<chapter>


<chapterinfo>
	<author>
		<firstname>Tim</firstname><surname>Potter</surname>
		<affiliation>
			<orgname>Samba Team</orgname>
			<address><email>tpot@linuxcare.com.au</email></address>
		</affiliation>
	</author>
	<author>
		<firstname>Andrew</firstname><surname>Trigdell</surname>
		<affiliation>
			<orgname>Samba Team</orgname>
			<address><email>tridge@linuxcare.com.au</email></address>
		</affiliation>
	</author>
	
		
	<pubdate>16 Oct 2000</pubdate>
</chapterinfo>

<title>Unifed Logons between Windows NT and UNIX using Winbind</title>

<sect1>
	<title>Abstract</title>

	<para>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 <emphasis>winbind
	</emphasis>, a component of the Samba suite of programs as a 
	solution to the unied 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.</para>
</sect1>


<sect1>
	<title>Introduction</title>
	
	<para>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.</para>
	
	<para>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 which
	can lead to synchronization problems between the UNIX and Windows 
	systems and confusion for users.</para>
	
	<para>We divide the unifed logon problem for UNIX machines into 
	three smaller problems:</para>
	
	<itemizedlist>
		<listitem><para>Obtaining Windows NT user and group information
		</para></listitem>
		
		<listitem><para>Authenticating Windows NT users
		</para></listitem>
		
		<listitem><para>Password changing for Windows NT users
		</para></listitem>
	</itemizedlist>


	<para>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 unifed logon 
	problem.</para>
</sect1>


<sect1>
	<title>What Winbind Provides</title>

	<para>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.</para>
	
	<para>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 specied 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.</para>
	
	<para>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.</para>
	
	<para>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.</para>
	
	<para>Additionally, Winbind provides a 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 as all passwords are stored in a single 
	location (on the domain controller).</para>
	
	<sect2>
		<title>Target Uses</title>
		
		<para>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 simplies 
		the administrative overhead of deploying UNIX workstations into 
		a NT based organization.</para>
		
		<para>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.</para>
	</sect2>
</sect1>



<sect1>
	<title>How Winbind Works</title>
	
	<para>The winbind system is designed around a client/server 
	architecture. A long running <command>winbindd</command> 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.</para>
	
	<para>The technologies used to implement winbind are described 
	in detail below.</para>
	
	<sect2>
		<title>Microsoft Remote Procedure Calls</title>
		
		<para>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.</para>
		
		<para>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.</para>
	</sect2>
	
	<sect2>
		<title>Name Service Switch</title>
		
		<para>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 dierent sources. For example, a standalone 
		UNIX workstation may resolve system information from a series of 
		flat files stored on the local lesystem. A networked workstation 
		may first attempt to resolve system information from local files, 
		then consult a NIS database for user information or a DNS server 
		for hostname information.</para>
		
		<para>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.</para>
		
		<para>The primary control le for NSS is <filename>/etc/nsswitch.conf
		</filename>. When a UNIX application makes a request to do a lookup 
		the C library looks in <filename>/etc/nsswitch.conf</filename> 
		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 andin what order. If the passwd 
		config line is:</para>

		<para><command>passwd: files example</command></para>

		<para>then the C library will first load a module called 
		<filename>/lib/libnss_files.so</filename> followed by
		the module <filename>/lib/libnss_example.so</filename>. 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.</para>
		
		<para>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 <filename>libnss_winbind.so</filename> in <filename>/lib/</filename> 
		then add "winbind" into <filename>/etc/nsswitch.conf</filename> at 
		the appropriate place. The C library will then call Winbind to 
		resolve user and group names.</para>
	</sect2>
	
	<sect2>
		<title>Pluggable Authentication Modules</title>
		
		<para>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 dierent 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.</para>
		
		<para>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 eect directly on the Primary Domain Controller.
		</para>
		
		<para>PAM is congured by providing control files in the directory 
		<filename>/etc/pam.d/</filename> 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 <filename>pam_winbind.so</filename> module 
		is copied to <filename>/lib/security/</filename> and the pam 
		control files for relevant services are updated to allow 
		authentication via winbind. See the PAM documentation
		for more details.</para>
	</sect2>
	
	
	<sect2>
		<title>User and Group ID Allocation</title>
		
		<para>When a user or group is created under Windows NT 
		is it allocated a numerical relative identier (RID). This is 
		slightly dierent to UNIX which has a range of numbers which 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 congured 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.</para>

		<para>The results of this mapping are stored persistently in 
		a ID mapping database held in a tdb database). This ensures that 
		RIDs are mapped to UNIX IDs in a consistent way.</para>
	</sect2>
	
	
	<sect2>
		<title>Result Caching</title>

		<para>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 modied. 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.</para>
	</sect2>
</sect1>


<sect1>
	<title>Installation and Configuration</title>

	<para>The easiest way to install winbind is by using the packages 
	provided in the <filename>pub/samba/appliance/</filename> 
	directory on your nearest 
	Samba mirror. These packages provide snapshots of the Samba source 
	code and binaries already setup to provide the full functionality 
	of winbind. This setup is a little more complex than a normal Samba 
	build as winbind needs a small amount of functionality from a 
	development code branch called SAMBA_TNG.</para>
	
	<para>Once you have installed the packages you should read 
	the <command>winbindd(8)</command> man page which will provide you 
	with conguration information and give you sample conguration files. 
	You may also wish to update the main Samba daemons smbd and nmbd) 
	with a more recent development release, such as the recently
	announced Samba 2.2 alpha release.</para>
</sect1>

<sect1>
	<title>Limitations</title>
	
	<para>Winbind has a number of limitations in its current 
	released version which we hope to overcome in future 
	releases:</para>

	<itemizedlist>
		<listitem><para>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.</para></listitem>
		
		<listitem><para>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.</para>
		</listitem>
		
		<listitem><para>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.</para></listitem>
		
		<listitem><para>Building winbind from source is currently 
		quite tedious as it requires combining source code from two Samba 
		branches. Work is underway to solve this by providing all 
		the necessary functionality in the main Samba code branch.</para>
		</listitem>
	</itemizedlist>
</sect1>


<sect1>
	<title>Conclusion</title>

	<para>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.</para>
	
</sect1>

</chapter>

