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documentation.suse.com / Authentication with PAM

Authentication with PAM

Publication Date: 12 Dec 2024
WHAT?

Linux uses PAM (pluggable authentication modules) in the authentication process as a layer that mediates between user and application. This article describes the PAM concept, structure of PAM configuration and usage of tools for configuring PAM.

WHY?

You want to know how to set up a PAM module and configure your system to use the U2F keys.

EFFORT

It takes approximately 20 minutes to read the article.

REQUIREMENTS

  • To set up the authentication using the U2F keys, you need to have either YubiKeys or Security Keys.

1 Introduction to PAM

 

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 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.

2 Structure of PAM configuration

 

PAM on SLE Micro comes with a so-called directory based configuration. The set of configuration files is stored in /etc/pam.d. Every service (or program) that relies on the PAM mechanism has its own configuration file in this directory. For example, the service for sshd can be found in the /etc/pam.d/sshd file.

Note
Note: File-based configuration (/etc/pam.conf) not used on SLE Micro

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

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.

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, if the module is located in the default directory /lib/security (for all 64-bit platforms supported by SUSE Linux Enterprise Micro, 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 calls a set of PAM functions, which then process the information in the 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.

2.1 An example of PAM configuration

 

To demonstrate a real use case example of PAM configuration, the configuration of sshd has been used in this section:

Example 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.

The first include file (common-auth) calls modules of the auth type: pam_env.so, pam_gnome_keyring.so and pam_unix.so. See Example 2, “Default configuration for the auth section (common-auth)”. Keep in mind that the modules may differ according to your installation.

Example 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, “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: Default configuration for the account section (common-account)
 
account  required  pam_unix.so  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.

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 Configuration of PAM modules

 

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.

3.1 pam_env.conf

 

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 4, “pam_env.conf”.

Example 4: 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.

3.2 limits.conf

 

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.

4 Configuring PAM using pam-config

 

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.

  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).

5 Manually configuring PAM

 

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.

6 Configuring SLE Micro to require U2F keys for local login

 

To provide more security during the local login to SLE Micro, you can configure two-factor authentication using the pam-u2f framework and the U2F feature on YubiKeys and Security Keys.

To set up U2F on your SLE Micro system, you need to associate your key with your account on SLE Micro. After that, configure your system to use the key. The procedure is described in the following sections.

6.1 Associating the U2F key with your account

 

To associate your U2F key with your account, proceed as follows:

  1. Log in to your machine.

  2. Insert your U2F key.

  3. Create a directory for the U2F key configuration: 

    > sudo mkdir -p ~/.config/Yubico

  4. Run the pamu2fcfg command that outputs configuration lines: 

    > sudo pamu2fcfg > ~/.config/Yubico/u2f_keys

  5. When your device begins flashing, touch the metal contact to confirm the association.

We recommend using a backup U2F device, which you can set up by running the following commands:

  1. Run: 

    > sudo pamu2fcfg -n >> ~/.config/Yubico/u2f_keys

  2. When your device begins flashing, touch the metal contact to confirm the association.

You can move the output file from the default location to a directory that requires the sudo permission to modify the file to increase security. For example, move it to the /etc directory. To do so, follow the steps:

  1. Create a directory in /etc: 

    > sudo mkdir /etc/Yubico

  2. Move the created file: 

    > sudo mv ~/.config/Yubico/u2f_keys /etc/Yubico/u2f_keys
Note
Note: Placing the u2f_keys to a non-default location

If you move the output file to a different directory than is the default ($HOME/.config/Yubico/u2f_keys), you need to add the path to the /etc/pam.d/login file as described in Section 6.2, “Updating the PAM configuration”.

6.2 Updating the PAM configuration

 

After you have created the U2F keys configuration, you need to adjust the PAM configuration on your system.

  1. Open the file /etc/pam.d/login.

  2. Add the line auth required pam_u2f.so to the file as follows: 

    #%PAM-1.0
auth      include    common-auth
auth      required   pam_u2f.so
account   include    common-account
password  include    common-password
session   optional   pam_keyinit.so revoke
session   include    common-session
#session  optional   pam_xauth.so

  3. If you placed the u2f_keys file to a different location than $HOME/.config/Yubico/u2f_keys, you need to use the authfile option in the /etc/pam.d/login PAM file as follows: 

    #%PAM-1.0 
auth     requisite pam_nologin.so 
auth     include   common-auth
auth  required pam_u2f.so authfile=<PATH_TO_u2f_keys>
...

    where <PATH_TO_u2f_keys> is the absolute path to the u2f_keys file.