How To Join CentOS Linux To An Active Directory Domain

Here we’ll show you how to add your Linux system to a Microsoft Windows Active Directory (AD) domain through the command line. This will allow us to SSH into the Linux server with user accounts in our AD domain, providing a central source of cross-platform authentication.

There are a number of ways to do this, however this is the easiest way that I’ve found to do it entirely through the command line.

In this example I am using CentOS 7 and Windows Server 2012 R2, however the version of Windows should not matter. We are assuming that our domain is already setup and configured, we’re simply joining our CentOS server to an existing domain.

Preparing CentOS

First we want to install all of the below packages in CentOS.

yum install sssd realmd oddjob oddjob-mkhomedir adcli samba-common samba-common-tools krb5-workstation openldap-clients policycoreutils-python -y

The CentOS server will need to be able to resolve the Active Directory domain in order to successfully join it. In this instance my DNS server in /etc/resolv.conf is set to one of the Active Directory servers hosting the example.com domain that I wish to join.

[[email protected] ~]# cat /etc/resolv.conf
search example.com
nameserver 192.168.1.2

Join CentOS To Windows Domain

Now that we’ve got that out of the way we can actually join the domain, this can be done with the ‘realm join’ command as shown below. You will need to specify the username of a user in the domain that has privileges to join a computer to the domain.

[[email protected] ~]# realm join --user=administrator example.com
Password for administrator:

Once you enter the password for your specific account, the /etc/sssd/sssd.conf and /etc/krb.conf files will be automatically configured. This is really great as editing these manually usually leads to all sorts of trivial problems when joining the domain. The /etc/krb5.keytab file is also created during this process.

If this fails, you can add -v to the end of the command for highly verbose output, which should give you more detailed information regarding the problem for further troubleshooting.

We can confirm that we’re in the realm (Linux terminology for the domain) by running the ‘realm list’ command, as shown below.

[[email protected] ~]# realm list
example.com
  type: kerberos
  realm-name: EXAMPLE.COM
  domain-name: example.com
  configured: kerberos-member
  server-software: active-directory
  client-software: sssd
  required-package: oddjob
  required-package: oddjob-mkhomedir
  required-package: sssd
  required-package: adcli
  required-package: samba-common-tools
  login-formats: [email protected]
  login-policy: allow-realm-logins

Once this has completed successfully, a computer object will be created in Active Directory in the default computers container as shown below.

Active Directory Users and Computers - Linux Computer Object

To keep things neat I like to move this into some other organizational unit (OU) for Linux servers rather than leaving things in the default computers container, however this doesn’t really matter for this exercise.

Now that our Linux server is a member of the Active Directory domain we can perform some tests. By default if we want to specify any users in the domain, we need to specify the domain name. For example with the ‘id’ command below, we get nothing back for ‘administrator’, however ‘[email protected]’ shows the UID for the account as well as all the groups the account is a member of in the Active Directory domain.

[[email protected] ~]# id administrator
id: administrator: no such user

[[email protected] ~]# id [email protected]
uid=1829600500([email protected]) gid=1829600513(domain [email protected]) groups=1829600513(domain [email protected]),1829600512(domain [email protected]),1829600572(denied rodc password replication [email protected]),1829600519(enterprise [email protected]),1829600518(schema [email protected]),1829600520(group policy creator [email protected])

We can change this behaviour by modifying the /etc/sssd/sssd.conf file, the following lines need to change from:

use_fully_qualified_names = True
fallback_homedir = /home/%u@%d

To the below, which does not require the fully qualified domain name (FQDN) to be specified. This also modifies the user directory in /home from having the FQDN specified after the username.

use_fully_qualified_names = False
fallback_homedir = /home/%u

To apply these changes, restart sssd.

[[email protected] ~]# systemctl restart sssd

Now we should be able to find user accounts without specifying the domain, as shown below this now works where it did not previously.

[[email protected] ~]# id administrator
uid=1829600500(administrator) gid=1829600513(domain users) groups=1829600513(domain users),1829600512(domain admins),1829600572(denied rodc password replication group),1829600520(group policy creator owners),1829600519(enterprise admins),1829600518(schema admins)

If this is still not correctly working for you, I suggest that you take a look at flushing your sssd cache.

Configuring SSH and Sudo Access

Now that we have successfully joined our CentOS server to the example.com domain, we can SSH in as any domain user from Active Directory with default settings.

[[email protected] ~]# ssh [email protected]
[email protected]'s password:
Creating home directory for user1.

We can further restrict SSH access by modifying the /etc/ssh/sshd_config file and make use of things like AllowUsers or AllowGroups to only allow certain user or groups from AD to have access. See our guide to the sshd_config file for further information. Don’t forget to restart sshd if you make any changes to this file in order to apply them.

We can also modify our sudoers configuration to allow our user account from the domain the desired level of access. I usually create an Active Directory group called something like ‘sudoers’, put my user in it, then allow this group sudo access by creating a file in /etc/sudoers.d/ which allows root access to be centrally controlled by AD.

Below is an example of this, the ‘sudoers’ group will have full root access.

[[email protected] ~]# cat /etc/sudoers.d/sudoers
%sudoers    ALL=(ALL)       ALL

This group only exists in Active Directory, our Linux server can see that user1 is a member of the sudoers group in Active Directory, and respects this group configuration and allows user1 root privileges as per the above configuration.

Active Directory Group Permissions

The username of Test Account is ‘user1’.

With this in place, our user1 account in the example.com Active Directory domain will now be able to use the sudo command to run commands with root privileges.

[[email protected] ~]$ sudo su
[sudo] password for user1:
[[email protected] user1]#
[[email protected] user1]# whoami
root

That’s all there is to it, we can now SSH to a Linux server with a user account from our Active Directory domain and even grant specific users or groups from AD specific levels of access.

Leaving The Domain

If you want to reverse the process and remove yourself from the domain, simply run the ‘realm leave’ command followed by the domain name, as shown below.

[[email protected] ~]# realm leave example.com

This will complete without any further user input. It will delete the computer object that was created in Active Directory, remove the keytab file, and set the sssd.conf and krb5.conf files back to default.

Summary

We have demonstrated how you can easily add your CentOS Linux system to a Microsoft Windows Active Directory domain, and then grant SSH or sudo access based on the user or group from the domain.

If you have a large number of Linux servers and an existing Windows domain you can easily use this process to add your Linux servers to the Windows domain, allowing for centralised user authentication which is far easier to manage when compared to having local user accounts spread out on every Linux server.

Leave a comment ?

22 Comments.

  1. Thank you Jarrod for the most useful article!

  2. Might want to add this.

    yum install samba-common-tools -y

    – of you wont be able to join the domain. ( error in missing packages.)

    Thanks

  3. Superb ! It worked for me.

  4. Hello, very usefull information.
    On a my active directory the OS information of my Linux box is empty. Does it the same for you ? How can I solve that?
    Thanks in advanced

    • I have the same thing, I haven’t been able to find a way through SSSD to populate that field. I believe with realmd it’s possible by specifying ‘os-name’, however I was not able to get that working in my test.

      • Hello, for info, I find a way to do it using realmd :
        realm join –user=xxxx –computer-ou=OU=LinuxOS –os-name=OracleLinux –os-version=”Red Hat Enterprise Linux 7.3″
        now I am looking on how to update the os-version & os-name fields when a linux box is already into the AD

  5. Super Article, A very convenient way of integration of Linux & Windows Server. Great Job, Jarrod

  6. Does this work for:

    CentOS 6x?

    How about HP-UX?

  7. Great article, Jarrod. It worked perfectly for me, using CentOS 7.

    Just a few comments:

    – /etc/krb.conf will be /etc/krb5.conf since we’re using krb5-workstation. At first I was concerned something went wrong but then I realized is was another filename.

    – I ran into some odd issues like not all users in domain being able to login, and id command not working for all users. So a colleague suggested installing winbind and it worked like a charm. My AD domain has a trust with another domain and I’m able to login with any user from both domains after installing winbind. I recommend you add it in the packages list.

    – You can add Domain Admins group in sudoers so every sysadmin can login in the Linux server and elevate. I used this line/syntax:

    %mydomain\\domain\ admins ALL=(ALL) ALL

    Any other AD groups with spaces in the name can also be added like this, using a single dash after the word preceding the space.

    – When using domains with trust, you may want to use FQDN to specify domain/user. So I can login and put in sudoers users like [email protected] and [email protected]. Winbind is essential for this.

    Thank you very much!

  8. Hello, there is a way for AD windows account to get log in over the file explorer to a specific shared folder on the linux? eg. get log in to her own $HOME on the linux from her Windows Computer?

  9. one of the best and easiest way to join AD.
    Thnx

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