Active Directory Authentication

Prerequisites¶

Some understanding of Active Directory
Some understanding of LDAP

Introduction¶

In most enterprises, Microsoft's Active Directory (AD) is the default 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.

While connecting Linux to an AD cluster cannot support all of the features mentioned, it can handle users, groups, and access control. It is 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

The domain name ad.company.local throughout this guide will represent the Active Directory domain. To follow this guide, replace it with your AD domain's actual domain name.

The first step to joining a Linux system into AD is to discover your AD cluster, to ensure 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

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

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¶

You should now 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

The relevant SRV records stored in your Active Directory DNS service will allow discovery.

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

Note

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 a relatively straightforward process, requiring 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

Do not 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.

If not yet installed, install the required package:

[user@host ~]$ sudo dnf install adcli

Test if you have ever joined an Active Directory domain:

[user@host ~]$ sudo adcli testjoin
Successfully validated join to domain ad.company.local

Get more advanced information about the domain:

[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:

Note

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 restart 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 ${DEBUG_LOGGER}
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

Author: Hayden Young

Contributors: Steven Spencer, Sambhav Saggi, Antoine Le Morvan, Krista Burdine, Ganna Zhyrnova, Neel Chauhan

Active Directory Authentication

Prerequisites¶

Introduction¶

Discovering and joining AD using SSSD¶

Preparation¶

Discovery¶

Joining¶

Attempting to authenticate¶

Setting the default domain¶

Restrict to certain users¶

Interact with the AD using adcli¶

Troubleshooting¶

Leaving the Active Directory¶

Interact with the AD using `adcli`¶