# Unified Logons between Windows NT and UNIX using Winbind

# Abstract

Integration of UNIX and Microsoft Windows NT through a unified logon has been considered a "holy grail" in heterogeneous computing environments for a long time. We present winbind, a component of the Samba suite of programs as a solution to the unified logon problem. Winbind uses a UNIX implementation of Microsoft RPC calls, Pluggable Authentication Modules, and the Name Service Switch to allow Windows NT domain users to appear and operate as UNIX users on a UNIX machine. This paper describes the winbind system, explaining the functionality it provides, how it is configured, and how it works internally.

# Introduction

It is well known that UNIX and Microsoft Windows NT have different models for representing user and group information and use different technologies for implementing them. This fact has made it difficult to integrate the two systems in a satisfactory manner.

One common solution in use today has been to create identically named user accounts on both the UNIX and Windows systems and use the Samba suite of programs to provide file and print services between the two. This solution is far from perfect however, as adding and deleting users on both sets of machines becomes a chore and two sets of passwords are required both of which can lead to synchronization problems between the UNIX and Windows systems and confusion for users.

We divide the unified logon problem for UNIX machines into three smaller problems:

- Obtaining Windows NT user and group information

- Authenticating Windows NT users

- Password changing for Windows NT users

Ideally, a prospective solution to the unified logon problem would satisfy all the above components without duplication of information on the UNIX machines and without creating additional tasks for the system administrator when maintaining users and groups on either system. The winbind system provides a simple and elegant solution to all three components of the unified logon problem.

# What Winbind Provides

Winbind unifies UNIX and Windows NT account management by allowing a UNIX box to become a full member of a NT domain. Once this is done the UNIX box will see NT users and groups as if they were native UNIX users and groups, allowing the NT domain to be used in much the same manner that NIS+ is used within UNIX-only environments.

The end result is that whenever any program on the UNIX machine asks the operating system to lookup a user or group name, the query will be resolved by asking the NT domain controller for the specified domain to do the lookup. Because Winbind hooks into the operating system at a low level (via the NSS name resolution modules in the C library) this redirection to the NT domain controller is completely transparent.

Users on the UNIX machine can then use NT user and group names as they would use "native" UNIX names. They can chown files so that they are owned by NT domain users or even login to the UNIX machine and run a UNIX X-Window session as a domain user.

The only obvious indication that Winbind is being used is that user and group names take the form DOMAIN\user and DOMAIN\group. This is necessary as it allows Winbind to determine that redirection to a domain controller is wanted for a particular lookup and which trusted domain is being referenced.

Additionally, Winbind provides an authentication service that hooks into the Pluggable Authentication Modules (PAM) system to provide authentication via a NT domain to any PAM enabled applications. This capability solves the problem of synchronizing passwords between systems since all passwords are stored in a single location (on the domain controller).

## Target Uses

Winbind is targeted at organizations that have an existing NT based domain infrastructure into which they wish to put UNIX workstations or servers. Winbind will allow these organizations to deploy UNIX workstations without having to maintain a separate account infrastructure. This greatly simplifies the administrative overhead of deploying UNIX workstations into a NT based organization.

Another interesting way in which we expect Winbind to be used is as a central part of UNIX based appliances. Appliances that provide file and print services to Microsoft based networks will be able to use Winbind to provide seamless integration of the appliance into the domain.

# How Winbind Works

The winbind system is designed around a client/server architecture. A long running winbindd daemon listens on a UNIX domain socket waiting for requests to arrive. These requests are generated by the NSS and PAM clients and processed sequentially.

The technologies used to implement winbind are described in detail below.

## Microsoft Remote Procedure Calls

Over the last two years, efforts have been underway by various Samba Team members to decode various aspects of the Microsoft Remote Procedure Call (MSRPC) system. This system is used for most network related operations between Windows NT machines including remote management, user authentication and print spooling. Although initially this work was done to aid the implementation of Primary Domain Controller (PDC) functionality in Samba, it has also yielded a body of code which can be used for other purposes.

Winbind uses various MSRPC calls to enumerate domain users and groups and to obtain detailed information about individual users or groups. Other MSRPC calls can be used to authenticate NT domain users and to change user passwords. By directly querying a Windows PDC for user and group information, winbind maps the NT account information onto UNIX user and group names.

## Name Service Switch

The Name Service Switch, or NSS, is a feature that is present in many UNIX operating systems. It allows system information such as hostnames, mail aliases and user information to be resolved from different sources. For example, a standalone UNIX workstation may resolve system information from a series of flat files stored on the local filesystem. A networked workstation may first attempt to resolve system information from local files, and then consult a NIS database for user information or a DNS server for hostname information.

The NSS application programming interface allows winbind to present itself as a source of system information when resolving UNIX usernames and groups. Winbind uses this interface, and information obtained from a Windows NT server using MSRPC calls to provide a new source of account enumeration. Using standard UNIX library calls, one can enumerate the users and groups on a UNIX machine running winbind and see all users and groups in a NT domain plus any trusted domain as though they were local users and groups.

The primary control file for NSS is `/etc/nsswitch.conf`. When a UNIX application makes a request to do a lookup the C library looks in `/etc/nsswitch.conf` for a line which matches the service type being requested, for example the "passwd" service type is used when user or group names are looked up. This config line species which implementations of that service should be tried and in what order. If the passwd config line is:

passwd: files example

then the C library will first load a module called `/lib/libnss_files.so` followed by the module `/lib/libnss_example.so`. The C library will dynamically load each of these modules in turn and call resolver functions within the modules to try to resolve the request. Once the request is resolved the C library returns the result to the application.

This NSS interface provides a very easy way for Winbind to hook into the operating system. All that needs to be done is to put `libnss_winbind.so` in `/lib/` then add "winbind" into `/etc/nsswitch.conf` at the appropriate place. The C library will then call Winbind to resolve user and group names.

## Pluggable Authentication Modules

Pluggable Authentication Modules, also known as PAM, is a system for abstracting authentication and authorization technologies. With a PAM module it is possible to specify different authentication methods for different system applications without having to recompile these applications. PAM is also useful for implementing a particular policy for authorization. For example, a system administrator may only allow console logins from users stored in the local password file but only allow users resolved from a NIS database to log in over the network.

Winbind uses the authentication management and password management PAM interface to integrate Windows NT users into a UNIX system. This allows Windows NT users to log in to a UNIX machine and be authenticated against a suitable Primary Domain Controller. These users can also change their passwords and have this change take effect directly on the Primary Domain Controller.

PAM is configured by providing control files in the directory `/etc/pam.d/` for each of the services that require authentication. When an authentication request is made by an application the PAM code in the C library looks up this control file to determine what modules to load to do the authentication check and in what order. This interface makes adding a new authentication service for Winbind very easy, all that needs to be done is that the `pam_winbind.so` module is copied to `/lib/security/` and the PAM control files for relevant services are updated to allow authentication via winbind. See the PAM documentation for more details.

## User and Group ID Allocation

When a user or group is created under Windows NT is it allocated a numerical relative identifier (RID). This is slightly different to UNIX which has a range of numbers that are used to identify users, and the same range in which to identify groups. It is winbind's job to convert RIDs to UNIX id numbers and vice versa. When winbind is configured it is given part of the UNIX user id space and a part of the UNIX group id space in which to store Windows NT users and groups. If a Windows NT user is resolved for the first time, it is allocated the next UNIX id from the range. The same process applies for Windows NT groups. Over time, winbind will have mapped all Windows NT users and groups to UNIX user ids and group ids.

The results of this mapping are stored persistently in an ID mapping database held in a tdb database). This ensures that RIDs are mapped to UNIX IDs in a consistent way.

## Result Caching

An active system can generate a lot of user and group name lookups. To reduce the network cost of these lookups winbind uses a caching scheme based on the SAM sequence number supplied by NT domain controllers. User or group information returned by a PDC is cached by winbind along with a sequence number also returned by the PDC. This sequence number is incremented by Windows NT whenever any user or group information is modified. If a cached entry has expired, the sequence number is requested from the PDC and compared against the sequence number of the cached entry. If the sequence numbers do not match, then the cached information is discarded and up to date information is requested directly from the PDC.

# Installation and Configuration

Many thanks to John Trostel jtrostel@snapserver.com for providing the HOWTO for this section.

This HOWTO describes how to get winbind services up and running to control access and authenticate users on your Linux box using the winbind services which come with SAMBA 2.2.2.

## Introduction

This HOWTO describes the procedures used to get winbind up and running on my RedHat 7.1 system. Winbind is capable of providing access and authentication control for Windows Domain users through an NT or Win2K PDC for 'regular' services, such as telnet a nd ftp, as well for SAMBA services.

This HOWTO has been written from a 'RedHat-centric' perspective, so if you are using another distribution, you may have to modify the instructions somewhat to fit the way your distribution works.

- Why should I to this?

  This allows the SAMBA administrator to rely on the authentication mechanisms on the NT/Win2K PDC for the authentication of domain members. NT/Win2K users no longer need to have separate accounts on the SAMBA server.

- Who should be reading this document?

  This HOWTO is designed for system administrators. If you are implementing SAMBA on a file server and wish to (fairly easily) integrate existing NT/Win2K users from your PDC onto the SAMBA server, this HOWTO is for you. That said, I am no NT or PAM expert, so you may find a better or easier way to accomplish these tasks.

## Requirements

If you have a samba configuration file that you are currently using... BACK IT UP! If your system already uses PAM, back up the `/etc/pam.d` directory contents! If you haven't already made a boot disk, MAKE ONE NOW!

Messing with the pam configuration files can make it nearly impossible to log in to yourmachine. That's why you want to be able to boot back into your machine in single user mode and restore your `/etc/pam.d` back to the original state they were in if you get frustrated with the way things are going. ;-)

The latest version of SAMBA (version 2.2.2 as of this writing), now includes a functioning winbindd daemon. Please refer to the main SAMBA web page or, better yet, your closest SAMBA mirror site for instructions on downloading the source code.

To allow Domain users the ability to access SAMBA shares and files, as well as potentially other services provided by your SAMBA machine, PAM (pluggable authentication modules) must be setup properly on your machine. In order to compile the winbind modules, you should have at least the pam libraries resident on your system. For recent RedHat systems (7.1, for instance), that means `pam-0.74-22`. For best results, it is helpful to also install the development packages in `pam-devel-0.74-22`.

## Testing Things Out

Before starting, it is probably best to kill off all the SAMBA related daemons running on your server. Kill off all smbd, nmbd, and winbindd processes that may be running. To use PAM, you will want to make sure that you have the standard PAM package (for RedHat) which supplies the `/etc/pam.d` directory structure, including the pam modules are used by pam-aware services, several pam libraries, and the `/usr/doc` and `/usr/man` entries for pam. Winbind built better in SAMBA if the pam-devel package was also installed. This package includes the header files needed to compile pam-aware applications. For instance, my RedHat system has both `pam-0.74-22` and `pam-devel-0.74-22` RPMs installed.

### Configure and compile SAMBA

The configuration and compilation of SAMBA is pretty straightforward. The first three steps may not be necessary depending upon whether or not you have previously built the Samba binaries.

    root# autoconf
    root# make clean
    root# rm config.cache
    root# ./configure --with-winbind
    root# make
    root# make install

This will, by default, install SAMBA in `/usr/local/samba`. See the main SAMBA documentation if you want to install SAMBA somewhere else. It will also build the winbindd executable and libraries.

### Configure `nsswitch.conf` and the winbind libraries

The libraries needed to run the winbindd daemon through nsswitch need to be copied to their proper locations, so

`root#` cp ../samba/source/nsswitch/libnss_winbind.so /lib

I also found it necessary to make the following symbolic link:

`root#` ln -s /lib/libnss_winbind.so /lib/libnss_winbind.so.2

Now, as root you need to edit `/etc/nsswitch.conf` to allow user and group entries to be visible from the winbindd daemon. My `/etc/nsswitch.conf` file look like this after editing:

    	passwd:     files winbind
    	shadow:     files
    	group:      files winbind

The libraries needed by the winbind daemon will be automatically entered into the ldconfig cache the next time your system reboots, but it is faster (and you don't need to reboot) if you do it manually:

`root#` /sbin/ldconfig -v | grep winbind

This makes `libnss_winbind` available to winbindd and echos back a check to you.

### Configure smb.conf

Several parameters are needed in the smb.conf file to control the behavior of winbindd. Configure `smb.conf` These are described in more detail in the winbindd(8) man page. My `smb.conf` file was modified to include the following entries in the [global] section:

    [global]
         <...>
         # separate domain and username with '+', like DOMAIN+username
         winbind separator = +
         # use uids from 10000 to 20000 for domain users
         winbind uid = 10000-20000
         # use gids from 10000 to 20000 for domain groups
         winbind gid = 10000-20000
         # allow enumeration of winbind users and groups
         winbind enum users = yes
         winbind enum groups = yes
         # give winbind users a real shell (only needed if they have telnet access)
         template homedir = /home/winnt/%D/%U
         template shell = /bin/bash

### Join the SAMBA server to the PDC domain

Enter the following command to make the SAMBA server join the PDC domain, where `DOMAIN` is the name of your Windows domain and `Administrator` is a domain user who has administrative privileges in the domain.

`root#` /usr/local/samba/bin/smbpasswd -j DOMAIN -r PDC -U Administrator

The proper response to the command should be: "Joined the domain `DOMAIN`" where `DOMAIN` is your DOMAIN name.

### Start up the winbindd daemon and test it!

Eventually, you will want to modify your smb startup script to automatically invoke the winbindd daemon when the other parts of SAMBA start, but it is possible to test out just the winbind portion first. To start up winbind services, enter the following command as root:

`root#` /usr/local/samba/bin/winbindd

I'm always paranoid and like to make sure the daemon is really running...

`root#` ps -ae | grep winbindd

This command should produce output like this, if the daemon is running

3025 ? 00:00:00 winbindd

Now... for the real test, try to get some information about the users on your PDC

`root#` /usr/local/samba/bin/wbinfo -u

This should echo back a list of users on your Windows users on your PDC. For example, I get the following response:

    CEO+Administrator
    CEO+burdell
    CEO+Guest
    CEO+jt-ad
    CEO+krbtgt
    CEO+TsInternetUser

Obviously, I have named my domain 'CEO' and my `winbindd separator` is '+'.

You can do the same sort of thing to get group information from the PDC:

    root# /usr/local/samba/bin/wbinfo -g
    CEO+Domain Admins
    CEO+Domain Users
    CEO+Domain Guests
    CEO+Domain Computers
    CEO+Domain Controllers
    CEO+Cert Publishers
    CEO+Schema Admins
    CEO+Enterprise Admins
    CEO+Group Policy Creator Owners

The function 'getent' can now be used to get unified lists of both local and PDC users and groups. Try the following command:

`root#` getent passwd

You should get a list that looks like your `/etc/passwd` list followed by the domain users with their new uids, gids, home directories and default shells.

The same thing can be done for groups with the command

`root#` getent group

### Fix the `/etc/rc.d/init.d/smb` startup files

The winbindd daemon needs to start up after the smbd and nmbd daemons are running. To accomplish this task, you need to modify the `/etc/init.d/smb` script to add commands to invoke this daemon in the proper sequence. My `/etc/init.d/smb` file starts up smbd, nmbd, and winbindd from the `/usr/local/samba/bin` directory directly. The 'start' function in the script looks like this:

    start() {
            KIND="SMB"
            echo -n $"Starting $KIND services: "
            daemon /usr/local/samba/bin/smbd $SMBDOPTIONS
            RETVAL=$?
            echo
            KIND="NMB"
            echo -n $"Starting $KIND services: "
            daemon /usr/local/samba/bin/nmbd $NMBDOPTIONS
            RETVAL2=$?
            echo
            KIND="Winbind"
            echo -n $"Starting $KIND services: "
            daemon /usr/local/samba/bin/winbindd
            RETVAL3=$?
            echo
            [ $RETVAL -eq 0 -a $RETVAL2 -eq 0 -a $RETVAL3 -eq 0 ] && touch /var/lock/subsys/smb || \
               RETVAL=1
            return $RETVAL
    }

The 'stop' function has a corresponding entry to shut down the services and look s like this:

    stop() {
            KIND="SMB"
            echo -n $"Shutting down $KIND services: "
            killproc smbd
            RETVAL=$?
            echo
            KIND="NMB"
            echo -n $"Shutting down $KIND services: "
            killproc nmbd
            RETVAL2=$?
            echo
            KIND="Winbind"
            echo -n $"Shutting down $KIND services: "
            killproc winbindd
            RETVAL3=$?
            [ $RETVAL -eq 0 -a $RETVAL2 -eq 0 -a $RETVAL3 -eq 0 ] && rm -f /var/lock/subsys/smb
            echo ""
            return $RETVAL
    }

If you restart the smbd, nmbd, and winbindd daemons at this point, you should be able to connect to the samba server as a domain member just as if you were a local user.

### Configure Winbind and PAM

If you have made it this far, you know that winbindd and samba are working together. If you want to use winbind to provide authentication for other services, keep reading. The pam configuration files need to be altered in this step. (Did you remember to make backups of your original `/etc/pam.d` files? If not, do it now.)

You will need a pam module to use winbindd with these other services. This module will be compiled in the `../source/nsswitch` directory by invoking the command

`root#` make nsswitch/pam_winbind.so

from the `../source` directory. The `pam_winbind.so` file should be copied to the location of your other pam security modules. On my RedHat system, this was the `/lib/security` directory.

`root#` cp ../samba/source/nsswitch/pam_winbind.so /lib/security

The `/etc/pam.d/samba` file does not need to be changed. I just left this fileas it was:

    auth    required        /lib/security/pam_stack.so service=system-auth
    account required        /lib/security/pam_stack.so service=system-auth

The other services that I modified to allow the use of winbind as an authentication service were the normal login on the console (or a terminal session), telnet logins, and ftp service. In order to enable these services, you may first need to change the entries in `/etc/xinetd.d` (or `/etc/inetd.conf`). RedHat 7.1 uses the new xinetd.d structure, in this case you need to change the lines in `/etc/xinetd.d/telnet` and `/etc/xinetd.d/wu-ftp` from

    enable = no

to

    enable = yes

For ftp services to work properly, you will also need to either have individual directories for the domain users already present on the server, or change the home directory template to a general directory for all domain users. These can be easily set using the `smb.conf` global entry template homedir.

The `/etc/pam.d/ftp` file can be changed to allow winbind ftp access in a manner similar to the samba file. My `/etc/pam.d/ftp` file was changed to look like this:

    auth       required     /lib/security/pam_listfile.so item=user sense=deny file=/etc/ftpusers onerr=succeed
    auth       sufficient   /lib/security/pam_winbind.so
    auth       required     /lib/security/pam_stack.so service=system-auth
    auth       required     /lib/security/pam_shells.so
    account    sufficient   /lib/security/pam_winbind.so
    account    required     /lib/security/pam_stack.so service=system-auth
    session    required     /lib/security/pam_stack.so service=system-auth

The `/etc/pam.d/login` file can be changed nearly the same way. It now looks like this:

    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    optional     /lib/security/pam_console.so

In this case, I added the auth sufficient /lib/security/pam_winbind.so lines as before, but also added the required pam_securetty.so above it, to disallow root logins over the network. I also added a sufficient /lib/security/pam_unix.so use_first_pass line after the winbind.so line to get rid of annoying double prompts for passwords.

# Limitations

Winbind has a number of limitations in its current released version that we hope to overcome in future releases:

- Winbind is currently only available for the Linux operating system, although ports to other operating systems are certainly possible. For such ports to be feasible, we require the C library of the target operating system to support the Name Service Switch and Pluggable Authentication Modules systems. This is becoming more common as NSS and PAM gain support among UNIX vendors.

- The mappings of Windows NT RIDs to UNIX ids is not made algorithmically and depends on the order in which unmapped users or groups are seen by winbind. It may be difficult to recover the mappings of rid to UNIX id mapping if the file containing this information is corrupted or destroyed.

- Currently the winbind PAM module does not take into account possible workstation and logon time restrictions that may be been set for Windows NT users.

# Conclusion

The winbind system, through the use of the Name Service Switch, Pluggable Authentication Modules, and appropriate Microsoft RPC calls have allowed us to provide seamless integration of Microsoft Windows NT domain users on a UNIX system. The result is a great reduction in the administrative cost of running a mixed UNIX and NT network.