docs/docbook/projdoc/winbind.sgml

   1 <chapter>
   2
   3
   4 <chapterinfo>
   5         <author>
   6                 <firstname>Tim</firstname><surname>Potter</surname>
   7                 <affiliation>
   8                         <orgname>Samba Team</orgname>
   9                         <address><email>tpot@linuxcare.com.au</email></address>
  10                 </affiliation>
  11         </author>
  12         <author>
  13                 <firstname>Andrew</firstname><surname>Trigdell</surname>
  14                 <affiliation>
  15                         <orgname>Samba Team</orgname>
  16                         <address><email>tridge@linuxcare.com.au</email></address>
  17                 </affiliation>
  18         </author>
  19
  20
  21         <pubdate>16 Oct 2000</pubdate>
  22 </chapterinfo>
  23
  24 <title>Unifed Logons between Windows NT and UNIX using Winbind</title>
  25
  26 <sect1>
  27         <title>Abstract</title>
  28
  29         <para>Integration of UNIX and Microsoft Windows NT through
  30         a unified logon has been considered a "holy grail" in heterogeneous
  31         computing environments for a long time. We present <emphasis>winbind
  32         </emphasis>, a component of the Samba suite of programs as a
  33         solution to the unied logon problem. Winbind uses a UNIX implementation
  34         of Microsoft RPC calls, Pluggable Authentication Modules, and the Name
  35         Service Switch to allow Windows NT domain users to appear and operate
  36         as UNIX users on a UNIX machine. This paper describes the winbind
  37         system, explaining the functionality it provides, how it is configured,
  38         and how it works internally.</para>
  39 </sect1>
  40
  41
  42 <sect1>
  43         <title>Introduction</title>
  44
  45         <para>It is well known that UNIX and Microsoft Windows NT have
  46         different models for representing user and group information and
  47         use different technologies for implementing them. This fact has
  48         made it difficult to integrate the two systems in a satisfactory
  49         manner.</para>
  50
  51         <para>One common solution in use today has been to create
  52         identically named user accounts on both the UNIX and Windows systems
  53         and use the Samba suite of programs to provide file and print services
  54         between the two. This solution is far from perfect however, as
  55         adding and deleting users on both sets of machines becomes a chore
  56         and two sets of passwords are required both of which which
  57         can lead to synchronization problems between the UNIX and Windows
  58         systems and confusion for users.</para>
  59
  60         <para>We divide the unifed logon problem for UNIX machines into
  61         three smaller problems:</para>
  62
  63         <itemizedlist>
  64                 <listitem><para>Obtaining Windows NT user and group information
  65                 </para></listitem>
  66
  67                 <listitem><para>Authenticating Windows NT users
  68                 </para></listitem>
  69
  70                 <listitem><para>Password changing for Windows NT users
  71                 </para></listitem>
  72         </itemizedlist>
  73
  74
  75         <para>Ideally, a prospective solution to the unified logon problem
  76         would satisfy all the above components without duplication of
  77         information on the UNIX machines and without creating additional
  78         tasks for the system administrator when maintaining users and
  79         groups on either system. The winbind system provides a simple
  80         and elegant solution to all three components of the unifed logon
  81         problem.</para>
  82 </sect1>
  83
  84
  85 <sect1>
  86         <title>What Winbind Provides</title>
  87
  88         <para>Winbind unifies UNIX and Windows NT account management by
  89         allowing a UNIX box to become a full member of a NT domain. Once
  90         this is done the UNIX box will see NT users and groups as if
  91         they were native UNIX users and groups, allowing the NT domain
  92         to be used in much the same manner that NIS+ is used within
  93         UNIX-only environments.</para>
  94
  95         <para>The end result is that whenever any
  96         program on the UNIX machine asks the operating system to lookup
  97         a user or group name, the query will be resolved by asking the
  98         NT domain controller for the specied domain to do the lookup.
  99         Because Winbind hooks into the operating system at a low level
 100         (via the NSS name resolution modules in the C library) this
 101         redirection to the NT domain controller is completely
 102         transparent.</para>
 103
 104         <para>Users on the UNIX machine can then use NT user and group
 105         names as they would use "native" UNIX names. They can chown files
 106         so that they are owned by NT domain users or even login to the
 107         UNIX machine and run a UNIX X-Window session as a domain user.</para>
 108
 109         <para>The only obvious indication that Winbind is being used is
 110         that user and group names take the form DOMAIN\user and
 111         DOMAIN\group. This is necessary as it allows Winbind to determine
 112         that redirection to a domain controller is wanted for a particular
 113         lookup and which trusted domain is being referenced.</para>
 114
 115         <para>Additionally, Winbind provides a authentication service
 116         that hooks into the Pluggable Authentication Modules (PAM) system
 117         to provide authentication via a NT domain to any PAM enabled
 118         applications. This capability solves the problem of synchronizing
 119         passwords between systems as all passwords are stored in a single
 120         location (on the domain controller).</para>
 121
 122         <sect2>
 123                 <title>Target Uses</title>
 124
 125                 <para>Winbind is targeted at organizations that have an
 126                 existing NT based domain infrastructure into which they wish
 127                 to put UNIX workstations or servers. Winbind will allow these
 128                 organizations to deploy UNIX workstations without having to
 129                 maintain a separate account infrastructure. This greatly simplies
 130                 the administrative overhead of deploying UNIX workstations into
 131                 a NT based organization.</para>
 132
 133                 <para>Another interesting way in which we expect Winbind to
 134                 be used is as a central part of UNIX based appliances. Appliances
 135                 that provide file and print services to Microsoft based networks
 136                 will be able to use Winbind to provide seamless integration of
 137                 the appliance into the domain.</para>
 138         </sect2>
 139 </sect1>
 140
 141
 142
 143 <sect1>
 144         <title>How Winbind Works</title>
 145
 146         <para>The winbind system is designed around a client/server
 147         architecture. A long running <command>winbindd</command> daemon
 148         listens on a UNIX domain socket waiting for requests
 149         to arrive. These requests are generated by the NSS and PAM
 150         clients and processed sequentially.</para>
 151
 152         <para>The technologies used to implement winbind are described
 153         in detail below.</para>
 154
 155         <sect2>
 156                 <title>Microsoft Remote Procedure Calls</title>
 157
 158                 <para>Over the last two years, efforts have been underway
 159                 by various Samba Team members to decode various aspects of
 160                 the Microsoft Remote Procedure Call (MSRPC) system. This
 161                 system is used for most network related operations between
 162                 Windows NT machines including remote management, user authentication
 163                 and print spooling. Although initially this work was done
 164                 to aid the implementation of Primary Domain Controller (PDC)
 165                 functionality in Samba, it has also yielded a body of code which
 166                 can be used for other purposes.</para>
 167
 168                 <para>Winbind uses various MSRPC calls to enumerate domain users
 169                 and groups and to obtain detailed information about individual
 170                 users or groups. Other MSRPC calls can be used to authenticate
 171                 NT domain users and to change user passwords. By directly querying
 172                 a Windows PDC for user and group information, winbind maps the
 173                 NT account information onto UNIX user and group names.</para>
 174         </sect2>
 175
 176         <sect2>
 177                 <title>Name Service Switch</title>
 178
 179                 <para>The Name Service Switch, or NSS, is a feature that is
 180                 present in many UNIX operating systems. It allows system
 181                 information such as hostnames, mail aliases and user information
 182                 to be resolved from dierent sources. For example, a standalone
 183                 UNIX workstation may resolve system information from a series of
 184                 flat files stored on the local lesystem. A networked workstation
 185                 may first attempt to resolve system information from local files,
 186                 then consult a NIS database for user information or a DNS server
 187                 for hostname information.</para>
 188
 189                 <para>The NSS application programming interface allows winbind
 190                 to present itself as a source of system information when
 191                 resolving UNIX usernames and groups.  Winbind uses this interface,
 192                 and information obtained from a Windows NT server using MSRPC
 193                 calls to provide a new source of account enumeration.  Using standard
 194                 UNIX library calls, one can enumerate the users and groups on
 195                 a UNIX machine running winbind and see all users and groups in
 196                 a NT domain plus any trusted domain as though they were local
 197                 users and groups.</para>
 198
 199                 <para>The primary control le for NSS is <filename>/etc/nsswitch.conf
 200                 </filename>. When a UNIX application makes a request to do a lookup
 201                 the C library looks in <filename>/etc/nsswitch.conf</filename>
 202                 for a line which matches the service type being requested, for
 203                 example the "passwd" service type is used when user or group names
 204                 are looked up. This     config line species which implementations
 205                 of that service should be tried andin what order. If the passwd
 206                 config line is:</para>
 207
 208                 <para><command>passwd: files example</command></para>
 209
 210                 <para>then the C library will first load a module called
 211                 <filename>/lib/libnss_files.so</filename> followed by
 212                 the module <filename>/lib/libnss_example.so</filename>. The
 213                 C library will dynamically load each of these modules in turn
 214                 and call resolver functions within the modules to try to resolve
 215                 the request. Once the request is resolved the C library returns the
 216                 result to the application.</para>
 217
 218                 <para>This NSS interface provides a very easy way for Winbind
 219                 to hook into the operating system. All that needs to be done
 220                 is to put <filename>libnss_winbind.so</filename> in <filename>/lib/</filename>
 221                 then add "winbind" into <filename>/etc/nsswitch.conf</filename> at
 222                 the appropriate place. The C library will then call Winbind to
 223                 resolve user and group names.</para>
 224         </sect2>
 225
 226         <sect2>
 227                 <title>Pluggable Authentication Modules</title>
 228
 229                 <para>Pluggable Authentication Modules, also known as PAM,
 230                 is a system for abstracting authentication and authorization
 231                 technologies. With a PAM module it is possible to specify different
 232                 authentication methods for dierent system applications without
 233                 having to recompile these applications. PAM is also useful
 234                 for implementing a particular policy for authorization. For example,
 235                 a system administrator may only allow console logins from users
 236                 stored in the local password file but only allow users resolved from
 237                 a NIS database to log in over the network.</para>
 238
 239                 <para>Winbind uses the authentication management and password
 240                 management PAM interface to integrate Windows NT users into a
 241                 UNIX system. This allows Windows NT users to log in to a UNIX
 242                 machine and be authenticated against a suitable Primary Domain
 243                 Controller. These users can also change their passwords and have
 244                 this change take eect directly on the Primary Domain Controller.
 245                 </para>
 246
 247                 <para>PAM is congured by providing control files in the directory
 248                 <filename>/etc/pam.d/</filename> for each of the services that
 249                 require authentication. When an authentication request is made
 250                 by an application the PAM code in the C library looks up this
 251                 control file to determine what modules to load to do the
 252                 authentication check and in what order. This interface makes adding
 253                 a new authentication service for Winbind very easy, all that needs
 254                 to be done is that the <filename>pam_winbind.so</filename> module
 255                 is copied to <filename>/lib/security/</filename> and the pam
 256                 control files for relevant services are updated to allow
 257                 authentication via winbind. See the PAM documentation
 258                 for more details.</para>
 259         </sect2>
 260
 261
 262         <sect2>
 263                 <title>User and Group ID Allocation</title>
 264
 265                 <para>When a user or group is created under Windows NT
 266                 is it allocated a numerical relative identier (RID). This is
 267                 slightly dierent to UNIX which has a range of numbers which are
 268                 used to identify users, and the same range in which to identify
 269                 groups. It is winbind's job to convert RIDs to UNIX id numbers and
 270                 vice versa.  When winbind is congured it is given part of the UNIX
 271                 user id space and a part of the UNIX group id space in which to
 272                 store Windows NT users and groups. If a Windows NT user is
 273                 resolved for the first time, it is allocated the next UNIX id from
 274                 the range. The same process applies for Windows NT groups. Over
 275                 time, winbind will have mapped all Windows NT users and groups
 276                 to UNIX user ids and group ids.</para>
 277
 278                 <para>The results of this mapping are stored persistently in
 279                 a ID mapping database held in a tdb database). This ensures that
 280                 RIDs are mapped to UNIX IDs in a consistent way.</para>
 281         </sect2>
 282
 283
 284         <sect2>
 285                 <title>Result Caching</title>
 286
 287                 <para>An active system can generate a lot of user and group
 288                 name lookups. To reduce the network cost of these lookups winbind
 289                 uses a caching scheme based on the SAM sequence number supplied
 290                 by NT domain controllers.  User or group information returned
 291                 by a PDC is cached by winbind along with a sequence number also
 292                 returned by the PDC. This sequence number is incremented by
 293                 Windows NT whenever any user or group information is modied. If
 294                 a cached entry has expired, the sequence number is requested from
 295                 the PDC and compared against the sequence number of the cached entry.
 296                 If the sequence numbers do not match, then the cached information
 297                 is discarded and up to date information is requested directly
 298                 from the PDC.</para>
 299         </sect2>
 300 </sect1>
 301
 302
 303 <sect1>
 304         <title>Installation and Configuration</title>
 305
 306         <para>The easiest way to install winbind is by using the packages
 307         provided in the <filename>pub/samba/appliance/</filename>
 308         directory on your nearest
 309         Samba mirror. These packages provide snapshots of the Samba source
 310         code and binaries already setup to provide the full functionality
 311         of winbind. This setup is a little more complex than a normal Samba
 312         build as winbind needs a small amount of functionality from a
 313         development code branch called SAMBA_TNG.</para>
 314
 315         <para>Once you have installed the packages you should read
 316         the <command>winbindd(8)</command> man page which will provide you
 317         with conguration information and give you sample conguration files.
 318         You may also wish to update the main Samba daemons smbd and nmbd)
 319         with a more recent development release, such as the recently
 320         announced Samba 2.2 alpha release.</para>
 321 </sect1>
 322
 323 <sect1>
 324         <title>Limitations</title>
 325
 326         <para>Winbind has a number of limitations in its current
 327         released version which we hope to overcome in future
 328         releases:</para>
 329
 330         <itemizedlist>
 331                 <listitem><para>Winbind is currently only available for
 332                 the Linux operating system, although ports to other operating
 333                 systems are certainly possible. For such ports to be feasible,
 334                 we require the C library of the target operating system to
 335                 support the Name Service Switch and Pluggable Authentication
 336                 Modules systems. This is becoming more common as NSS and
 337                 PAM gain        support among UNIX vendors.</para></listitem>
 338
 339                 <listitem><para>The mappings of Windows NT RIDs to UNIX ids
 340                 is not made algorithmically and depends on the order in which
 341                 unmapped users or groups are seen by winbind. It may be difficult
 342                 to recover the mappings of rid to UNIX id mapping if the file
 343                 containing this information is corrupted or destroyed.</para>
 344                 </listitem>
 345
 346                 <listitem><para>Currently the winbind PAM module does not take
 347                 into account possible workstation and logon time restrictions
 348                 that may be been set for Windows NT users.</para></listitem>
 349
 350                 <listitem><para>Building winbind from source is currently
 351                 quite tedious as it requires combining source code from two Samba
 352                 branches. Work is underway to solve this by providing all
 353                 the necessary functionality in the main Samba code branch.</para>
 354                 </listitem>
 355         </itemizedlist>
 356 </sect1>
 357
 358
 359 <sect1>
 360         <title>Conclusion</title>
 361
 362         <para>The winbind system, through the use of the Name Service
 363         Switch, Pluggable Authentication Modules, and appropriate
 364         Microsoft RPC calls have allowed us to provide seamless
 365         integration of Microsoft Windows NT domain users on a
 366         UNIX system. The result is a great reduction in the administrative
 367         cost of running a mixed UNIX and NT network.</para>
 368
 369 </sect1>
 370
 371 </chapter>
 372