Join Ubuntu 18.04 LXC container to Active Directory

March 5, 2020
LXC AD

Even in small businesses you may find that using Active Directory domain accounts for authentication on Linux servers can save significant time. If you need a shared Linux workstations with a lot of users and already have a domain controller in your network, you can join your servers to the domain instead of creating accounts and different access levels on each standalone host.

There are plenty of tutorials out there how to do this on Linux. But if you prefer running LXC containers as workstations rather than regular VMs, you may need to tweak the uid/gid mapping after registering your host to the domain controller.

This short tutorial will show you how to join an Ubuntu 18 LXC container to an Active Directory and how to configure SSH to login with your domain account.

I will assume that you already:

Launch a new container first:

lxc launch ubuntu:18.04 ad-join-test
Creating ad-join-test
Starting ad-join-test

Get a shell inside:

lxc exec ad-join-test -- /bin/bash
root@ad-join-test:~#

In Ubuntu, we can use System Security Services Daemon (sssd) for connecting to remote authentication providers. The documentation recommends using an automated tool to configure it and here I am going to use the realm command-line tool for quick enrollment.

Install realmd and some other necessary packages:

root@ad-join-test:~# apt update && apt install realmd sssd-tools sssd libnss-sss libpam-sss adcli packagekit

Try to discover your realm:

root@ad-join-test:~# realm discover eilabs.local

In case of no answer here, make sure that you can resolve the name of domain controller:

root@ad-join-test:~# nslookup -type=srv _ldap._tcp.eilabs.local

I couldn’t resolve the domain name of my domain controller in the first place, because we used .local as top level domain, which is nowadays intended to use in multicast DNS, so I had to change the systemd-resolver configuration.

If realm can discover the ldap service, use your administrator account to join. Realm is a silent program, so I am going to use –verbose to see the details of the process:

root@ad-join-test:~# realm join eilabs.local --user=david.jenei --verbose

Now try to check connection to the domain controller and look up your user passwd entry:

root@ad-join-test:~# getent passwd david.jenei@eilabs.local

If you try to login now, you see that the system is using the pam_sss module now as it should, but there is this error in the auth.log.

 pam_sss(login:auth): received for user david.jenei@eilabs.local: 4 (System error)

Changing the debug log level in the sssd reveals that there is problem with the uid. If you would like see the error message, put the debug directive under your domain’s section in the sssd configuration (/etc/sssd/sssd.conf) and restart sssd.

Here you can see, that my uid is a huge number, and the default id mapping in sssd can’t map those into the allocated uid map:

root@ad-join-test:~# cat /proc/self/uid_map
         0     100000      65536
root@ad-join-test:~# getent passwd david.jenei@eilabs.local
david.jenei@eilabs.local:*:1149001181:1149000513:Jenei, David:/home/david.jenei@eilabs.local:/bin/bash

Add these lines to the sssd configuration for your domain to limit idmap range and map uid/gid into the available space.

ldap_id_mapping = True
ldap_idmap_range_min = 10000
ldap_idmap_range_max = 50000
ldap_idmap_range_size = 1000

I also changed the backup homedir line to a new home directory format and enabled login without fully qualified names:

use_fully_qualified_names = False
override_homedir = /home/%d/%u

If you restart sssd after changing the configuration, it will fail, because we also need to manually clear the cache:

root@ad-join-test:~# rm -rf /var/lib/sss/db/*

Restart sssd service and login with your account:

root@ad-join-test:~# systemctl restart sssd
root@ad-join-test:~# login
ad-join-test login: david.jenei
Password:
Welcome to Ubuntu 18.04.3 LTS (GNU/Linux 4.15.0-88-generic x86_64)

Run pam-auth-update and enable home directory creation on login. Next time you login, your home directory will be created automatically.

root@ad-join-test:~# pam-auth-update

Finally edit /etc/ssh/sshd_config and enable password login:

PasswordAuthentication yes

Restart the ssh daemon and now you can use SSH with your AD user name to login to your container.

root@ad-join-test:~# systemctl restart sshd
root@ad-join-test:~# ssh david.jenei@localhost

For more information, you can check out the details of uid mapping in LXD here.