One of the problems with running a network that contains both Linux and Windows systems is maintaining multiple account databases. One way to integrate these disparate systems is to use the Windows account database maintained on a Windows NT domain controller (or a Windows Active Directory controller or a Samba server) for both Windows and Linux systems. Unifying accounts is fairly easy for the Windows systems, but for Linux, you must make several configuration changes. However, the result can work reasonably well, and greatly simplifies cross-platform account maintenance.

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Using Winbind Authentication

One of the problems with running a network that contains both Linux and Windows systems is maintaining multiple account databases. One way to integrate these disparate systems is to use the Windows account database maintained on a Windows NT domain controller (or a Windows Active Directory controller or a Samba server) for both Windows and Linux systems. Unifying accounts is fairly easy for the Windows systems, but for Linux, you must make several configuration changes. However, the result can work reasonably well, and greatly simplifies cross-platform account maintenance.
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Tuesday, February 1st, 2005

Before proceeding further, be aware that modifying how Linux deals with accounts can be dangerous. A mistake can make it impossible to log in, even as root! When changing Linux accounts, it’s best to leave a root login running at all times on a virtual terminal or from a remote system. Moreover, always save a copy of each system configuration file before making any changes. That way, you can undo any mistakes you might make.

What Does Winbind Do?

Linux authentication is a multi-layered thing. Two components are particularly important:

*The Pluggable Authentication Modules (PAM) system is a way to modularize password verification, account setup, and similar features. Using PAM, programs that need to authenticate users (such as the login tool) call PAM rather than checking /etc/passwd themselves. With PAM acting as a layer of abstraction, you can reconfigure PAM to use a new authentication tool (such as an NT domain controller rather than /etc/passwd and /etc/shadow) without modifying any user programs.

*The Name Service Switch (NSS) system verifies the existence of an account (among other things). By telling NSS to use an NT domain controller, you won’t need to maintain non-password account information in /etc/passwd. If NSS doesn’t know about the NT domain controller, PAM could still use the domain controller for authentication, but you’d still need valid account data in /etc/passwd, even if you didn’t store passwords locally.

Winbind is a set of libraries that works with PAM and NSS to link the Linux authentication system to an NT domain controller (be it a Windows NT system, a Windows Active Directory controller, or a Samba server configured to function as a domain controller.) To make Winbind work, you must install Winbind, join the domain, and then configure both PAM and NSS.

Installing Winbind is usually straightforward, as it’s typically installed as part of the main Samba package. Look for the file pam_winbind.so in /lib/security/ or /usr/lib/security/, as well as /lib/libnss_winbind.so and /usr/sbin/winbindd. If any of these files aren’t present, peruse your distribution’s Samba packages. Or, if you installed Samba from source code, you must use the ––with-pam configure option. Then, after building or rebuilding Samba, check the source/nsswitch/ subdirectory of the Samba source tree and copy the files to appropriate locations. Sometimes, /lib/libnss_windbind.so is a symbolic link to /lib/libnss_winbind.so.2. If both files aren’t present, create a symbolic link.

Joining a Domain

Once you’ve confirmed that all the necessary files are present, you can configure your system to use Winbind. This task begins with making some changes to smb.conf (usually located in /etc/samba/). Listing One shows typical options for Winbind, all of which appear in the [global] section of the file.

Listing One: smb.conf options for Winbind

workgroup= MYWG

security= Domain

encrypt passwords= Yes

password server= 192.168.1.1

winbind use default domain= Yes

idmap uid= 2000-25000

idmap gid= 2000-25000

template shell= /bin/bash

template homedir= /home/% U

The first four lines in Listing One are fairly straightforward, and might appear on any Samba server on the network. They set the workgroup/domain name, tell Samba to use domain-level security, enable encrypted passwords, and specify the password server system (that is, the domain controller). The remaining lines in this listing set Winbind-specific options.

*The idmap uid and idmap gid options set the range of UID and GID numbers that Winbind (its NSS components, specifically) may assign. These UID and GID values should not be used by local users, but you can change them from the values set in Listing One, if you like. These options are necessary because NT domain controllers don’t maintain Linux-style UID and GID numbers, so Winbind must make these values up itself.

*The template shell and template homedir options set the default shell and home directory. The %U in the latter option stands in for the username. As with idmap uid and idmap gid, these options are necessary because NT domain controllers don’t maintain the information.

While you’ve now told your Linux system how to find the domain controller and manage accounts, you must still join the domain — that is, notify the domain controller about the new member. This can be done using the net command:

# net join member –U adminuser

When you type this command, adminuser is the username of an administrative user on the domain controller. On Windows systems, this is likely to be Administrator. On domain controllers that use Linux and Samba, it could be something else, so check your domain controller configuration. Samba domain controllers may also need a machine trust account that’s been prepared on the domain controller itself. (Samba domain controller configuration is well beyond the scope of this article.)

Running the Daemon

At this point, you can start running the Winbind daemon, winbindd:

# /usr/sbin/winbindd –i

This command runs the daemon and (because of the –i option) sends log information to standard output rather than to a log file. Launching the daemon in this way works well for testing, but in the long term, you’re better off putting this command (without the –i option) in a startup script. In fact, if you installed Winbind from a Linux package, it should have come with a System V- like startup script to start Winbind, so look for such a script and use your distribution’s System V package management utilities (such as chkconfig or rc-update) to activate it in your default runlevel.

The Winbind daemon manages the actual connection to the domain controller. PAM and NSS then consult this daemon to do their jobs. You can check basic operations using the wbinfo command. The –t option causes this program to check the basic connection of Winbind to the domain controller. It should return a message like this:

$ wbinfo –t
checking the trust secret via RPC calls
succeeded

You can also use the –u option to obtain a list of accounts managed by the domain controller. If one or both of these calls fail, review your configuration and consult your log files for clues about what’s going wrong.

Configuring PAM

PAM is controlled through files in /etc/pam.d/. For the most part, these files control how specific programs interact with PAM.

For instance, /etc/pam.d/login tells the login program how to use PAM. These configurations vary greatly from one distribution to another, but they all consist of a series of stacks — auth, account, session, and password. Each stack consists of one or more lines that begin with the relevant keyword. Each stack manages a particular sub-task, such as authentication (auth) or verifying account accessibility (account).

Modifying a PAM configuration to include a new authentication tool, such as Winbind, is a matter of adding lines to one or more of the auth and account stacks, and possibly modifying other lines. Listing Two shows a typical PAM configuration file with Winbind support added. New lines or amendments to existing lines are highlighted in bold.

Listing Two: A typical PAM configuration file with Winbind support

auth requisite pam_securetty.so
auth requisite pam_nologin.so
auth required pam_env.so
B
auth required pam_unix.so nullok B


account requisite pam_time.so
B
account required pam_unix.so

session required pam_unix.so
session optional pam_lastlog.so
session optional pam_motd.so
session optional pam_mail.so standard noenv

password required pam_unix.so nullok min=6 max=255 md5

This configuration adds lines to the auth and account stacks, inserting a call to pam_winbind.so just before a call to pam_unix.so. These calls are marked as sufficient, meaning that if Winbind gives its OK, subsequent modules need not succeed. This is very important when combining multiple authentication tools, such as Winbind and pam_unix.so (which is the standard tool that validates users against /etc/passwd and /etc/shadow).

Other modules called in these stacks don’t actually verify passwords; instead, they perform additional checks, such as verifying that root isn’t logging in via telnet. You might optionally want to add another line to the end of the session stack:

session required pam_mkhomedir.so
skel=/etc/skel umask=0027

(This line has been split for publication purposes, and should be recombined into a single line if you add it.) This automatically creates a home directory for the user if one doesn’t exist. This can be handy if you want users to be able to log into the Linux system without your having to manually create home directories for them.

On some distributions, you must change the PAM configuration files for all of the services that you want to use Winbind. For instance, if you want to use domain accounts for text-mode console logins, logins via the GNOME Display Manager (GDM), for X screensaver password prompts, and for POP mail retrieval, you would have to modify the login, gdm, xscreensaver, and pop files in /etc/pam.d/. This can be tedious, so some distributions use a module called pam_stack.so instead of pam_unix.so. The pam_stack.so module calls an entire stack of PAM modules itself, as specified in the file defined by the service= option to this module (typically /etc/pam.d/system-auth). The end result is that, if your system uses pam_stack.so, you can probably modify system-auth rather than all of the other files. This can be a great time-saver, but if you want to use Winbind for some services but not others, you’ll still have to modify the individual files.

One service requires a special note: passwd. This service (and its /etc/pam.d/passwd configuration file) controls how the passwd command interacts with PAM. For a Winbind configuration, it’s probably best to leave this configuration alone. Users can then use the passwd command to change their local passwords (if they exist), and use smbpasswd to change their passwords on the domain controller. Alternatively, if you add a call to pam_winbind.so to the password stack, then the passwd command changes the password on the domain controller.

If a server or other program is running, you may need to restart it before you can use any new authentication tools you’ve defined in PAM. In the case of many login tools, logging in and then logging out again does the trick. You may need to restart some servers via their startup scripts, though.

Configuring NSS

At this point, your system should be able to use the NT domain controller for authenticating users; however, they must still have accounts defined locally, in /etc/passwd and /etc/shadow. Thus, implementing this system isn’t likely to save a lot of effort.

The final step is to link NSS to Winbind. You can do this by editing the /etc/nsswitch.conf file. Look for two lines in this file that begin with passwd and group, and add the string winbind to these lines. These two lines are ordinarily separated by one called shadow, but you don’t modify that line. The result might look something like this:

passwd: files winbind
shadow: files
group: files winbind

Some distributions use other options instead of or in addition to files; compat is one popular alternative. The key is to add winbind to the passwd and shadow lines, rather than use precisely the configuration shown here.

When you’re done with this, NSS will use both its original configuration and Winbind for the purposes for which /etc/passwd and /etc/shadow are normally used. This will enable you to use your normal Linux-only local accounts and groups, such as root and any users you want to define locally, without the help of the domain controller.

While you’re modifying /etc/nsswitch.conf, you might want to change one other line: hosts. This line tells the system what tools to use to resolve hostnames. If you add wins to this line, Linux will use NetBIOS name resolution methods in addition to any other methods (such as /etc/hosts and DNS). The order of items on this line defines the order Linux uses.

For instance, you might end up with a line like this:

hosts: files wins dns

This configuration isn’t strictly necessary, and it requires its own library (libnss_wins.so, installed much like libnss_winbind.so, as described earlier). Still, it can be handy if your system is running on a network that uses NetBIOS names locally and you don’t want to maintain all your local names in /etc/hosts or run a local DNS server.

You needn’t restart anything to have NSS begin using the new configuration you’ve specified in /etc/nsswitch.conf. You may want to check that the NSS portion of the configuration is working by using getent. This command returns information on user and group database entries. In particular, typing getent passwd returns user information, and getent group returns group information. On Linux systems with default configurations, these commands’ outputs are similar to what you’d get by typing cat /etc/passwd or cat /etc/group. On a system with a working Winbind NSS configuration, you should see the contents of these files plus accounts maintained by the NT domain controller. If you don’t see these accounts, review your configuration and consult your log files (on both the Linux system and the domain controller) for clues.

Testing the Configuration

At this point, everything should be working, and you should have tested the Winbind and NSS subsystems. To test PAM and everything else, you should try an ordinary login using a domain account — that is, one that’s defined on the domain controller but not on the local system. You can do this via whatever login methods you chose to configure in PAM, and in fact you should test all of these login methods, to be sure there’s not a problem with some of them but not others.

Be sure to test both valid and invalid logins, that is, correct usernames and passwords, correct usernames and incorrect passwords, and incorrect usernames. Some configurations will enable anybody to log in, using correct or incorrect passwords. Presumably that’s not what you want to do! You should also test your local accounts while you’re at it — some types of configurations will disable those accounts, but you should leave them enabled. If nothing else, root should be defined locally, not via the domain controller.

Roderick W. Smith is the author or co-author of over a dozen books, including Linux in a Windows World and The Definitive Guide to Samba 3. He can be reached at class="emailaddress">rodsmith@rodsbooks.com.

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