Enrolling an Active Directory RHEL-6 client machine using adcli

If you're adding a modern Linux client to an Active Directory domain, you really should be using realmd. It's easy to use, secure and does the right thing by default.

If you haven't heard about realmd already, check out the documentation. In a nutshell, realmd makes the client enrollment as easy as:

# realm join

However, realmd depends on some software that is not available on stable platforms used in production, like RHEL-6 and its derivatives. Still, it's possible to use some of the components realmd builds on separately and have a reasonably user-friendly experience. In this blog post, I’ll show you how, using a package called adcli, that is usually just a building block of realmd.

My test AD server domain is called win.example.com and the server that runs the domain is called server.win.example.com. For the test, I've used a mostly default CentOS 6.5 VM.

Typically, you'll want to point your Linux machine to the AD server for DNS:

# cat /etc/resolv.conf
# host domain name pointer server.win.example.com.

Start the setup by enabling the EPEL repository and installing the 'adcli'

# yum install adcli

You can type just 'adcli' to get an overview of what commands adcli supports. We're interested in joining the client to the AD domain in order to be able to log in as users from Active Directory.

Now you should be able to find your domain already:

# adcli info win.example.com
domain-name = WIN.EXAMPLE.COM
domain-short = WIN
domain-forest = WIN.EXAMPLE.COM
domain-controller = SERVER.WIN.EXAMPLE.COM
domain-controller-site = Default-First-Site-Name
domain-controller-flags = pdc gc ldap ds kdc timeserv closest writable good-timeserv full-secret ads-web
domain-controller-usable = yes
domain-controllers = SERVER.WIN.EXAMPLE.COM
computer-site = Default-First-Site-Name

As you can see, adcli was able to discover quite a few details about my test domain, so it’s time to join the client:

# adcli join win.example.com
Password for Administrator@WIN.EXAMPLE.COM:

You’ll be prompted for the Administrator password by default, but it’s possible to specify another user with the -U option. See the adcli man page for full list of details.

The join operation creates a keytab the machine will authenticate with. When you inspect the with klist -kt, you should see several entries that contain you client hostname in some form. Here are the keytab contents on my test system:

# klist -k | head
Keytab name: FILE:/etc/krb5.keytab
KVNO Principal
---- --------------------------------------------------------------------------

It’s recommeded to also configure /etc/krb5.conf to use the AD domain:

default_realm = WIN.EXAMPLE.COM
dns_lookup_realm = true
dns_lookup_kdc = true
ticket_lifetime = 24h
renew_lifetime = 7d
forwardable = true

kdc = server.win.example.com
admin_server = server.win.example.com

.win.example.com = WIN.EXAMPLE.COM
win.example.com = WIN.EXAMPLE.COM

The final step is setting up the SSSD (or Winbind if you like) to actually make use of the keytab to resolve users. I’ll show how to use the AD back end of SSSD as an example. Make sure sssd and authconfig are installed:

# yum install authconfig sssd

Unfortunately, authconfig in RHEL-6 doesn’t support configuring the AD back end directly, so you’ll have to do a bit of manual configuration. We can still use authconfig to set up the Name Service Switch and PAM stacks:

# authconfig --enablesssd --enablesssdauth --update

Now you should see ‘sss’ being present in /etc/nsswitch.conf and the pam stack configuration:

# grep sss /etc/nsswitch.conf
passwd: files sss
shadow: files sss
group: files sss
services: files sss
netgroup: files sss

The final step is to configure the SSSD itself. Open /etc/sssd/sssd.conf and define a single domain:

services = nss, pam, ssh, autofs
config_file_version = 2

id_provider = ad
# Uncomment if service discovery is not working
# ad_server = server.win.example.com

Start the SSSD and make sure it’s up after reboots:

# service sssd start
# chkconfig sssd on

You should now be able to log in as an AD user just fine:

su - administrator@win.example.com
-sh-4.1$ id
uid=388000500(administrator) gid=388000513(domain users) groups=388000513(domain users),388000512(domain admins),388000518(schema admins),388000519(enterprise admins),388000520(group policy creator owners),388000572(denied rodc password replication group),388001123(supergroup) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

For more information on the SSSD ad provider, see the SSSD wiki page or just sssd-ad the manual page.