowing a more secure centralized deployment. It is included with every SUSE Linux Enterprise subscription and is therefore fully supported.

The RMT provides an alternative to the default configuration, which requires opening the firewall to outbound connections for each device to receive updates. That requirement often violates corporate security policies and can be seen as a threat to regulatory compliance by some organizations. Through its integration with SUSE Customer Center, the RMT ensures that each device can receive its appropriate updates without the need to open the firewall, and without any redundant bandwidth requirements.

The RMT also enables customers to locally track their SUSE Linux Enterprise devices (that is servers, desktops, or Point of Service terminals) throughout their enterprise. Now they can easily determine how many entitlements are in need of renewal at the end of a billing cycle without having to physically walk through the data center to manually update spreadsheets.

The RMT informs the SUSE Linux Enterprise devices of any available software updates. Each device then obtains the required software updates from the RMT. The introduction of the RMT improves the interaction among SUSE Linux Enterprise devices within the network and simplifies how they receive their system updates. The RMT enables an infrastructure for several hundred SUSE Linux Enterprise devices per instance of each installation (depending on the specific usage profile). This offers more accurate and efficient server tracking.

In a nutshell, the Repository Mirroring Tool for SUSE Linux Enterprise provides customers with:

  • Assurance of firewall and regulatory compliance

  • Reduced bandwidth usage during software updates

  • Full support under active subscription from SUSE

  • Maintenance of existing customer interface with SUSE Customer Center

  • Accurate server entitlement tracking and effective measurement of subscription usage

  • Automated process to easily tally entitlement totals (no more spreadsheets!)

  • Simple installation process that automatically synchronizes server entitlement with SUSE Customer Center

12.2.4 SUSE Manager Edit source

SUSE Manager automates Linux server management, allowing you to provision and maintain your servers faster and more accurately. It monitors the health of each Linux server from a single console so you can identify server performance issues before they impact your business. And it lets you comprehensively manage your Linux servers across physical, virtual and cloud environments while improving data center efficiency. SUSE Manager delivers complete lifecycle management for Linux:

  • Asset management

  • Provisioning

  • Package management

  • Patch management

  • Configuration management

  • Redeployment

For more information on SUSE Manager refer to https://www.suse.com/products/suse-manager/.

13 File Management Edit source

13.1 Disk Partitions Edit source

Servers should have separate file systems for at least /, /boot, /usr, /var, /tmp, and /home. This prevents, for example, that logging space and temporary space under /var and /tmp fill up the root partition. Third-party applications should be on separate file systems as well, for example under /opt.

Another advantage of separate file systems is the possibility to choose special mount options that are only suitable for certain regions in the file system hierarchy. A number of interesting mount options are:

  • noexec: prevents execution of files.

  • nodev: prevents character or block special devices from being usable.

  • nosuid: prevents the set-user-ID or set-group-ID bits from being effective.

  • ro: mounts the file system read-only.

Each of these options needs to be carefully considered before applying it to a partition mount. Applications may stop working, or the support status may be violated. When applied correctly, mount options can help against some types of security attacks or misconfigurations. For example, there should be no need for set-user-ID binaries to be placed in /tmp.

You are advised to review Chapter 2, Common Criteria. It is important to understand the need to separate the partitions that could impact a running system (for example, log files filling up /var/log are a good reason to separate /var from the / partition). Another thing to keep in mind is that you will likely need to leverage LVM or another volume manager or at the very least the extended partition type to work around the limit of four primary partitions on PC class systems.

Another capability in SUSE Linux Enterprise Server is encrypting a partition or even a single directory or file as a container. Refer to Chapter 14, Encrypting Partitions and Files for details.

13.2 Checking File Permissions and Ownership Edit source

The following sections deal with some ways the default permissions and file settings can be modified to enhance the security of a host. It is important to note that the use of the default SUSE Linux Enterprise Server utilities like seccheck - can be run to lock down and improve the general file security and user environment. However, it is beneficial to understand how to modify these things.

SUSE Linux Enterprise Server hosts include three default settings for file permissions: permissions.easy, permissions.secure, and permissions.paranoid, all located in the /etc directory. The purpose of these files is to define special permissions, such as world-writable directories or, for files, the setuser ID bit (programs with the setuser ID bit set do not run with the permissions of the user that has launched it, but with the permissions of the file owner, usually root).

Administrators can use the file /etc/permissions.local to add their own settings. The easiest way to implement one of the default permission rule-sets above is to use the Local Security module in YaST.

Each of the following topics will be modified by a selected rule-set, but the information is important to understand on its own.

13.3 Default umask Edit source

The umask (user file-creation mode mask) command is a shell built-in command which determines the default file permissions for newly created files and directories. This can be overwritten by system calls but many programs and utilities use umask. By default, umask is set to 022. You can modify this globally by changing the value in /etc/profile or for each user in the startup files of the shell.

To determine the active umask, use the umask command:

tux > umask
022

The umask is subtracted from the access mode 777 if at least one bit is set.

With the default umask you see the behavior most users expect to see on a Linux system.

tux > touch a
tux > mkdir b
tux > ls -on
total 16
-rw-r--r--. 1 17086    0 Nov 29 15:05 a
drwxr-xr-x. 2 17086 4096 Nov 29 15:05 b

You can specify arbitrary umask values, depending on your needs.

tux > umask 111
tux > touch c
tux > mkdir d
tux > ls -on
total 16
-rw-rw-rw-. 1 17086    0 Nov 29 15:05 c
drw-rw-rw-. 2 17086 4096 Nov 29 15:05 d

Based on your thread model you can use a stricter umask like 037 to prevent accidental data leakage.

tux > umask 037
tux > touch e
tux > mkdir f
tux > ls -on
total 16
-rw-r-----. 1 17086    0 Nov 29 15:06 e
drwxr-----. 2 17086 4096 Nov 29 15:06 f

13.4 SUID/SGID Files Edit source

When the SUID (set user ID) or SGID (set group ID) bits are set on an executable, it executes with the UID or GID of the owner of the executable rather than that of the person executing it. This means that, for example, all executables that have the SUID bit set and are owned by root are executed with the UID of root. A good example is the passwd command that allows ordinary users to update the password field in the /etc/shadow file which is owned by root.

But SUID/SGID bits can be misused when the executable has a security hole. Therefore, you should search the entire system for SUID/SGID executables and document it. To search the entire system for SUID or SGID files, you can run the following command:

root # find /bin /boot /etc /home /lib /lib64 /opt /root /sbin /srv /tmp /usr /var -type f -perm '/6000' -ls

You might need to extend the list of directories that are searched if you have a different file system structure.

SUSE only sets the SUID/SGID bit on binary if it is really necessary. Ensure that code developers do not set SUID/SGID bits on their programs if it is not an absolute requirement. Very often you can use workarounds like removing the executable bit for world/others. However, a better approach is to change the design of the software or use capabilities.

SUSE Linux Enterprise Server supports file capabilities to allow more fine grained privileges to be given to programs rather than the full power of root:

root # getcap -v /usr/bin/ping
      /usr/bin/ping = cap_new_raw+eip

The previous command only grants the CAP_NET_RAW capability to whoever executes ping. In case of vulnerabilities inside ping, an attacker can gain at most this capability in contrast with full root. Whenever possible, file capabilities should be chosen in favor of the SUID bit. But this only applies when the binary is suid to root, not to other users such as news, lp and similar.

13.5 World-Writable Files Edit source

World-writable files are a security risk since they can be modified by any user on the system. Additionally, world-writable directories allow anyone to add or delete files. To locate world-writable files and directories, you can use the following command:

root # find /bin /boot /etc /home /lib /lib64 /opt /root /sbin /srv /tmp /usr /var -type f -perm -2 ! -type l -ls

You might need to extend the list of directories that are searched if you have a different file system structure.

The ! -type l parameter skips all symbolic links since symbolic links are always world-writable. However, this is not a problem as long as the target of the link is not world-writable, which is checked by the above find command.

World-writable directories with the sticky bit such as the /tmp directory do not allow anyone except the owner of a file to delete or rename it in this directory. The sticky bit makes files stick to the user who created it and it prevents other users from deleting and renaming the files. Therefore, depending on the purpose of the directory, world-writable directories with sticky are usually not an issue. An example is the /tmp directory:

tux > ls -ld /tmp
drwxrwxrwt 18 root root 16384 Dec 23 22:20 /tmp

The t mode bit in the output denotes the sticky bit.

13.6 Orphaned or Unowned Files Edit source

Files not owned by any user or group might not necessarily be a security problem in itself. However, unowned files could pose a security problem in the future. For example, if a new user is created and the new users happens to get the same UID as the unowned files have, then this new user will automatically become the owner of these files.

To locate files not owned by any user or group, use the following command:

root # find /bin /boot /etc /home /lib /lib64 /opt /root /sbin /srv /tmp /usr /var -nouser -o -nogroup

You might need to extend the list of directories that are searched if you have a different file system structure.

A different problem is files that were not installed via the packaging system and therefore don't receive updates. You can check for such files with the following command:

tux > find /bin /lib /lib64 /usr -path /usr/local -prune -o -type f -a -exec /bin/sh -c "rpm -qf {} &> /dev/null || echo {}" \;

Run this command as an untrusted user (for example nobody) since crafted file names might lead to command execution. This shouldn't be a problem since these directories should only be writeable by root, but it's still a good security precaution.

This will show you all files under /bin, /lib, /lib64 and /usr (with the exception of files in /usr/local) that are not tracked by the package manager. These files might not represent a security issue, but you should be aware of what is not tracked and take the necessary precautions to keep these files up to date.

14 Encrypting Partitions and Files Edit source

Encrypting files, partitions, and entire disks prevents unauthorized access to your data and protects your confidential files and documents.

You can choose between the following encryption options:

Encrypting a Hard Disk Partition

It is possible to create an encrypted partition with YaST during installation or in an already installed system. For further info, see Section 14.1.1, “Creating an Encrypted Partition during Installation” and Section 14.1.2, “Creating an Encrypted Partition on a Running System”. This option can also be used for removable media, such as external hard disks, as described in Section 14.1.3, “Encrypting the Content of Removable Media”.

Encrypting Single Files with GPG

To quickly encrypt one or more files, you can use the GPG tool. See Section 14.2, “Encrypting Files with GPG” for more information.

Warning
Warning: Encryption Offers Limited Protection

Encryption methods described in this chapter cannot protect your running system from being compromised. After the encrypted volume is successfully mounted, everybody with appropriate permissions can access it. However, encrypted media are useful in case of loss or theft of your computer, or to prevent unauthorized individuals from reading your confidential data.

14.1 Setting Up an Encrypted File System with YaST Edit source

Use YaST to encrypt partitions or parts of your file system during installation or in an already installed system. However, encrypting a partition in an already-installed system is more difficult, because you need to resize and change existing partitions. In such cases, it may be more convenient to create an encrypted file of a defined size, in which to store other files or parts of your file system. To encrypt an entire partition, dedicate a partition for encryption in the partition layout. The standard partitioning proposal, as suggested by YaST, does not include an encrypted partition by default. Add an encrypted partition manually in the partitioning dialog.

14.1.1 Creating an Encrypted Partition during Installation Edit source

Warning
Warning: Password Input

Make sure to memorize the password for your encrypted partitions well. Without that password, you cannot access or restore the encrypted data.

The YaST expert dialog for partitioning offers the options needed for creating an encrypted partition. To create a new encrypted partition proceed as follows:

  1. Run the YaST Expert Partitioner with System › Partitioner.

  2. Select a hard disk, click Add, and select a primary or an extended partition.

  3. Select the partition size or the region to use on the disk.

  4. Select the file system, and mount point of this partition.

  5. Activate the Encrypt device check box.

    Note
    Note: Additional Software Required

    After checking Encrypt device, a pop-up window asking for installing additional software may appear. Confirm to install all the required packages to ensure that the encrypted partition works well.

  6. If the encrypted file system needs to be mounted only when necessary, enable Do not mount partition in the Fstab Options. otherwise enable Mount partition and enter the mount point.

  7. Click Next and enter a password which is used to encrypt this partition. This password is not displayed. To prevent typing errors, you need to enter the password twice.

  8. Complete the process by clicking Finish. The newly-encrypted partition is now created.

During the boot process, the operating system asks for the password before mounting any encrypted partition which is set to be auto-mounted in /etc/fstab. Such a partition is then available to all users when it has been mounted.

To skip mounting the encrypted partition during start-up, press Enter when prompted for the password. Then decline the offer to enter the password again. In this case, the encrypted file system is not mounted and the operating system continues booting, blocking access to your data.

To mount an encrypted partition which is not mounted during the boot process, open a file manager and click the partition entry in the pane listing common places on your file system. You will be prompted for a password and the partition will be mounted.

When you are installing your system on a machine where partitions already exist, you can also decide to encrypt an existing partition during installation. In this case follow the description in Section 14.1.2, “Creating an Encrypted Partition on a Running System” and be aware that this action destroys all data on the existing partition.

14.1.2 Creating an Encrypted Partition on a Running System Edit source

Warning
Warning: Activating Encryption on a Running System

It is also possible to create encrypted partitions on a running system. However, encrypting an existing partition destroys all data on it, and requires re-sizing and restructuring of existing partitions.

On a running system, select System › Partitioner in the YaST control center. Click Yes to proceed. In the Expert Partitioner, select the partition to encrypt and click Edit. The rest of the procedure is the same as described in Section 14.1.1, “Creating an Encrypted Partition during Installation”.

14.1.3 Encrypting the Content of Removable Media Edit source

YaST treats removable media (like external hard disks or flash disks) the same as any other storage device. Virtual disks or partitions on external media can be encrypted as described above. However, you should disable mounting at boot time, because removable media is usually connected only when the system is up and running.

If you encrypted your removable device with YaST, the GNOME desktop automatically recognizes the encrypted partition and prompts for the password when the device is detected. If you plug in a FAT-formatted removable device when running GNOME, the desktop user entering the password automatically becomes the owner of the device. For devices with a file system other than FAT, change the ownership explicitly for users other than root to give them read-write access to the device.

14.2 Encrypting Files with GPG Edit source

The GPG encryption software can be used to encrypt individual files and documents.

To encrypt a file with GPG, you need to generate a key pair first. To do this, run the gpg --gen-key and follow the on-screen instructions. When generating the key pair, GPG creates a user ID (UID) to identify the key based on your real name, comments, and email address. You need this UID (or just a part of it like your first name or email address) to specify the key you want to use to encrypt a file. To find the UID of an existing key, use the gpg --list-keys command. To encrypt a file use the following command:

tux > gpg -e -r UID
  FILE

Replace UID with part of the UID (for example, your first name) and FILE with the file you want to encrypt. For example:

tux > gpg -e -r Tux secret.txt

This command creates an encrypted version of the specified file recognizable by the .gpg file extension (in this example, it is secret.txt.gpg).

To decrypt an encrypted file, use the following command:

tux > gpg -d -o DECRYPTED_FILE
  ENCRYPTED_FILE

Replace DECRYPTED_FILE with the desired name for the decrypted file and ENCRYPTED_FILE with the encrypted file you want to decrypt.

Keep in mind that the encrypted file can be only decrypted using the same key that was used for encryption. If you want to share an encrypted file with another person, you have to use that person's public key to encrypt the file.

15 User Management Edit source

15.1 Various Account Checks Edit source

15.1.1 Unlocked Accounts Edit source

It is important that all system and vendor accounts that are not used for logins are locked. To get a list of unlocked accounts on your system, you can check for accounts that do not have an encrypted password string starting with ! or * in the /etc/shadow file. If you lock an account using passwd -l, it will put a !! in front of the encrypted password, effectively disabling the password. If you lock an account using usermod -L, it will put a ! in front of the encrypted password. Many system and shared accounts are usually locked by default by having a * or !! in the password field which renders the encrypted password into an invalid string. Hence, to get a list of all unlocked (encryptable) accounts, run (egrep is used to allow use of regular-expressions):

root # egrep -v ':\*|:\!' /etc/shadow | awk -F: '{print $1}'

Also make sure all accounts have a x in the password field in /etc/passwd. The following command lists all accounts that do not have a x in the password field:

root # grep -v ':x:' /etc/passwd

An x in the password fields means that the password has been shadowed, for example the encrypted password needs to be looked up in the /etc/shadow file. If the password field in /etc/passwd is empty, then the system will not look up the shadow file and it will not prompt the user for a password at the login prompt.

15.1.2 Unused Accounts Edit source

All system or vendor accounts that are not being used by users, applications, by the system or by daemons should be removed from the system. You can use the following command to find out if there are any files owned by a specific account:

root # find / -path /proc -prune -o -user ACCOUNT -ls

The -prune option in this example is used to skip the /proc file system. If you are sure that an account can be deleted, you can remove the account using the following command:

root # userdel -r ACCOUNT

Without the -r option userdel will not delete the user's home directory and mail spool (/var/spool/mail/USER). Note that many system accounts have no home directory.

15.2 Enabling Password Aging Edit source

Password expiration is a general best practice—but might need to be excluded for some system and shared accounts (for example Oracle, etc.). Expiring password on those accounts could lead to system outages if the application account expires.

Typically a corporate policy should be developed that dictates rules/procedures regarding password changes for system and shared accounts. However, normal user account passwords should expire automatically. The following example shows how password expiration can be set up for individual user accounts.

The following files and parameters in the table can be used when a new account is created with the useradd command. Settings such as these are stored for each user account in the /etc/shadow file. If using the YaST tool (User and Group Management) to add users, the settings are available on a per-user basis. Here are the various settings—some of which can also be system-wide (for example modification of /etc/login.defs and /etc/default/useradd):

/etc/login.defs

PASS_MAX_DAYS

Maximum number of days a password is valid.

/etc/login.defs

PASS_MIN_DAYS

Minimum number of days before a user can change the password since the last change.

/etc/login.defs

PASS_WARN_AGE

Number of days when the password change reminder starts.

/etc/default/useradd

INACTIVE

Number of days after password expiration that account is disabled.

/etc/default/useradd

EXPIRE

Account expiration date in the format YYYY-MM-DD.

Note
Note

Users created prior to these modifications will not be affected.

Ensure that the above parameters are changed in the /etc/login.defs and /etc/default/useradd files. Review of the /etc/shadow file will show how these settings get stored after adding a user.

To create a new user account, execute the following command:

root # useradd -c "TEST_USER" -g USERS TEST

The -g option specifies the primary group for this account:

root # id TEST
uid=509(test) gid=100(users) groups=100(users)

The settings in /etc/login.defs and /etc/default/useradd are recorded for the test user in the /etc/shadow file as follows:

root # grep TEST /etc/shadow
test:!!:12742:7:60:7:14::

Password aging can be modified at any time by use of the chage command. To disable password aging for system and shared accounts, you can run the following chage command:

root # chage -M -1 SYSTEM_ACCOUNT_NAME

To get password expiration information:

root # chage -l SYSTEM_ACCOUNT_NAME

For example:

root # chage -l TEST
Minimum: 7
Maximum: 60
Warning: 7
Inactive: 14
Last Change: Jan 11, 2015
Password Expires: Mar 12, 2015
Password Inactive: Mar 26, 2015
Account Expires: Never

15.3 Stronger Password Enforcement Edit source

On an audited system it is important to restrict people from using simple passwords that can be cracked too easily. Writing down complex passwords is all right as long as they are stored securely. Some will argue that strong passwords protect you against dictionary attacks and those type of attacks can be defeated by locking accounts after a few failed attempts. However, this is not always an option. If set up like this, locking system accounts could bring down your applications and systems which would be nothing short of a denial of service attack – another issue.

At any rate, it is important to practice effective password management safety. Most companies require that passwords have at the very least a number, one lowercase letter, and one uppercase letter. Policies vary, but maintaining a balance between password strength/complexity and management is sometimes difficult.

15.4 Password and Login Management with PAM Edit source

Linux-PAM (Pluggable Authentication Modules for Linux) is a suite of shared libraries that enable the local system administrator to choose how applications authenticate users.

It is strongly recommended to familiarize oneself with the capabilities of PAM and how this architecture can be leveraged to provide the best authentication setup for an environment. This configuration can be done once – and implemented across all systems (a standard) or can be enhanced for individual hosts (enhanced security – by host / service / application). The key is to realize how flexible the architecture is.

To learn more about the PAM architecture, find PAM documentation in the /usr/share/doc/packages/pam directory (in a variety of formats).

The following discussions are examples of how to modify the default PAM stacks—specifically around password policies—for example password strength, password re-use and account locking. While these are only a few of the possibilities, they serve as a good start and demonstrate PAM's flexibility.

Important
Important: pam-config Limitations

The pam-config tool can be used to configure the common-{account,auth,password,session} PAM configuration files, which contain global options. These files include the following comment:

# This file is autogenerated by pam-config. All changes
# will be overwritten.

Individual service files, such as login, password, sshd, and su must be edited directly. You may elect to edit all files directly, and not use pam-config, though pam-config includes useful features such as converting an older configuration, updating your current configuration, and sanity checks. For more information see man 8 pam-config.

15.4.1 Password Strength Edit source

SUSE Linux Enterprise Server can leverage the pam_cracklib library to test for weak passwords – and to suggest using a stronger one if it determines obvious weakness. The following parameters represent an example that could be part of a corporate password policy or something required because of audit constraints.

The PAM libraries follow a defined flow. The best way to design the perfect stack usually is to consider all of the requirements and policies and draw out a flow chart.

Table 15.1: Sample rules/constraints for password enforcement

pam_cracklib.so

minlen=8

Minimum length of password is 8

pam_cracklib.so

lcredit=-1

Minimum number of lowercase letters is 1

pam_cracklib.so

ucredit=-1

Minimum number of uppercase letters is 1

pam_cracklib.so

dcredit=-1

Minimum number of digits is 1

pam_cracklib.so

ocredit=-1

Minimum number of other characters is 1

To set up these password restrictions, use the pam-config tool to specify the parameters you want to configure. For example, the minimum length parameter could be modified like this:

tux > sudo pam-config -a --cracklib-minlen=8 --cracklib-retry=3 \
--cracklib-lcredit=-1 --cracklib-ucredit=-1 --cracklib-dcredit=-1 \
--cracklib-ocredit=-1 --cracklib

Now verify that the new password restrictions work for new passwords. Simply login to a non-root account and change the password using the passwd command. Note that the above requirements are not enforced if you run the passwd command under root.

15.4.2 Restricting Use of Previous Passwords Edit source

The pam_pwhistory module can be used to configure the number of previous passwords that cannot be reused. The following command implements password restrictions on a system so that a password cannot be reused for at least six months:

tux > sudo pam-config -a --pwhistory --pwhistory-remember=26

Recall that in the section Section 15.2, “Enabling Password Aging” we set PASS_MIN_DAYS to 7, which specifies the minimum number of days allowed between password changes. Therefore, if pam_unix is configured to remember 26 passwords, then the previously used passwords cannot be reused for at least six months (26*7 days).

The PAM configuration (/etc/pam.d/common-auth) resulting from the pam-config command looks like the following:

auth      required   pam_env.so
auth      required   pam_unix.so     try_first_pass
account   required   pam_unix.so     try_first_pass
password  requisit   pam_cracklib.so
password  required   pam_pwhistory.so        remember=26
password  optional   pam_gnome_keyring.so    use_authtok
password  required   pam_unix.so     use_authtok nullok shadow try_first_pass
session   required   pam_limits.so
session   required   pam_unix.so     try_first_pass
session   optional   pam_umask.so

15.4.3 Locking User Accounts After Too Many Login Failures Edit source

Locking accounts after a defined number of failed ssh, login, su, or sudo attempts is a common security practice. However, this could lead to outages if an application, admin, or root user is locked out. In effect this makes it easy to cause denial-of-service attacks by deliberately creating login failures. Fortunately, controlling this with PAM is straightforward.

By default, PAM allows all root logins. Use pam_tally2 to control failed login behavior for all other users, including human and system users. Add the following line to the top of /etc/pam.d/login to lockout all users, (except root) after 6 failed logins, and to automatically unlock the account after ten minutes:

auth required pam_tally2.so deny=6 unlock_time=600

This is an example of a complete /etc/pam.d/login file:

#%PAM-1.0
auth     requisite      pam_nologin.so
auth     include        common-auth
auth     required       pam_tally2.so deny=6 unlock_time=600
account  include        common-account
account  required       pam_tally2.so
password include        common-password
session  required       pam_loginuid.so
session  include        common-session
#session  optional       pam_lastlog.so nowtmp showfailed
session  optional       pam_mail.so standard

You may also lock out root, though obviously you must be very certain you want to do this:

auth required pam_tally2.so deny=6 even_deny_root unlock_time=600

You may define a different lockout time for root:

auth required pam_tally2.so deny=6 root_unlock_time=120  unlock_time=600

If you want to require the administrator to unlock accounts, leave off the unlock_time option. The next two example commands display the number of failed login attempts, and how to unlock a user account:

root # pam_tally2 -u username
Login           Failures Latest failure     From
username            6    12/17/19 13:49:43  pts/1

root # pam_tally2 -r -u username

The default location for attempted accesses is recorded in /var/log/tallylog.

If the user succeeds in logging in after the login timeout expires, or after the administrator resets their account, the counter resets to to 0.

Configure other login services to use pam_tally2 in their individual configuration files in /etc/pam.d/: sshd, su, sudo, sudo-i, and su-l.

15.5 Restricting root Logins Edit source

By default, the root user is assigned a password and can log in using various methods—for example, on a local terminal, in a graphical session, or remotely via SSH. These methods should be restricted as far as possible. Shared usage of the root account should be avoided. Instead, individual administrators should use tools such as su or sudo (for more information, type man 1 su or man 8 sudo) to obtain elevated privileges. This allows associating root logins with particular users. This also adds another layer of security; not only the root password, but both the root and the password of an an administrator's regular account would need to be compromised to gain full root access. This section explains how to limit direct root logins on the different levels of the system.

15.5.1 Restricting Local Text Console Logins Edit source

TTY devices provide text-mode system access via the console. For desktop systems these are accessed via the local keyboard or—in case of server systems—via input devices connected to a KVM switch or a remote management card (ILO, DRAC, etc). By default, Linux offers 6 different consoles, that can be switched to via the key combinations AltF1 to AltF6, when running in text mode, or CtrlAltF1 to CtrlAltF6 when running in a graphical session. The associated terminal devices are named tty1 .. tty6 accordingly.

The following steps restrict root access to the first TTY. Even this access method is only meant for emergency access to the system and should never be used for everyday system administration tasks.

Note
Note

The steps shown here are tailored towards PC architectures (x86 and AMD64/Intel 64). On architectures such as POWER, different terminal device names than tty1 may be used. Be careful not to lock yourself out completely by specifying wrong terminal device names. You can determine the device name of the terminal you are currently logged into by running the tty command. Be careful not to do this in a virtual terminal, such as via SSH or in a graphical session (device names /dev/pts/N), but only from an actual login terminal reachable via AltFN.

Procedure 15.1: Restricting root logins on local TTYs
  1. Ensure that the PAM stack configuration file /etc/pam.d/login contains the pam_securetty module in the auth block:

    auth     requisite      pam_nologin.so
     auth     [user_unknown=ignore success=ok ignore=ignore auth_err=die default=bad] pam_securetty.so noconsole
     auth     include        common-auth

    This will include the pam_securetty module during the authentication process on local consoles, which restricts root to logging-in only on TTY devices that are listed in the file /etc/securetty.

  2. Remove all entries from /etc/securetty except one. This limits the access to TTY devices for root.

    #
    # This file contains the device names of tty lines (one per line,
    # without leading /dev/) on which root is allowed to login.
    #
    tty1
  3. Check whether logins to other terminals will be rejected for root. A login on tty2, for example, should be rejected immediately, without even querying the account password. Also make sure that you can still successfully login to tty1 and thus that root is not locked out of the system completely.

Important
Important

Do not add the pam_securetty module to the /etc/pam.d/common-auth file. This would break the su and sudo commands, because these tools would then also reject root authentications.

Important
Important

These configuration changes will also cause root logins on serial consoles such as /dev/ttyS0 to be denied. In case you require such use cases you need to list the respective tty devices additionally in the /etc/securetty file.

15.5.2 Restricting Graphical Session Logins Edit source

To improve security on your server, avoid using graphical environments at all. Graphical programs are often not designed to be run as root and are more likely to contain security issues than console programs. If you require a graphical login, use a non-root login. Configure your system to disallow root from logging in to graphical sessions.

To prevent root from logging in to graphical sessions, you can apply the same basic steps as outlined in Section 15.5.1, “Restricting Local Text Console Logins”. Just add the pam_securetty module to the PAM stack file belonging to the display manager—for example, /etc/pam.d/gdm for GDM. The graphical session also runs on a TTY device: by default, tty7. Therefore, if you restrict root logins to tty1, then root will be denied login in the graphical session.

15.5.3 Restricting SSH Logins Edit source

By default, the root user is also allowed to log into a machine remotely via the SSH network protocol (if the SSH port is not blocked by the firewall). To restrict this, make the following change to the OpenSSH configuration:

  1. Edit /etc/ssh/sshd_config and adjust the following parameter:

    PermitRootLogin no
  2. Restart the sshd service to make the changes effective:

    systemctl restart sshd.service
Note
Note

Using the PAM pam_securetty module is not suitable in case of OpenSSH, because not all SSH logins go through the PAM stack during authorization (for example, when using SSH public-key authentication). In addition, an attacker could differentiate between a wrong password and a successful login that was only rejected later on by policy.

15.6 Setting an Inactivity Timeout for Interactive Shell Sessions Edit source

It can be a good idea to terminate an interactive shell session after a certain period of inactivity. For example, to revent open, unguarded sessions or to avoid waste of system resources.

By default, there is no inactivity timeout for shells. Nothing will happen if a shell stays open and unused for days or even years. It is possible to configure most shells in a way that idle sessions terminate automatically after a certain amount of time. The following example shows how to set an inactivity timeout for a number of common types of shells.

The inactivity timeout can be configured for login shells only or for all interactive shells. In the latter case the inactivity timeout is running individually for each shell instance. This means that timeouts will accumulate. When a sub- or child-shell is started, then a new timeout begins for the sub- or child-shell, and only afterwards will the timeout of the parent continue running.

The following table contains configuration details for a selection of common shells shipped with SUSE Linux Enterprise Server:

packageshell personalitiesshell variabletime unitreadonly settingconfig path (only login shell)config path (all shells)

bash

bash, sh

TMOUT

seconds

readonly TMOUT=

/etc/profile.local, /etc/profile.d/

/etc/bash.bashrc

mksh

ksh, lksh, mksh, pdksh

TMOUT

seconds

readonly TMOUT=

/etc/profile.local, /etc/profile.d/

/etc/ksh.kshrc.local

tcsh

csh, tcsh

autologout

minutes

set -r autologout=

/etc/csh.login.local

/etc/csh.cshrc.local

zsh

zsh

TMOUT

seconds

readonly TMOUT=

/etc/profile.local, /etc/profile.d/

/etc/zsh.zshrc.local

Every listed shell supports an internal timeout shell variable that can be set to a specific time value to cause the inactivity timeout. If you want to prevent users from overriding the timeout setting, you can mark the corresponding shell timeout variable as readonly. The corresponding variable declaration syntax is also found in the table above.

Note
Note

This feature is only helpful for avoiding risks if a user is forgetful or follows unsafe practices. It does not protect against hostile users. The timeout only applies to interactive wait states of a shell. A malicious user can always find ways to circumvent the timeout and keep their session open regardless.

To configure the inactivity timeout, you need to add the matching timeout variable declaration to each shell's startup script. Use either the path for login shells only, or the one for all shells, as listed in the table. The following example uses paths and settings that are suitable for bash and ksh to setup a read-only login shell timeout that cannot be overridden by users. Create the file /etc/profile.d/timeout.sh with the following content:

# /etc/profile.d/timeout.sh for SUSE Linux
#
# Timeout in seconds until the bash/ksh session is terminated
# in case of inactivity.
# 24h = 86400 sec
readonly TMOUT=86400
Tip
Tip

We recommend using the screen tool in order to detach sessions before logging out. screen sessions are not terminated and can be re-attached whenever required. An active session can be locked without logging out (read about Ctrlax / lockscreen in man screen for details).

15.7 Preventing Accidental Denial of Service Edit source

Linux allows you to set limits on the amount of system resources that users and groups can consume. This is also very handy if bugs in programs cause them to use up too much resources (for example memory leaks), slow down the machine, or even render the system unusable. Incorrect settings can allow programs to use too many resources which may make the server unresponsive to new connections or even local logins (for example if a program uses up all available file handles on the host). This can also be a security concern if someone is allowed to consume all system resources and therefore cause a denial of service attack – either unplanned or worse, planned. Setting resource limits for users and groups may be an effective way to protect systems, depending on the environment.

15.7.1 Example for Restricting System Resources Edit source

The following example demonstrates the practical usage of setting or restricting system resource consumption for an Oracle user account. For a list of system resource settings, see /etc/security/limits.conf or man limits.conf.

Most shells like Bash provide control over various resources (for example the maximum allowable number of open file descriptors or the maximum number of processes) that are available on a per/user basis. To examine all current limits in the shell execute:

root # ulimit -a

For more information on ulimit for the Bash shell, examine the Bash man pages.

Important
Important: Setting Limits for SSH Sessions

Setting "hard" and "soft" limits might not behave as expected when using an SSH session. To see valid behavior it may be necessary to login as root and then su to the id with limits (for example, oracle in these examples). Resource limits should also work assuming the application has been started automatically during the boot process. It may be necessary to set UsePrivilegeSeparation in /etc/ssh/sshd_config to "no" and restart the SSH daemon (systemctl restart sshd) if it seems that the changes to resource limits are not working (via SSH). However this is not generally recommended – as it weakens a systems security.

Tip
Tip: Disabling Password Logins via ssh

You can add some extra security to your server by disabling password authentication for SSH. Remember that you need to have SSH keys configured, otherwise you cannot access the server. To disable password login, add the following lines to /etc/ssh/sshd_config:

UseLogin no
UsePAM no
PasswordAuthentication no
PubkeyAuthentication yes

In this example, a change to the number of file handles or open files that the user oracle can use is made by editing /etc/security/limits.conf as root making the following changes:

oracle           soft    nofile          4096
oracle           hard    nofile          63536

The soft limit in the first line defines the limit on the number of file handles (open files) that the oracle user will have after login. If the user sees error messages about running out of file handles, then the user can increase the number of file handles like in this example up to the hard limit (in this example 63536) by executing:

root # ulimit -n 63536

You can set the soft and hard limits higher if necessary.

Note
Note

It is important to be judicious with the usage of ulimits. Allowing a "hard" limit for nofile for a user that equals the kernel limit (/proc/sys/fs/file-max) is very bad! If the user consumes all the available file handles, the system cannot initiate new logins as accessing (opening) PAM modules which are required for performing a login will not be possible.

You also need to ensure that pam_limits is either configured globally in /etc/pam.d/common-auth, or for individual services like SSH, su, login, and telnet in:

/etc/pam.d/sshd (for SSH)
/etc/pam.d/su (for su)
/etc/pam.d/login (local logins and telnet)

If you do not want to enable it for all logins, there is a specific PAM module that will read the /etc/security/limits.conf file. Entries in pam configuration directives will have entries like:

session     required      /lib/security/pam_limits.so
session     required      /lib/security/pam_unix.so

It is important to note that changes are not immediate and require a new login session:

root # su – oracle
tux > ulimit -n
4096

Note that these examples are specific to the Bash shell - ulimit options are different for other shells. The default limit for the user oracle is 4096. To increase the number of file handles the user oracle can use to 63536, do:

root # su – oracle
tux > ulimit -n
4096
tux > ulimit -n 63536
tux > ulimit -n
63536

Making this permanent requires the addition of the setting, ulimit -n 63536, (again, for Bash) to the users profile (~/.bashrc or ~/.profile file) which is the user start-up file for the Bash shell on SUSE Linux Enterprise Server (to verify your shell run: echo $SHELL). To do this you could simply type (or copy/paste – if you are reading this on the system) the following commands for the Bash shell of the user oracle:

root # su - oracle
tux > cat >> ~oracle/.bash_profile << EOF
ulimit -n 63536
EOF

15.8 Displaying Login Banners Edit source

It is often necessary to place a banner on login screens on all servers for legal/audit policy reasons or to give security instructions to users.

If you want to print a login banner after a user logs in on a text based terminal for example using SSH or on a local console, you can leverage the file /etc/motd (motd = message of the day). The file exists by default on SUSE Linux Enterprise Server, but it is empty. Simply add content to the file that is applicable/required by the organization.

Note
Note: Banner Length

Try to keep the login banner content to a single terminal page (or less), as it will scroll the screen if it does not fit, making it more difficult to read.

You can also have a login banner printed before a user logs in on a text based terminal. For local console logins you can edit the /etc/issue file, which will cause the banner to be displayed before the login prompt. For logins via SSH you can edit the Banner parameter in the /etc/ssh/sshd_config file, which will then appropriately display the banner text before the SSH login prompt.

For graphical logins via GDM, you can follow the GNOME admin guide to set up a login banner. Furthermore you can make the following changes to require a user to acknowledge the legal banner by selecting Yes or No. Edit the /etc/gdm/Xsession file and add the following lines at the beginning of the script:

if ! /usr/bin/gdialog --yesno '\nThis system is classified...\n' 10 10; then
    /usr/bin/gdialog --infobox 'Aborting login'
    exit 1;
fi

The text This system is classified... needs to be replaced with the desired banner text. It is important to note that this dialog will not prevent a login from progressing. For more information about GDM scripting, refer to the GDM Admin Manual.

15.9 Connection Accounting Utilities Edit source

Here is a list of commands you can use to get data about user logins:

who Lists currently logged in users.

w Shows who is logged in and what they are doing.

last Shows a list of last logged in users, including login time, logout time, login IP address, etc.

lastb Same as last, except that by default it shows /var/log/btmp, which contains all the bad login attempts.

lastlog This command reports data maintained in /var/log/lastlog, which is a record of the last time a user logged in.

ac Available after installing the acct package. Prints the connect time in hours on a per-user basis or daily basis, etc. This command reads /var/log/wtmp.

dump-utmp Converts raw data from /var/run/utmp or /var/log/wtmp into ASCII-parseable format.

Also check the /var/log/messages file, or the output of journalctl if no logging facility is running. See Book “Administration Guide”, Chapter 17 “journalctl: Query the systemd Journal” for more information on the systemd journal.

16 Spectre/Meltdown Checker Edit source

spectre-meltdown-checker is a shell script to test if your system is vulnerable to the several speculative execution vulnerabilities that are present in nearly all CPUs manufactured in the past 20 years. This is a hardware flaw that potentially allows an attacker to read all data on the system. On cloud computing services, where multiple virtual machines are on a single physical host, an attacker can gain access to all virtual machines. Fixing these vulnerabilities requires re-designing and replacing CPUs. Until this happens, there are several software patches that mitigate these vulnerabilities. If you have kept your SUSE systems updated, all of these patches should already be installed.

spectre-meltdown-checker generates a detailed report. It is impossible to guarantee that your system is secure, but it shows you which mitigations are in place, and potential vulnerabilities

16.1 Using spectre-meltdown-checker Edit source

Install the script, and then run it as root without any options:

root # zypper in spectre-meltdown-checker
root # spectre-meltdown-checker.sh

You will see colorful output like Figure 16.1, “Output From spectre-meltdown-checker” :

Partial output of spectre-meltdown-checker.sh
Figure 16.1: Output From spectre-meltdown-checker

spectre-meltdown-checker.sh --help lists all options. It is useful to pipe plain text output, with no colors, to a file:

root # spectre-meltdown-checker.sh --no-color| tee filename.txt

The previous examples are on a running system, which is the default. You may also run spectre-meltdown-checker offline by specifying the paths to the kernel, config, and System.map files :

root # cd /boot
root # spectre-meltdown-checker.sh \
--no-color \
--kernel vmlinuz-4.12.14-lp151.28.13-default \
--config config-4.12.14-lp151.28.13-default \
--map System.map-4.12.14-lp151.28.13-default| tee filename.txt

Other useful options are:

--verbose, -v

Increase verbosity; repeat for more verbosity, for example -v -v -v

--explain

Print human-readable explanations

--batch [short] [json] [nrpe] [prometheus]

Format output in various machine-readable formats

Important
Important: --disclaimer Option

spectre-meltdown-checker.sh --disclaimer provides important information about what the script does, and does not do.

16.2 Additional Information about Spectre/Meltdown Edit source

For more information, see the following references:

17 Configuring Security Settings with YaST Edit source

The YaST module Security Center and Hardening offers a central clearinghouse to configure security-related settings for SUSE Linux Enterprise Server. Use it to configure security aspects such as settings for the login procedure and for password creation, for boot permissions, user creation or for default file permissions. Launch it from the YaST control center by Security and Users › Security Center and Hardening. The Security Center dialog always starts with the Security Overview, and other configuration dialogs are available from the right pane.

17.1 Security Overview Edit source

The Security Overview displays a comprehensive list of the most important security settings for your system. The security status of each entry in the list is clearly visible. A green check mark indicates a secure setting while a red cross indicates an entry as being insecure. Click Help to open an overview of the setting and information on how to make it secure. To change a setting, click the corresponding link in the Status column. Depending on the setting, the following entries are available:

Enabled/Disabled

Click this entry to toggle the status of the setting to either enabled or disabled.

Configure

Click this entry to launch another YaST module for configuration. You will return to the Security Overview when leaving the module.

Unknown

A setting's status is set to unknown when the associated service is not installed. Such a setting does not represent a potential security risk.

YaST Security Center and Hardening: Security Overview
Figure 17.1: YaST Security Center and Hardening: Security Overview

17.2 Predefined Security Configurations Edit source

SUSE Linux Enterprise Server comes with three Predefined Security Configurations. These configurations affect all the settings available in the Security Center module. Each configuration can be modified to your needs using the dialogs available from the right pane changing its state to Custom Settings:

Workstation

A configuration for a workstation with any kind of network connection (including a connection to the Internet).

Roaming Device

This setting is designed for a laptop or tablet that connects to different networks.

Network Server

Security settings designed for a machine providing network services such as a Web server, file server, name server, etc. This set provides the most secure configuration of the predefined settings.

Custom Settings

A pre-selected Custom Settings (when opening the Predefined Security Configurations dialog) indicates that one of the predefined sets has been modified. Actively choosing this option does not change the current configuration—you will need to change it using the Security Overview.

17.3 Password Settings Edit source

Passwords that are easy to guess are a major security issue. The Password Settings dialog provides the means to ensure that only secure passwords can be used.

Check New Passwords

By activating this option, a warning will be issued if new passwords appear in a dictionary, or if they are proper names (proper nouns).

Minimum Acceptable Password Length

If the user chooses a password with a length shorter than specified here, a warning will be issued.

Number of Passwords to Remember

When password expiration is activated (via Password Age), this setting stores the given number of a user's previous passwords, preventing their reuse.

Password Encryption Method

Choose a password encryption algorithm. Normally there is no need to change the default (Blowfish).

Password Age

Activate password expiration by specifying a minimum and a maximum time limit (in days). By setting the minimum age to a value greater than 0 days, you can prevent users from immediately changing their passwords again (and in doing so circumventing the password expiration). Use the values 0 and 99999 to deactivate password expiration.

Days Before Password Expires Warning

When a password expires, the user receives a warning in advance. Specify the number of days prior to the expiration date that the warning should be issued.

17.4 Boot Settings Edit source

Configure which users can shut down the machine via the graphical login manager in this dialog. You can also specify how CtrlAltDel will be interpreted and who can hibernate the system.

17.5 Login Settings Edit source

This dialog lets you configure security-related login settings:

Delay after Incorrect Login Attempt

To make it difficult to guess a user's password by repeatedly logging in, it is recommended to delay the display of the login prompt that follows an incorrect login. Specify the value in seconds. Make sure that users who have mistyped their passwords do not need to wait too long.

Allow Remote Graphical Login

When checked, the graphical login manager (GDM) can be accessed from the network. This is a potential security risk.

17.6 User Addition Edit source

Set minimum and maximum values for user and group IDs. These default settings would rarely need to be changed.

17.7 Miscellaneous Settings Edit source

Other security settings that do not fit the above-mentioned categories are listed here:

File Permissions

SUSE Linux Enterprise Server comes with three predefined sets of file permissions for system files. These permission sets define whether a regular user may read log files or start certain programs. Easy file permissions are suitable for stand-alone machines. These settings allow regular users to, for example, read most system files. See the file /etc/permissions.easy for the complete configuration. The Secure file permissions are designed for multiuser machines with network access. A thorough explanation of these settings can be found in /etc/permissions.secure. The Paranoid settings are the most restrictive ones and should be used with care. See /etc/permissions.paranoid for more information.

User Launching updatedb

The program updatedb scans the system and creates a database of all file locations which can be queried with the command locate. When updatedb is run as user nobody, only world-readable files will be added to the database. When run as user root, almost all files (except the ones root is not allowed to read) will be added.

Enable Magic SysRq Keys

The magic SysRq key is a key combination that enables you to have some control over the system even when it has crashed. The complete documentation can be found at https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html.

18 Authorization with PolKit Edit source

PolKit (formerly known as PolicyKit) is an application framework that acts as a negotiator between the unprivileged user session and the privileged system context. Whenever a process from the user session tries to carry out an action in the system context, PolKit is queried. Based on its configuration—specified in a so-called policy—the answer could be yes, no, or needs authentication. Unlike classical privilege authorization programs such as sudo, PolKit does not grant root permissions to an entire session, but only to the action in question.

18.1 Conceptual Overview Edit source

PolKit works by limiting specific actions by users, by group, or by name. It then defines how those users are allowed to perform this action.

18.1.1 Available Authentication Agents Edit source

When a user starts a session (using the graphical environment or on the console), each session consists of the authority and an authentication agent. The authority is implemented as a service on the system message bus, whereas the authentication agent is used to authenticate the current user, which started the session. The current user needs to prove their authenticity, for example, using a passphrase.

Each desktop environment has its own authentication agent. Usually it is started automatically, whatever environment you choose.

18.1.2 Structure of PolKit Edit source

PolKit's configuration depends on actions and authorization rules:

Actions (file extension *.policy)

Written as XML files and located in /usr/share/polkit-1/actions. Each file defines one or more actions, and each action contains descriptions and default permissions. Although a system administrator can write their own rules, PolKit's files must not be edited.

Authorization Rules (file extension *.rules)

Written as JavaScript files and located in two places: /usr/share/polkit-1/rules.d is used for third party packages and /etc/polkit-1/rules.d for local configurations. Each rule file refers to the action specified in the action file. A rule determines what restrictions are allowed to a subset of users. For example, a rule file could overrule a restrictive permission and allow some users to allow it.

18.1.3 Available Commands Edit source

PolKit contains several commands for specific tasks (see also the specific man page for further details):

pkaction

Get details about a defined action. See Section 18.3, “Querying Privileges” for more information.

pkcheck

Checks whether a process is authorized, specified by either --process or --system-bus-name.

pkexec

Allows an authorized user to execute the specific program as another user.

pkttyagent

Starts a textual authentication agent. This agent is used if a desktop environment does not have its own authentication agent.

18.1.4 Available Policies and Supported Applications Edit source

At the moment, not all applications requiring privileges use PolKit. Find the most important policies available on SUSE® Linux Enterprise Server below, sorted into the categories where they are used.

PulseAudio
Set scheduling priorities for the PulseAudio daemon
CUPS
Add, remove, edit, enable or disable printers
GNOME
Modify system and mandatory values with GConf
Change the system time
libvirt
Manage and monitor local virtualized systems
PolKit
Read and change privileges for other users
Modify defaults
PackageKit
Update and remove packages
Change and refresh repositories
Install local files
Rollback
Import repository keys
Accepting EULAs
Setting the network proxy
System
Wake on LAN
Mount or unmount fixed, hotpluggable and encrypted devices
Eject and decrypt removable media
Enable or disable WLAN
Enable or disable Bluetooth
Device access
Stop, suspend, hibernate and restart the system
Undock a docking station
Change power-management settings
YaST
Register product
Change the system time and language

18.2 Authorization Types Edit source

Every time a PolKit-enabled process carries out a privileged operation, PolKit is asked whether this process is entitled to do so. PolKit answers according to the policy defined for this process. The answers can be yes, no, or authentication needed. By default, a policy contains implicit privileges, which automatically apply to all users. It is also possible to specify explicit privileges which apply to a specific user.

18.2.1 Implicit Privileges Edit source

Implicit privileges can be defined for any active and inactive sessions. An active session is the one in which you are currently working. It becomes inactive when you switch to another console for example. When setting implicit privileges to no, no user is authorized, whereas yes authorizes all users. However, usually it is useful to demand authentication.

A user can either authorize by authenticating as root or by authenticating as self. Both authentication methods exist in four variants:

Authentication

The user always needs to authenticate.

One Shot Authentication

The authentication is bound to the instance of the program currently running. After the program is restarted, the user is required to authenticate again.

Keep Session Authentication

The authentication dialog offers a check button Remember authorization for this session. If checked, the authentication is valid until the user logs out.

Keep Indefinitely Authentication

The authentication dialog offers a check button Remember authorization. If checked, the user needs to authenticate only once.

18.2.2 Explicit Privileges Edit source

Explicit privileges can be granted to specific users. They can either be granted without limitations, or, when using constraints, limited to an active session and/or a local console.

It is not only possible to grant privileges to a user, a user can also be blocked. Blocked users cannot carry out an action requiring authorization, even though the default implicit policy allows authorization by authentication.

18.2.3 Default Privileges Edit source

Each application supporting PolKit comes with a default set of implicit policies defined by the application's developers. Those policies are the so-called upstream defaults. The privileges defined by the upstream defaults are not necessarily the ones that are activated by default on SUSE systems. SUSE Linux Enterprise Server comes with a predefined set of privileges that override the upstream defaults:

/etc/polkit-default-privs.standard

Defines privileges suitable for most desktop systems

/etc/polkit-default-privs.restrictive

Designed for machines administrated centrally. It is active by default.

To switch between the two sets of default privileges, adjust the value of POLKIT_DEFAULT_PRIVS to either restrictive or standard in /etc/sysconfig/security. Then run the command set_polkit_default_privs as root.

Do not modify the two files in the list above. To define your own custom set of privileges, use /etc/polkit-default-privs.local. For details, refer to Section 18.4.3, “Modifying Configuration Files for Implicit Privileges”.

18.3 Querying Privileges Edit source

To query privileges use the command pkaction included in PolKit.

PolKit comes with command line tools for changing privileges and executing commands as another user (see Section 18.1.3, “Available Commands” for a short overview). Each existing policy has a speaking, unique name with which it can be identified. List all available policies with the command pkaction. See man pkaction for more information.

If you want to display the needed authorization for a given policy (for example, org.freedesktop.login1.reboot) use pkaction as follows:

tux > pkaction -v --action-id=org.freedesktop.login1.reboot
org.freedesktop.login1.reboot:
  description:       Reboot the system
  message:           Authentication is required to allow rebooting the system
  vendor:            The systemd Project
  vendor_url:        http://www.freedesktop.org/wiki/Software/systemd
  icon:
  implicit any:      auth_admin_keep
  implicit inactive: auth_admin_keep
  implicit active:   yes

The keyword auth_admin_keep means that users need to enter a passphrase.

Note
Note: Restrictions of pkaction on SUSE Linux Enterprise Server

pkaction always operates on the upstream defaults. Therefore it cannot be used to list or restore the defaults shipped with SUSE Linux Enterprise Server. To do so, refer to Section 18.5, “Restoring the Default Privileges”.

18.4 Modifying Configuration Files Edit source

Adjusting privileges by modifying configuration files is useful when you want to deploy the same set of policies to different machines, for example to the computers of a specific team. It is possible to change implicit and explicit privileges by modifying configuration files.

18.4.1 Adding Action Rules Edit source

The available actions depend on what additional packages you have installed on your system. For a quick overview, use pkaction to list all defined rules.

To get an idea, the following example describes how the command gparted (GNOME Partition Editor) is integrated into PolKit.

The file /usr/share/polkit-1/actions/org.opensuse.policykit.gparted.policy contains the following content:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policyconfig PUBLIC
 "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd">
<policyconfig> 1

  <action id="org-opensuse-policykit-gparted"> 2
    <message>Authentication is required to run the GParted Partition Editor</message>
    <icon_name>gparted</icon_name>
    <defaults> 3
      <allow_any>auth_admin</allow_any>
      <allow_inactive>auth_admin</allow_inactive>
     < allow_active>auth_admin</allow_active>
    </defaults>
    <annotate 4
      key="org.freedesktop.policykit.exec.path">/usr/sbin/gparted</annotate>
    <annotate 4
      key="org.freedesktop.policykit.exec.allow_gui">true</annotate>
  </action>

</policyconfig>

1

Root element of the policy file.

2

Contains one single action.

3

The defaults element contains several permissions used in remote sessions like SSH, VNC (element allow_inactive), when logged directly into the machine on a TTY or X display (element allow_active), or for both (element allow_any). The value auth_admin indicates authentication is required as an administrative user.

4

The annotate element contains specific information regarding how PolKit performs an action. In this case, it contains the path to the executable and states whether a GUI is allowed to open a X display.

To add your own policy, create a .policy file with the structure above, add the appropriate value into the id attribute, and define the default permissions.

18.4.2 Adding Authorization Rules Edit source

Your own authorization rules overrule the default settings. To add your own settings, store your files under /etc/polkit-1/rules.d/.

The files in this directory start with a two-digit number, followed by a descriptive name, and end with .rules. Functions inside these files are executed in the order they have been sorted in. For example, 00-foo.rules is sorted (and hence executed) before 60-bar.rules or even 90-default-privs.rules.

Inside the file, the script checks for the specified action ID, which is defined in the .policy file. For example, if you want to allow the command gparted to be executed by any member of the admin group, check for the action ID org.opensuse.policykit.gparted:

/* Allow users in admin group to run GParted without authentication */
polkit.addRule(function(action, subject) {
    if (action.id == "org.opensuse.policykit.gparted" &&
        subject.isInGroup("admin")) {
        return polkit.Result.YES;
    }
});

Find the description of all classes and methods of the functions in the PolKit API at http://www.freedesktop.org/software/polkit/docs/latest/ref-api.html.

18.4.3 Modifying Configuration Files for Implicit Privileges Edit source

SUSE Linux Enterprise Server ships with two sets of default authorizations, located in /etc/polkit-default-privs.standard and /etc/polkit-default-privs.restrictive. For more information, refer to Section 18.2.3, “Default Privileges”.

Custom privileges are defined in /etc/polkit-default-privs.local. Privileges defined here will always take precedence over the ones defined in the other configuration files. To define your custom set of privileges, do the following:

  1. Open /etc/polkit-default-privs.local. To define a privilege, add a line for each policy with the following format:

    <privilege_identifier>     <any session>:<inactive session>:<active session>

    For example:

    org.freedesktop.policykit.modify-defaults     auth_admin_keep_always

    The following values are valid for the SESSION placeholders:

    yes

    grant privilege

    no

    block

    auth_self

    user needs to authenticate with own password every time the privilege is requested

    auth_self_keep_session

    user needs to authenticate with own password once per session, privilege is granted for the whole session

    auth_self_keep_always

    user needs to authenticate with own password once, privilege is granted for the current and for future sessions

    auth_admin

    user needs to authenticate with root password every time the privilege is requested

    auth_admin_keep_session

    user needs to authenticate with root password once per session, privilege is granted for the whole session

    auth_admin_keep_always

    user needs to authenticate with root password once, privilege is granted for the current and for future sessions

  2. Run as root for changes to take effect:

    # /sbin/set_polkit_default_privs
  3. Optionally check the list of all privilege identifiers with the command pkaction.

18.5 Restoring the Default Privileges Edit source

SUSE Linux Enterprise Server comes with a predefined set of privileges that is activated by default and thus overrides the upstream defaults. For details, refer to Section 18.2.3, “Default Privileges”.

Since the graphical PolKit tools and the command line tools always operate on the upstream defaults, SUSE Linux Enterprise Server includes an additional command-line tool, set_polkit_default_privs. It resets privileges to the values defined in /etc/polkit-default-privs.*. However, the command set_polkit_default_privs will only reset policies that are set to the upstream defaults.

Procedure 18.1: Restoring the SUSE Linux Enterprise Server Defaults
  1. Make sure /etc/polkit-default-privs.local does not contain any overrides of the default policies.

    Important
    Important: Custom Policy Configuration

    Policies defined in /etc/polkit-default-privs.local will be applied on top of the defaults during the next step.

  2. To reset all policies to the upstream defaults first and then apply the SUSE Linux Enterprise Server defaults:

    tux > sudo rm -f /var/lib/polkit/* && set_polkit_default_privs

19 Access Control Lists in Linux Edit source

POSIX ACLs (access control lists) can be used as an expansion of the traditional permission concept for file system objects. With ACLs, permissions can be defined more flexibly than with the traditional permission concept.

The term POSIX ACL suggests that this is a true POSIX (portable operating system interface) standard. The respective draft standards POSIX 1003.1e and POSIX 1003.2c have been withdrawn for several reasons. Nevertheless, ACLs (as found on many systems belonging to the Unix family) are based on these drafts and the implementation of file system ACLs (as described in this chapter) follows these two standards.

19.1 Traditional File Permissions Edit source

The permissions of all files included in SUSE Linux Enterprise Server are carefully chosen. When installing additional software or files, take great care when setting the permissions. Always use the -l option with the command ls to detect any incorrect file permissions immediately. An incorrect file attribute does not only mean that files could be changed or deleted. Modified files could be executed by root or services could be hijacked by modifying configuration files. This significantly increases the danger of an attack.

A SUSE® Linux Enterprise Server system includes the files permissions, permissions.easy, permissions.secure, and permissions.paranoid, all in the directory /etc. The purpose of these files is to define special permissions, such as world-writable directories or, for files, the setuser ID bit. Programs with the setuser ID bit set do not run with the permissions of the user that launched it, but with the permissions of the file owner, usually root. An administrator can use the file /etc/permissions.local to add their own settings.

To define one of the available profiles, select Local Security in the Security and Users section of YaST. To learn more about the topic, read the comments in /etc/permissions or consult man chmod.

Find detailed information about the traditional file permissions in the GNU Coreutils Info page, Node File permissions (info coreutils "File permissions"). More advanced features are the setuid, setgid, and sticky bit.

19.1.1 The setuid Bit Edit source

In certain situations, the access permissions may be too restrictive. Therefore, Linux has additional settings that enable the temporary change of the current user and group identity for a specific action. For example, the passwd program normally requires root permissions to access /etc/passwd. This file contains some important information, like the home directories of users and user and group IDs. Thus, a normal user would not be able to change passwd, because it would be too dangerous to grant all users direct access to this file. A possible solution to this problem is the setuid mechanism. setuid (set user ID) is a special file attribute that instructs the system to execute programs marked accordingly under a specific user ID. Consider the passwd command:

-rwsr-xr-x  1 root shadow 80036 2004-10-02 11:08 /usr/bin/passwd

You can see the s that denotes that the setuid bit is set for the user permission. By means of the setuid bit, all users starting the passwd command execute it as root.

19.1.2 The setgid Bit Edit source

The setuid bit applies to users. However, there is also an equivalent property for groups: the setgid bit. A program for which this bit was set runs under the group ID under which it was saved, no matter which user starts it. Therefore, in a directory with the setgid bit, all newly created files and subdirectories are assigned to the group to which the directory belongs. Consider the following example directory:

drwxrws--- 2 tux archive 48 Nov 19 17:12  backup

You can see the s that denotes that the setgid bit is set for the group permission. The owner of the directory and members of the group archive may access this directory. Users that are not members of this group are mapped to the respective group. The effective group ID of all written files will be archive. For example, a backup program that runs with the group ID archive can access this directory even without root privileges.

19.1.3 The Sticky Bit Edit source

There is also the sticky bit. It makes a difference whether it belongs to an executable program or a directory. If it belongs to a program, a file marked in this way is loaded to RAM to avoid needing to get it from the hard disk each time it is used. This attribute is used rarely, because modern hard disks are fast enough. If this bit is assigned to a directory, it prevents users from deleting each other's files. Typical examples include the /tmp and /var/tmp directories:

drwxrwxrwt 2 root root 1160 2002-11-19 17:15 /tmp

19.2 Advantages of ACLs Edit source

Traditionally, three permission sets are defined for each file object on a Linux system. These sets include the read (r), write (w), and execute (x) permissions for each of three types of users—the file owner, the group, and other users. In addition to that, it is possible to set the set user id, the set group id, and the sticky bit. This lean concept is fully adequate for most practical cases. However, for more complex scenarios or advanced applications, system administrators formerly needed to use several workarounds to circumvent the limitations of the traditional permission concept.

ACLs can be used as an extension of the traditional file permission concept. They allow the assignment of permissions to individual users or groups even if these do not correspond to the original owner or the owning group. Access control lists are a feature of the Linux kernel and are currently supported by Ext2, Ext3, Ext4, JFS, and XFS. Using ACLs, complex scenarios can be realized without implementing complex permission models on the application level.

The advantages of ACLs are evident if you want to replace a Windows server with a Linux server. Some connected workstations may continue to run under Windows even after the migration. The Linux system offers file and print services to the Windows clients with Samba. With Samba supporting access control lists, user permissions can be configured both on the Linux server and in Windows with a graphical user interface (only Windows NT and later). With winbindd, part of the Samba suite, it is even possible to assign permissions to users only existing in the Windows domain without any account on the Linux server.

19.3 Definitions Edit source

User Class

The conventional POSIX permission concept uses three classes of users for assigning permissions in the file system: the owner, the owning group, and other users. Three permission bits can be set for each user class, giving permission to read (r), write (w), and execute (x).

ACL

The user and group access permissions for all kinds of file system objects (files and directories) are determined by means of ACLs.

Default ACL

Default ACLs can only be applied to directories. They determine the permissions a file system object inherits from its parent directory when it is created.

ACL Entry

Each ACL consists of a set of ACL entries. An ACL entry contains a type, a qualifier for the user or group to which the entry refers, and a set of permissions. For some entry types, the qualifier for the group or users is undefined.

19.4 Handling ACLs Edit source

Table 19.1, “ACL Entry Types” summarizes the six possible types of ACL entries, each defining permissions for a user or a group of users. The owner entry defines the permissions of the user owning the file or directory. The owning group entry defines the permissions of the file's owning group. The superuser can change the owner or owning group with chown or chgrp, in which case the owner and owning group entries refer to the new owner and owning group. Each named user entry defines the permissions of the user specified in the entry's qualifier field. Each named group entry defines the permissions of the group specified in the entry's qualifier field. Only the named user and named group entries have a qualifier field that is not empty. The other entry defines the permissions of all other users.

The mask entry further limits the permissions granted by named user, named group, and owning group entries by defining which of the permissions in those entries are effective and which are masked. If permissions exist in one of the mentioned entries and in the mask, they are effective. Permissions contained only in the mask or only in the actual entry are not effective—meaning the permissions are not granted. All permissions defined in the owner and owning group entries are always effective. The example in Table 19.2, “Masking Access Permissions” demonstrates this mechanism.

There are two basic classes of ACLs: A minimum ACL contains only the entries for the types owner, owning group, and other, which correspond to the conventional permission bits for files and directories. An extended ACL goes beyond this. It must contain a mask entry and may contain several entries of the named user and named group types.

Table 19.1: ACL Entry Types

Type

Text Form

owner

user::rwx

named user

user:name:rwx

owning group

group::rwx

named group

group:name:rwx

mask

mask::rwx

other

other::rwx

Table 19.2: Masking Access Permissions

Entry Type

Text Form

Permissions

named user

user:geeko:r-x

r-x

mask

mask::rw-

rw-

effective permissions:

r--

19.4.1 ACL Entries and File Mode Permission Bits Edit source

Figure 19.1, “Minimum ACL: ACL Entries Compared to Permission Bits” and Figure 19.2, “Extended ACL: ACL Entries Compared to Permission Bits” illustrate the two cases of a minimum ACL and an extended ACL. The figures are structured in three blocks—the left block shows the type specifications of the ACL entries, the center block displays an example ACL, and the right block shows the respective permission bits according to the conventional permission concept (for example, as displayed by ls -l). In both cases, the owner class permissions are mapped to the ACL entry owner. Other class permissions are mapped to the respective ACL entry. However, the mapping of the group class permissions is different in the two cases.

Minimum ACL: ACL Entries Compared to Permission Bits
Figure 19.1: Minimum ACL: ACL Entries Compared to Permission Bits

In the case of a minimum ACL—without mask—the group class permissions are mapped to the ACL entry owning group. This is shown in Figure 19.1, “Minimum ACL: ACL Entries Compared to Permission Bits”. In the case of an extended ACL—with mask—the group class permissions are mapped to the mask entry. This is shown in Figure 19.2, “Extended ACL: ACL Entries Compared to Permission Bits”.

Extended ACL: ACL Entries Compared to Permission Bits
Figure 19.2: Extended ACL: ACL Entries Compared to Permission Bits

This mapping approach ensures the smooth interaction of applications, regardless of whether they have ACL support. The access permissions that were assigned by means of the permission bits represent the upper limit for all other fine adjustments made with an ACL. Changes made to the permission bits are reflected by the ACL and vice versa.

19.4.2 A Directory with an ACL Edit source

With getfacl and setfacl on the command line, you can access ACLs. The usage of these commands is demonstrated in the following example.

Before creating the directory, use the umask command to define which access permissions should be masked each time a file object is created. The command umask 027 sets the default permissions by giving the owner the full range of permissions (0), denying the group write access (2), and giving other users no permissions (7). umask actually masks the corresponding permission bits or turns them off. For details, consult the umask man page.

mkdir mydir creates the mydir directory with the default permissions as set by umask. Use ls -dl mydir to check whether all permissions were assigned correctly. The output for this example is:

drwxr-x--- ... tux project3 ... mydir

With getfacl mydir, check the initial state of the ACL. This gives information like:

# file: mydir
# owner: tux
# group: project3
user::rwx
group::r-x
other::---

The first three output lines display the name, owner, and owning group of the directory. The next three lines contain the three ACL entries owner, owning group, and other. In fact, in the case of this minimum ACL, the getfacl command does not produce any information you could not have obtained with ls.

Modify the ACL to assign read, write, and execute permissions to an additional user geeko and an additional group mascots with:

root # setfacl -m user:geeko:rwx,group:mascots:rwx mydir

The option -m prompts setfacl to modify the existing ACL. The following argument indicates the ACL entries to modify (multiple entries are separated by commas). The final part specifies the name of the directory to which these modifications should be applied. Use the getfacl command to take a look at the resulting ACL.

# file: mydir
# owner: tux
# group: project3
user::rwx
user:geeko:rwx
group::r-x
group:mascots:rwx
mask::rwx
other::---

In addition to the entries initiated for the user geeko and the group mascots, a mask entry has been generated. This mask entry is set automatically so that all permissions are effective. setfacl automatically adapts existing mask entries to the settings modified, unless you deactivate this feature with -n. The mask entry defines the maximum effective access permissions for all entries in the group class. This includes named user, named group, and owning group. The group class permission bits displayed by ls -dl mydir now correspond to the mask entry.

drwxrwx---+ ... tux project3 ... mydir

The first column of the output contains an additional + to indicate that there is an extended ACL for this item.

According to the output of the ls command, the permissions for the mask entry include write access. Traditionally, such permission bits would mean that the owning group (here project3) also has write access to the directory mydir.

However, the effective access permissions for the owning group correspond to the overlapping portion of the permissions defined for the owning group and for the mask—which is r-x in our example (see Table 19.2, “Masking Access Permissions”). As far as the effective permissions of the owning group in this example are concerned, nothing has changed even after the addition of the ACL entries.

Edit the mask entry with setfacl or chmod. For example, use chmod g-w mydir. ls -dl mydir then shows:

drwxr-x---+ ... tux project3 ... mydir

getfacl mydir provides the following output:

# file: mydir
# owner: tux
# group: project3
user::rwx
user:geeko:rwx          # effective: r-x
group::r-x
group:mascots:rwx       # effective: r-x
mask::r-x
other::---

After executing chmod to remove the write permission from the group class bits, the output of ls is sufficient to see that the mask bits must have changed accordingly: write permission is again limited to the owner of mydir. The output of the getfacl confirms this. This output includes a comment for all those entries in which the effective permission bits do not correspond to the original permissions, because they are filtered according to the mask entry. The original permissions can be restored at any time with chmod g+w mydir.

19.4.3 A Directory with a Default ACL Edit source

Directories can have a default ACL, which is a special kind of ACL defining the access permissions that objects in the directory inherit when they are created. A default ACL affects both subdirectories and files.

19.4.3.1 Effects of a Default ACL Edit source

There are two ways in which the permissions of a directory's default ACL are passed to the files and subdirectories:

  • A subdirectory inherits the default ACL of the parent directory both as its default ACL and as an ACL.

  • A file inherits the default ACL as its ACL.

All system calls that create file system objects use a mode parameter that defines the access permissions for the newly created file system object. If the parent directory does not have a default ACL, the permission bits as defined by the umask are subtracted from the permissions as passed by the mode parameter, with the result being assigned to the new object. If a default ACL exists for the parent directory, the permission bits assigned to the new object correspond to the overlapping portion of the permissions of the mode parameter and those that are defined in the default ACL. The umask is disregarded in this case.

19.4.3.2 Application of Default ACLs Edit source

The following three examples show the main operations for directories and default ACLs:

  1. Add a default ACL to the existing directory mydir with:

    tux > setfacl -d -m group:mascots:r-x mydir

    The option -d of the setfacl command prompts setfacl to perform the following modifications (option -m) in the default ACL.

    Take a closer look at the result of this command:

    tux > getfacl mydir
    
    # file: mydir
    # owner: tux
    # group: project3
    user::rwx
    user:geeko:rwx
    group::r-x
    group:mascots:rwx
    mask::rwx
    other::---
    default:user::rwx
    default:group::r-x
    default:group:mascots:r-x
    default:mask::r-x
    default:other::---

    getfacl returns both the ACL and the default ACL. The default ACL is formed by all lines that start with default. Although you merely executed the setfacl command with an entry for the mascots group for the default ACL, setfacl automatically copied all other entries from the ACL to create a valid default ACL. Default ACLs do not have an immediate effect on access permissions. They only come into play when file system objects are created. These new objects inherit permissions only from the default ACL of their parent directory.

  2. In the next example, use mkdir to create a subdirectory in mydir, which inherits the default ACL.

    tux > mkdir mydir/mysubdir
    
    getfacl mydir/mysubdir
    
    # file: mydir/mysubdir
    # owner: tux
    # group: project3
    user::rwx
    group::r-x
    group:mascots:r-x
    mask::r-x
    other::---
    default:user::rwx
    default:group::r-x
    default:group:mascots:r-x
    default:mask::r-x
    default:other::---

    As expected, the newly-created subdirectory mysubdir has the permissions from the default ACL of the parent directory. The ACL of mysubdir is an exact reflection of the default ACL of mydir. The default ACL that this directory will hand down to its subordinate objects is also the same.

  3. Use touch to create a file in the mydir directory, for example, touch mydir/myfile. ls -l mydir/myfile then shows:

    -rw-r-----+ ... tux project3 ... mydir/myfile

    The output of getfacl mydir/myfile is:

    # file: mydir/myfile
    # owner: tux
    # group: project3
    user::rw-
    group::r-x          # effective:r--
    group:mascots:r-x   # effective:r--
    mask::r--
    other::---

    touch uses a mode with the value 0666 when creating new files, which means that the files are created with read and write permissions for all user classes, provided no other restrictions exist in umask or in the default ACL (see Section 19.4.3.1, “Effects of a Default ACL”). In effect, this means that all access permissions not contained in the mode value are removed from the respective ACL entries. Although no permissions were removed from the ACL entry of the group class, the mask entry was modified to mask permissions not set in mode.

    This approach ensures the smooth interaction of applications (such as compilers) with ACLs. You can create files with restricted access permissions and subsequently mark them as executable. The mask mechanism guarantees that the right users and groups can execute them as desired.

19.4.4 The ACL Check Algorithm Edit source

A check algorithm is applied before any process or application is granted access to an ACL-protected file system object. As a basic rule, the ACL entries are examined in the following sequence: owner, named user, owning group or named group, and other. The access is handled in accordance with the entry that best suits the process. Permissions do not accumulate.

Things are more complicated if a process belongs to more than one group and would potentially suit several group entries. An entry is randomly selected from the suitable entries with the required permissions. It is irrelevant which of the entries triggers the final result access granted. Likewise, if none of the suitable group entries contain the required permissions, a randomly selected entry triggers the final result access denied.

19.5 ACL Support in Applications Edit source

ACLs can be used to implement very complex permission scenarios that meet the requirements of modern applications. The traditional permission concept and ACLs can be combined in a smart manner. The basic file commands (cp, mv, ls, etc.) support ACLs, as do Samba and Nautilus.

Vi/Vim and emacs both fully support ACLs by preserving the permissions on writing files including backups. Unfortunately, many other editors and file managers still lack ACL support. When modifying files with an editor, the ACLs of files are sometimes preserved and sometimes not, depending on the backup mode of the editor used. If the editor writes the changes to the original file, the ACL is preserved. If the editor saves the updated contents to a new file that is subsequently renamed to the old file name, the ACLs may be lost, unless the editor supports ACLs. Except for the star archiver, there are currently no backup applications that preserve ACLs.

19.6 For More Information Edit source

For more information about ACLs, see the man pages for getfacl(1), acl(5), and setfacl(1).

20 Certificate Store Edit source

Certificates play an important role in the authentication of companies and individuals. Usually certificates are administered by the application itself. In some cases, it makes sense to share certificates between applications. The certificate store is a common ground for Firefox, Evolution, and NetworkManager. This chapter explains some details.

The certificate store is a common database for Firefox, Evolution, and NetworkManager at the moment. Other applications that use certificates are not covered but may be in the future. If you have such an application, you can continue to use its private, separate configuration.

20.1 Activating Certificate Store Edit source

The configuration is mostly done in the background. To activate it, proceed as follows:

  1. Decide if you want to activate the certificate store globally (for every user on your system) or specifically to a certain user:

    • For every user.  Use the file /etc/profile.local

    • For a specific user.  Use the file ~/.profile

  2. Open the file from the previous step and insert the following line:

    export NSS_USE_SHARED_DB=1

    Save the file

  3. Log out of and log in to your desktop.

All the certificates are stored under $HOME/.local/var/pki/nssdb/.

20.2 Importing Certificates Edit source

To import a certificate into the certificate store, do the following:

  1. Start Firefox.

  2. Open the dialog from Edit › Preferences. Change to Advanced › Encryption and click View Certificates.

  3. Import your certificate depending on your type: use Servers to import server certificate, People to identify other, and Your Certificates to identify yourself.

21 Intrusion Detection with AIDE Edit source

Securing your systems is a mandatory task for any mission-critical system administrator. Because it is impossible to always guarantee that the system is not compromised, it is very important to do extra checks regularly (for example with cron) to ensure that the system is still under your control. This is where AIDE, the Advanced Intrusion Detection Environment, comes into play.

21.1 Why Use AIDE? Edit source

An easy check that often can reveal unwanted changes can be done by means of RPM. The package manager has a built-in verify function that checks all the managed files in the system for changes. To verify of all files, run the command rpm -Va. However, this command will also display changes in configuration files and you will need to do some filtering to detect important changes.

An additional problem to the method with RPM is that an intelligent attacker will modify rpm itself to hide any changes that might have been done by some kind of root-kit which allows the attacker to mask its intrusion and gain root privilege. To solve this, you should implement a secondary check that can also be run completely independent of the installed system.

21.2 Setting Up an AIDE Database Edit source

Important
Important: Initialize AIDE Database After Installation

Before you install your system, verify the checksum of your medium (see Book “Deployment Guide”, Chapter 12 “Troubleshooting”, Section 12.1 “Checking Media”) to make sure you do not use a compromised source. After you have installed the system, initialize the AIDE database. To make sure that all went well during and after the installation, do an installation directly on the console, without any network attached to the computer. Do not leave the computer unattended or connected to any network before AIDE creates its database.

AIDE is not installed by default on SUSE Linux Enterprise Server. To install it, either use Computer › Install Software, or enter zypper install aide on the command line as root.

To tell AIDE which attributes of which files should be checked, use the /etc/aide.conf configuration file. It must be modified to become the actual configuration. The first section handles general parameters like the location of the AIDE database file. More relevant for local configurations are the Custom Rules and the Directories and Files sections. A typical rule looks like the following:

Binlib     = p+i+n+u+g+s+b+m+c+md5+sha1

After defining the variable Binlib, the respective check boxes are used in the files section. Important options include the following:

Table 21.1: Important AIDE Check Boxes

Option

Description

p

Check for the file permissions of the selected files or directories.

i

Check for the inode number. Every file name has a unique inode number that should not change.

n

Check for the number of links pointing to the relevant file.

u

Check if the owner of the file has changed.

g

Check if the group of the file has changed.

s

Check if the file size has changed.

b

Check if the block count used by the file has changed.

m

Check if the modification time of the file has changed.

c

Check if the files access time has changed.

S

Check for a changed file size.

I

Ignore changes of the file name.

md5

Check if the md5 checksum of the file has changed. We recommend to use sha256 or sha512.

sha1

Check if the sha1 (160 Bit) checksum of the file has changed. We recommend to use sha256 or sha512.

sha256

Check if the sha256 checksum of the file has changed.

sha512

Check if the sha512 checksum of the file has changed.

This is a configuration that checks for all files in /sbin with the options defined in Binlib but omits the /sbin/conf.d/ directory:

/sbin  Binlib
!/sbin/conf.d

To create the AIDE database, proceed as follows:

  1. Open /etc/aide.conf.

  2. Define which files should be checked with which check boxes. For a complete list of available check boxes, see /usr/share/doc/packages/aide/manual.html. The definition of the file selection needs some knowledge about regular expressions. Save your modifications.

  3. To check whether the configuration file is valid, run:

    root # aide --config-check

    Any output of this command is a hint that the configuration is not valid. For example, if you get the following output:

    root # aide --config-check
    35:syntax error:!
    35:Error while reading configuration:!
    Configuration error

    The error is to be expected in line 36 of /etc/aide.conf. Note that the error message contains the last successfully read line of the configuration file.

  4. Initialize the AIDE database. Run the command:

    root # aide -i
  5. Copy the generated database to a save location like a CD-R or DVD-R, a remote server or a flash disk for later use.

    Important
    Important:

    This step is essential as it avoids compromising your database. It is recommended to use a medium which can be written only once to prevent the database being modified. Never leave the database on the computer which you want to monitor.

21.3 Local AIDE Checks Edit source

To perform a file system check, proceed as follows:

  1. Rename the database:

    root # mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db
  2. After any configuration change, you always need to re-initialize the AIDE database and subsequently move the newly generated database. It is also a good idea to make a backup of this database. See Section 21.2, “Setting Up an AIDE Database” for more information.

  3. Perform the check with the following command:

    root # aide --check

If the output is empty, everything is fine. If AIDE found changes, it displays a summary of changes, for example:

root # aide --check
AIDE found differences between database and filesystem!!

Summary:
  Total number of files:        1992
  Added files:                  0
  Removed files:                0
  Changed files:                1

To learn about the actual changes, increase the verbose level of the check with the parameter -V. For the previous example, this could look like the following:

root # aide --check -V
AIDE found differences between database and filesystem!!
Start timestamp: 2009-02-18 15:14:10

Summary:
  Total number of files:        1992
  Added files:                  0
  Removed files:                0
  Changed files:                1


---------------------------------------------------
Changed files:
---------------------------------------------------

changed: /etc/passwd

--------------------------------------------------
Detailed information about changes:
---------------------------------------------------


File: /etc/passwd
  Mtime    : 2009-02-18 15:11:02              , 2009-02-18 15:11:47
  Ctime    : 2009-02-18 15:11:02              , 2009-02-18 15:11:47

In this example, the file /etc/passwd was touched to demonstrate the effect.

21.4 System Independent Checking Edit source

To avoid risk, it is advisable to also run the AIDE binary from a trusted source. This excludes the risk that some attacker also modified the aide binary to hide its traces.

To accomplish this task, AIDE must be run from a rescue system that is independent of the installed system. With SUSE Linux Enterprise Server it is relatively easy to extend the rescue system with arbitrary programs, and thus add the needed functionality.

Before you can start using the rescue system, you need to provide two packages to the system. These are included with the same syntax as you would add a driver update disk to the system. For a detailed description about the possibilities of linuxrc that are used for this purpose, see https://en.opensuse.org/SDB:Linuxrc. In the following, one possible way to accomplish this task is discussed.

Procedure 21.1: Starting a Rescue System with AIDE
  1. Provide an FTP server as a second machine.

  2. Copy the packages aide and mhash to the FTP server directory, in our case /srv/ftp/. Replace the placeholders ARCH and VERSION with the corresponding values:

    root # cp DVD1/suse/ARCH/aideVERSION.ARCH.rpm /srv/ftp
    root # cp DVD1/suse/ARCH/mhashVERSION.ARCH.rpm /srv/ftp
  3. Create an info file /srv/ftp/info.txt that provides the needed boot parameters for the rescue system:

    dud:ftp://ftp.example.com/aideVERSION.ARCH.rpm
    dud:ftp://ftp.example.com/mhashVERSION.ARCH.rpm

    Replace your FTP domain name, VERSION and ARCH with the values used on your system.

  4. Restart the server that needs to go through an AIDE check with the Rescue system from your DVD. Add the following string to the boot parameters:

    info=ftp://ftp.example.com/info.txt

    This parameter tells linuxrc to also read in all information from the info.txt file.

After the rescue system has booted, the AIDE program is ready for use.

21.5 For More Information Edit source

Information about AIDE is available at the following places:

Part III Network Security Edit source

22 X Window System and X Authentication

As mentioned at the beginning, network transparency is one of the central characteristics of a Unix system. X, the windowing system of Unix operating systems, can use this feature in an impressive way. With X, it is no problem to log in to a remote host and start a graphical program that is then sen…

23 SSH: Secure Network Operations

In networked environments, it is often necessary to access hosts from a remote location. If a user sends login and password strings for authentication purposes as plain text, they could be intercepted and misused to gain access to that user account. This would open all the user's files to an attacker and the illegal account could be used to obtain administrator or root access, or to penetrate other systems. In the past, remote connections were established with telnet, rsh or rlogin, which offered no guards against eavesdropping in the form of encryption or other security mechanisms. There are other unprotected communication channels, like the traditional FTP protocol and some remote copying programs like rcp.

24 Masquerading and Firewalls

Whenever Linux is used in a network environment, you can use the kernel functions that allow the manipulation of network packets to maintain a separation between internal and external network areas. The Linux netfilter framework provides the means to establish an effective firewall that keeps differ…

25 Configuring a VPN Server

Today, Internet connections are cheap and available almost everywhere. However, not all connections are secure. Using a Virtual Private Network (VPN), you can create a secure network within an insecure network such as the Internet or Wi-Fi. It can be implemented in different ways and serves several purposes. In this chapter, we focus on the OpenVPN implementation to link branch offices via secure wide area networks (WANs).

26 Enabling FIPS 140-2

The Federal Information Processing Standard 140-2 (FIPS 140-2) is a security standard for cryptographic modules. Modules are certified by the National Institute of Standards and Technology (NIST, see https://csrc.nist.gov/projects/cryptographic-module-validation-program). See https://www.suse.com/support/security/certifications/ for a list of certified modules.

22 X Window System and X Authentication Edit source

As mentioned at the beginning, network transparency is one of the central characteristics of a Unix system. X, the windowing system of Unix operating systems, can use this feature in an impressive way. With X, it is no problem to log in to a remote host and start a graphical program that is then sent over the network to be displayed on your computer.

When an X client needs to be displayed remotely using an X server, the latter should protect the resource managed by it (the display) from unauthorized access. In more concrete terms, certain permissions must be given to the client program. With the X Window System, there are two ways to do this, called host-based access control and cookie-based access control. The former relies on the IP address of the host where the client should run. The program to control this is xhost. xhost enters the IP address of a legitimate client into a database belonging to the X server. However, relying on IP addresses for authentication is not very secure. For example, if there were a second user working on the host sending the client program, that user would have access to the X server as well—like someone spoofing the IP address. Because of these shortcomings, this authentication method is not described in more detail here, but you can learn about it with man xhost.

In the case of cookie-based access control, a character string is generated that is only known to the X server and to the legitimate user, like an ID card of some kind. This cookie is stored on login in the file .Xauthority in the user's home directory and is available to any X client wanting to use the X server to display a window. The file .Xauthority can be examined by the user with the tool xauth. If you rename .Xauthority, or if you delete the file from your home directory by accident, you will not be able to open any new windows or X clients.

SSH (secure shell) can be used to encrypt a network connection and forward it to an X server transparently. This is also called X forwarding. X forwarding is achieved by simulating an X server on the server side and setting a DISPLAY variable for the shell on the remote host. Further details about SSH can be found in Chapter 23, SSH: Secure Network Operations.

Warning
Warning: X Forwarding Can Be Insecure

If you do not consider the computer where you log in to be a secure host, do not use X forwarding. If X forwarding is enabled, an attacker could authenticate via your SSH connection. The attacker could then intrude on your X server and, for example, read your keyboard input.

23 SSH: Secure Network Operations Edit source

In networked environments, it is often necessary to access hosts from a remote location. If a user sends login and password strings for authentication purposes as plain text, they could be intercepted and misused to gain access to that user account. This would open all the user's files to an attacker and the illegal account could be used to obtain administrator or root access, or to penetrate other systems. In the past, remote connections were established with telnet, rsh or rlogin, which offered no guards against eavesdropping in the form of encryption or other security mechanisms. There are other unprotected communication channels, like the traditional FTP protocol and some remote copying programs like rcp.

The SSH suite provides the necessary protection by encrypting the authentication strings (usually a login name and a password) and all the other data exchanged between the hosts. With SSH, the data flow could still be recorded by a third party, but the contents are encrypted and cannot be reverted to plain text unless the encryption key is known. So SSH enables secure communication over insecure networks, such as the Internet. The SSH implementation coming with SUSE Linux Enterprise Server is OpenSSH.

SUSE Linux Enterprise Server installs the OpenSSH package by default providing the commands ssh, scp, and sftp. In the default configuration, remote access of a SUSE Linux Enterprise Server system is only possible with the OpenSSH utilities, and only if the sshd is running and the firewall permits access.

SSH on SUSE Linux Enterprise Server uses cryptographic hardware acceleration if available. As a result, the transfer of large quantities of data through an SSH connection is considerably faster than without cryptographic hardware. As an additional benefit, the CPU will see a significant reduction in load.

23.1 ssh—Secure Shell Edit source

With ssh it is possible to log in to remote systems and to work interactively. To log in to the host sun as user tux enter one of the following commands:

tux > ssh tux@sun
tux > ssh -l tux sun

If the user name is the same on both machines, you can omit it. Using ssh sun is sufficient. The remote host prompts for the remote user's password. After a successful authentication, you can work on the remote command line or use interactive applications, such as YaST in text mode.

Furthermore, ssh offers the possibility to run non-interactive commands on remote systems using ssh HOST COMMAND. COMMAND needs to be properly quoted. Multiple commands can be concatenated as on a local shell.

tux > ssh root@sun "dmesg -T | tail -n 25"
tux > ssh root@sun "cat /etc/issue && uptime"

23.1.1 Starting X Applications on a Remote Host Edit source

SSH also simplifies the use of remote X applications. If you run ssh with the -X option, the DISPLAY variable is automatically set on the remote machine and all X output is exported to the local machine over the existing SSH connection. At the same time, X applications started remotely cannot be intercepted by unauthorized individuals.

23.1.2 Agent Forwarding Edit source

By adding the -A option, the ssh-agent authentication mechanism is carried over to the next machine. This way, you can work from different machines without having to enter a password, but only if you have distributed your public key to the destination hosts and properly saved it there. Refer to Section 23.5.2, “Copying an SSH Key” for details.

This mechanism is deactivated in the default settings, but can be permanently activated at any time in the system-wide configuration file /etc/ssh/sshd_config by setting AllowAgentForwarding yes.

23.2 scp—Secure Copy Edit source

scp copies files to or from a remote machine. If the user name on jupiter is different than the user name on sun, specify the latter using the USER_NAME@host format. If the file should be copied into a directory other than the remote user's home directory, specify it as sun:DIRECTORY. The following examples show how to copy a file from a local to a remote machine and vice versa.

tux > scp ~/MyLetter.tex tux@sun:/tmp 1
tux > scp tux@sun:/tmp/MyLetter.tex ~ 2

1

local to remote

2

remote to local

Tip
Tip: The -l Option

With the ssh command, the option -l can be used to specify a remote user (as an alternative to the USER_NAME@host format). With scp the option -l is used to limit the bandwidth consumed by scp.

After the correct password is entered, scp starts the data transfer. It displays a progress bar and the time remaining for each file that is copied. Suppress all output with the -q option.

scp also provides a recursive copying feature for entire directories. The command

tux > scp -r src/ sun:backup/

copies the entire contents of the directory src including all subdirectories to the ~/backup directory on the host sun. If this subdirectory does not exist, it is created automatically.

The -p option tells scp to leave the time stamp of files unchanged. -C compresses the data transfer. This minimizes the data volume to transfer, but creates a heavier burden on the processors of both machines.

23.3 sftp—Secure File Transfer Edit source

23.3.1 Using sftp Edit source

If you want to copy several files from or to different locations, sftp is a convenient alternative to scp. It opens a shell with a set of commands similar to a regular FTP shell. Type help at the sftp-prompt to get a list of available commands. More details are available from the sftp man page.

tux > sftp sun
Enter passphrase for key '/home/tux/.ssh/id_rsa':
Connected to sun.
sftp> help
Available commands:
bye                                Quit sftp
cd path                            Change remote directory to 'path'
[...]

23.3.2 Setting Permissions for File Uploads Edit source

As with a regular FTP server, a user cannot only download, but also upload files to a remote machine running an SFTP server by using the put command. By default the files will be uploaded to the remote host with the same permissions as on the local host. There are two options to automatically alter these permissions:

Setting a umask

A umask works as a filter against the permissions of the original file on the local host. It can only withdraw permissions:

Table 23.1:

permissions original

umask

permissions uploaded

0666

0002

0664

0600

0002

0600

0775

0025

0750

To apply a umask on an SFTP server, edit the file /etc/ssh/sshd_configuration. Search for the line beginning with Subsystem sftp and add the -u parameter with the desired setting, for example:

Subsystem sftp /usr/lib/ssh/sftp-server -u 0002
Explicitly Setting the Permissions

Explicitly setting the permissions sets the same permissions for all files uploaded via SFTP. Specify a three-digit pattern such as 600, 644, or 755 with -u. When both -m and -u are specified, -u is ignored.

To apply explicit permissions for uploaded files on an SFTP server, edit the file /etc/ssh/sshd_configuration. Search for the line beginning with Subsystem sftp and add the -m parameter with the desired setting, for example:

Subsystem sftp /usr/lib/ssh/sftp-server -m 600

23.4 The SSH Daemon (sshd) Edit source

To work with the SSH client programs ssh and scp, a server (the SSH daemon) must be running in the background, listening for connections on TCP/IP port 22. The daemon generates three key pairs when starting for the first time. Each key pair consists of a private and a public key. Therefore, this procedure is called public key-based. To guarantee the security of the communication via SSH, access to the private key files must be restricted to the system administrator. The file permissions are set accordingly by the default installation. The private keys are only required locally by the SSH daemon and must not be given to anyone else. The public key components (recognizable by the name extension .pub) are sent to the client requesting the connection. They are readable for all users.

A connection is initiated by the SSH client. The waiting SSH daemon and the requesting SSH client exchange identification data to compare the protocol and software versions, and to prevent connections through the wrong port. Because a child process of the original SSH daemon replies to the request, several SSH connections can be made simultaneously.

For the communication between SSH server and SSH client, OpenSSH supports versions 1 and 2 of the SSH protocol. Version 2 of the SSH protocol is used by default. Override this to use version 1 of protocol with the -1 option.

When using version 1 of SSH, the server sends its public host key and a server key, which is regenerated by the SSH daemon every hour. Both allow the SSH client to encrypt a freely chosen session key, which is sent to the SSH server. The SSH client also tells the server which encryption method (cipher) to use. Version 2 of the SSH protocol does not require a server key. Both sides use an algorithm according to Diffie-Hellman to exchange their keys.

The private host and server keys are absolutely required to decrypt the session key and cannot be derived from the public parts. Only the contacted SSH daemon can decrypt the session key using its private keys. This initial connection phase can be watched closely by turning on verbose debugging using the -v option of the SSH client.

Tip
Tip: Viewing the SSH Daemon Log File

To watch the log entries from the sshd use the following command:

tux > sudo journalctl -u sshd

23.4.1 Maintaining SSH Keys Edit source

It is recommended to back up the private and public keys stored in /etc/ssh/ in a secure, external location. In this way, key modifications can be detected or the old ones can be used again after having installed a new system.

Tip
Tip: Existing SSH Host Keys

If you install SUSE Linux Enterprise Server on a machine with existing Linux installations, the installation routine automatically imports the SSH host key with the most recent access time from an existing installation.

When establishing a secure connection with a remote host for the first time, the client stores all public host keys in ~/.ssh/known_hosts. This prevents any man-in-the-middle attacks—attempts by foreign SSH servers to use spoofed names and IP addresses. Such attacks are detected either by a host key that is not included in ~/.ssh/known_hosts, or by the server's inability to decrypt the session key in the absence of an appropriate private counterpart.

If the public keys of a host have changed (that needs to be verified before connecting to such a server), the offending keys can be removed with ssh-keygen -r HOSTNAME.

23.4.2 Rotating Host Keys Edit source

As of version 6.8, OpenSSH comes with a protocol extension that supports host key rotation. It makes sense to replace keys, if you are still using weak keys such as 1024-bit RSA keys. It is strongly recommended to replace such a key and go for 2048-bit DSA keys or something even better. The client will then use the best host key.

Tip
Tip: Restarting sshd

After installing new host keys on the server, restart sshd.

This protocol extension can inform a client of all the new host keys on the server, if the user initiates a connection with ssh. Then, the software on the client updates ~/.ssh/known_hosts, and the user is not required to accept new keys of previously known and trusted hosts manually. The local known_hosts file will contain all the host keys of the remote hosts, in addition to the one that authenticated the host during this session.

Once the administrator of the server knows that all the clients have fetched the new keys, they can remove the old keys. The protocol extension ensures that the obsolete keys will be removed from the client's configuration, too. The key removal occurs while initiating an ssh session.

For more information, see:

23.5 SSH Authentication Mechanisms Edit source

In its simplest form, authentication is done by entering the user's password just as if logging in locally. However, having to memorize passwords of several users on remote machines is inefficient. What is more, these passwords may change. On the other hand—when granting root access—an administrator needs to be able to quickly revoke such a permission without having to change the root password.

To accomplish a login that does not require to enter the remote user's password, SSH uses another key pair, which needs to be generated by the user. It consists of a public (id_rsa.pub or id_dsa.pub) and a private key (id_rsa or id_dsa).

To be able to log in without having to specify the remote user's password, the public key of the SSH user must be in ~/.ssh/authorized_keys. This approach also ensures that the remote user has got full control: adding the key requires the remote user's password and removing the key revokes the permission to log in from remote.

For maximum security such a key should be protected by a passphrase which needs to be entered every time you use ssh, scp, or sftp. Contrary to the simple authentication, this passphrase is independent from the remote user and therefore always the same.

An alternative to the key-based authentication described above, SSH also offers a host-based authentication. With host-based authentication, users on a trusted host can log in to another host on which this feature is enabled using the same user name. SUSE Linux Enterprise Server is set up for using key-based authentication, covering setting up host-based authentication on SUSE Linux Enterprise Server is beyond the scope of this manual.

Note
Note: File Permissions for Host-Based Authentication

If the host-based authentication is to be used, the file /usr/lib/ssh/ssh-keysign should have the setuid bit set, which is not the default setting in SUSE Linux Enterprise Server. In such case, set the file permissions manually. You should use /etc/permissions.local for this purpose, to make sure that the setuid bit is preserved after security updates of openssh.

23.5.1 Generating an SSH Key Edit source

  1. To generate a key with default parameters (RSA, 2048 bits), enter the command ssh-keygen.

  2. Accept the default location to store the key (~/.ssh/id_rsa) by pressing Enter (strongly recommended) or enter an alternative location.

  3. Enter a passphrase consisting of 10 to 30 characters. The same rules as for creating safe passwords apply. It is strongly advised to refrain from specifying no passphrase.

You should make absolutely sure that the private key is not accessible by anyone other than yourself (always set its permissions to 0600). The private key must never fall into the hands of another person.

To change the password of an existing key pair, use the command ssh-keygen -p.

23.5.2 Copying an SSH Key Edit source

To copy a public SSH key to ~/.ssh/authorized_keys of a user on a remote machine, use the command ssh-copy-id. To copy your personal key stored under ~/.ssh/id_rsa.pub you may use the short form. To copy DSA keys or keys of other users, you need to specify the path:

tux > ~/.ssh/id_rsa.pub
ssh-copy-id -i tux@sun

tux > ~/.ssh/id_dsa.pub
ssh-copy-id -i ~/.ssh/id_dsa.pub  tux@sun

tux > ~notme/.ssh/id_rsa.pub
ssh-copy-id -i ~notme/.ssh/id_rsa.pub  tux@sun

To successfully copy the key, you need to enter the remote user's password. To remove an existing key, manually edit ~/.ssh/authorized_keys.

23.5.3 Using the ssh-agent Edit source

When doing lots of secure shell operations it is cumbersome to type the SSH passphrase for each such operation. Therefore, the SSH package provides another tool, ssh-agent, which retains the private keys for the duration of an X or terminal session. All other windows or programs are started as clients to the ssh-agent. By starting the agent, a set of environment variables is set, which will be used by ssh, scp, or sftp to locate the agent for automatic login. See the ssh-agent man page for details.

After the ssh-agent is started, you need to add your keys by using ssh-add. It will prompt for the passphrase. After the password has been provided once, you can use the secure shell commands within the running session without having to authenticate again.

23.5.3.1 Using ssh-agent in an X Session Edit source

On SUSE Linux Enterprise Server, the ssh-agent is automatically started by the GNOME display manager. To also invoke ssh-add to add your keys to the agent at the beginning of an X session, do the following:

  1. Log in as the desired user and check whether the file ~/.xinitrc exists.

  2. If it does not exist, use an existing template or copy it from /etc/skel:

    if [ -f ~/.xinitrc.template ]; then mv ~/.xinitrc.template ~/.xinitrc; \
    else cp /etc/skel/.xinitrc.template ~/.xinitrc; fi
  3. If you have copied the template, search for the following lines and uncomment them. If ~/.xinitrc already existed, add the following lines (without comment signs).

    # if test -S "$SSH_AUTH_SOCK" -a -x "$SSH_ASKPASS"; then
    #       ssh-add < /dev/null
    # fi
  4. When starting a new X session, you will be prompted for your SSH passphrase.

23.5.3.2 Using ssh-agent in a Terminal Session Edit source

In a terminal session you need to manually start the ssh-agent and then call ssh-add afterward. There are two ways to start the agent. The first example given below starts a new Bash shell on top of your existing shell. The second example starts the agent in the existing shell and modifies the environment as needed.

tux > ssh-agent -s /bin/bash
eval $(ssh-agent)

After the agent has been started, run ssh-add to provide the agent with your keys.

23.6 Port Forwarding Edit source

ssh can also be used to redirect TCP/IP connections. This feature, also called SSH tunneling, redirects TCP connections to a certain port to another machine via an encrypted channel.

With the following command, any connection directed to jupiter port 25 (SMTP) is redirected to the SMTP port on sun. This is especially useful for those using SMTP servers without SMTP-AUTH or POP-before-SMTP features. From any arbitrary location connected to a network, e-mail can be transferred to the home mail server for delivery.

root # ssh -L 25:sun:25 jupiter

Similarly, all POP3 requests (port 110) on jupiter can be forwarded to the POP3 port of sun with this command:

root # ssh -L 110:sun:110 jupiter

Both commands must be executed as root, because the connection is made to privileged local ports. E-mail is sent and retrieved by normal users in an existing SSH connection. The SMTP and POP3 host must be set to localhost for this to work. Additional information can be found in the manual pages for each of the programs described above and in the OpenSSH package documentation under /usr/share/doc/packages/openssh.

23.7 Adding and Removing Public Keys on an Installed System Edit source

In some environments, it is convenient or necessary to log in over SSH. As such, the user needs to provide a public SSH key. To add or remove an SSH key, proceed as follows:

  1. Open YaST.

  2. Under Security and Users, open the User and Group Management module.

  3. Select the user you want to change and press Edit.

  4. Switch to the SSH Public Key tab.

  5. Add or remove your public key(s). If you add a public SSH key, look for the file extension .pub.

  6. Confirm with Ok.

Your public SSH key is saved in ~/.ssh/authorized_keys.

23.8 For More Information Edit source

https://www.openssh.com

The home page of OpenSSH

https://en.wikibooks.org/wiki/OpenSSH

The OpenSSH Wikibook

man sshd

The man page of the OpenSSH daemon

man ssh_config

The man page of the OpenSSH SSH client configuration files

man scp , man sftp , man slogin , man ssh , man ssh-add , man ssh-agent , man ssh-copy-id , man ssh-keyconvert , man ssh-keygen , man ssh-keyscan

Man pages of several binary files to securely copy files (scp, sftp), to log in (slogin, ssh), and to manage keys.

/usr/share/doc/packages/openssh/README.SUSE , /usr/share/doc/packages/openssh/README.FIPS

SUSE package specific documentation; changes in defaults with respect to upstream, notes on FIPS mode etc.

24 Masquerading and Firewalls Edit source

Whenever Linux is used in a network environment, you can use the kernel functions that allow the manipulation of network packets to maintain a separation between internal and external network areas. The Linux netfilter framework provides the means to establish an effective firewall that keeps different networks apart. Using iptables—a generic table structure for the definition of rule sets—precisely controls the packets allowed to pass a network interface. Such a packet filter can be set up using firewalld and its graphical interface firewall-config.

SUSE Linux Enterprise 15 introduces firewalld as the new software firewall, replacing SuSEfirewall2. SuSEfirewall2 has not been removed from SUSE Linux Enterprise 15 and is still in the Main repository. However, it is not included in the default installation, and firewalld is installed by default in new installations. This chapter provides guidance for configuring firewalld, and migrating from SuSEfirewall2 for users who have upgraded from older SUSE Linux Enterprise releases.

24.1 Packet Filtering with iptables Edit source

This section discusses the low-level details of packet filtering. The components netfilter and iptables are responsible for the filtering and manipulation of network packets and for network address translation (NAT). The filtering criteria and any actions associated with them are stored in chains, which must be matched one after another by individual network packets as they arrive. The chains to match are stored in tables. The iptables command allows you to alter these tables and rule sets.

The Linux kernel maintains three tables, each for a particular category of functions of the packet filter:

filter

This table holds the bulk of the filter rules, because it implements the packet filtering mechanism in the stricter sense, which determines whether packets are let through (ACCEPT) or discarded (DROP), for example.

nat

This table defines any changes to the source and target addresses of packets. Using these functions also allows you to implement masquerading, which is a special case of NAT used to link a private network with the Internet.

mangle

The rules held in this table make it possible to manipulate values stored in IP headers (such as the type of service).

These tables contain several predefined chains to match packets:

PREROUTING

This chain is applied to all incoming packets.

INPUT

This chain is applied to packets destined for the system's internal processes.

FORWARD

This chain is applied to packets that are only routed through the system.

OUTPUT

This chain is applied to packets originating from the system itself.

POSTROUTING

This chain is applied to all outgoing packets.

Figure 24.1, “iptables: A Packet's Possible Paths” illustrates the paths along which a network packet may travel on a given system. For the sake of simplicity, the figure lists tables as parts of chains, but in reality these chains are held within the tables themselves.

In the simplest case, an incoming packet destined for the system itself arrives at the eth0 interface. The packet is first referred to the PREROUTING chain of the mangle table then to the PREROUTING chain of the nat table. The following step, concerning the routing of the packet, determines that the actual target of the packet is a process of the system itself. After passing the INPUT chains of the mangle and the filter table, the packet finally reaches its target, provided that the rules of the filter table allow this.

iptables: A Packet's Possible Paths
Figure 24.1: iptables: A Packet's Possible Paths

24.2 Masquerading Basics Edit source

Masquerading is the Linux-specific form of NAT (network address translation) and can be used to connect a small LAN with the Internet. LAN hosts use IP addresses from the private range (see Book “Administration Guide”, Chapter 19 “Basic Networking”, Section 19.1.2 “Netmasks and Routing”) and on the Internet official IP addresses are used. To be able to connect to the Internet, a LAN host's private address is translated to an official one. This is done on the router, which acts as the gateway between the LAN and the Internet. The underlying principle is a simple one: The router has more than one network interface, typically a network card and a separate interface connecting with the Internet. While the latter links the router with the outside world, one or several others link it with the LAN hosts. With these hosts in the local network connected to the network card (such as eth0) of the router, they can send any packets not destined for the local network to their default gateway or router.

Important
Important: Using the Correct Network Mask

When configuring your network, make sure both the broadcast address and the netmask are the same for all local hosts. Failing to do so prevents packets from being routed properly.

As mentioned, whenever one of the LAN hosts sends a packet destined for an Internet address, it goes to the default router. However, the router must be configured before it can forward such packets. For security reasons, this is not enabled in a default installation. To enable it, add the line net.ipv4.ip_forward = 1 in the file /etc/sysctl.conf. Alternatively do this via YaST, for example by calling yast routing ip-forwarding on.

The target host of the connection can see your router, but knows nothing about the host in your internal network where the packets originated. This is why the technique is called masquerading. Because of the address translation, the router is the first destination of any reply packets. The router must identify these incoming packets and translate their target addresses, so packets can be forwarded to the correct host in the local network.

With the routing of inbound traffic depending on the masquerading table, there is no way to open a connection to an internal host from the outside. For such a connection, there would be no entry in the table. In addition, any connection already established has a status entry assigned to it in the table, so the entry cannot be used by another connection.

As a consequence of all this, you might experience some problems with several application protocols, such as ICQ, cucme, IRC (DCC, CTCP), and FTP (in PORT mode). Web browsers, the standard FTP program, and many other programs use the PASV mode. This passive mode is much less problematic as far as packet filtering and masquerading are concerned.

24.3 Firewalling Basics Edit source

Firewall is probably the term most widely used to describe a mechanism that controls the data flow between networks. Strictly speaking, the mechanism described in this section is called a packet filter. A packet filter regulates the data flow according to certain criteria, such as protocols, ports, and IP addresses. This allows you to block packets that, according to their addresses, are not supposed to reach your network. To allow public access to your Web server, for example, explicitly open the corresponding port. However, a packet filter does not scan the contents of packets with legitimate addresses, such as those directed to your Web server. For example, if incoming packets were intended to compromise a CGI program on your Web server, the packet filter would still let them through.

A more effective but more complex mechanism is the combination of several types of systems, such as a packet filter interacting with an application gateway or proxy. In this case, the packet filter rejects any packets destined for disabled ports. Only packets directed to the application gateway are accepted. This gateway or proxy pretends to be the actual client of the server. In a sense, such a proxy could be considered a masquerading host on the protocol level used by the application. One example for such a proxy is Squid, an HTTP and FTP proxy server. To use Squid, the browser must be configured to communicate via the proxy. Any HTTP pages or FTP files requested are served from the proxy cache and objects not found in the cache are fetched from the Internet by the proxy.

The following section focuses on the packet filter that comes with SUSE Linux Enterprise Server. For further information about packet filtering and firewalling, read the Firewall HOWTO.

24.4 firewalld Edit source

Note
Note: firewalld Replaces SuSEfirewall2

SUSE Linux Enterprise 15 introduces firewalld, replacing SuSEfirewall2. SuSEfirewall2 has not been removed from SUSE Linux Enterprise 15 and is still in the Main repository. However, it is not included in the default installation, and firewalld is installed by default in new installations. If you are upgrading from a SUSE Linux Enterprise release older than SLE 15.0, SuSEfirewall2 will be unchanged and you must manually upgrade to firewalld (see Section 24.5, “Migrating From SuSEfirewall2”).

firewalld is a daemon that maintains the system's iptables rules and offers a D-Bus interface for operating on them. It comes with a command line utility firewall-cmd and a graphical user interface firewall-config for interacting with it. Since firewalld is running in the background and provides a well defined interface it allows other applications to request changes to the iptables rules, for example to set up virtual machine networking.

firewalld implements different security zones. A number of predefined zones like internal and public exist. The administrator can define additional custom zones if desired. Each zone contains its own set of iptables rules. Each network interface is a member of exactly one zone. Individual connections can also be assigned to a zone based on the source addresses.

Each zone represents a certain level of trust. For example the public zone is not trusted, because other computers in this network are not under your control (suitable for Internet or wireless hotspot connections). On the other hand the internal zone is used for networks that are under your control, like a home or company network. By utilizing zones this way, a host can offer different kinds of services to trusted networks and untrusted networks in a defined way.

For more information about the predefined zones and their meaning in firewalld, refer to its manual at http://www.firewalld.org/documentation/zone/predefined-zones.html.

Note
Note: No Zone Assigned Behavior

The initial state for network interfaces is to be assigned to no zone at all. In this case the network interface will be implicitly handled in the default zone, which can be determined by calling firewall-cmd --get-default-zone. If not configured otherwise, the default zone is the public zone.

The firewalld packet filtering model allows any outgoing connections to pass. Outgoing connections are connections that are actively established by the local host. Incoming connections that are established by remote hosts are blocked if the respective service is not allowed in the zone in question. Therefore, each of the interfaces with incoming traffic must be placed in a suitable zone to allow for the desired services to be accessible. For each of the zones, define the services or protocols you need.

An important concept of firewalld is the distinction between two separate configurations: the runtime and the permanent configuration. The runtime configuration represents the currently active rules, while the permanent configuration represents the saved rules that will be applied when restarting firewalld. This allows to add temporary rules that will be discarded after restarting firewalld, or to experiment with new rules while being able to revert back to the original state. When you are changing the configuration, you need to be aware of which configuration you're editing. How this is done is discussed in Section 24.4.3.2, “Runtime Versus Permanent Configuration”.

If you want to perform the firewalld configuration using the graphical user interface firewall-config then refer to its documentation. In the following section we will be looking at how to perform typical firewalld configuration tasks using firewall-cmd on the command line.

24.4.1 Configuring the Firewall with NetworkManager Edit source

The NetworkManager supports a basic configuration of firewalld by selecting zones.

When editing a wired or wireless connection, go to the Identity tab in the configuration window and use the Firewall Zone drop-down box.

24.4.2 Configuring the Firewall with YaST Edit source

The yast firewall module supports a basic configuration of firewalld. It provides a zone selector, services selector, and ports selector. It does not support creating custom iptables rules, and limits zone creation and customization to selecting services and ports.

24.4.3 Configuring the Firewall on the Command Line Edit source

24.4.3.1 Firewall Startup Edit source

firewalld will be installed and enabled by default. It is a regular systemd service that can be configured via systemctl or the YaST Services Manager.

Important
Important: Automatic Firewall Configuration

After the installation, YaST automatically starts firewalld and leaves all interfaces in the default public zone. If a server application is configured and activated on the system, YaST can adjust the firewall rules via the options Open Ports on Selected Interface in Firewall or Open Ports on Firewall in the server configuration modules. Some server module dialogs include a Firewall Details button for activating additional services and ports.

24.4.3.2 Runtime Versus Permanent Configuration Edit source

By default all firewall-cmd commands operate on the runtime configuration. You can apply most operations to the permanent configuration only by adding the --permanent parameter. When doing so the change will only affect the permanent configuration and will not be effective immediately in the runtime configuration. There is currently no way to add a rule to both runtime and permanent configurations in a single invocation. To achieve this you can apply all necessary changes to the runtime configuration and when all is working as expected issue the following command:

root # firewall-cmd --runtime-to-permanent

This will write all current runtime rules into the permanent configuration. Any temporary modifications you or other programs may have made to the firewall in other contexts are made permanent this way. If you're unsure about this, you can also take the opposite approach to be on the safe side: Add new rules to the permanent configuration and reload firewalld to make them active.

Note
Note

Some configuration items, like the default zone, are shared by both the runtime and permanent configurations. Changing them will reflect in both configurations at once.

To revert the runtime configuration to the permanent configuration and thereby discard any temporary changes, two possibilities exist, either via the firewalld command line interface or via systemd:

root # firewall-cmd --reload
root # systemctl reload firewalld

For brevity the examples in the following sections will always operate on the runtime configuration, if applicable. Adjust them accordingly if you want to make them permanent.

24.4.3.3 Assignment of Interfaces to Zones Edit source

You can list all network interfaces currently assigned to a zone like this:

root # firewall-cmd --zone=public --list-interfaces
eth0

Similarly you can query which zone a specific interface is assigned to:

root # firewall-cmd --get-zone-of-interface=eth0
public

The following command lines assign an interface to a zone. The variant using --add-interface will only work if eth0 is not already assigned to another zone. The variant using --change-interface will always work, removing eth0 from its current zone if necessary:

root # firewall-cmd --zone=internal --add-interface=eth0
root # firewall-cmd --zone=internal --change-interface=eth0

Any operations without an explicit --zone argument will implicitly operate on the default zone. This pair of commands can be used for getting and setting the default zone assignment:

root # firewall-cmd --get-default-zone
dmz
root # firewall-cmd --set-default-zone=public
Important
Important

Any network interfaces not explicitly assigned to a zone will be automatically part of the default zone. Changing the default zone will reassign all those network interfaces immediately for the permanent and runtime configurations. You should never use a trusted zone like internal as the default zone, to avoid unexpected exposure to threats. For example hotplugged network interfaces like USB Ethernet interfaces would automatically become part of the trusted zone in such cases.

Also note that interfaces that are not explicitly part of any zone will not appear in the zone interface list. There is currently no command to list unassigned interfaces. Due to this it is best to avoid unassigned network interfaces during regular operation.

24.4.3.4 Making Network Services Accessible Edit source

firewalld has a concept of services. A service consists of definitions of ports and protocols. These definitions logically belong together in the context of a given network service like a Web or mail server protocol. The following commands can be used to get information about predefined services and their details:

root # firewall-cmd --get-services
[...] dhcp dhcpv6 dhcpv6-client dns docker-registry [...]
root # firewall-cmd --info-service dhcp
dhcp
  ports: 67/udp
  protocols:
  source-ports:
  modules:
  destination:

These service definitions can be used for easily making the associated network functionality accessible in a zone. This command line will open the HTTP Web server port in the internal zone, for example:

root # firewall-cmd --add-service=http --zone=internal

The removal of a service from a zone is performed using the counterpart command --remove-service. You can also define custom services using the --new-service subcommand. Refer to http://www.firewalld.org/documentation/howto/add-a-service.html for more details on how to do this.

If you just want to open a single port by number, you can use the following approach. This will open TCP port 8000 in the internal zone:

root # firewall-cmd --add-port=8000/tcp --zone=internal

For removal use the counterpart command --remove-port.

Tip
Tip: Temporarily Opening a Service or Port

firewalld supports a --timeout parameter that allows to open a service or port for a limited time duration. This can be helpful for quick testing and makes sure that closing the service or port will not be forgotten. To allow the imap service in the internal zone for 5 minutes, you would call

root # firewall-cmd --add-service=imap --zone=internal --timeout=5m

24.4.3.5 Lockdown Mode Edit source

firewalld offers a lockdown mode that prevents changes to the firewall rules while it is active. Since applications can automatically change the firewall rules via the D-Bus interface, and depending on the PolicyKit rules regular users may be able to do the same, it can be helpful to prevent changes in some situations. You can find more information about this at https://fedoraproject.org/wiki/Features/FirewalldLockdown.

It is important to understand that the lockdown mode feature provides no real security, but merely protection against accidental or benign attempts to change the firewall. The way the lockdown mode is currently implemented in firewalld provides no security against malicious intent. as is pointed out at http://seclists.org/oss-sec/2017/q3/139.

24.4.3.6 Adding Custom iptables Rules Edit source

firewalld claims exclusive control over the host's netfilter rules. You should never modify firewall rules using other tools like iptables. Doing so could confuse firewalld and break security or functionality.

If you need to add custom firewall rules that aren't covered by firewalld features then there are two ways to do so. To directly pass raw iptables syntax you can use the --direct option. It expects the table, chain, and priority as initial arguments and the rest of the command line is passed as is to iptables. The following example adds a connection tracking rule for the forwarding filter table:

root # firewall-cmd  --direct --add-rule ipv4 filter FORWARD 0 -i eth0 -o eth1 \
    -p tcp --dport 80 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT

Additionally, firewalld implements so called rich rules, an extended syntax for specifying iptables rules in an easier way. You can find the syntax specification at http://www.firewalld.org/documentation/man-pages/firewalld.richlanguage.html. The following example drops all IPv4 packets originating from a certain source address:

root # firewall-cmd --zone=public --add-rich-rule='rule family="ipv4" \
    source address="192.168.2.4" drop'

24.4.3.7 Routing, Forwarding, and Masquerading Edit source

firewalld is not designed to run as a fully fledged router. The basic functionality for typical home router setups is available. For a corporate production router you should not use firewalld, however, but use dedicated router and firewall devices instead. The following provides just a few pointers on what to look for to utilize routing in firewalld:

  • First of all IP forwarding needs to be enabled as outlined in Section 24.2, “Masquerading Basics”.

  • To enable IPv4 masquerading, for example in the internal zone, issue the following command.

    root # firewall-cmd --zone=internal --add-masquerade
  • firewalld can also enable port forwarding. The following command will forward local TCP connections on port 80 to another host:

    root # firewall-cmd --zone=public \
        --add-forward-port=port=80:proto=tcp:toport=80:toaddr=192.168.1.10

24.4.4 Accessing Services Listening on Dynamic Ports Edit source

Some network services do not listen on predefined port numbers. Instead they operate based on the portmapper or rpcbind protocol. We will use the term rpcbind from here on. When one of these services starts, it chooses a random local port and talks to rpcbind to make the port number known. rpcbind itself is listening on a well known port. Remote systems can then query rpcbind about the network services it knows about and on which ports they are listening. Not many programs use this approach anymore today. Popular examples are Network Information Services (NIS; ypserv and ypbind) and the Network File System (NFS) version 3.

Note
Note: About NFSv4

The newer NFSv4 only requires the single well known TCP port 2049. For protocol version 4.0 the kernel parameter fs.nfs.nfs_callback_tcpport may need to be set to a static port (see Example 24.1, “Callback Port Configuration for the nfs Kernel Module in /etc/modprobe.d/60-nfs.conf). Starting with protocol version 4.1 this setting has also become unnecessary.

The dynamic nature of the rpcbind protocol makes it difficult to make the affected services behind the firewall accessible. firewalld does not support these services by itself. For manual configuration, see Section 24.4.4.1, “Configuring Static Ports”. Alternatively, SUSE Linux Enterprise Server provides a helper script. For details, see Section 24.4.4.2, “Using firewall-rpcbind-helper for Configuring Static Ports”.

24.4.4.1 Configuring Static Ports Edit source

One possibility is to configure all involved network services to use fixed port numbers. Once this is done, the fixed ports can be opened in firewalld and everything should work. The actual port numbers used are at your discretion but should not clash with any well known port numbers assigned to other services. See Table 24.1, “Important Sysconfig Variables for Static Port Configuration” for a list of the available configuration items for NIS and NFSv3 services. Note that depending on your actual NIS or NFS configuration, not all of these ports may be required for your setup.

Table 24.1: Important Sysconfig Variables for Static Port Configuration

File Path

Variable Name

Example Value

/etc/sysconfig/nfs MOUNTD_PORT 21001
STATD_PORT 21002
LOCKD_TCPPORT 21003
LOCKD_UDPPORT 21003
RQUOTAD_PORT 21004
/etc/sysconfig/ypbind YPBIND_OPTIONS -p 24500
/etc/sysconfig/ypserv YPXFRD_ARGS -p 24501
YPSERV_ARGS -p 24502
YPPASSWDD_ARGS --port 24503

You will need to restart any related services that are affected by these static port configurations for the changes to take effect. You can see the currently assigned rpcbind ports by using the command rpcinfo -p. On success only the statically configured ports should show up there.

Apart from the port configuration for network services running in userspace there are also ports that are used by the Linux kernel directly when it comes to NFS. One of these ports is nfs_callback_tcpport. It is only required for NFS protocol versions older than 4.1. There is a sysctl named fs.nfs.nfs_callback_tcpport to configure this port. This sysctl node only appears dynamically when NFS mounts are active. Therefore it is best to configure the port via kernel module parameters. This can be achieved by creating a file as shown in Example 24.1, “Callback Port Configuration for the nfs Kernel Module in /etc/modprobe.d/60-nfs.conf.

Example 24.1: Callback Port Configuration for the nfs Kernel Module in /etc/modprobe.d/60-nfs.conf
options nfs callback_tcpport=21005

To make this change effective it is easiest to reboot the machine. Otherwise all NFS services need to be stopped and the nfs kernel module needs to be reloaded. To verify the active NFS callback port, check the output of cat /sys/module/nfs/parameters/callback_tcpport.

For easy handling of the now statically configured RPC ports, it is useful to create a new firewalld service definition. This service definition will group all related ports and, for example, makes it easy to make them accessible in a specific zone. In Example 24.2, “Commands to Define a new firewalld RPC Service for NFS” this is done for the NFS ports as they have been configured in the accompanying examples.

Example 24.2: Commands to Define a new firewalld RPC Service for NFS
root # firewall-cmd --permanent --new-service=nfs-rpc
root # firewall-cmd --permanent --service=nfs-rpc --set-description="NFS related, statically configured RPC ports"
# add UDP and TCP ports for the given sequence
root # for port in 21001 21002 21003 21004; do
    firewall-cmd --permanent --service=nfs-rpc --add-port ${port}/udp --add-port ${port}/tcp
done
# the callback port is TCP only
root # firewall-cmd --permanent --service=nfs-rpc --add-port 21005/tcp

# show the complete definition of the new custom service
root # firewall-cmd --info-service=nfs-rpc --permanent -v
nfs-rpc
  summary:
  description: NFS and related, statically configured RPC ports
  ports: 4711/tcp 21001/udp 21001/tcp 21002/udp 21002/tcp 21003/udp 21003/tcp 21004/udp 21004/tcp
  protocols:
  source-ports:
  modules:
  destination:

# reload firewalld to make the new service definition available
root # firewall-cmd --reload

# the new service definition can now be used to open the ports for example in the internal zone
root # firewall-cmd --add-service=nfs-rpc --zone=internal

24.4.4.2 Using firewall-rpcbind-helper for Configuring Static Ports Edit source

The steps to configure static ports as shown in the previous section can be simplified by using the SUSE helper tool firewall-rpc-helper.py. Install it with zypper in firewalld-rpcbind-helper.

The tool allows interactive configuration of the service patterns discussed in the previous section. It can also display current port assignments and can be used for scripting. For details, see firewall-rpc-helper.py --help.

24.5 Migrating From SuSEfirewall2 Edit source

Note
Note: Creating a firewalld Configuration for AutoYaST

See the Firewall Configuration section of the AutoYaST Guide to learn how to create a firewalld configuration for AutoYaST.

When upgrading from SUSE Linux Enterprise 12.x to SUSE Linux Enterprise 15 SP1, SuSEfirewall2 is not changed and remains active. There is no automatic migration, so you must migrate to firewalld manually. firewalld includes a helper migration script, susefirewall2-to-firewalld. Depending on the complexity of your SuSEfirewall2 configuration the script may perform a perfect migration, or it may fail. Most likely it will partially succeed and you will have to review your new firewalld configuration and make adjustments.

The resulting configuration will make firewalld behave somewhat like SuSEfirewall2. To take full advantage of firewalld's features you may elect to create a new configuration, rather than trying to migrate your old configuration. It is safe to run the susefirewall2-to-firewalld script with no options, as it makes no permanent changes to your system. However, if you are administering the system remotely you could get locked out.

Install and run susefirewall2-to-firewalld:

root # zypper in susefirewall2-to-firewalld
root # susefirewall2-to-firewalld
INFO: Reading the /etc/sysconfig/SuSEfirewall2 file
INFO: Ensuring all firewall services are in a well-known state.
INFO: This will start/stop/restart firewall services and it's likely
INFO: to cause network disruption.
INFO: If you do not wish for this to happen, please stop the script now!
5...4...3...2...1...Lets do it!
INFO: Stopping firewalld
INFO: Restarting SuSEfirewall2_init
INFO: Restarting SuSEfirewall2
INFO: DIRECT: Adding direct rule="ipv4 -t filter -A INPUT -p udp -m udp --dport 5353 -m pkttype
  --pkt-type multicast -j ACCEPT"
[...]
INFO: Enabling direct rule=ipv6 -t filter -A INPUT -p udp -m udp --dport 546 -j ACCEPT
INFO: Enabling direct rule=ipv6 -t filter -A INPUT -p udp -m udp --dport 5353 -m pkttype
  --pkt-type multicast -j ACCEPT
INFO: Enable logging for denied packets
INFO: ##########################################################
INFO:
INFO: The dry-run has been completed. Please check the above output to ensure
INFO: that everything looks good.
INFO:
INFO: ###########################################################
INFO: Stopping firewalld
INFO: Restarting SuSEfirewall2_init
INFO: Restarting SuSEfirewall2

This results in a lot of output, which you may wish to direct to a file for easier review:

root # susefirewall2-to-firewalld | tee newfirewallrules.txt

The script supports these options:

-c

Commit changes. The script will make changes to the system so make sure you only use this option if you are really happy with the proposed changes. This *will* reset your current firewalld configuration so make sure you make backups!

-d

Super noisy. Use it to file bug reports but be careful to mask sensitive information.

-h

This message.

-q

No output. Errors will not be printed either!

-v

Verbose mode. It will print warnings and other informative messages.

24.6 For More Information Edit source

The most up-to-date information and other documentation about the firewalld package is found in /usr/share/doc/packages/firewalld. The home page of the netfilter and iptables project, http://www.netfilter.org, provides a large collection of documents about iptables in general in many languages.

25 Configuring a VPN Server Edit source

Today, Internet connections are cheap and available almost everywhere. However, not all connections are secure. Using a Virtual Private Network (VPN), you can create a secure network within an insecure network such as the Internet or Wi-Fi. It can be implemented in different ways and serves several purposes. In this chapter, we focus on the OpenVPN implementation to link branch offices via secure wide area networks (WANs).

25.1 Conceptual Overview Edit source

This section defines some terms regarding VPN and gives a brief overview of some scenarios.

25.1.1 Terminology Edit source

Endpoint

The two ends of a tunnel, the source or destination client.

Tap Device

A tap device simulates an Ethernet device (layer 2 packets in the OSI model, such as Ethernet frames). A tap device is used for creating a network bridge. It works with Ethernet frames.

Tun Device

A tun device simulates a point-to-point network (layer 3 packets in the OSI model, such as IP packets). A tun device is used with routing and works with IP frames.

Tunnel

Linking two locations through a primarily public network. From a more technical viewpoint, it is a connection between the client's device and the server's device. Usually a tunnel is encrypted, but it does need to be by definition.

25.1.2 VPN Scenarios Edit source

Whenever you set up a VPN connection, your IP packets are transferred over a secured tunnel. A tunnel can use either a tun or tap device. They are virtual network kernel drivers which implement the transmission of Ethernet frames or IP frames/packets.

Any user space program, such as OpenVPN, can attach itself to a tun or tap device to receive packets sent by your operating system. The program is also able to write packets to the device.

There are many solutions to set up and build a VPN connection. This section focuses on the OpenVPN package. Compared to other VPN software, OpenVPN can be operated in two modes:

Routed VPN

Routing is an easy solution to set up. It is more efficient and scales better than a bridged VPN. Furthermore, it allows the user to tune MTU (Maximum Transfer Unit) to raise efficiency. However, in a heterogeneous environment, if you do not have a Samba server on the gateway, NetBIOS broadcasts do not work. If you need IPv6, the drivers for the tun devices on both ends must support this protocol explicitly. This scenario is depicted in Figure 25.1, “Routed VPN”.

Routed VPN
Figure 25.1: Routed VPN
Bridged VPN

Bridging is a more complex solution. It is recommended when you need to browse Windows file shares across the VPN without setting up a Samba or WINS server. Bridged VPN is also needed to use non-IP protocols (such as IPX) or applications relying on network broadcasts. However, it is less efficient than routed VPN. Another disadvantage is that it does not scale well. This scenario is depicted in the following figures.

Bridged VPN - Scenario 1
Figure 25.2: Bridged VPN - Scenario 1
Bridged VPN - Scenario 2
Figure 25.3: Bridged VPN - Scenario 2
Bridged VPN - Scenario 3
Figure 25.4: Bridged VPN - Scenario 3

The major difference between bridging and routing is that a routed VPN cannot IP-broadcast while a bridged VPN can.

25.2 Setting Up a Simple Test Scenario Edit source

In the following example, we will create a point-to-point VPN tunnel. The example demonstrates how to create a VPN tunnel between one client and a server. It is assumed that your VPN server will use private IP addresses like IP_OF_SERVER and your client will use the IP address IP_OF_CLIENT. Make sure you select addresses which do not conflict with other IP addresses.

Warning
Warning: Use Only for Testing

This following scenario is provided as an example meant for familiarizing yourself with VPN technology. Do not use this as a real world scenario, as it can compromise the security and safety of your IT infrastructure!

Tip
Tip: Names for Configuration File

To simplify working with OpenVPN configuration files, we recommend the following:

  • Place your OpenVPN configuration files in the directory /etc/openvpn.

  • Name your configuration files MY_CONFIGURATION.conf.

  • If there are multiple files that belong to the same configuration, place them in a subdirectory like /etc/openvpn/MY_CONFIGURATION.

25.2.1 Configuring the VPN Server Edit source

To configure a VPN server, proceed as follows:

Procedure 25.1: VPN Server Configuration
  1. Install the package openvpn on the machine that will later become your VPN server.

  2. Open a shell, become root and create the VPN secret key:

    root # openvpn --genkey --secret /etc/openvpn/secret.key
  3. Copy the secret key to your client:

    root # scp /etc/openvpn/secret.key root@IP_OF_CLIENT:/etc/openvpn/
  4. Create the file /etc/openvpn/server.conf with the following content:

    dev tun
    ifconfig IP_OF_SERVER IP_OF_CLIENT
    secret secret.key
  5. Set up a tun device configuration by creating a file called /etc/sysconfig/network/ifcfg-tun0 with the following content:

    STARTMODE='manual'
    BOOTPROTO='static'
    TUNNEL='tun'
    TUNNEL_SET_OWNER='nobody'
    TUNNEL_SET_GROUP='nobody'
    LINK_REQUIRED=no
    PRE_UP_SCRIPT='systemd:openvpn@server'
    PRE_DOWN_SCRIPT='systemd:openvpn@service'

    The notation openvpn@server points to the OpenVPN server configuration file located at /etc/openvpn/server.conf. For more information, see /usr/share/doc/packages/openvpn/README.SUSE.

  6. If you use a firewall, open the required ports by allowing the openvpn service:

          tux > sudo firewall-cmd --add-service openvpn
          tux > sudo firewall-cmd --permanent --add-service openvpn

    Use sudo firewall-cmd --list-services to check whether the commands were successful. openvpn needs to appear in the list of services.

  7. Start the OpenVPN server service by setting the tun device to up:

    tux > sudo wicked ifup tun0

    You should see the confirmation:

    tun0            up

25.2.2 Configuring the VPN Clients Edit source

To configure the VPN client, do the following:

Procedure 25.2: VPN Client Configuration
  1. Install the package openvpn on your client VPN machine.

  2. Create /etc/openvpn/client.conf with the following content:

    remote DOMAIN_OR_PUBLIC_IP_OF_SERVER
    dev tun
    ifconfig IP_OF_CLIENT IP_OF_SERVER
    secret secret.key

    Replace the placeholder IP_OF_CLIENT in the first line with either the domain name, or the public IP address of your server.

  3. Set up a tun device configuration by creating a file called /etc/sysconfig/network/ifcfg-tun0 with the following content:

    STARTMODE='manual'
    BOOTPROTO='static'
    TUNNEL='tun'
    TUNNEL_SET_OWNER='nobody'
    TUNNEL_SET_GROUP='nobody'
    LINK_REQUIRED=no
    PRE_UP_SCRIPT='systemd:openvpn@client'
    PRE_DOWN_SCRIPT='systemd:openvpn@client'
  4. If you use a firewall, open the respective port (UDP 1194) by allowing the openvpn service:

          tux > sudo firewall-cmd --add-service openvpn
          tux > sudo firewall-cmd --permanent --add-service openvpn

    Use sudo firewall-cmd --list-services to check whether the commands were successful. openvpn needs to appear in the list of services.

  5. Start the OpenVPN service by setting the tun device to up:

    tux > sudo wicked ifup tun0

    You should see the confirmation:

    tun0            up

25.2.3 Testing the VPN Example Scenario Edit source

After OpenVPN has successfully started, test the availability of the tun device with the following command:

ip addr show tun0

To verify the VPN connection, use ping on both client and server side to see if they can reach each other. Ping the server from the client:

ping -I tun0 IP_OF_SERVER

Ping the client from the server:

ping -I tun0 IP_OF_CLIENT

25.3 Setting Up Your VPN Server Using a Certificate Authority Edit source

The example in Section 25.2 is useful for testing, but not for daily work. This section explains how to build a VPN server that allows more than one connection at the same time. This is done with a public key infrastructure (PKI). A PKI consists of a pair of public and private keys for the server and each client, and a master certificate authority (CA), which is used to sign every server and client certificate.

This setup involves the following basic steps:

25.3.1 Creating Certificates Edit source

Before a VPN connection can be established, the client must authenticate the server certificate. Conversely, the server must also authenticate the client certificate. This is called mutual authentication.

Creating certificates is not supported on SUSE Linux Enterprise Server. The following assumes you have created a CA certificate, a server certificate and a client certificate on another system.

The server certificate is required in the PEM and unencrypted key in PEM formats. Copy the PEM version to /etc/openvpn/server_crt.pem on the VPN server. The unencrypted version needs to go to /etc/openvpn/server_key.pem.

Client certificates need to be of the format PKCS12 (preferred) or PEM. The certificate in PKCS12 format needs to contain the CA chain and needs to be copied to /etc/openvpn/CLIENT.p12. In case you have client certificates in PEM format containing the CA chain, copy them to /etc/openvpn/CLIENT.pem. In case you have split the PEM certificates into client certificate (*.ca, client key (*.key), and the CA certificate (*.ca), copy these files to /etc/openvpn/ on each client.

The CA certificate needs to be copied to /etc/openvpn/vpn_ca.pem on the server and each client.

Important
Important: Splitting Client Certificates

If you split client certificates into client certificate, client key, and the CA certificate, you need to provide the respective filenames in the OpenVPN configuration file on the respective clients (see Example 25.1, “VPN Server Configuration File”).

25.3.2 Configuring the VPN Server Edit source

As the basis of your configuration file, copy /usr/share/doc/packages/openvpn/sample-config-files/server.conf to /etc/openvpn/. Then customize it to your needs.

Example 25.1: VPN Server Configuration File
# /etc/openvpn/server.conf
port 1194 1
proto udp 2
dev tun0 3

# Security 4

ca    vpn_ca.pem
cert  server_crt.pem
key   server_key.pem

# ns-cert-type server 
remote-cert-tls client 5
dh   server/dh2048.pem 6

server 192.168.1.0 255.255.255.0 7
ifconfig-pool-persist /var/run/openvpn/ipp.txt 8

# Privileges 9
user nobody
group nobody

# Other configuration 10
keepalive 10 120
comp-lzo
persist-key
persist-tun
# status      /var/log/openvpn-status.tun0.log 11
# log-append  /var/log/openvpn-server.log 12
verb 4

1

The TCP/UDP port on which OpenVPN listens. The standard port for VPN is UDP 1194, so you can usually leave that as it is. If you change it, you may need to adjust your firefall configuration as well.

2

The protocol, either UDP or TCP.

3

The tun or tap device. For the difference between these, see Section 25.1.1, “Terminology”.

4

The following lines contain the relative or absolute path to the root server CA certificate (ca), the root CA key (cert), and the private server key (key). These were generated in Section 25.3.1, “Creating Certificates”.

5

Require that peer certificates have been signed with an explicit key usage and extended key usage based on RFC3280 TLS rules.

6

The Diffie-Hellman parameters. Create the required file with the following command:

openssl dhparam -out /etc/openvpn/dh2048.pem 2048

7

Supplies a VPN subnet. The server can be reached by 192.168.1.1.

8

Records a mapping of clients and its virtual IP address in the given file. Useful when the server goes down and (after the restart) the clients get their previously assigned IP address.

9

For security reasons, run the OpenVPN daemon with reduced privileges. To do so, specify that it should use the group and user nobody.

10

Several other configuration options—see the comment in the example configuration file: /usr/share/doc/packages/openvpn/sample-config-files.

11

Enable this option to write short status updates with statistical data (operational status dump) to the named file. By default, this is not enabled.

All output is written to the system journal, which can be displayed with journalctl. If you have more than one configuration file (for example, one for home and another for work), it is recommended to include the device name into the file name. This avoids overwriting output files accidentally. In this case, it is tun0, taken from the dev directive—see 3.

12

By default, log messages go to syslog. Overwrite this behavior by removing the hash character. In that case, all messages go to /var/log/openvpn-server.log. Do not forget to configure a logrotate service. See man 8 logrotate for further details.

After having completed this configuration, you can see log messages of your OpenVPN server under /var/log/openvpn.log. After having started it for the first time, it should finish with:

... Initialization Sequence Completed

If you do not see this message, check the log carefully for any hints of what is wrong in your configuration file.

25.3.3 Configuring the VPN Clients Edit source

As the basis of your configuration file, copy /usr/share/doc/packages/openvpn/sample-config-files/client.conf to /etc/openvpn/. Then customize it to your needs.

Example 25.2: VPN Client Configuration File
# /etc/openvpn/client.conf
client 1
dev tun 2
proto udp 3
remote IP_OR_HOST_NAME 1194 4
resolv-retry infinite
nobind

remote-cert-tls server 5

# Privileges 6
user nobody
group nobody

# Try to preserve some state across restarts.
persist-key
persist-tun

# Security 7
pkcs12 client1.p12

comp-lzo 8

1

Specifies that this machine is a client.

2

The network device. Both clients and server must use the same device.

3

The protocol. Use the same settings as on the server.

5

This is security option for clients which ensures that the host they connect to is a designated server.

4

Replace the placeholder IP_OR_HOST_NAME with the respective host name or IP address of your VPN server. After the host name, the port of the server is given. You can have multiple lines of remote entries pointing to different VPN servers. This is useful for load balancing between different VPN servers.

6

For security reasons, run the OpenVPN daemon with reduced privileges. To do so, specify that it should use the group and user nobody.

7

Contains the client files. For security reasons, use a separate pair of files for each client.

8

Turn on compression. Only use this parameter if compression is enabled on the server as well.

25.4 Setting Up a VPN Server or Client Using YaST Edit source

You can also use YaST to set up a VPN server. However, the YaST module does not support OpenVPN. Instead, it provides support for the IPsec protocol (as implemented in the software StrongSwan). Like OpenVPN, IPsec is a widely supported VPN scheme.

Procedure 25.3: Setting Up an IPsec Server
  1. To start the YaST VPN module, select Applications › VPN Gateways and Clients.

  2. Under Global Configuration, activate Enable VPN Daemon.

  3. To create a new VPN, click New VPN, then enter a name for the connection.

  4. Under Type, select Gateway (Server).

  5. Then choose the scenario:

    • The scenarios Secure communication with a pre-shared key and Secure communication with a certificate are best suited to Linux client setups.

    • The scenario Provide access to Android, iOS, Mac OS X clients sets up a configuration that is natively supported by modern versions of Android, iOS, and macOS. It is based on a pre-shared key setup with an additional user name and password authentication.

    • The scenario Provide access to Windows 7, Windows 8 clients is a configuration that is natively supported by Windows and BlackBerry devices. It is based on a certificate setup with an additional user name and password authentication.

    For this example, choose Secure communication with a pre-shared key.

  6. To specify the key, click Edit Credentials. Activate Show key, then type the secret key. Confirm with OK.

  7. Choose whether and how to limit access within your VPN under Provide VPN clients access to. To enable only certain IP ranges, specify these in CIDR format, separated by commas in Limited CIDRs. For more information about the CIDR format, see https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing.

  8. Under Clients' address pool, specify the format of IP addresses your VPN should provide to its clients.

  9. To finish, click OK. The YaST VPN module will now automatically add and enable firewall rules to allow clients to connect to the new VPN.

    To view the connection status, in the following confirmation window, click Yes. You will then see the output of systemctl status for your VPN, which allows you to check if the VPN is running and configured correctly.

25.5 For More Information Edit source

For more information about VPN in general, see:

  • https://openvpn.net: the OpenVPN home page

  • man openvpn

  • /usr/share/doc/packages/openvpn/sample-config-files/: example configuration files for different scenarios.

  • /usr/src/linux/Documentation/networking/tuntap.txt, to install the kernel-source package.

26 Enabling FIPS 140-2 Edit source

The Federal Information Processing Standard 140-2 (FIPS 140-2) is a security standard for cryptographic modules. Modules are certified by the National Institute of Standards and Technology (NIST, see https://csrc.nist.gov/projects/cryptographic-module-validation-program). See https://www.suse.com/support/security/certifications/ for a list of certified modules.

26.1 Enabling FIPS Edit source

Enabling FIPS takes a few steps. First, read the /usr/share/doc/packages/openssh-common/README.SUSE file, from the openssh-common package. This contains important information about FIPS on SUSE Linux Enterprise.

Check if it is already enabled:

tux > sudo sysctl -a | grep fips
crypto.fips_enabled = 0

crypto.fips_enabled = 0 indicates that it is not enabled. A return value of 1 means that it is enabled.

To enable FIPS, install the fips pattern:

tux > sudo zypper in -t pattern fips

Then edit /etc/default/grub. If /boot is not on a separate partition, add fips=1 to GRUB_CMDLINE_LINUX_DEFAULT, like the following example:

GRUB_CMDLINE_LINUX_DEFAULT="splash=silent mitigations=auto quiet fips=1"

If /boot is on a separate partition, specify which partition, like the following example, substituting the name of your boot partition:

GRUB_CMDLINE_LINUX_DEFAULT="splash=silent mitigations=auto quiet fips=1 boot=/dev/sda1"

Save your changes, and rebuild your GRUB configuration and initramfs image:

tux > sudo grub2-mkconfig -o /boot/grub2/grub.cfg
tux > sudo mkinitrd

Reboot, then verify your changes. The following example shows that FIPS is enabled:

tux > sudo sysctl -a | grep fips
crypto.fips_enabled = 1

After enabling FIPS it is possible that your system will not boot. If this happens, reboot to bring up the GRUB menu. Press E to edit your boot entry, and delete fips=1 from the linux line. Save your changes and boot. This is a temporary change, so you can find the error and correct it.

Part IV Confining Privileges with AppArmor Edit source

27 Introducing AppArmor

Many security vulnerabilities result from bugs in trusted programs. A trusted program runs with privileges that attackers want to possess. The program fails to keep that trust if there is a bug in the program that allows the attacker to acquire said privilege.

28 Getting Started

Prepare a successful deployment of AppArmor on your system by carefully considering the following items:

29 Immunizing Programs

Effective hardening of a computer system requires minimizing the number of programs that mediate privilege, then securing the programs as much as possible. With AppArmor, you only need to profile the programs that are exposed to attack in your environment, which drastically reduces the amount of wor…

30 Profile Components and Syntax

Building AppArmor profiles to confine an application is very straightforward and intuitive. AppArmor ships with several tools that assist in profile creation. It does not require you to do any programming or script handling. The only task that is required of the administrator is to determine a polic…

31 AppArmor Profile Repositories

AppArmor ships with a set of profiles enabled by default. These are created by the AppArmor developers, and are stored in /etc/apparmor.d. In addition to these profiles, SUSE Linux Enterprise Server ships profiles for individual applications together with the relevant application. These profiles are…

32 Building and Managing Profiles with YaST

YaST provides a basic way to build profiles and manage AppArmor® profiles. It provides two interfaces: a graphical one and a text-based one. The text-based interface consumes less resources and bandwidth, making it a better choice for remote administration, or for times when a local graphical enviro…

33 Building Profiles from the Command Line

AppArmor® provides the user the ability to use a command line interface rather than a graphical interface to manage and configure the system security. Track the status of AppArmor and create, delete, or modify AppArmor profiles using the AppArmor command line tools.

34 Profiling Your Web Applications Using ChangeHat

An AppArmor® profile represents the security policy for an individual program instance or process. It applies to an executable program, but if a portion of the program needs different access permissions than other portions, the program can “change hats” to use a different security context, distincti…

35 Confining Users with pam_apparmor

An AppArmor profile applies to an executable program; if a portion of the program needs different access permissions than other portions need, the program can change hats via change_hat to a different role, also known as a subprofile. The pam_apparmor PAM module allows applications to confine authen…

36 Managing Profiled Applications

After creating profiles and immunizing your applications, SUSE® Linux Enterprise Server becomes more efficient and better protected as long as you perform AppArmor® profile maintenance (which involves analyzing log files, refining your profiles, backing up your set of profiles and keeping it up-to-d…

37 Support

This chapter outlines maintenance-related tasks. Learn how to update AppArmor® and get a list of available man pages providing basic help for using the command line tools provided by AppArmor. Use the troubleshooting section to learn about some common problems encountered with AppArmor and their sol…

38 AppArmor Glossary

See profile foundation classes below.

27 Introducing AppArmor Edit source

Many security vulnerabilities result from bugs in trusted programs. A trusted program runs with privileges that attackers want to possess. The program fails to keep that trust if there is a bug in the program that allows the attacker to acquire said privilege.

AppArmor® is an application security solution designed specifically to apply privilege confinement to suspect programs. AppArmor allows the administrator to specify the domain of activities the program can perform by developing a security profile. A security profile is a listing of files that the program may access and the operations the program may perform. AppArmor secures applications by enforcing good application behavior without relying on attack signatures, so it can prevent attacks even if previously unknown vulnerabilities are being exploited.

27.1 AppArmor Components Edit source

AppArmor consists of:

  • A library of AppArmor profiles for common Linux* applications, describing what files the program needs to access.

  • A library of AppArmor profile foundation classes (profile building blocks) needed for common application activities, such as DNS lookup and user authentication.

  • A tool suite for developing and enhancing AppArmor profiles, so that you can change the existing profiles to suit your needs and create new profiles for your own local and custom applications.

  • Several specially modified applications that are AppArmor enabled to provide enhanced security in the form of unique subprocess confinement (including Apache).

  • The AppArmor-related kernel code and associated control scripts to enforce AppArmor policies on your SUSE® Linux Enterprise Server system.

27.2 Background Information on AppArmor Profiling Edit source

For more information about the science and security of AppArmor, refer to the following papers:

SubDomain: Parsimonious Server Security by Crispin Cowan, Steve Beattie, Greg Kroah-Hartman, Calton Pu, Perry Wagle, and Virgil Gligor

Describes the initial design and implementation of AppArmor. Published in the proceedings of the USENIX LISA Conference, December 2000, New Orleans, LA. This paper is now out of date, describing syntax and features that are different from the current AppArmor product. This paper should be used only for background, and not for technical documentation.

Defcon Capture the Flag: Defending Vulnerable Code from Intense Attack by Crispin Cowan, Seth Arnold, Steve Beattie, Chris Wright, and John Viega

A good guide to strategic and tactical use of AppArmor to solve severe security problems in a very short period of time. Published in the Proceedings of the DARPA Information Survivability Conference and Expo (DISCEX III), April 2003, Washington, DC.

AppArmor for Geeks by Seth Arnold

This document tries to convey a better understanding of the technical details of AppArmor. It is available at https://en.opensuse.org/SDB:AppArmor_geeks.

28 Getting Started Edit source

Prepare a successful deployment of AppArmor on your system by carefully considering the following items:

  1. Determine the applications to profile. Read more on this in Section 28.3, “Choosing Applications to Profile”.

  2. Build the needed profiles as roughly outlined in Section 28.4, “Building and Modifying Profiles”. Check the results and adjust the profiles when necessary.

  3. Update your profiles whenever your environment changes or you need to react to security events logged by the reporting tool of AppArmor. Refer to Section 28.5, “Updating Your Profiles”.

28.1 Installing AppArmor Edit source

AppArmor is installed and running on any installation of SUSE® Linux Enterprise Server by default, regardless of what patterns are installed. The packages listed below are needed for a fully-functional instance of AppArmor:

  • apparmor-docs

  • apparmor-parser

  • apparmor-profiles

  • apparmor-utils

  • audit

  • libapparmor1

  • perl-libapparmor

  • yast2-apparmor

Tip
Tip

If AppArmor is not installed on your system, install the pattern apparmor for a complete AppArmor installation. Either use the YaST Software Management module for installation, or use Zypper on the command line:

tux > sudo zypper in -t pattern apparmor

28.2 Enabling and Disabling AppArmor Edit source

AppArmor is configured to run by default on any fresh installation of SUSE Linux Enterprise Server. There are two ways of toggling the status of AppArmor:

Using YaST Services Manager

Disable or enable AppArmor by removing or adding its boot script to the sequence of scripts executed on system boot. Status changes are applied on reboot.

Using AppArmor Configuration Window

Toggle the status of AppArmor in a running system by switching it off or on using the YaST AppArmor Control Panel. Changes made here are applied instantaneously. The Control Panel triggers a stop or start event for AppArmor and removes or adds its boot script in the system's boot sequence.

To disable AppArmor permanently (by removing it from the sequence of scripts executed on system boot) proceed as follows:

  1. Start YaST.

  2. Select System › Services Manager.

  3. Mark apparmor by clicking its row in the list of services, then click Enable/Disable in the lower part of the window. Check that Enabled changed to Disabled in the apparmor row.

  4. Confirm with OK.

AppArmor will not be initialized on reboot, and stays inactive until you re-enable it. Re-enabling a service using the YaST Services Manager tool is similar to disabling it.

Toggle the status of AppArmor in a running system by using the AppArmor Configuration window. These changes take effect when you apply them and survive a reboot of the system. To toggle the status of AppArmor, proceed as follows:

  1. Start YaST, select AppArmor Configuration, and click Settings in the main window.

  2. Enable AppArmor by checking Enable AppArmor or disable AppArmor by deselecting it.

  3. Click Done in the AppArmor Configuration window.

28.3 Choosing Applications to Profile Edit source

You only need to protect the programs that are exposed to attacks in your particular setup, so only use profiles for those applications you actually run. Use the following list to determine the most likely candidates:

Network Agents
Web Applications
Cron Jobs

To find out which processes are currently running with open network ports and might need a profile to confine them, run aa-unconfined as root.

Example 28.1: Output of aa-unconfined
19848 /usr/sbin/cupsd not confined
19887 /usr/sbin/sshd not confined
19947 /usr/lib/postfix/master not confined
1328 /usr/sbin/smbd confined by '/usr/sbin/smbd (enforce)'

Each of the processes in the above example labeled not confined might need a custom profile to confine it. Those labeled confined by are already protected by AppArmor.

Tip
Tip: For More Information

For more information about choosing the right applications to profile, refer to Section 29.2, “Determining Programs to Immunize”.

28.4 Building and Modifying Profiles Edit source

AppArmor on SUSE Linux Enterprise Server ships with a preconfigured set of profiles for the most important applications. In addition, you can use AppArmor to create your own profiles for any application you want.

There are two ways of managing profiles. One is to use the graphical front-end provided by the YaST AppArmor modules and the other is to use the command line tools provided by the AppArmor suite itself. The main difference is that YaST supports only basic functionality for AppArmor profiles, while the command line tools let you update/tune the profiles in a more fine-grained way.

For each application, perform the following steps to create a profile:

  1. As root, let AppArmor create a rough outline of the application's profile by running aa-genprof PROGRAM_NAME.

    or

    Outline the basic profile by running YaST › Security and Users › AppArmor Configuration › Manually Add Profile and specifying the complete path to the application you want to profile.

    A new basic profile is outlined and put into learning mode, which means that it logs any activity of the program you are executing, but does not yet restrict it.

  2. Run the full range of the application's actions to let AppArmor get a very specific picture of its activities.

  3. Let AppArmor analyze the log files generated in Step 2 by typing S in aa-genprof.

    AppArmor scans the logs it recorded during the application's run and asks you to set the access rights for each event that was logged. Either set them for each file or use globbing.

  4. Depending on the complexity of your application, it might be necessary to repeat Step 2 and Step 3. Confine the application, exercise it under the confined conditions, and process any new log events. To properly confine the full range of an application's capabilities, you might be required to repeat this procedure often.

  5. When you finish aa-genprof, your profile is set to enforce mode. The profile is applied and AppArmor restricts the application according to it.

    If you started aa-genprof on an application that had an existing profile that was in complain mode, this profile remains in learning mode upon exit of this learning cycle. For more information about changing the mode of a profile, refer to Section 33.7.3.2, “aa-complain—Entering Complain or Learning Mode” and Section 33.7.3.6, “aa-enforce—Entering Enforce Mode”.

Test your profile settings by performing every task you need with the application you confined. Normally, the confined program runs smoothly and you do not notice AppArmor activities. However, if you notice certain misbehavior with your application, check the system logs and see if AppArmor is too tightly confining your application. Depending on the log mechanism used on your system, there are several places to look for AppArmor log entries:

/var/log/audit/audit.log
The command journalctl | grep -i apparmor
The command dmesg -T

To adjust the profile, analyze the log messages relating to this application again as described in Section 33.7.3.9, “aa-logprof—Scanning the System Log”. Determine the access rights or restrictions when prompted.

Tip
Tip: For More Information

For more information about profile building and modification, refer to Chapter 30, Profile Components and Syntax, Chapter 32, Building and Managing Profiles with YaST, and Chapter 33, Building Profiles from the Command Line.

28.5 Updating Your Profiles Edit source

Software and system configurations change over time. As a result, your profile setup for AppArmor might need some fine-tuning from time to time. AppArmor checks your system log for policy violations or other AppArmor events and lets you adjust your profile set accordingly. Any application behavior that is outside of any profile definition can be addressed by aa-logprof. For more information, see Section 33.7.3.9, “aa-logprof—Scanning the System Log”.

29 Immunizing Programs Edit source

Effective hardening of a computer system requires minimizing the number of programs that mediate privilege, then securing the programs as much as possible. With AppArmor, you only need to profile the programs that are exposed to attack in your environment, which drastically reduces the amount of work required to harden your computer. AppArmor profiles enforce policies to make sure that programs do what they are supposed to do, but nothing else.

AppArmor provides immunization technologies that protect applications from the inherent vulnerabilities they possess. After installing AppArmor, setting up AppArmor profiles, and rebooting the computer, your system becomes immunized because it begins to enforce the AppArmor security policies. Protecting programs with AppArmor is called immunizing.

Administrators need only concern themselves with the applications that are vulnerable to attacks, and generate profiles for these. Hardening a system thus comes down to building and maintaining the AppArmor profile set and monitoring any policy violations or exceptions logged by AppArmor's reporting facility.

Users should not notice AppArmor. It runs behind the scenes and does not require any user interaction. Performance is not noticeably affected by AppArmor. If some activity of the application is not covered by an AppArmor profile or if some activity of the application is prevented by AppArmor, the administrator needs to adjust the profile of this application.

AppArmor sets up a collection of default application profiles to protect standard Linux services. To protect other applications, use the AppArmor tools to create profiles for the applications that you want protected. This chapter introduces the philosophy of immunizing programs. Proceed to Chapter 30, Profile Components and Syntax, Chapter 32, Building and Managing Profiles with YaST, or Chapter 33, Building Profiles from the Command Line if you are ready to build and manage AppArmor profiles.

AppArmor provides streamlined access control for network services by specifying which files each program is allowed to read, write, and execute, and which type of network it is allowed to access. This ensures that each program does what it is supposed to do, and nothing else. AppArmor quarantines programs to protect the rest of the system from being damaged by a compromised process.

AppArmor is a host intrusion prevention or mandatory access control scheme. Previously, access control schemes were centered around users because they were built for large timeshare systems. Alternatively, modern network servers largely do not permit users to log in, but instead provide a variety of network services for users (such as Web, mail, file, and print servers). AppArmor controls the access given to network services and other programs to prevent weaknesses from being exploited.

Tip
Tip: Background Information for AppArmor

To get a more in-depth overview of AppArmor and the overall concept behind it, refer to Section 27.2, “Background Information on AppArmor Profiling”.

29.1 Introducing the AppArmor Framework Edit source

This section provides a very basic understanding of what is happening behind the scenes (and under the hood of the YaST interface) when you run AppArmor.

An AppArmor profile is a plain text file containing path entries and access permissions. See Section 30.1, “Breaking an AppArmor Profile into Its Parts” for a detailed reference profile. The directives contained in this text file are then enforced by the AppArmor routines to quarantine the process or program.

The following tools interact in the building and enforcement of AppArmor profiles and policies:

aa-status

aa-status reports various aspects of the current state of the running AppArmor confinement.

aa-unconfined

aa-unconfined detects any application running on your system that listens for network connections and is not protected by an AppArmor profile. Refer to Section 33.7.3.12, “aa-unconfined—Identifying Unprotected Processes” for detailed information about this tool.

aa-autodep

aa-autodep creates a basic framework of a profile that needs to be fleshed out before it is put to use in production. The resulting profile is loaded and put into complain mode, reporting any behavior of the application that is not (yet) covered by AppArmor rules. Refer to Section 33.7.3.1, “aa-autodep—Creating Approximate Profiles” for detailed information about this tool.

aa-genprof

aa-genprof generates a basic profile and asks you to refine this profile by executing the application and generating log events that need to be taken care of by AppArmor policies. You are guided through a series of questions to deal with the log events that have been triggered during the application's execution. After the profile has been generated, it is loaded and put into enforce mode. Refer to Section 33.7.3.8, “aa-genprof—Generating Profiles” for detailed information about this tool.

aa-logprof

aa-logprof interactively scans and reviews the log entries generated by an application that is confined by an AppArmor profile in both complain and enforced modes. It assists you in generating new entries in the profile concerned. Refer to Section 33.7.3.9, “aa-logprof—Scanning the System Log” for detailed information about this tool.

aa-easyprof

aa-easyprof provides an easy-to-use interface for AppArmor profile generation. aa-easyprof supports the use of templates and policy groups to quickly profile an application. Note that while this tool can help with policy generation, its utility is dependent on the quality of the templates, policy groups and abstractions used. aa-easyprof may create a profile that is less restricted than creating the profile with aa-genprof and aa-logprof.

aa-complain

aa-complain toggles the mode of an AppArmor profile from enforce to complain. Violations to rules set in a profile are logged, but the profile is not enforced. Refer to Section 33.7.3.2, “aa-complain—Entering Complain or Learning Mode” for detailed information about this tool.

aa-enforce

aa-enforce toggles the mode of an AppArmor profile from complain to enforce. Violations to rules set in a profile are logged and not permitted—the profile is enforced. Refer to Section 33.7.3.6, “aa-enforce—Entering Enforce Mode” for detailed information about this tool.

aa-disable

aa-disable disables the enforcement mode for one or more AppArmor profiles. This command will unload the profile from the kernel and prevent it from being loaded on AppArmor start-up. The aa-enforce and aa-complain utilities may be used to change this behavior.

aa-exec

aa-exec launches a program confined by the specified AppArmor profile and/or namespace. If both a profile and namespace are specified, the command will be confined by the profile in the new policy namespace. If only a namespace is specified, the profile name of the current confinement will be used. If neither a profile or namespace is specified, the command will be run using standard profile attachment—as if run without aa-exec.

aa-notify

aa-notify is a handy utility that displays AppArmor notifications in your desktop environment. You can also configure it to display a summary of notifications for the specified number of recent days. For more information, see Section 33.7.3.13, “aa-notify”.

29.2 Determining Programs to Immunize Edit source

Now that you have familiarized yourself with AppArmor, start selecting the applications for which to build profiles. Programs that need profiling are those that mediate privilege. The following programs have access to resources that the person using the program does not have, so they grant the privilege to the user when used:

cron Jobs

Programs that are run periodically by cron. Such programs read input from a variety of sources and can run with special privileges, sometimes with as much as root privilege. For example, cron can run /usr/sbin/logrotate daily to rotate, compress, or even mail system logs. For instructions for finding these types of programs, refer to Section 29.3, “Immunizing cron Jobs”.

Web Applications

Programs that can be invoked through a Web browser, including CGI Perl scripts, PHP pages, and more complex Web applications. For instructions for finding these types of programs, refer to Section 29.4.1, “Immunizing Web Applications”.

Network Agents

Programs (servers and clients) that have open network ports. User clients, such as mail clients and Web browsers mediate privilege. These programs run with the privilege to write to the user's home directory and they process input from potentially hostile remote sources, such as hostile Web sites and e-mailed malicious code. For instructions for finding these types of programs, refer to Section 29.4.2, “Immunizing Network Agents”.

Conversely, unprivileged programs do not need to be profiled. For example, a shell script might invoke the cp program to copy a file. Because cp does not by default have its own profile or subprofile, it inherits the profile of the parent shell script. Thus cp can copy any files that the parent shell script's profile can read and write.

29.3 Immunizing cron Jobs Edit source

To find programs that are run by cron, inspect your local cron configuration. Unfortunately, cron configuration is rather complex, so there are numerous files to inspect. Periodic cron jobs are run from these files:

/etc/crontab
/etc/cron.d/*
/etc/cron.daily/*
/etc/cron.hourly/*
/etc/cron.monthly/*
/etc/cron.weekly/*

The crontab command lists/edits the current user's crontab. To manipulate root's cron jobs, first become root, and then edit the tasks with crontab -e or list them with crontab -l.

29.4 Immunizing Network Applications Edit source

An automated method for finding network server daemons that should be profiled is to use the aa-unconfined tool.

The aa-unconfined tool uses the command netstat -nlp to inspect open ports from inside your computer, detect the programs associated with those ports, and inspect the set of AppArmor profiles that you have loaded. aa-unconfined then reports these programs along with the AppArmor profile associated with each program, or reports none (if the program is not confined).

Note
Note

If you create a new profile, you must restart the program that has been profiled to have it be effectively confined by AppArmor.

Below is a sample aa-unconfined output:

37021 /usr/sbin/sshd2 confined
   by '/usr/sbin/sshd3 (enforce)'
4040 /usr/sbin/smbd confined by '/usr/sbin/smbd (enforce)'
4373 /usr/lib/postfix/master confined by '/usr/lib/postfix/master (enforce)'
4505 /usr/sbin/httpd2-prefork confined by '/usr/sbin/httpd2-prefork (enforce)'
646 /usr/lib/wicked/bin/wickedd-dhcp4 not confined
647 /usr/lib/wicked/bin/wickedd-dhcp6 not confined
5592 /usr/bin/ssh not confined
7146 /usr/sbin/cupsd confined by '/usr/sbin/cupsd (complain)'

1

The first portion is a number. This number is the process ID number (PID) of the listening program.

2

The second portion is a string that represents the absolute path of the listening program

3

The final portion indicates the profile confining the program, if any.

Note
Note

aa-unconfined requires root privileges and should not be run from a shell that is confined by an AppArmor profile.

aa-unconfined does not distinguish between one network interface and another, so it reports all unconfined processes, even those that might be listening to an internal LAN interface.

Finding user network client applications is dependent on your user preferences. The aa-unconfined tool detects and reports network ports opened by client applications, but only those client applications that are running at the time the aa-unconfined analysis is performed. This is a problem because network services tend to be running all the time, while network client applications tend only to be running when the user is interested in them.

Applying AppArmor profiles to user network client applications is also dependent on user preferences. Therefore, we leave the profiling of user network client applications as an exercise for the user.

To aggressively confine desktop applications, the aa-unconfined command supports a --paranoid option, which reports all processes running and the corresponding AppArmor profiles that might or might not be associated with each process. The user can then decide whether each of these programs needs an AppArmor profile.

If you have new or modified profiles, you can submit them to the <> mailing list along with a use case for the application behavior that you exercised. The AppArmor team reviews and may submit the work into SUSE Linux Enterprise Server. We cannot guarantee that every profile will be included, but we make a sincere effort to include as much as possible.

29.4.1 Immunizing Web Applications Edit source

To find Web applications, investigate your Web server configuration. The Apache Web server is highly configurable and Web applications can be stored in many directories, depending on your local configuration. SUSE Linux Enterprise Server, by default, stores Web applications in /srv/www/cgi-bin/. To the maximum extent possible, each Web application should have an AppArmor profile.

Once you find these programs, you can use the aa-genprof and aa-logprof tools to create or update their AppArmor profiles.

Because CGI programs are executed by the Apache Web server, the profile for Apache itself, usr.sbin.httpd2-prefork for Apache2 on SUSE Linux Enterprise Server, must be modified to add execute permissions to each of these programs. For example, adding the line /srv/www/cgi-bin/my_hit_counter.pl rPx grants Apache permission to execute the Perl script my_hit_counter.pl and requires that there be a dedicated profile for my_hit_counter.pl. If my_hit_counter.pl does not have a dedicated profile associated with it, the rule should say /srv/www/cgi-bin/my_hit_counter.pl rix to cause my_hit_counter.pl to inherit the usr.sbin.httpd2-prefork profile.

Some users might find it inconvenient to specify execute permission for every CGI script that Apache might invoke. Instead, the administrator can grant controlled access to collections of CGI scripts. For example, adding the line /srv/www/cgi-bin/*.{pl,py,pyc} rix allows Apache to execute all files in /srv/www/cgi-bin/ ending in .pl (Perl scripts) and .py or .pyc (Python scripts). As above, the ix part of the rule causes Python scripts to inherit the Apache profile, which is appropriate if you do not want to write individual profiles for each CGI script.

Note
Note

If you want the subprocess confinement module (apache2-mod-apparmor) functionality when Web applications handle Apache modules (mod_perl and mod_php), use the ChangeHat features when you add a profile in YaST or at the command line. To take advantage of the subprocess confinement, refer to Section 34.2, “Managing ChangeHat-Aware Applications”.

Profiling Web applications that use mod_perl and mod_php requires slightly different handling. In this case, the program is a script interpreted directly by the module within the Apache process, so no exec happens. Instead, the AppArmor version of Apache calls change_hat() using a subprofile (a hat) corresponding to the name of the URI requested.

Note
Note

The name presented for the script to execute might not be the URI, depending on how Apache has been configured for where to look for module scripts. If you have configured your Apache to place scripts in a different place, the different names appear in the log file when AppArmor complains about access violations. See Chapter 36, Managing Profiled Applications.

For mod_perl and mod_php scripts, this is the name of the Perl script or the PHP page requested. For example, adding this subprofile allows the localtime.php page to execute and access to the local system time and locale files:

/usr/bin/httpd2-prefork {
  # ...
  ^/cgi-bin/localtime.php {
    /etc/localtime                  r,
    /srv/www/cgi-bin/localtime.php  r,
    /usr/lib/locale/**              r,
  }
}

If no subprofile has been defined, the AppArmor version of Apache applies the DEFAULT_URI hat. This subprofile is sufficient to display a Web page. The DEFAULT_URI hat that AppArmor provides by default is the following:

^DEFAULT_URI {
    /usr/sbin/suexec2                  mixr,
    /var/log/apache2/**                rwl,
    @{HOME}/public_html                r,
    @{HOME}/public_html/**             r,
    /srv/www/htdocs                    r,
    /srv/www/htdocs/**                 r,
    /srv/www/icons/*.{gif,jpg,png}     r,
    /srv/www/vhosts                    r,
    /srv/www/vhosts/**                 r,
    /usr/share/apache2/**              r,
    /var/lib/php/sess_*                rwl
}

To use a single AppArmor profile for all Web pages and CGI scripts served by Apache, a good approach is to edit the DEFAULT_URI subprofile. For more information on confining Web applications with Apache, see Chapter 34, Profiling Your Web Applications Using ChangeHat.

29.4.2 Immunizing Network Agents Edit source

To find network server daemons and network clients (such as fetchmail or Firefox) that need to be profiled, you should inspect the open ports on your machine. Also consider the programs that are answering on those ports, and provide profiles for as many of those programs as possible. If you provide profiles for all programs with open network ports, an attacker cannot get to the file system on your machine without passing through an AppArmor profile policy.

Scan your server for open network ports manually from outside the machine using a scanner (such as nmap), or from inside the machine using the netstat --inet -n -p command as root. Then, inspect the machine to determine which programs are answering on the discovered open ports.

Tip
Tip

Refer to the man page of the netstat command for a detailed reference of all possible options.

30 Profile Components and Syntax Edit source

Building AppArmor profiles to confine an application is very straightforward and intuitive. AppArmor ships with several tools that assist in profile creation. It does not require you to do any programming or script handling. The only task that is required of the administrator is to determine a policy of strictest access and execute permissions for each application that needs to be hardened.

Updates or modifications to the application profiles are only required if the software configuration or the desired range of activities changes. AppArmor offers intuitive tools to handle profile updates and modifications.

You are ready to build AppArmor profiles after you select the programs to profile. To do so, it is important to understand the components and syntax of profiles. AppArmor profiles contain several building blocks that help build simple and reusable profile code:

Include Files

Include statements are used to pull in parts of other AppArmor profiles to simplify the structure of new profiles.

Abstractions

Abstractions are include statements grouped by common application tasks.

Program Chunks

Program chunks are include statements that contain chunks of profiles that are specific to program suites.

Capability Entries

Capability entries are profile entries for any of the POSIX.1e http://en.wikipedia.org/wiki/POSIX#POSIX.1 Linux capabilities allowing a fine-grained control over what a confined process is allowed to do through system calls that require privileges.

Network Access Control Entries

Network Access Control Entries mediate network access based on the address type and family.

Local Variable Definitions

Local variables define shortcuts for paths.

File Access Control Entries

File Access Control Entries specify the set of files an application can access.

rlimit Entries

rlimit entries set and control an application's resource limits.

For help determining the programs to profile, refer to Section 29.2, “Determining Programs to Immunize”. To start building AppArmor profiles with YaST, proceed to Chapter 32, Building and Managing Profiles with YaST. To build profiles using the AppArmor command line interface, proceed to Chapter 33, Building Profiles from the Command Line.

For more details about creating AppArmor profiles, see man 5 apparmor.

30.1 Breaking an AppArmor Profile into Its Parts Edit source

The easiest way of explaining what a profile consists of and how to create one is to show the details of a sample profile, in this case for a hypothetical application called /usr/bin/foo:

#include <tunables/global>1

# a comment naming the application to confine
/usr/bin/foo2 {3
   #include <abstractions/base>4

   capability setgid5,
   network inet tcp6,

   link /etc/sysconfig/foo -> /etc/foo.conf,7
   /bin/mount            ux,
   /dev/{,u}8random     r,
   /etc/ld.so.cache      r,
   /etc/foo/*            r,
   /lib/ld-*.so*         mr,
   /lib/lib*.so*         mr,
   /proc/[0-9]**         r,
   /usr/lib/**           mr,
   /tmp/                 r,9
   /tmp/foo.pid          wr,
   /tmp/foo.*            lrw,
   /@{HOME}10/.foo_file   rw,
   /@{HOME}/.foo_lock    kw,
   owner11 /shared/foo/** rw,
   /usr/bin/foobar       Cx,12
   /bin/**               Px -> bin_generic,13

   # a comment about foo's local (children) profile for /usr/bin/foobar.

   profile /usr/bin/foobar14 {
      /bin/bash          rmix,
      /bin/cat           rmix,
      /bin/more          rmix,
      /var/log/foobar*   rwl,
      /etc/foobar        r,
   }

  # foo's hat, bar.
   ^bar15 {
    /lib/ld-*.so*         mr,
    /usr/bin/bar          px,
    /var/spool/*          rwl,
   }
}

1

This loads a file containing variable definitions.

2

The normalized path to the program that is confined.

3

The curly braces ({}) serve as a container for include statements, subprofiles, path entries, capability entries, and network entries.

4

This directive pulls in components of AppArmor profiles to simplify profiles.

5

Capability entry statements enable each of the 29 POSIX.1e draft capabilities.

6

A directive determining the kind of network access allowed to the application. For details, refer to Section 30.5, “Network Access Control”.

7

A link pair rule specifying the source and the target of a link. See Section 30.7.6, “Link Pair” for more information.

8

The curly braces ({}) here allow for each of the listed possibilities, one of which is the empty string.

9

A path entry specifying what areas of the file system the program can access. The first part of a path entry specifies the absolute path of a file (including regular expression globbing) and the second part indicates permissible access modes (for example r for read, w for write, and x for execute). A whitespace of any kind (spaces or tabs) can precede the path name, but must separate the path name and the mode specifier. Spaces between the access mode and the trailing comma are optional. Find a comprehensive overview of the available access modes in Section 30.7, “File Permission Access Modes”.

10

This variable expands to a value that can be changed without changing the entire profile.

11

An owner conditional rule, granting read and write permission on files owned by the user. Refer to Section 30.7.8, “Owner Conditional Rules” for more information.

12

This entry defines a transition to the local profile /usr/bin/foobar. Find a comprehensive overview of the available execute modes in Section 30.12, “Execute Modes”.

13

A named profile transition to the profile bin_generic located in the global scope. See Section 30.12.7, “Named Profile Transitions” for details.

14

The local profile /usr/bin/foobar is defined in this section.

15

This section references a hat subprofile of the application. For more details on AppArmor's ChangeHat feature, refer to Chapter 34, Profiling Your Web Applications Using ChangeHat.

When a profile is created for a program, the program can access only the files, modes, and POSIX capabilities specified in the profile. These restrictions are in addition to the native Linux access controls.

Example:  To gain the capability CAP_CHOWN, the program must have both access to CAP_CHOWN under conventional Linux access controls (typically, be a root-owned process) and have the capability chown in its profile. Similarly, to be able to write to the file /foo/bar the program must have both the correct user ID and mode bits set in the files attributes and have /foo/bar w in its profile.

Attempts to violate AppArmor rules are recorded in /var/log/audit/audit.log if the audit package is installed, or in /var/log/messages, or only in journalctl if no traditional syslog is installed. Often AppArmor rules prevent an attack from working because necessary files are not accessible and, in all cases, AppArmor confinement restricts the damage that the attacker can do to the set of files permitted by AppArmor.

30.2 Profile Types Edit source

AppArmor knows four different types of profiles: standard profiles, unattached profiles, local profiles and hats. Standard and unattached profiles are stand-alone profiles, each stored in a file under /etc/apparmor.d/. Local profiles and hats are children profiles embedded inside of a parent profile used to provide tighter or alternate confinement for a subtask of an application.

30.2.1 Standard Profiles Edit source

The default AppArmor profile is attached to a program by its name, so a profile name must match the path to the application it is to confine.

/usr/bin/foo {
...
}

This profile will be automatically used whenever an unconfined process executes /usr/bin/foo.

30.2.2 Unattached Profiles Edit source

Unattached profiles do not reside in the file system namespace and therefore are not automatically attached to an application. The name of an unattached profile is preceded by the keyword profile. You can freely choose a profile name, except for the following limitations: the name must not begin with a : or . character. If it contains a whitespace, it must be quoted. If the name begins with a /, the profile is considered to be a standard profile, so the following two profiles are identical:

profile /usr/bin/foo {
...
}
/usr/bin/foo {
...
}

Unattached profiles are never used automatically, nor can they be transitioned to through a Px rule. They need to be attached to a program by either using a named profile transition (see Section 30.12.7, “Named Profile Transitions”) or with the change_profile rule (see Section 30.2.5, “Change rules”).

Unattached profiles are useful for specialized profiles for system utilities that generally should not be confined by a system-wide profile (for example, /bin/bash). They can also be used to set up roles or to confine a user.

30.2.3 Local Profiles Edit source

Local profiles provide a convenient way to provide specialized confinement for utility programs launched by a confined application. They are specified like standard profiles, except that they are embedded in a parent profile and begin with the profile keyword:

/parent/profile {
   ...
   profile /local/profile {
      ...
   }
}

To transition to a local profile, either use a cx rule (see Section 30.12.2, “Discrete Local Profile Execute Mode (Cx)”) or a named profile transition (see Section 30.12.7, “Named Profile Transitions”).

30.2.4 Hats Edit source

AppArmor "hats" are a local profiles with some additional restrictions and an implicit rule allowing for change_hat to be used to transition to them. Refer to Chapter 34, Profiling Your Web Applications Using ChangeHat for a detailed description.

30.2.5 Change rules Edit source

AppArmor provides change_hat and change_profile rules that control domain transitioning. change_hat are specified by defining hats in a profile, while change_profile rules refer to another profile and start with the keyword change_profile:

change_profile -> /usr/bin/foobar,

Both change_hat and change_profile provide for an application directed profile transition, without having to launch a separate application. change_profile provides a generic one way transition between any of the loaded profiles. change_hat provides for a returnable parent child transition where an application can switch from the parent profile to the hat profile and if it provides the correct secret key return to the parent profile at a later time.

change_profile is best used in situations where an application goes through a trusted setup phase and then can lower its privilege level. Any resources mapped or opened during the start-up phase may still be accessible after the profile change, but the new profile will restrict the opening of new resources, and will even limit some resources opened before the switch. Specifically, memory resources will still be available while capability and file resources (as long as they are not memory mapped) can be limited.

change_hat is best used in situations where an application runs a virtual machine or an interpreter that does not provide direct access to the applications resources (for example Apache's mod_php). Since change_hat stores the return secret key in the application's memory the phase of reduced privilege should not have direct access to memory. It is also important that file access is properly separated, since the hat can restrict accesses to a file handle but does not close it. If an application does buffering and provides access to the open files with buffering, the accesses to these files might not be seen by the kernel and hence not restricted by the new profile.

Warning
Warning: Safety of Domain Transitions

The change_hat and change_profile domain transitions are less secure than a domain transition done through an exec because they do not affect a process's memory mappings, nor do they close resources that have already been opened.

30.3 Include Statements Edit source

Include statements are directives that pull in components of other AppArmor profiles to simplify profiles. Include files retrieve access permissions for programs. By using an include, you can give the program access to directory paths or files that are also required by other programs. Using includes can reduce the size of a profile.

Include statements normally begin with a hash (#) sign. This is confusing because the same hash sign is used for comments inside profile files. Because of this, #include is treated as an include only if there is no preceding # (##include is a comment) and there is no whitespace between # and include (# include is a comment).

You can also use include without the leading #.

include "/etc/apparmor.d/abstractions/foo"

is the same as using

#include "/etc/apparmor.d/abstractions/foo"
Note
Note: No Trailing ','

Note that because includes follow the C pre-processor syntax, they do not have a trailing ',' like most AppArmor rules.

By slight changes in syntax, you can modify the behavior of include. If you use "" around the including path, you instruct the parser to do an absolute or relative path lookup.

include "/etc/apparmor.d/abstractions/foo"   # absolute path
include "abstractions/foo"   # relative path to the directory of current file

Note that when using relative path includes, when the file is included, it is considered the new current file for its includes. For example, suppose you are in the /etc/apparmor.d/bar file, then

include "abstractions/foo"

includes the file /etc/apparmor.d/abstractions/foo. If then there is

include "example"

inside the /etc/apparmor.d/abstractions/foo file, it includes /etc/apparmor.d/abstractions/example.

The use of <> specifies to try the include path (specified by -I, defaults to the /etc/apparmor.d directory) in an ordered way. So assuming the include path is

-I /etc/apparmor.d/ -I /usr/share/apparmor/

then the include statement

include <abstractions/foo>

will try /etc/apparmor.d/abstractions/foo, and if that file does not exist, the next try is /usr/share/apparmor/abstractions/foo.

Tip
Tip

The default include path can be overridden manually by passing -I to the apparmor_parser, or by setting the include paths in /etc/apparmor/parser.conf:

Include /usr/share/apparmor/
Include /etc/apparmor.d/

Multiple entries are allowed, and they are taken in the same order as when they are when using -I or --Include from the apparmor_parser command line.

If an include ends with '/', this is considered a directory include, and all files within the directory are included.

To assist you in profiling your applications, AppArmor provides three classes of includes: abstractions, program chunks and tunables.

30.3.1 Abstractions Edit source

Abstractions are includes that are grouped by common application tasks. These tasks include access to authentication mechanisms, access to name service routines, common graphics requirements, and system accounting. Files listed in these abstractions are specific to the named task. Programs that require one of these files usually also require other files listed in the abstraction file (depending on the local configuration and the specific requirements of the program). Find abstractions in /etc/apparmor.d/abstractions.

30.3.2 Program Chunks Edit source

The program-chunks directory (/etc/apparmor.d/program-chunks) contains some chunks of profiles that are specific to program suites and not generally useful outside of the suite, thus are never suggested for use in profiles by the profile wizards (aa-logprof and aa-genprof). Currently, program chunks are only available for the postfix program suite.

30.3.3 Tunables Edit source

The tunables directory (/etc/apparmor.d/tunables) contains global variable definitions. When used in a profile, these variables expand to a value that can be changed without changing the entire profile. Add all the tunables definitions that should be available to every profile to /etc/apparmor.d/tunables/global.

30.4 Capability Entries (POSIX.1e) Edit source

Capability rules are simply the word capability followed by the name of the POSIX.1e capability as defined in the capabilities(7) man page. You can list multiple capabilities in a single rule, or grant all implemented capabilities with the bare keyword capability.

capability dac_override sys_admin,   # multiple capabilities
capability,                          # grant all capabilities

30.5 Network Access Control Edit source

AppArmor allows mediation of network access based on the address type and family. The following illustrates the network access rule syntax:

network [[<domain>1][<type2>][<protocol3>]]

1

Supported domains: inet, ax25, ipx, appletalk, netrom, bridge, x25, inet6, rose, netbeui, security, key, packet, ash, econet, atmsvc, sna, pppox, wanpipe, bluetooth, unix, atmpvc,netlink, llc, can, tipc, iucv, rxrpc, isdn, phonet, ieee802154, caif, alg, nfc, vsock

2

Supported types: stream, dgram, seqpacket, rdm, raw, packet

3

Supported protocols: tcp, udp, icmp

The AppArmor tools support only family and type specification. The AppArmor module emits only network DOMAIN TYPE in ACCESS DENIED messages. And only these are output by the profile generation tools, both YaST and command line.

The following examples illustrate possible network-related rules to be used in AppArmor profiles. Note that the syntax of the last two are not currently supported by the AppArmor tools.

network1,
network inet2,
network inet63,
network inet stream4,
network inet tcp5,
network tcp6,

1

Allow all networking. No restrictions applied with regard to domain, type, or protocol.

2

Allow general use of IPv4 networking.

3

Allow general use of IPv6 networking.

4

Allow the use of IPv4 TCP networking.

5

Allow the use of IPv4 TCP networking, paraphrasing the rule above.

6

Allow the use of both IPv4 and IPv6 TCP networking.

30.6 Profile Names, Flags, Paths, and Globbing Edit source

A profile is usually attached to a program by specifying a full path to the program's executable. For example in the case of a standard profile (see Section 30.2.1, “Standard Profiles”), the profile is defined by

/usr/bin/foo { ... }

The following sections describe several useful techniques that can be applied when naming a profile or putting a profile in the context of other existing ones, or specifying file paths.

AppArmor explicitly distinguishes directory path names from file path names. Use a trailing / for any directory path that needs to be explicitly distinguished:

/some/random/example/* r

Allow read access to files in the /some/random/example directory.

/some/random/example/ r

Allow read access to the directory only.

/some/**/ r

Give read access to any directories below /some (but not /some/ itself).

/some/random/example/** r

Give read access to files and directories under /some/random/example (but not /some/random/example/ itself).

/some/random/example/**[^/] r

Give read access to files under /some/random/example. Explicitly exclude directories ([^/]).

Globbing (or regular expression matching) is when you modify the directory path using wild cards to include a group of files or subdirectories. File resources can be specified with a globbing syntax similar to that used by popular shells, such as csh, Bash, and zsh.

*

Substitutes for any number of any characters, except /.

Example: An arbitrary number of file path elements.

**

Substitutes for any number of characters, including /.

Example: An arbitrary number of path elements, including entire directories.

?

Substitutes for any single character, except /.

[abc]

Substitutes for the single character a, b, or c.

Example: a rule that matches /home[01]/*/.plan allows a program to access .plan files for users in both /home0 and /home1.

[a-c]

Substitutes for the single character a, b, or c.

{ab,cd}

Expands to one rule to match ab and one rule to match cd.

Example: a rule that matches /{usr,www}/pages/** grants access to Web pages in both /usr/pages and /www/pages.

[^a]

Substitutes for any character except a.

30.6.1 Profile Flags Edit source

Profile flags control the behavior of the related profile. You can add profile flags to the profile definition by editing it manually, see the following syntax:

/path/to/profiled/binary flags=(list_of_flags) {
  [...]
}

You can use multiple flags separated by a comma ',' or space ' '. There are three basic types of profile flags: mode, relative, and attach flags.

Mode flag is complain (illegal accesses are allowed and logged). If it is omitted, the profile is in enforce mode (enforces the policy).

Tip
Tip

A more flexible way of setting the whole profile into complain mode is to create a symbolic link from the profile file inside the /etc/apparmor.d/force-complain/ directory.

ln -s /etc/apparmor.d/bin.ping /etc/apparmor.d/force-complain/bin.ping

Relative flags are chroot_relative (states that the profile is relative to the chroot instead of namespace) or namespace_relative (the default, with the path being relative to outside the chroot). They are mutually exclusive.

Attach flags consist of two pairs of mutually exclusive flags: attach_disconnected or no_attach_disconnected (determine if path names resolved to be outside of the namespace are attached to the root, which means they have the '/' character at the beginning), and chroot_attach or chroot_no_attach (control path name generation when in a chroot environment while a file is accessed that is external to the chroot but within the namespace).

30.6.2 Using Variables in Profiles Edit source

AppArmor allows to use variables holding paths in profiles. Use global variables to make your profiles portable and local variables to create shortcuts for paths.

A typical example of when global variables come in handy are network scenarios in which user home directories are mounted in different locations. Instead of rewriting paths to home directories in all affected profiles, you only need to change the value of a variable. Global variables are defined under /etc/apparmor.d/tunables and need to be made available via an include statement. Find the variable definitions for this use case (@{HOME} and @{HOMEDIRS}) in the /etc/apparmor.d/tunables/home file.

Local variables are defined at the head of a profile. This is useful to provide the base of for a chrooted path, for example:

@{CHROOT_BASE}=/tmp/foo
/sbin/rsyslogd {
...
# chrooted applications
@{CHROOT_BASE}/var/lib/*/dev/log w,
@{CHROOT_BASE}/var/log/** w,
...
}

In the following example, while @{HOMEDIRS} lists where all the user home directories are stored, @{HOME} is a space-separated list of home directories. Later on, @{HOMEDIRS} is expanded by two new specific places where user home directories are stored.

@{HOMEDIRS}=/home/
@{HOME}=@{HOMEDIRS}/*/ /root/
[...]
@{HOMEDIRS}+=/srv/nfs/home/ /mnt/home/
Note
Note

With the current AppArmor tools, variables can only be used when manually editing and maintaining a profile.

30.6.3 Pattern Matching Edit source

Profile names can contain globbing expressions allowing the profile to match against multiple binaries.

The following example is valid for systems where the foo binary resides either in /usr/bin or /bin.

/{usr/,}bin/foo { ... }

In the following example, when matching against the executable /bin/foo, the /bin/foo profile is an exact match so it is chosen. For the executable /bin/fat, the profile /bin/foo does not match, and because the /bin/f* profile is more specific (less general) than /bin/**, the /bin/f* profile is chosen.

/bin/foo { ... }

/bin/f*  { ... }

/bin/**  { ... }

For more information on profile name globbing examples, see the man page of AppArmor, man 5 apparmor.d,, section Globbing.

30.6.4 Namespaces Edit source

Namespaces are used to provide different profiles sets. Say one for the system, another for a chroot environment or container. Namespaces are hierarchical—a namespace can see its children but a child cannot see its parent. Namespace names start with a colon : followed by an alphanumeric string, a trailing colon : and an optional double slash //, such as

:childNameSpace://

Profiles loaded to a child namespace will be prefixed with their namespace name (viewed from a parent's perspective):

:childNameSpace://apache

Namespaces can be entered via the change_profile API, or named profile transitions:

/path/to/executable px -> :childNameSpace://apache

30.6.5 Profile Naming and Attachment Specification Edit source

Profiles can have a name, and an attachment specification. This allows for profiles with a logical name that can be more meaningful to users/administrators than a profile name that contains pattern matching (see Section 30.6.3, “Pattern Matching”). For example, the default profile

/** { ... }

can be named

profile default /** { ... }

Also, a profile with pattern matching can be named. For example:

/usr/lib64/firefox*/firefox-*bin { ... }

can be named

profile firefox /usr/lib64/firefox*/firefox-*bin { ... }

30.6.6 Alias Rules Edit source

Alias rules provide an alternative way to manipulate profile path mappings to site specific layouts. They are an alternative form of path rewriting to using variables, and are done post variable resolution. The alias rule says to treat rules that have the same source prefix as if the rules are at target prefix.

alias /home/ -> /usr/home/

All the rules that have a prefix match to /home/ will provide access to /usr/home/. For example

/home/username/** r,

allows as well access to

/usr/home/username/** r,

Aliases provide a quick way of remapping rules without the need to rewrite them. They keep the source path still accessible—in our example, the alias rule keeps the paths under /home/ still accessible.

With the alias rule, you can point to multiple targets at the same time.

alias /home/ -> /usr/home/
alias /home/ -> /mnt/home/
Note
Note

With the current AppArmor tools, alias rules can only be used when manually editing and maintaining a profile.

Tip
Tip

Insert global alias definitions in the file /etc/apparmor.d/tunables/alias.

30.7 File Permission Access Modes Edit source

File permission access modes consist of combinations of the following modes:

r

Read mode

w

Write mode (mutually exclusive to a)

a

Append mode (mutually exclusive to w)

k

File locking mode

l

Link mode

link FILE -> TARGET

Link pair rule (cannot be combined with other access modes)

30.7.1 Read Mode (r) Edit source

Allows the program to have read access to the resource. Read access is required for shell scripts and other interpreted content and determines if an executing process can core dump.

30.7.2 Write Mode (w) Edit source

Allows the program to have write access to the resource. Files must have this permission if they are to be unlinked (removed).

30.7.3 Append Mode (a) Edit source

Allows a program to write to the end of a file. In contrast to the w mode, the append mode does not include the ability to overwrite data, to rename, or to remove a file. The append permission is typically used with applications who need to be able to write to log files, but which should not be able to manipulate any existing data in the log files. As the append permission is a subset of the permissions associated with the write mode, the w and a permission flags cannot be used together and are mutually exclusive.

30.7.4 File Locking Mode (k) Edit source

The application can take file locks. Former versions of AppArmor allowed files to be locked if an application had access to them. By using a separate file locking mode, AppArmor makes sure locking is restricted only to those files which need file locking and tightens security as locking can be used in several denial of service attack scenarios.

30.7.7 Optional allow and file Rules Edit source

The allow prefix is optional, and it is idiomatically implied if not specified and the deny (see Section 30.7.9, “Deny Rules”) keyword is not used.

allow file /example r,
allow /example r,
allow network,

You can also use the optional file keyword. If you omit it and there are no other rule types that start with a keyword, such as network or mount, it is automatically implied.

file /example/rule r,

is equivalent to

/example/rule r,

The following rule grants access to all files:

file,

which is equal to

/** rwmlk,

File rules can use leading or trailing permissions. The permissions should not be specified as a trailing permission, but rather used at the start of the rule. This is important in that it makes file rules behave like any other rule types.

/path rw,            # old style
rw /path,            # leading permission
file rw /path,       # with explicit 'file' keyword
allow file rw /path, # optional 'allow' keyword added

30.7.8 Owner Conditional Rules Edit source

The file rules can be extended so that they can be conditional upon the user being the owner of the file (the fsuid needs to match the file's uid). For this purpose the owner keyword is put in front of the rule. Owner conditional rules accumulate like regular file rules do.

owner /home/*/** rw

When using file ownership conditions with link rules the ownership test is done against the target file so the user must own the file to be able to link to it.

Note
Note: Precedence of Regular File Rules

Owner conditional rules are considered a subset of regular file rules. If a regular file rule overlaps with an owner conditional file rule, the rules are merged. Consider the following example.

/foo r,
owner /foo rw,  # or w,

The rules are merged—it results in r for everybody, and w for the owner only.

Tip
Tip

To address everybody but the owner of the file, use the keyword other.

owner /foo rw,
other /foo r,

30.7.9 Deny Rules Edit source

Deny rules can be used to annotate or quiet known rejects. The profile generating tools will not ask about a known reject treated with a deny rule. Such a reject will also not show up in the audit logs when denied, keeping the log files lean. If this is not desired, put the keyword audit in front of the deny entry.

It is also possible to use deny rules in combination with allow rules. This allows you to specify a broad allow rule, and then subtract a few known files that should not be allowed. Deny rules can also be combined with owner rules, to deny files owned by the user. The following example allows read/write access to everything in a users directory except write access to the .ssh/ files:

deny /home/*/.ssh/** w,
owner /home/*/** rw,

The extensive use of deny rules is generally not encouraged, because it makes it much harder to understand what a profile does. However a judicious use of deny rules can simplify profiles. Therefore the tools only generate profiles denying specific files and will not use globbing in deny rules. Manually edit your profiles to add deny rules using globbing. Updating such profiles using the tools is safe, because the deny entries will not be touched.

30.8 Mount Rules Edit source

AppArmor can limit mount and unmount operations, including file system types and mount flags. The rule syntax is based on the mount command syntax and starts with one of the keywords mount, remount, or umount. Conditions are optional and unspecified conditionals are assumed to match all entries. For example, not specifying a file system means that all file systems are matched.

Conditionals can be specified by specifying conditionals with options= or options in.

options= specifies conditionals that have to be met exactly. The rule

mount options=ro /dev/foo -E /mnt/,

matches

root # mount -o ro /dev/foo /mnt

but does not match

root # mount -o ro,atime /dev/foo /mnt
root # mount -o rw /dev/foo /mnt

options in requires that at least one of the listed mount options is used. The rule

mount options in (ro,atime) /dev/foo -> /mnt/,

matches

root # mount -o ro /dev/foo /mnt
root # mount -o ro,atime /dev/foo /mnt
root # mount -o atime /dev/foo /mnt

but does not match

root # mount -o ro,sync /dev/foo /mnt
root # mount -o ro,atime,sync /dev/foo /mnt
root # mount -o rw /dev/foo /mnt
root # mount -o rw,noatime /dev/foo /mnt
root # mount /dev/foo /mnt

With multiple conditionals, the rule grants permission for each set of options. The rule

mount options=ro options=atime

matches

root # mount -o ro /dev/foo /mnt
root # mount -o atime /dev/foo /mnt

but does not match

root # mount -o ro,atime /dev/foo /mnt

Separate mount rules are distinct and the options do not accumulate. The rules

mount options=ro,
mount options=atime,

are not equivalent to

mount options=(ro,atime),
mount options in (ro,atime),

The following rule allows mounting /dev/foo on /mnt/ read only and using inode access times or allows mounting /dev/foo on /mnt/ with some combination of 'nodev' and 'user'.

mount options=(ro, atime) options in (nodev, user) /dev/foo -> /mnt/,

allows

root # mount -o ro,atime /dev/foo /mnt
root # mount -o nodev /dev/foo /mnt
root # mount -o user /dev/foo /mnt
root # mount -o nodev,user /dev/foo /mnt

30.9 Pivot Root Rules Edit source

AppArmor can limit changing the root file system. The syntax is

pivot_root [oldroot=OLD_ROOT] NEW_ROOT

The paths specified in 'pivot_root' rules must end with '/' since they are directories.

# Allow any pivot
pivot_root,

# Allow pivoting to any new root directory and putting the old root
# directory at /mnt/root/old/
pivot_root oldroot=/mnt/root/old/,

# Allow pivoting the root directory to /mnt/root/
pivot_root /mnt/root/,

# Allow pivoting to /mnt/root/ and putting the old root directory at
# /mnt/root/old/
pivot_root oldroot=/mnt/root/old/ /mnt/root/,

# Allow pivoting to /mnt/root/, putting the old root directory at
# /mnt/root/old/ and transition to the /mnt/root/sbin/init profile
pivot_root oldroot=/mnt/root/old/ /mnt/root/ -> /mnt/root/sbin/init,

30.10 PTrace Rules Edit source

AppArmor supports limiting ptrace system calls. ptrace rules are accumulated so that the granted ptrace permissions are the union of all the listed ptrace rule permissions. If a rule does not specify an access list, permissions are implicitly granted.

The trace and tracedby permissions control ptrace(2); read and readby control proc(5) file system access, kcmp(2), futexes (get_robust_list(2)) and perf trace events.

For a ptrace operation to be allowed, the tracing and traced processes need the correct permissions. This means that the tracing process needs the trace permission and the traced process needs the tracedby permission.

Example AppArmor PTrace rules:

# Allow all PTrace access
ptrace,

# Explicitly allow all PTrace access,
ptrace (read, readby, trace, tracedby),

# Explicitly deny use of ptrace(2)
deny ptrace (trace),

# Allow unconfined processes (eg, a debugger) to ptrace us
ptrace (readby, tracedby) peer=unconfined,

# Allow ptrace of a process running under the /usr/bin/foo profile
ptrace (trace) peer=/usr/bin/foo,

30.11 Signal Rules Edit source

AppArmor supports limiting inter-process signals. AppArmor signal rules are accumulated so that the granted signal permissions are the union of all the listed signal rule permissions. AppArmor signal permissions are implied when a rule does not explicitly state an access list.

The sending and receiving process must both have the correct permissions.

Example signal rules:

# Allow all signal access
signal,

# Explicitly deny sending the HUP and INT signals
deny signal (send) set=(hup, int),

# Allow unconfined processes to send us signals
signal (receive) peer=unconfined,

# Allow sending of signals to a process running under the /usr/bin/foo
# profile
signal (send) peer=/usr/bin/foo,

# Allow checking for PID existence
signal (receive, send) set=("exists"),

# Allow us to signal ourselves using the built-in @{profile_name} variable
signal peer=@{profile_name},

# Allow two real-time signals
signal set=(rtmin+0 rtmin+32),

30.12 Execute Modes Edit source

Execute modes, also named profile transitions, consist of the following modes:

Px

Discrete profile execute mode

Cx

Discrete local profile execute mode

Ux

Unconfined execute mode

ix

Inherit execute mode

m

Allow PROT_EXEC with mmap(2) calls

30.12.1 Discrete Profile Execute Mode (Px) Edit source

This mode requires that a discrete security profile is defined for a resource executed at an AppArmor domain transition. If there is no profile defined, the access is denied.

Incompatible with Ux,