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]

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 2.6, “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.

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 34 “Samba”, Section 34.5 “Configuring Clients”.

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.

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.

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:

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

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

# 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
   [...]

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:

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

You will use the dscreate command to create new 389 Directory Server instances, and the dsctl command to cleanly remove them.

There are two ways to configure and create a new instance: from a custom configuration file, and from an auto-generated template file. You can use the auto-generated template without changes for a test instance, though for a production system you must carefully review it and make any necessary changes.

Then you will set up administration credentials, manage users and groups, and configure identity services.

The 389 Directory Server is controlled by three primary commands:

Follow these steps to set up a simple instance for testing and development, populated with a small set of sample entries.

You can create a new 389 Directory Server instance from a simple custom configuration file. This file must be in the INF format, and you can name it anything you like.

The default instance name is localhost. The instance name cannot be changed after it has been created. It is better to create your own instance name, rather than using the default, to avoid confusion and to enable a better understanding of how it all works. The following examples use the LDAP1 instance name, and a suffix of dc=LDAP1,dc=COM.

Example 5.2 shows an example configuration file that you can use to create a new 389 Directory Server instance. You can copy and use this file without changes.

If you forget the name of your instance, use dsctl to list all instances:

> sudo dsctl -l
slapd-LDAP1

You can auto-create a template for a new 389 Directory Server instance with the dscreate command. This creates a template that you can use without making any changes, for testing. For production systems, review and change it to suit your own requirements. All of the defaults are documented in the template file, and commented out. To make changes, uncomment the default and enter your own value. All options are well documented.

The following example prints the template to stdout:

> sudo dscreate create-template

This is good for a quick review of the template, but you must create a file to use in creating your new 389 Directory Server instance. You can name this file anything you want:

> sudo dscreate create-template TEMPLATE.txt

This is a snippet from the new file:

# full_machine_name (str)
# Description: Sets the fully qualified hostname (FQDN) of this system. When
# installing this instance with GSSAPI authentication behind a load balancer, set
# this parameter to the FQDN of the load balancer and, additionally, set
# "strict_host_checking" to "false".
# Default value: ldapserver1.test.net
;full_machine_name = ldapserver1.test.net

# selinux (bool)
# Description: Enables SELinux detection and integration during the installation
# of this instance. If set to "True", dscreate auto-detects whether SELinux is
# enabled. Set this parameter only to "False" in a development environment.
# Default value: True
;selinux = True

It automatically configures some options from your existing environment, for example, the system's fully-qualified domain name, which is called full_machine_name in the template. Use this file with no changes to create a new instance:

> sudo dscreate from-file TEMPLATE.txt

This creates a new instance named localhost, and automatically starts it after creation:

> sudo dsctl localhost status
Instance "localhost" is running

The default values create a fully operational instance, but there are some values you might want to change.

The instance name cannot be changed after it has been created. It is better to create your own instance name, rather than using the default, to avoid confusion and to enable a better understanding of how it all works. To do this, uncomment the ;instance_name = localhost line and change localhost to your chosen name. In the following examples, the instance name is LDAP1.

Another useful change is to populate your new instance with sample users and groups. Uncomment ;sample_entries = no and change no to yes. This creates the demo_user and demo_group.

Set your own password by uncommenting ;root_password, and replacing the default password with your own.

The template does not create a default suffix, so you should configure your own on the suffix line, like the following example:

suffix = dc=LDAP1,dc=COM

You can cleanly remove any instance and start over with dsctl:

> sudo dsctl LDAP1 remove --do-it

The following examples use LDAP1 as the instance name. Use systemd to manage your 389 Directory Server instance. Get the status of your instance:

> systemctl status --no-pager --full dirsrv@LDAP1.service
   ● dirsrv@LDAP1.service - 389 Directory Server LDAP1.
     Loaded: loaded (/usr/lib/systemd/system/dirsrv@.service; enabled; vendor preset: disabled)
     Active: active (running) since Thu 2021-03-11 08:55:28 PST; 2h 7min ago
    Process: 4451 ExecStartPre=/usr/lib/dirsrv/ds_systemd_ask_password_acl
       /etc/dirsrv/slapd-LDAP1/dse.ldif (code=exited, status=0/SUCCESS)
   Main PID: 4456 (ns-slapd)
     Status: "slapd started: Ready to process requests"
      Tasks: 26
     CGroup: /system.slice/system-dirsrv.slice/dirsrv@LDAP1.service
             └─4456 /usr/sbin/ns-slapd -D /etc/dirsrv/slapd-LDAP1 -i /run/dirsrv/slapd-LDAP1.pid

Start, stop, and restart your LDAP server:

> sudo systemctl start dirsrv@LDAP1.service
> sudo systemctl stop dirsrv@LDAP1.service
> sudo systemctl restart dirsrv@LDAP1.service

See Book “Administration Guide”, Chapter 15 “The systemd daemon” for more information on using systemctl.

The dsctl command also starts and stops your server:

> sudo dsctl LDAP1 status
> sudo dsctl LDAP1 stop
> sudo dsctl LDAP1 restart
> sudo dsctl LDAP1 start

For local administration of the 389 Directory Server, you can create a .dsrc configuration file in the /root directory, allowing root and sudo users to administer the server without typing connection details with every command. Example 5.3 shows an example for local administration on the server, using LDAP1 and com for the suffix.

After creating your /root/.dsrc file, try a few administration commands, such as creating new users (see Section 5.5, “Managing LDAP users and groups”).

# /root/.dsrc file for administering the LDAP1 instance

[LDAP1] 1

uri = ldapi://%%2fvar%%2frun%%2fslapd-LDAP1.socket 2
basedn = dc=LDAP1,dc=COM
binddn = cn=Directory Manager

Important: New negation feature in sudoers.ldap

In sudo versions older than 1.9.9, negation in sudoers.ldap does not work for the sudoUser, sudoRunAsUser, or sudoRunAsGroup attributes. For example:

 # does not match all but joe
# instead, it does not match anyone
sudoUser: !joe

# does not match all but joe
# instead, it matches everyone including Joe
sudoUser: ALL
sudoUser: !joe

In sudo version 1.9.9 and higher, negation is enabled for the sudoUser attribute. See man 5 sudoers.ldap for more information.

Your LDAP server configuration is in the directory /etc/dirsrv/slapd-INSTANCE_NAME. This directory contains certificates, keys, and the dse.ldif file. Make a compressed backup of this directory with the tar command:

> sudo tar caf \
config_slapd-INSTANCE_NAME-$(date +%Y-%m-%d_%H-%M-%S).tar.gz \
/etc/dirsrv/slapd-INSTANCE_NAME/

When running tar, you may see the harmless informational message tar: Removing leading `/' from member names.

To restore a previous configuration, unpack it to the same directory:

The dsctl command makes offline backups. Stop the server:

> sudo dsctl INSTANCE_NAME stop
Instance "INSTANCE_NAME" has been stopped

Then make the backup using your instance name. The following example creates a backup archive at /var/lib/dirsrv/slapd-INSTANCE_NAME/bak/INSTANCE_NAME-DATE:

> sudo dsctl INSTANCE_NAME db2bak
db2bak successful

For example, on a test instance named ldap1 it looks like this:

/var/lib/dirsrv/slapd-ldap1/bak/ldap1-2021_10_25_13_03_17

Restore from this backup, naming the directory containing the backup archive:

> sudo dsctl INSTANCE_NAME bak2db \
/var/lib/dirsrv/slapd-INSTANCE_NAME/bak/INSTANCE_NAME-DATE/
bak2db successful

Then start the server:

> sudo dsctl INSTANCE_NAME start
Instance "INSTANCE_NAME" has been started

You can also create LDIF backups:

> sudo dsctl INSTANCE_NAME db2ldif --replication userRoot
ldiffile: /var/lib/dirsrv/slapd-INSTANCE_NAME/ldif/INSTANCE_NAME-userRoot-DATE.ldif
db2ldif successful

Restore an LDIF backup with the name of the archive, then start the server:

> sudo dsctl ldif2db userRoot \
/var/lib/dirsrv/slapd-INSTANCE_NAME/ldif/INSTANCE_NAME-userRoot-DATE.ldif
> sudo dsctl INSTANCE_NAME start

Use the dsconf to make an online backup of your LDAP database:

> sudo dsconf INSTANCE_NAME backup create
The backup create task has finished successfully

This creates /var/lib/dirsrv/slapd-INSTANCE_NAME/bak/INSTANCE_NAME-DATE.

Restore it:

> sudo dsconf INSTANCE_NAME backup restore \
/var/lib/dirsrv/slapd-INSTANCE_NAME/bak/INSTANCE_NAME-DATE

The following examples show how to list your existing users and groups. The examples use the instance name LDAP1. Replace this with your instance name:

> sudo dsidm LDAP1 user list
> sudo dsidm LDAP1 group list

List all information on a single user:

> sudo dsidm LDAP1 user get USER

List all information on a single group:

> sudo dsidm LDAP1 group get GROUP

List members of a group:

> sudo dsidm LDAP1 group members GROUP

In the following example we create one user, wilber. The example server instance is named LDAP1, and the instance's suffix is dc=LDAP1,dc=COM.

After creating users, you can create groups, and then assign users to them. In the following examples, we create a group, server_admins, and assign the user wilber to this group. The example server instance is named LDAP1, and the instance's suffix is dc=LDAP1,dc=COM.

Use the dsidm command to delete users, remove users from groups, and delete groups. The following example removes our example user wilber from the server_admins group:

> sudo dsidm LDAP1 group remove_member SERVER_ADMINS \
uid=wilber,ou=people,dc=LDAP1,dc=COM

Delete a user:

> sudo dsidm LDAP1 user delete \
uid=wilber,ou=people,dc=LDAP1,dc=COM

Delete a group:

> sudo dsidm LDAP1 group delete SERVER_ADMINS

The following are not supported as configuration values in dse.ldif for the settings nsslapd-rootpwstoragescheme or passwordStorageScheme, or as a value of passwordStorageScheme in the account policy objects:

Note

Database imports that contain these values are supported if nsslapd-enable-upgrade-hash is set to on (defaults to on).

The following plug-ins are not supported on 389 Directory Server:

389 DS manages replication differently than other databases. Replication is asynchronous, and eventually consistent. This means:

In general, as LDAP is "low-write", these factors mean that all servers are at least up to a common baseline of a known consistent state. Small changes occur on top of this baseline, so many of these aspects of delayed replication are not perceived in day to day usage.

Consider the following factors when you are designing your replication topology.

These factors all affect how you may create your topology. (See Section 5.9.3, “Example replication topologies” for some topology examples.)

The following sections provide examples of replication topologies, using two to six 389 Directory Server nodes. The maximum number of supported supplier replicas in a topology is twenty. Operational experience shows the optimal number for replication efficiency is a maximum of eight.

5.9.3.1 Two replicas #

  

Example 5.4: Two supplier replicas #

  

┌────┐       ┌────┐
│ S1 │◀─────▶│ S2 │
└────┘       └────┘

In Example 5.4, “Two supplier replicas” there are two replicas, S1 and S2, which replicate bi-directionally between each other, so they are both suppliers and consumers. S1 and S2 could be in separate data centers, or in the same data center. Clients can balance across the servers using LDAP URIs, a load balancer, or DNS SRV records. This is the simplest topology for high availability. Note that each server needs to be able to provide 100% of client load, in case the other server is offline for any reason. A two-node replication is generally not adequate for horizontal read scaling, as a single node will handle all read requests if the other node is offline.

Note: Default topology

The two-node topology should be considered the default topology, because it is the simplest to manage. You can expand your toplogy, over time, as necessary.

┌────┐       ┌────┐
│ S1 │◀─────▶│ S2 │
└────┘       └────┘
   ▲            ▲
   │            │
   ▼            ▼
┌────┐       ┌────┐
│ S3 │◀─────▶│ S4 │
└────┘       └────┘

                  ┌────┐       ┌────┐
                  │ S1 │◀─────▶│ S2 │
                  └────┘       └────┘
                     ▲            ▲
                     │            │
   ┌────────────┬────┴────────────┴─────┬────────────┐
   │            │                       │            │
   ▼            ▼                       ▼            ▼
┌────┐       ┌────┐                  ┌────┐       ┌────┐
│ S3 │◀─────▶│ S4 │                  │ S5 │◀─────▶│ S6 │
└────┘       └────┘                  └────┘       └────┘

             ┌────┐       ┌────┐
             │ S1 │◀─────▶│ S2 │
             └────┘       └────┘
                │            │
                │            │
   ┌────────────┼────────────┼────────────┐
   │            │            │            │
   ▼            ▼            ▼            ▼
┌────┐       ┌────┐       ┌────┐       ┌────┐
│ S3 │       │ S4 │       │ S5 │       │ S6 │
└────┘       └────┘       └────┘       └────┘

In the example topologies we have seen that 389 DS can take on a number of roles in a topology. The following list clarifies the terminology.

The first example sets up a two node bi-directional replication with a single read-only server, as a minimal starting example. In the following examples, the host names of the two read-write nodes are RW1 and RW2, and the read-only server is RO1. (Of course you must use your own host names.)

All servers should have a backend with an identical suffix. Only one server, RW1, needs an initial copy of the database.

5.9.5.1 Configuring two-node replication #

  

The following commands configure the read-write replicas in a two-node setup (Example 5.4, “Two supplier replicas”), with the hostnames RW1 and RW2. (Remember to use your own hostnames.)

Warning: Create a strong replication manager password

The replication manager should be considered equivalent to the directory manager, in terms of security and access, and should have a very strong password.

If you create different replication manager passwords for each server, be sure to keep track of which password belongs to which server. For example, when you configure the outbound connection in RW1's replication agreement, you need to set the replication manager password to the RW2 replication manager password.

First, configure RW1:

> sudo dsconf INSTANCE-NAME replication create-manager
> sudo dsconf INSTANCE-NAME replication enable \
--suffix dc=example,dc=com \
--role supplier --replica-id 1 --bind-dn "cn=replication manager,cn=config"

Configure RW2:

> sudo dsconf INSTANCE-NAME replication create-manager
> sudo dsconf INSTANCE-NAME replication enable \
--suffix dc=example,dc=com \
--role supplier --replica-id 2 --bind-dn "cn=replication manager,cn=config"

This will create the replication metadata required on RW1 and RW2. Note the difference in the replica-id between the two servers. This also creates the replication manager account, which is an account with replication rights for authenticating between the two nodes.

RW1 and RW2 are now both configured to have replication metadata. The next step is to create the first agreement for outbound data from RW1 to RW2.

> sudo dsconf INSTANCE-NAME repl-agmt create \
--suffix dc=example,dc=com \
--host=RW2 --port=636 --conn-protocol LDAPS --bind-dn "cn=replication manager,cn=config" \
--bind-passwd PASSWORD --bind-method SIMPLE RW1_to_RW2

Data will not flow from RW1 to RW2 until after a full synchronization of the database, which is called an initialization or reinit. This will reset all database content on RW2 to match the content of RW1. Run the following command to trigger a reinit of the data:

> sudo dsconf INSTANCE-NAME repl-agmt init \
--suffix dc=example,dc=com RW1_to_RW2

Check the status by running this command on RW1:

> sudo dsconf INSTANCE-NAME repl-agmt init-status \
--suffix dc=example,dc=com RW1_to_RW2

When it is finished, you should see a "Agreement successfully initialized" message. If you get an error message, check the errors log. Otherwise, you should see the identical content from RW1 on RW2.

Finally, to make this bi-directional, configure a replication agreement from RW2 outbound to RW1:

> sudo dsconf INSTANCE-NAME repl-agmt create \
--suffix dc=example,dc=com \
--host=RW1 --port=636 --conn-protocol LDAPS \
--bind-dn "cn=replication manager,cn=config" --bind-passwd PASSWORD \
--bind-method SIMPLE RW2_to_RW1

Changes made on either RW1 or RW2 will now be replicated to the other. Check replication status on either server with the following command:

> sudo dsconf INSTANCE-NAME repl-agmt status \
--suffix dc=example,dc=com \
--bind-dn "cn=replication manager,cn=config" \
--bind-passwd PASSWORD RW2_to_RW1

5.9.5.2 Configuring a read-only node #

  

To create a read-only node, start by creating the replication manager account and metadata. The hostname of the example server is RO3:

Warning: Create a strong replication manager password

The replication manager should be considered equivalent to the directory manager, in terms of security and access, and should have a very strong password.

If you create different replication manager passwords for each server, be sure to keep track of which password belongs to which server. For example, when you configure the outbound connection in RW1's replication agreement, you need to set the replication manager password to the RW2 replication manager password.

> sudo dsconf INSTANCE_NAME replication create-manager
> sudo  dsconf INSTANCE_NAME \
replication enable --suffix dc=EXAMPLE,dc=COM \
--role consumer --bind-dn "cn=replication manager,cn=config"

Note that for a read-only replica you do not provide a replica-id, and the role is set to consumer. This allocates a special read-only replica-id for all read-only replicas. After the read-only replica is created, add the replication agreements from RW1 and RW2 to the read-only instance. The following example is on RW1:

> sudo dsconf INSTANCE_NAME \
repl-agmt create --suffix dc=EXAMPLE,dc=COM \
--host=RO3 --port=636 --conn-protocol LDAPS \
--bind-dn "cn=replication manager,cn=config" --bind-passwd PASSWORD
--bind-method SIMPLE RW1_to_RO3

The following example, on RW2, configures the replication agreement between RW2 and RO3:

> sudo dsconf INSTANCE_NAME repl-agmt create \
--suffix dc=EXAMPLE,dc=COM \
--host=RO3 --port=636 --conn-protocol LDAPS \
--bind-dn "cn=replication manager,cn=config" --bind-passwd PASSWORD \
--bind-method SIMPLE RW2_to_RO3

After these steps are completed, you can use either RW1 or RW2 to perform the initialization of the database on RO3. The following example initalizes RO3 from RW2:

> sudo dsconf INSTANCE_NAME repl-agmt init
--suffix dc=EXAMPLE,dc=COM RW2_to_RO3

The dsconf command includes a monitoring option. You can check the status of each replica status directly on the replicas, or from other hosts. The following example commands are run on RW1, checking the status on two remote replicas, and then on itself:

> sudo dsconf -D "cn=Directory Manager" ldap://RW2 replication monitor
> sudo dsconf -D "cn=Directory Manager" ldap://RO3 replication monitor
> sudo dsconf -D "cn=Directory Manager" ldap://RW1 replication monitor

The dsctl command has a healthcheck option. The following example runs a replication healthcheck on the local 389 DS instance:

> sudo dsctl INSTANCE_NAME healthcheck --check replication

Use the -v option for verbosity, to see what the healthcheck examines:

> sudo dsctl -v INSTANCE_NAME healthcheck --check replication

Run dsctl INSTANCE_NAME healthcheck with no options for a general health check.

Run the following command to see a list of the checks that healthcheck performs:

> sudo dsctl INSTANCE_NAME healthcheck --list-checks
config:hr_timestamp
config:passwordscheme
backends:userroot:cl_trimming
backends:userroot:mappingtree
backends:userroot:search
backends:userroot:virt_attrs
encryption:check_tls_version
fschecks:file_perms
[...]

You can run one or more of the individual checks:

> sudo dsctl INSTANCE_NAME healthcheck \
--check monitor-disk-space:disk_space tls:certificate_expiration

When replication is enabled you need to adjust your 389 Directory Server backup strategy (see Section 5.4, “Backing up and restoring 389 Directory Server” to learn about making backups). If you are using db2ldif you must add the --replication flag to ensure that replication metadata is backed up. You should backup all servers in the topology. When restoring from backup, start by restoring a single node of the topology, then reinitialize all other nodes as new instances.

You can pause replication during maintenance windows, or anytime you need to stop it. A node of the topology can only be offline for a maximum of days up to the limit of the changelog (see Section 5.9.9, “ Changelog max-age”).

Use the repl-agmt command to pause replication. The following example is on RW2:

> sudo dsconf INSTANCE_NAME repl-agmt disable \
--suffix dc=EXAMPLE,dc=COM RW2_to_RW1

The following example re-enables replication:

> sudo dsconf INSTANCE_NAME repl-agmt enable \
--suffix dc=EXAMPLE,dc=COM RW2_to_RW1

A replica can be offline for up to the length of time defined by the changelog max-age option. max-age defines the maximum age of any entry in the changelog. Any items older than the max-age value are automatically removed.

After the replica comes back online it will synchronize with the other replicas. If it is offline for longer than the max-age value, the replica will need to be re-initialised, and will refuse to accept or provide changes to other nodes, as they may be inconsistent. The following example sets the max-age to seven days:

> sudo dsconf INSTANCE_NAME \
replication set-changelog --max-age 7d \
--suffix dc=EXAMPLE,dc=COM

To remove a replica, first fence the node to prevent any incoming changes or reads. Then, find all servers that have incoming replication agreements with the node you are removing, and remove them. The following example removes RW2. Start by disabling the outbound replication agreement on RW1:

> sudo dsconf INSTANCE_NAME repl-agmt delete \
--suffix dc=EXAMPLE,dc=COM RW1_to_RW2

On the replica you are removing, which in the following example is RW2, remove all outbound agreements:

> sudo dsconf INSTANCE_NAME repl-agmt delete \
--suffix dc=EXAMPLE,dc=COM RW2_to_RW1
> sudo dsconf INSTANCE_NAME repl-agmt delete \
--suffix dc=EXAMPLE,dc=COM RW2_to_RO3

Stop the instance on RW2:

> sudo systemctl stop dirsrv@INSTANCE_NAME.service

Then run the cleanallruv command to remove the replica ID from the topology. The following example is run on RW1:

> sudo dsconf INSTANCE_NAME repl-tasks cleanallruv \
--suffix dc=EXAMPLE,dc=COM --replica-id 2
> sudo dsconf INSTANCE_NAME repl-tasks list-cleanruv-tasks

The use of 389 Directory Server is supported within the following replication limits:

Due to how the synchronization works, only a single 389 Directory Server server and Active Directory server are involved. The Active Directory server must be a full Domain Controller, and not a Read Only Domain Controller (RODC). The Global Catalog is not required on the DC that is synchronized, as 389 DS only replicates the content of a single forest in a domain.

You must first chooose the direction of your data flow. There are three options: from AD to 389 DS, from 389 DS to AD, or bi- directional.

Note: No password synchronization

Passwords cannot be synchronised between 389 DS and Active Directory. This may change in the future, to support Active Directory to 389 DS password flow.

Your topology will look like the following diagram. The 389 Directory Server and Active Directory topologies may differ, but the most important factor is to have only a single connection between 389 DS and Active Directory. It is very important to account for this in your disaster recovery and backup plans for both 389 DS and AD, to ensure that you correctly restore only a single replication connection between these topologies.

┌────────┐     ┌────────┐         ┌────────┐     ┌────────┐
│        │     │        │         │        │     │        │
│ 389-ds │◀───▶│ 389-ds │◀ ─ ─ ─ ▶│   AD   │◀───▶│   AD   │
│        │     │        │         │        │     │        │
└────────┘     └────────┘         └────────┘     └────────┘
    ▲               ▲                  ▲             ▲        
    │               │                  │             │        
    ▼               ▼                  ▼             ▼        
┌────────┐     ┌────────┐         ┌────────┐     ┌────────┐
│        │     │        │         │        │     │        │
│ 389-ds │◀───▶│ 389-ds │         │   AD   │◀───▶│   AD   │
│        │     │        │         │        │     │        │
└────────┘     └────────┘         └────────┘     └────────┘

A security group that is granted the "Replicating Directory Changes" permission is required. For example, you have created a group named "Directory Server Sync". Follow the steps in the "How to grant the 'Replicating Directory Changes' permission for the Microsoft Metadirectory Services ADMA service account" (https://docs.microsoft.com/en-us/troubleshoot/windows-server/windows-security/grant-replicating-directory-changes-permission-adma-service to set this up.

Warning: Strong security needed

You should consider members of this group to be of equivalent security importance to Domain Administrators. Members of this group have the ability to read sensitive content from the Active Directory environment, so you should use strong, randomly-generated service account passwords for these accounts, and carefully audit membership to this group.

You should also create a service account that is a member of this group.

Your Active Directory environment must have certificates configured for LDAPS to ensure that authentication between 389 DS and AD is secure. Authentication with Generic Security Services API/Kerberos (GSSAPI/KRB) cannot be used.

The 389 Directory Server server must have a backend database already configured with Organization Units (OUs) for entries to be synchronised into.

The 389 Directory Server server must have a replica ID configured as though the server is a read-write replica. (For details about setting up replication see Section 5.9, “Setting up replication”).

The following example command, which is run on the 389 Directory Server server, creates a replication agreement from Active Directory to 389 Directory Server:

> sudo dsconf INSTANCE-NAME repl-winsync-agmt create --suffix dc=example,dc=com \
  --host AD-HOSTNAME --port 636 --conn-protocol LDAPS \
  --bind-dn "cn=SERVICE-ACCOUNT,cn=USERS,dc=AD,dc=EXAMPLE,dc=COM" \
  --bind-passwd "PASSWORD" --win-subtree "cn=USERS,dc=AD,dc=EXAMPLE,dc=COM" \
  --ds-subtree ou=AD,dc=EXAMPLE,dc=COM --one-way-sync fromWindows \
  --sync-users=on --sync-groups=on --move-action delete \
  --win-domain AD-DOMAIN adsync_agreement

Once the agreement has been created, you must perform an initial resynchronization:

> sudo dsconf INSTANCE-NAME repl-winsync-agmt init --suffix dc=example,dc=com adsync_agreement

Use the following command to check the status of the initialization:

> sudo dsconf INSTANCE-NAME repl-winsync-agmt init-status --suffix dc=example,dc=com adsync_agreement

Note: Some entries are not synchronized

In some cases, an entry may not be synchronized, even if the init status reports success. Check your 389 DS log files in /var/log/dirsrv/slapd-INSTANCE-NAME/errors.

Check the status of the agreement with the following command:

> sudo dsconf INSTANCE-NAME repl-winsync-agmt status --suffix dc=example,dc=com adsync_agreement

Whe you are performing maintenance on the Active Directory or 389 Directory Server topology, you can pause the agreement with the following command:

> sudo dsconf INSTANCE-NAME repl-winsync-agmt disable --suffix dc=example,dc=com adsync_agreement

Resume the agreement with the following command:

> sudo dsconf INSTANCE-NAME repl-winsync-agmt enable --suffix dc=example,dc=com adsync_agreement

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:

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.

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:

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.

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.

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:

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 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.

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

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.

Figure 6.1: Kerberos Network Topology #

  

Tip: Configuring Subnet Routing

For a setup similar to the one in Figure 6.1, “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.

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.

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:

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 30 “Time Synchronization with NTP”, Section 30.1 “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 6.5.6.3, “Adjusting the Clock Skew”.

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

[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

> sudo kdb5_util create -r EXAMPLE.COM -s

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

> kadmin.local

kadmin> listprincs

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

> kadmin.local

kadmin> ank suzanne

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

> sudo systemctl start krb5kdc
sudo systemctl start kadmind

> sudo systemctl enable krb5kdc kadmind

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.

[libdefaults]
        default_realm = EXAMPLE.COM

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

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

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

[libdefaults]
        clockskew = 60

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:

suzanne/admin              *

Replace the user name suzanne 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:

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

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

kadmin:  getprinc suzanne
Principal: suzanne@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" suzanne
Principal "suzanne@EXAMPLE.COM" modified.
kadmin:  getprinc suzanne
Principal: suzanne@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 (suzanne/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.

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 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:

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

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.

> 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:

> 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.

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: Additional Information

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

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).

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.

> kinit suzanne@EXAMPLE.COM

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

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 several 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. 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.

Usually 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.

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 does not 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.

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 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.

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:

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.

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 7.2, “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):

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 7.3, “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.

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).