Join Linux to Active Directory With Winbind

Tuesday May 3rd 2005 by Carla Schroder

From time synchronization to wrestling with PAM, there are a lot of ins and outs to join your Linux systems with Active Directory. Here's how to manage it.

Two weeks ago we gave the high-level view of what windbind is for. Today we'll walk through using winbind to provide a single sign-on for Linux servers and workstations joined to a Windows Active Directory domain.

Join Samba Servers to Active Directory

See Join Samba 3 to Your Active Directory Domain for how to do this. Additionally, there is one more step you should take after editing your configuration files. You should delete all .tdb files to get rid of stale data. You may want to back them up first; look for /etc/samba/secrets.tdb (which may not exist) and in /var/lib/samba.

Another important step is to make sure all systems are keeping the same time; Kerberos is especially sensitive to time synchronization. Setting up a local time server on Linux is easy, see Keeping Accurate Time on Linux. Configuring a local time server is easier than ever -- instead of listing individual time servers as the article says to do, configure /etc/ntp.conf to use the NTP server pool:

server pool.ntp.org
server pool.ntp.org
server pool.ntp.org

Listing it three times creates performance redundancy -- if you hit a bad server, it will quickly try a different one. Then configure the local clients to point to your local NTP server.

Join Linux Workstations to Active Directory: PAM Fun

Samba and winbind provide authentication and identity resolution for Linux hosts that are part of an Active Directory domain, since Active Directory does not deign to provide a method for authenticating them directly. Follow the steps for joining a Samba server to AD. Then comes the hairy part -- if your Linux users require access to network services that require authentication, you'll have to configure PAM (pluggable authentication modules). This can be a bit vexing, but the advantage is it saves users from having to manage multiple logins. And it allows you to control access to services very precisely.

In the olden days there was but a single /etc/pam.conf file. Then it was improved and gained all kinds of flexibility, using a single file for each service in /etc/pam.d. Adding to the fun is Red Hat, SuSE, Debian, and doubtless other distributions configure PAM a little differently, bless their individualistic little souls. Note to distribution maintainers: just because you can be different doesn't mean you have to.

Your very first job is to make a backup of /etc/pam.d, because any mistake can prevent you from being able to login. So keep a bootable rescue disk handy or boot to single-user if you get in trouble, and restore your original configuration.

Stop both the smbd and windbindd services, if they are running. At the very least you must configure winbind authentication in /etc/pam.d/login. Here is a sample configuration that works on Debian:

auth       required     /lib/security/pam_securetty.so
auth       sufficient   /lib/security/pam_winbind.so
auth       sufficient   /lib/security/pam_unix.so use_first_pass
auth       required     /lib/security/pam_nologin.so
account    sufficient   /lib/security/pam_winbind.so
session    required     /lib/security/pam_mkhomedir.so skel=/etc/skel
@include common-auth
@include common-account
@include common-session
session    optional     /lib/security/pam_console.so
@include common-password

The order of the directives is important. Make sure that pam_winbind.so is either copied or linked to /lib/security. PAM automatically looks in /lib/security for modules so you don't have to spell out the full path, but it's a good habit to get into anyway. The files common-auth, common-account, common-session, and common-password define common settings for all services.

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The use_first_pass argument tells PAM to re-use the previously entered password. This works only for auth and password modules.

session required /lib/security/pam_mkhomedir.so skel=/etc/skel umask=0022 is a slick little PAM feature that creates home directories for users on the fly.

This does the same thing on Red Hat:

auth       required     /lib/security/pam_securetty.so
auth       sufficient   /lib/security/pam_winbind.so
auth       sufficient   /lib/security/pam_unix.so use_first_pass
auth       required     /lib/security/pam_stack.so service=system-auth
auth       required     /lib/security/pam_nologin.so
account    sufficient   /lib/security/pam_winbind.so
account    required     /lib/security/pam_stack.so service=system-auth
password   required     /lib/security/pam_stack.so service=system-auth
session    required     /lib/security/pam_stack.so service=system-auth
session    required      /lib/security/pam_mkhomedir.so skel=/etc/skel
session    optional     /lib/security/pam_console.so
What if you want to authenticate SSH logins via PAM? Do this in /etc/pam.d/ssh:

auth       required     /lib/security/pam_securetty.so
auth       sufficient   /lib/security/pam_winbind.so
auth       sufficient   /lib/security/pam_unix.so
auth       required     /lib/security/pam_pwdb.so use_first_pass
account    sufficient   /lib/security/pam_unix.so
account    required    /lib/security/ pam_winbind.so
session    required     /lib/security/pam_unix.so
session    required     /lib/security/pam_winbind.so
password   required     /lib/security/pam_unix.so
password   required     /lib/security/pam_winbind.so

How to configure other services? As a general rule, stick your pam_winbind.so module next to any existing line that references a standard Linux auth, account, session, or password module. I don't promise that this will always work, but it's a good starting point. Or you can study the PAM documentation. Or wait for my detailed PAM howto.

Make sure that you do not have more than one account that has UID=0 in the password database. If there are two accounts in the passdb backend that have the same UID, winbind will break.

Now you can restart smbd and windbindd and try logging in from a Linux workstation. If you run into trouble look for help in Resources. The Samba mail list archives contain a wealth of excellent information.


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