Active Directory Authentication¶
Microsoft’s Active Directory (AD) is, in most enterprises, the de facto authentication system for Windows systems and for external, LDAP-connected services. It allows you to configure users and groups, access control, permissions, auto-mounting, and more.
Now, while connecting Linux to an AD cluster cannot support all of the features mentioned, it can handle users, groups, and access control. It is even possible (through some configuration tweaks on the Linux side and some advanced options on the AD side) to distribute SSH keys using AD.
This guide, however, will just cover configuring authentication against Active Directory, and will not include any extra configuration on the Windows side.
Discovering and joining AD using SSSD¶
Throughout this guide, the domain name ad.company.local will be used to represent the Active Directory domain. To follow this guide, replace it with the actual domain name your AD domain uses.
The first step along the way to join a Linux system into AD is to discover your AD cluster, to ensure that the network configuration is correct on both sides.
Preparation¶
- Ensure the following ports are open to your Linux host on your domain controller:
- Ensure you have configured your AD domain controller as a DNS server on your Rocky Linux host:
With NetworkManager:
# where your primary NetworkManager connection is 'System eth0' and your AD # server is accessible on the IP address 10.0.0.2. [root@host ~]$ nmcli con mod 'System eth0' ipv4.dns 10.0.0.2 To check the time on Rocky Linux:
[user@host ~]$ date Wed 22 Sep 17:11:35 BST 2021 [user@host ~]$ sudo dnf install realmd oddjob oddjob-mkhomedir sssd adcli krb5-workstation Discovery¶
Now, you should be able to successfully discover your AD server(s) from your Linux host.
[user@host ~]$ realm discover ad.company.local ad.company.local type: kerberos realm-name: AD.COMPANY.LOCAL domain-name: ad.company.local configured: no server-software: active-directory client-software: sssd required-package: oddjob required-package: oddjob-mkhomedir required-package: sssd required-package: adcli required-package: samba-common This will be discovered using the relevant SRV records stored in your Active Directory DNS service.
Joining¶
Once you have successfully discovered your Active Directory installation from the Linux host, you should be able to use realmd to join the domain, which will orchestrate the configuration of sssd using adcli and some other such tools.
[user@host ~]$ sudo realm join ad.company.local If this process complains about encryption with KDC has no support for encryption type , try updating the global crypto policy to allow older encryption algorithms:
[user@host ~]$ sudo update-crypto-policies --set DEFAULT:AD-SUPPORT If this process succeeds, you should now be able to pull passwd information for an Active Directory user.
[user@host ~]$ sudo getent passwd administrator@ad.company.local administrator@ad.company.local:*:1450400500:1450400513:Administrator:/home/administrator@ad.company.local:/bin/bash getent get entries from Name Service Switch libraries (NSS). It means that, contrary to passwd or dig for example, it will query different databases, including /etc/hosts for getent hosts or from sssd in the getent passwd case.
realm provides some interesting options that you can use:
| Option | Observation |
|---|---|
| —computer-ou=’OU=LINUX,OU=SERVERS,dc=ad,dc=company.local’ | The OU where to store the server account |
| —os-name=’rocky’ | Specify the OS name stored in the AD |
| —os-version=’8′ | Specify the OS version stored in the AD |
| -U admin_username | Specify an admin account |
Attempting to Authenticate¶
Now your users should be able to authenticate to your Linux host against Active Directory.
On Windows 10: (which provides its own copy of OpenSSH)
C:\Users\John.Doe> ssh -l john.doe@ad.company.local linux.host Password for john.doe@ad.company.local: Activate the web console with: systemctl enable --now cockpit.socket Last login: Wed Sep 15 17:37:03 2021 from 10.0.10.241 [john.doe@ad.company.local@host ~]$ If this succeeds, you have successfully configured Linux to use Active Directory as an authentication source.
Setting the default domain¶
In a completely default setup, you will need to log in with your AD account by specifying the domain in your username (e.g., john.doe@ad.company.local ). If this is not the desired behavior, and you instead want to be able to omit the domain name at authentication time, you can configure SSSD to default to a specific domain.
This is actually a relatively simple process, and just requires a configuration tweak in your SSSD configuration file.
[user@host ~]$ sudo vi /etc/sssd/sssd.conf [sssd] . default_domain_suffix = ad.company.local By adding the default_domain_suffix , you are instructing SSSD to (if no other domain is specified) infer that the user is trying to authenticate as a user from the ad.company.local domain. This allows you to authenticate as something like john.doe instead of john.doe@ad.company.local .
To make this configuration change take effect, you must restart the sssd.service unit with systemctl .
[user@host ~]$ sudo systemctl restart sssd In the same way, if you don’t want your home directories to be suffixed by the domain name, you can add those options into your configuration file /etc/sssd/sssd.conf :
[domain/ad.company.local] use_fully_qualified_names = False override_homedir = /home/%u Don’t forget to restart the sssd service.
Restrict to certain users¶
There are various methods to restrict access to the server to a limited list of users, but this, as the name suggests, is certainly the simplest:
Add those options into your configuration file /etc/sssd/sssd.conf and restart the service:
access_provider = simple simple_allow_groups = group1, group2 simple_allow_users = user1, user2 Now, only users from group1 and group2, or user1 and user2 will be able to connect to the server using sssd!
Interact with the AD using adcli ¶
adcli is a CLI to perform actions on an Active Directory domain.
[user@host ~]$ sudo dnf install adcli [user@host ~]$ sudo adcli testjoin Successfully validated join to domain ad.company.local [user@host ~]$ adcli info ad.company.local [domain] domain-name = ad.company.local domain-short = AD domain-forest = ad.company.local domain-controller = dc1.ad.company.local domain-controller-site = site1 domain-controller-flags = gc ldap ds kdc timeserv closest writable full-secret ads-web domain-controller-usable = yes domain-controllers = dc1.ad.company.local dc2.ad.company.local [computer] computer-site = site1 - More than a consulting tool, you can use adcli to interact with your domain: manage users or groups, change password, etc.
Example: use adcli to get information about a computer:
This time we will provide an admin username thanks to the -U option
[user@host ~]$ adcli show-computer pctest -U admin_username Password for admin_username@AD: sAMAccountName: pctest$ userPrincipalName: - not set - msDS-KeyVersionNumber: 9 msDS-supportedEncryptionTypes: 24 dNSHostName: pctest.ad.company.local servicePrincipalName: RestrictedKrbHost/pctest.ad.company.local RestrictedKrbHost/pctest host/pctest.ad.company.local host/pctest operatingSystem: Rocky operatingSystemVersion: 8 operatingSystemServicePack: - not set - pwdLastSet: 133189248188488832 userAccountControl: 69632 description: - not set - Example: use adcli to change user’s password:
[user@host ~]$ adcli passwd-user user_test -U admin_username Password for admin_username@AD: Password for user_test: [user@host ~]$ Troubleshooting¶
Sometimes, the network service will start after SSSD, that cause trouble with authentication.
No AD users will be able to connect until you restarted the service.
In that case, you will have to override the systemd’s service file to manage this problem.
Copy this content into /etc/systemd/system/sssd.service :
[Unit] Description=System Security Services Daemon # SSSD must be running before we permit user sessions Before=systemd-user-sessions.service nss-user-lookup.target Wants=nss-user-lookup.target After=network-online.target [Service] Environment=DEBUG_LOGGER=--logger=files EnvironmentFile=-/etc/sysconfig/sssd ExecStart=/usr/sbin/sssd -i $ Type=notify NotifyAccess=main PIDFile=/var/run/sssd.pid [Install] WantedBy=multi-user.target The next reboot, the service will start after its requirements, and everything will go well.
Leaving the Active Directory¶
Sometimes, it’s necessary to leave the AD.
You can, once again, proceed with realm and then remove the packages that are no longer required:
[user@host ~]$ sudo realm leave ad.company.local [user@host ~]$ sudo dnf remove realmd oddjob oddjob-mkhomedir sssd adcli krb5-workstation Contributors: Steven Spencer, Sambhav Saggi, Antoine Le Morvan, Krista Burdine