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<HTML
><HEAD
><TITLE
>Unifed Logons between Windows NT and UNIX using Winbind</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.57"></HEAD
><BODY
CLASS="ARTICLE"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#0000FF"
VLINK="#840084"
ALINK="#0000FF"
><DIV
CLASS="ARTICLE"
><DIV
CLASS="TITLEPAGE"
><H1
CLASS="TITLE"
><A
NAME="AEN1"
>Unifed Logons between Windows NT and UNIX using Winbind</A
></H1
><HR></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN3"
>Abstract</A
></H1
><P
>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 <I
CLASS="EMPHASIS"
>winbind
</I
>, 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.</P
></DIV
><DIV
CLASS="SECT1"
><HR><H1
CLASS="SECT1"
><A
NAME="AEN7"
>Introduction</A
></H1
><P
>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.</P
><P
>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.</P
><P
>We divide the unifed logon problem for UNIX machines into
three smaller problems:</P
><P
></P
><UL
><LI
><P
>Obtaining Windows NT user and group information
</P
></LI
><LI
><P
>Authenticating Windows NT users
</P
></LI
><LI
><P
>Password changing for Windows NT users
</P
></LI
></UL
><P
>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.</P
></DIV
><DIV
CLASS="SECT1"
><HR><H1
CLASS="SECT1"
><A
NAME="AEN20"
>What Winbind Provides</A
></H1
><P
>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.</P
><P
>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.</P
><P
>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.</P
><P
>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.</P
><P
>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).</P
><DIV
CLASS="SECT2"
><HR><H2
CLASS="SECT2"
><A
NAME="AEN27"
>Target Uses</A
></H2
><P
>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.</P
><P
>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.</P
></DIV
></DIV
><DIV
CLASS="SECT1"
><HR><H1
CLASS="SECT1"
><A
NAME="AEN31"
>How Winbind Works</A
></H1
><P
>The winbind system is designed around a client/server
architecture. A long running <B
CLASS="COMMAND"
>winbindd</B
> 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.</P
><P
>The technologies used to implement winbind are described
in detail below.</P
><DIV
CLASS="SECT2"
><HR><H2
CLASS="SECT2"
><A
NAME="AEN36"
>Microsoft Remote Procedure Calls</A
></H2
><P
>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.</P
><P
>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.</P
></DIV
><DIV
CLASS="SECT2"
><HR><H2
CLASS="SECT2"
><A
NAME="AEN40"
>Name Service Switch</A
></H2
><P
>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.</P
><P
>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.</P
><P
>The primary control le for NSS is <TT
CLASS="FILENAME"
>/etc/nsswitch.conf
</TT
>. When a UNIX application makes a request to do a lookup
the C library looks in <TT
CLASS="FILENAME"
>/etc/nsswitch.conf</TT
>
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:</P
><P
><B
CLASS="COMMAND"
>passwd: files example</B
></P
><P
>then the C library will first load a module called
<TT
CLASS="FILENAME"
>/lib/libnss_files.so</TT
> followed by
the module <TT
CLASS="FILENAME"
>/lib/libnss_example.so</TT
>. 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.</P
><P
>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 <TT
CLASS="FILENAME"
>libnss_winbind.so</TT
> in <TT
CLASS="FILENAME"
>/lib/</TT
>
then add "winbind" into <TT
CLASS="FILENAME"
>/etc/nsswitch.conf</TT
> at
the appropriate place. The C library will then call Winbind to
resolve user and group names.</P
></DIV
><DIV
CLASS="SECT2"
><HR><H2
CLASS="SECT2"
><A
NAME="AEN56"
>Pluggable Authentication Modules</A
></H2
><P
>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.</P
><P
>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.
</P
><P
>PAM is congured by providing control files in the directory
<TT
CLASS="FILENAME"
>/etc/pam.d/</TT
> 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 <TT
CLASS="FILENAME"
>pam_winbind.so</TT
> module
is copied to <TT
CLASS="FILENAME"
>/lib/security/</TT
> and the pam
control files for relevant services are updated to allow
authentication via winbind. See the PAM documentation
for more details.</P
></DIV
><DIV
CLASS="SECT2"
><HR><H2
CLASS="SECT2"
><A
NAME="AEN64"
>User and Group ID Allocation</A
></H2
><P
>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.</P
><P
>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.</P
></DIV
><DIV
CLASS="SECT2"
><HR><H2
CLASS="SECT2"
><A
NAME="AEN68"
>Result Caching</A
></H2
><P
>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.</P
></DIV
></DIV
><DIV
CLASS="SECT1"
><HR><H1
CLASS="SECT1"
><A
NAME="AEN71"
>Installation and Configuration</A
></H1
><P
>The easiest way to install winbind is by using the packages
provided in the <TT
CLASS="FILENAME"
>pub/samba/appliance/</TT
>
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.</P
><P
>Once you have installed the packages you should read
the <B
CLASS="COMMAND"
>winbindd(8)</B
> 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.</P
></DIV
><DIV
CLASS="SECT1"
><HR><H1
CLASS="SECT1"
><A
NAME="AEN77"
>Limitations</A
></H1
><P
>Winbind has a number of limitations in its current
released version which we hope to overcome in future
releases:</P
><P
></P
><UL
><LI
><P
>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.</P
></LI
><LI
><P
>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.</P
></LI
><LI
><P
>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.</P
></LI
><LI
><P
>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.</P
></LI
></UL
></DIV
><DIV
CLASS="SECT1"
><HR><H1
CLASS="SECT1"
><A
NAME="AEN89"
>Conclusion</A
></H1
><P
>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.</P
></DIV
></DIV
></BODY
></HTML
>
|
