Linux authentication with Active Directory using Kerberos 5
Section by Section
The configuration files are divided into sections for the four PAM module types: auth, account, password, and session. The auth section defines two alternative authentication methods in order to determine whether the users are who they claim to be (lines 2 and 3). PAM prompts the user for a password once only, and Kerberos checks the credentials (line 2). If this step is successful, pam_krb5.so requests a TGT with a set forward-capable bit to use the ticket on a remote system.
If successful, the authentication process tags pam_krb5.so sufficient and terminates without processing any additional modules.
Second Chance
If authentication fails, PAM calls the second module, pam_unix.so, to check that the user account exists locally (line 3). If PAM fails to reach the authentication server, root can still log in. The module argument use_first_pass, means the second authentication method reuses the password entered by the user instead of prompting again. Thanks to nullok_secure, there is no need to set a password in the local password file. Although you can log in with a blank password, you can only do so on terminals listed in /etc/securetty.
After PAM has finished processing the auth type modules, it goes on to execute the next include. The common-account file again contains two modules, pam_krb5.so and pam_unix, and is responsible for handing system access.
The PAM account service checks the user's password to see if it is still valid or if user access to the system is restricted with respect to time, resource usage, or location.
If the user exists and is allowed to log in, PAM turns to the next stack of modules, which is described by the common-password file. These modules give users the ability to change their passwords. In contrast to the normal procedure on Linux, which only allows users to change their own passwords, the Kerberos module gives any user the ability to change any other user's password. However, to do so, the user needs to know the current password for the account they are changing.
Session Management
Finally, PAM calls the modules configured in common-session. The session type is responsible for all the additional authentication chores. The remaining steps can include setting variables or mounting directories. Within the scope of this PAM service, pam_krb5 fulfills a very important task: it deletes a user's tickets when the user logs off.
After completing the configuration, any PAM-capable program can use PAM to access Active Directory. For example, the GNU Display Manager (GDM) offers both local and domain user accounts as login candidates and launches the required session after authentication with the Kerberos server (Figure 5).

As a final step, Active Directory users will need a home directory. If the NSS does not have any details on the directory, or if the directory does not exist, Linux will either send the user off to the root directory or it will not let the user access the system – despite successful authentication – because a desktop environment such as KDE needs to read and write to certain files that just do not exist.
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Find SysAdmin Jobs
News
-
KDE Plasma 5.27 Beta is Ready for Testing
The latest beta iteration of the KDE Plasma desktop is now available and includes some important additions and fixes.
-
Netrunner OS 23 Is Now Available
The latest version of this Linux distribution is now based on Debian Bullseye and is ready for installation and finally hits the KDE 5.20 branch of the desktop.
-
New Linux Distribution Built for Gamers
With a Gnome desktop that offers different layouts and a custom kernel, PikaOS is a great option for gamers of all types.
-
System76 Beefs Up Popular Pangolin Laptop
The darling of open-source-powered laptops and desktops will soon drop a new AMD Ryzen 7-powered version of their popular Pangolin laptop.
-
Nobara Project Is a Modified Version of Fedora with User-Friendly Fixes
If you're looking for a version of Fedora that includes third-party and proprietary packages, look no further than the Nobara Project.
-
Gnome 44 Now Has a Release Date
Gnome 44 will be officially released on March 22, 2023.
-
Nitrux 2.6 Available with Kernel 6.1 and a Major Change
The developers of Nitrux have officially released version 2.6 of their Linux distribution with plenty of new features to excite users.
-
Vanilla OS Initial Release Is Now Available
A stock GNOME experience with on-demand immutability finally sees its first production release.
-
Critical Linux Vulnerability Found to Impact SMB Servers
A Linux vulnerability with a CVSS score of 10 has been found to affect SMB servers and can lead to remote code execution.
-
Linux Mint 21.1 Now Available with Plenty of Look and Feel Changes
Vera has arrived and although it is still using kernel 5.15, there are plenty of improvements sure to please everyone.