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Active Directory Authentication

Prerequisites

  • Some understanding of Active Directory
  • Some understanding of LDAP

Introduction

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

Note

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:
Service Port(s) Notes
DNS 53 (TCP+UDP)
Kerberos 88, 464 (TCP+UDP) Used by kadmin for setting & updating passwords
LDAP 389 (TCP+UDP)
LDAP-GC 3268 (TCP) LDAP Global Catalog - allows you to source user IDs from AD
  • 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

Manually editing the /etc/resolv.conf: 

# Edit the resolv.conf file
[user@host ~]$ sudo vi /etc/resolv.conf
search lan
nameserver 10.0.0.2
nameserver 1.1.1.1 # replace this with your preferred public DNS (as a backup)

# Make the resolv.conf file unwritable, preventing NetworkManager from
# overwriting it.
[user@host ~]$ sudo chattr +i /etc/resolv.conf

  • Ensure that the time on both sides (AD host and Linux system) is synchronized

To check the time on Rocky Linux: 

[user@host ~]$ date
Wed 22 Sep 17:11:35 BST 2021

  • Install the required packages for AD connection on the Linux side:
[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

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 behaviour, 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

Author: Hayden Young

Contributors: Steven Spencer, Sambhav Saggi

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