xdm - X Display Manager with support for XDMCP, host chooser
xdm [ -config configuration_file ] [ -nodaemon ] [
  -debug debug_level ] [ -error error_log_file ] [
  -resources resource_file ] [ -server server_entry
  ] [ -session session_program ]
Xdm manages a collection of X displays, which may be on the local host or
  remote servers. The design of xdm was guided by the needs of X
  terminals as well as The Open Group standard XDMCP, the X Display Manager
  Control Protocol. Xdm provides services similar to those provided
  by init, getty and login on character terminals:
  prompting for login name and password, authenticating the user, and running a
  ``session.''
A ``session'' is defined by the lifetime of a particular process;
    in the traditional character-based terminal world, it is the user's login
    shell. In the xdm context, it is an arbitrary session manager. This
    is because in a windowing environment, a user's login shell process does not
    necessarily have any terminal-like interface with which to connect. When a
    real session manager is not available, a window manager or terminal emulator
    is typically used as the ``session manager,'' meaning that termination of
    this process terminates the user's session.
When the session is terminated, xdm resets the X server and
    (optionally) restarts the whole process.
When xdm receives an Indirect query via XDMCP, it can run a
    chooser process to perform an XDMCP BroadcastQuery (or an XDMCP Query
    to specified hosts) on behalf of the display and offer a menu of possible
    hosts that offer XDMCP display management. This feature is useful with X
    terminals that do not offer a host menu themselves.
Xdm can be configured to ignore BroadcastQuery messages
    from selected hosts. This is useful when you don't want the host to appear
    in menus produced by chooser or X terminals themselves.
Because xdm provides the first interface that users will
    see, it is designed to be simple to use and easy to customize to the needs
    of a particular site. Xdm has many options, most of which have
    reasonable defaults. Browse through the various sections of this manual,
    picking and choosing the things you want to change. Pay particular attention
    to the Session Program section, which will describe how to set up the
    style of session desired.
xdm is highly configurable, and most of its behavior can be controlled by
  resource files and shell scripts. The names of these files themselves are
  resources read from the file xdm-config or the file named by the
  -config option.
xdm offers display management two different ways. It can
    manage X servers running on the local machine and specified in
    Xservers, and it can manage remote X servers (typically X terminals)
    using XDMCP (the XDM Control Protocol) as specified in the Xaccess
    file.
The resources of the X clients run by xdm outside the
    user's session, including xdm's own login window, can be affected by
    setting resources in the Xresources file.
For X terminals that do not offer a menu of hosts to get display
    management from, xdm can collect willing hosts and run the
    chooser program to offer the user a menu. For X displays attached to
    a host, this step is typically not used, as the local host does the display
    management.
After resetting the X server, xdm runs the Xsetup
    script to assist in setting up the screen the user sees along with the
    xlogin widget.
The xlogin widget, which xdm presents, offers the
    familiar login and password prompts.
After the user logs in, xdm runs the Xstartup script
    as root.
Then xdm runs the Xsession script as the user. This
    system session file may do some additional startup and typically runs the
    .xsession script in the user's home directory. When the
    Xsession script exits, the session is over.
At the end of the session, the Xreset script is run to
    clean up, the X server is reset, and the cycle starts over.
The file  /var/log/xdm.log will contain error messages from
    xdm and anything output to stderr by Xsetup, Xstartup,
    Xsession or Xreset. When you have trouble getting xdm
    working, check this file to see if xdm has any clues to the
  trouble.
All of these options, except -config itself, specify values that can also
  be specified in the configuration file as resources.
  - -config configuration_file
- Names the configuration file, which specifies resources to control the
      behavior of xdm. /etc/X11/xdm/xdm-config is the default. See
      the section Configuration File.
- -nodaemon
- Specifies ``false'' as the value for the DisplayManager.daemonMode
      resource. This suppresses the normal daemon behavior, which is for
      xdm to close all file descriptors, disassociate itself from the
      controlling terminal, and put itself in the background when it first
      starts up.
- -debug debug_level
- Specifies the numeric value for the DisplayManager.debugLevel
      resource. A non-zero value causes xdm to print lots of debugging
      statements to the terminal; it also disables the
      DisplayManager.daemonMode resource, forcing xdm to run
      synchronously. To interpret these debugging messages, a copy of the source
      code for xdm is almost a necessity. No attempt has been made to
      rationalize or standardize the output.
- -error error_log_file
- Specifies the value for the DisplayManager.errorLogFile resource.
      This file contains errors from xdm as well as anything written to
      stderr by the various scripts and programs run during the progress of the
      session.
- -resources resource_file
- Specifies the value for the DisplayManager*resources resource. This
      file is loaded using xrdb(1) to specify configuration parameters
      for the authentication widget.
- -server server_entry
- Specifies the value for the DisplayManager.servers resource. See
      the section Local Server Specification for a description of this
      resource.
- -udpPort port_number
- Specifies the value for the DisplayManager.requestPort resource.
      This sets the port-number which xdm will monitor for XDMCP
      requests. If set to 0, xdm will not listen for XDMCP or Chooser requests.
      As XDMCP uses the registered well-known UDP port 177, this resource should
      not be changed to a value other than 0, except for debugging.
- -session session_program
- Specifies the value for the DisplayManager*session resource. This
      indicates the program to run as the session after the user has logged
    in.
- -xrm resource_specification
- Allows an arbitrary resource to be specified, as in most X Toolkit
      applications.
At many stages the actions of xdm can be controlled through the use of
  its configuration file, which is in the X resource format. Some resources
  modify the behavior of xdm on all displays, while others modify its
  behavior on a single display. Where actions relate to a specific display, the
  display name is inserted into the resource name between ``DisplayManager'' and
  the final resource name segment.For local displays, the resource name and class are as read from
    the Xservers file.
For remote displays, the resource name is what the network address
    of the display resolves to. See the removeDomain resource. The name
    must match exactly; xdm is not aware of all the network aliases that
    might reach a given display. If the name resolve fails, the address is used.
    The resource class is as sent by the display in the XDMCP Manage
  request.
Because the resource manager uses colons to separate the name of
    the resource from its value and dots to separate resource name parts,
    xdm substitutes underscores for both dots and colons when generating
    the resource name. For example, DisplayManager.expo_x_org_0.startup
    is the name of the resource which defines the startup shell file for the
    ``expo.x.org:0'' display.
  - DisplayManager.servers
- This resource either specifies a file name full of server entries, one per
      line (if the value starts with a slash), or a single server entry. See the
      section Local Server Specification for the details.
- DisplayManager.requestPort
- This indicates the UDP port number which xdm uses to listen for
      incoming XDMCP requests. Unless you need to debug the system, leave this
      with its default value of 177.
- DisplayManager.errorLogFile
- Error output is normally directed at the system console. To redirect it,
      set this resource to a file name. A method to send these messages to
      syslog should be developed for systems which support it; however,
      the wide variety of interfaces precludes any system-independent
      implementation. This file also contains any output directed to stderr by
      the Xsetup, Xstartup, Xsession and Xreset files, so it will
      contain descriptions of problems in those scripts as well.
- DisplayManager.debugLevel
- If the integer value of this resource is greater than zero, reams of
      debugging information will be printed. It also disables daemon mode, which
      would redirect the information into the bit-bucket, and allows non-root
      users to run xdm, which would normally not be useful.
- DisplayManager.daemonMode
- Normally, xdm attempts to make itself into a daemon process
      unassociated with any terminal. This is accomplished by forking and
      leaving the parent process to exit, then closing file descriptors and
      releasing the controlling terminal. In some environments this is not
      desired (in particular, when debugging). Setting this resource to
      ``false'' will disable this feature.
- DisplayManager.pidFile
- The filename specified will be created to contain an ASCII representation
      of the process-id of the main xdm process. Xdm also uses
      file locking on this file to attempt to eliminate multiple daemons running
      on the same machine, which would cause quite a bit of havoc.
- DisplayManager.lockPidFile
- This is the resource which controls whether xdm uses file locking
      to keep multiple display managers from running amok. On System V, this
      uses the lockf library call, while on BSD it uses
    flock.
- DisplayManager.authDir
- This names a directory under which xdm stores authorization files
      while initializing the session. The default value is  /var/db/xdm.
      Can be overridden for specific displays by
      DisplayManager.DISPLAY.authFile.
- DisplayManager.autoRescan
- This boolean controls whether xdm rescans the configuration,
      servers, access control and authentication keys files after a session
      terminates and the files have changed. By default it is ``true.'' You can
      force xdm to reread these files by sending a SIGHUP to the main
      process.
- DisplayManager.removeDomainname
- When computing the display name for XDMCP clients, the name resolver will
      typically create a fully qualified host name for the terminal. As this is
      sometimes confusing, xdm will remove the domain name portion of the
      host name if it is the same as the domain name of the local host when this
      variable is set. By default the value is ``true.''
- DisplayManager.keyFile
- XDM-AUTHENTICATION-1 style XDMCP authentication requires that a private
      key be shared between xdm and the terminal. This resource specifies
      the file containing those values. Each entry in the file consists of a
      display name and the shared key.
- DisplayManager.accessFile
- To prevent unauthorized XDMCP service and to allow forwarding of XDMCP
      IndirectQuery requests, this file contains a database of hostnames which
      are either allowed direct access to this machine, or have a list of hosts
      to which queries should be forwarded to. The format of this file is
      described in the section XDMCP Access Control.
- DisplayManager.exportList
- A list of additional environment variables, separated by white space, to
      pass on to the Xsetup, Xstartup, Xsession, and
      Xreset programs.
- DisplayManager.randomDevice
- A file to read 8 bytes from to generate the seed of authorization keys.
      The default is  /dev/urandom . If this file cannot be read, or if a
      read blocks for more than 5 seconds, xdm falls back to using a checksum of
      DisplayManager.randomFile to generate the seed.
- DisplayManager.greeterLib
- On systems that support a dynamically-loadable greeter library, the name
      of the library. The default is  /usr/X11R7/lib/libXdmGreet.so.
- DisplayManager.choiceTimeout
- Number of seconds to wait for display to respond after user has selected a
      host from the chooser. If the display sends an XDMCP IndirectQuery within
      this time, the request is forwarded to the chosen host. Otherwise, it is
      assumed to be from a new session and the chooser is offered again. Default
      is 15.
- DisplayManager.sourceAddress
- Use the numeric IP address of the incoming connection on multihomed hosts
      instead of the host name. This is to avoid trying to connect on the wrong
      interface which might be down at this time.
- DisplayManager.willing
- This specifies a program which is run (as) root when an an XDMCP
      BroadcastQuery is received and this host is configured to offer XDMCP
      display management. The output of this program may be displayed on a
      chooser window. If no program is specified, the string Willing to
      manage is sent.
- DisplayManager.DISPLAY.resources
- This resource specifies the name of the file to be loaded by xrdb
      as the resource database onto the root window of screen 0 of the display.
      The Xsetup program, the Login widget, and chooser will use
      the resources set in this file. This resource data base is loaded just
      before the authentication procedure is started, so it can control the
      appearance of the login window. See the section Authentication
      Widget, which describes the various resources that are appropriate to
      place in this file. There is no default value for this resource, but 
      /etc/X11/xdm/Xresources is the conventional name.
- DisplayManager.DISPLAY.chooser
- Specifies the program run to offer a host menu for Indirect queries
      redirected to the special host name CHOOSER. 
      /usr/X11R7/libexec/chooser  is the default. See the sections XDMCP
      Access Control and Chooser.
- DisplayManager.DISPLAY.xrdb
- Specifies the program used to load the resources. By default, xdm
      uses  /usr/X11R7/bin/xrdb.
- DisplayManager.DISPLAY.setup
- This specifies a program which is run (as root) before offering the Login
      window. This may be used to change the appearance of the screen around the
      Login window or to put up other windows (e.g., you may want to run
      xconsole here). By default, no program is run. The conventional
      name for a file used here is Xsetup. See the section Setup
      Program.
- DisplayManager.DISPLAY.startup
- This specifies a program which is run (as root) after the authentication
      process succeeds. By default, no program is run. The conventional name for
      a file used here is Xstartup. See the section Startup
      Program.
- DisplayManager.DISPLAY.session
- This specifies the session to be executed (not running as root). By
      default,  /usr/X11R7/bin/xterm is run. The conventional name is
      Xsession. See the section Session Program.
- DisplayManager.DISPLAY.reset
- This specifies a program which is run (as root) after the session
      terminates. By default, no program is run. The conventional name is
      Xreset. See the section Reset Program.
- DisplayManager.DISPLAY.openDelay
- DisplayManager.DISPLAY.openRepeat
- DisplayManager.DISPLAY.openTimeout
- DisplayManager.DISPLAY.startAttempts
- DisplayManager.DISPLAY.reservAttempts
- These numeric resources control the behavior of xdm when attempting
      to open intransigent servers. openDelay is the length of the pause
      in seconds between successive attempts, openRepeat is the number of
      attempts to make, openTimeout is the amount of time to wait while
      actually attempting the open (i.e., the maximum time spent in the
      connect(2) system call) and startAttempts is the number of
      times this entire process is done before giving up on the server. After
      openRepeat attempts have been made, or if openTimeout
      seconds elapse in any particular attempt, xdm terminates and
      restarts the server, attempting to connect again. This process is repeated
      startAttempts times, at which point the display is declared dead
      and disabled. Although this behavior may seem arbitrary, it has been
      empirically developed and works quite well on most systems. The bound
      reservAttempts is the number of times a successful connect is
      allowed to be followed by a fatal error. When reached, the display is
      disabled. The default values are openDelay: 15, openRepeat:
      5, openTimeout: 120, startAttempts: 4 and
      reservAttempts: 2.
- DisplayManager.DISPLAY.pingInterval
- DisplayManager.DISPLAY.pingTimeout
- To discover when remote displays disappear, xdm occasionally pings
      them, using an X connection and XSync calls. pingInterval
      specifies the time (in minutes) between each ping attempt,
      pingTimeout specifies the maximum amount of time (in minutes) to
      wait for the terminal to respond to the request. If the terminal does not
      respond, the session is declared dead and terminated. By default, both are
      set to 5 minutes. If you frequently use X terminals which can become
      isolated from the managing host, you may wish to increase this value. The
      only worry is that sessions will continue to exist after the terminal has
      been accidentally disabled. xdm will not ping local displays.
      Although it would seem harmless, it is unpleasant when the workstation
      session is terminated as a result of the server hanging for NFS service
      and not responding to the ping.
- DisplayManager.DISPLAY.terminateServer
- This boolean resource specifies whether the X server should be terminated
      when a session terminates (instead of resetting it). This option can be
      used when the server tends to grow without bound over time, in order to
      limit the amount of time the server is run. The default value is
      ``false.''
- DisplayManager.DISPLAY.userPath
- Xdm sets the PATH environment variable for the session to this
      value. It should be a colon separated list of directories; see
      sh(1) for a full description. The default value is
      ``/sbin:/usr/sbin:/bin:/usr/bin:/usr/games:/usr/pkg/sbin:/usr/pkg/bin:/usr/local/sbin:/usr/local/bin:/usr/X11R7/bin''.
- DisplayManager.DISPLAY.systemPath
- Xdm sets the PATH environment variable for the startup and reset
      scripts to the value of this resource. The default for this resource is
      ``/sbin:/usr/sbin:/bin:/usr/bin:/usr/X11R7/bin''. Note the absence of
      ``.'' from this entry. This is a good practice to follow for root; it
      avoids many common Trojan Horse system penetration schemes.
- DisplayManager.DISPLAY.systemShell
- Xdm sets the SHELL environment variable for the startup and reset
      scripts to the value of this resource. It is /bin/sh by
    default.
- DisplayManager.DISPLAY.failsafeClient
- If the default session fails to execute, xdm will fall back to this
      program. This program is executed with no arguments, but executes using
      the same environment variables as the session would have had (see the
      section Session Program). By default,  /usr/X11R7/bin/xterm
      is used.
- DisplayManager.DISPLAY.grabServer
- DisplayManager.DISPLAY.grabTimeout
- To improve security, xdm grabs the server and keyboard while
      reading the login name and password. The grabServer resource
      specifies if the server should be held for the duration of the
      name/password reading. When ``false,'' the server is ungrabbed after the
      keyboard grab succeeds, otherwise the server is grabbed until just before
      the session begins. The default is ``false.'' The grabTimeout
      resource specifies the maximum time xdm will wait for the grab to
      succeed. The grab may fail if some other client has the server grabbed, or
      possibly if the network latencies are very high. This resource has a
      default value of 3 seconds; you should be cautious when raising it, as a
      user can be spoofed by a look-alike window on the display. If the grab
      fails, xdm kills and restarts the server (if possible) and the
      session.
- DisplayManager.DISPLAY.authorize
- DisplayManager.DISPLAY.authName
- authorize is a boolean resource which controls whether xdm
      generates and uses authorization for the local server connections. If
      authorization is used, authName is a list of authorization
      mechanisms to use, separated by white space. XDMCP connections dynamically
      specify which authorization mechanisms are supported, so authName
      is ignored in this case. When authorize is set for a display and
      authorization is not available, the user is informed by having a different
      message displayed in the login widget. By default, authorize is
      ``true,'' authName is ``MIT-MAGIC-COOKIE-1,'' or, if
      XDM-AUTHORIZATION-1 is available,
      ``XDM-AUTHORIZATION-1 MIT-MAGIC-COOKIE-1.''
- DisplayManager.DISPLAY.authFile
- This file is used to communicate the authorization data from xdm to
      the server, using the -auth server command line option. It should
      be kept in a directory which is not world-writable as it could easily be
      removed, disabling the authorization mechanism in the server. If not
      specified, a name is generated from DisplayManager.authDir and the name of
      the display.
- DisplayManager.DISPLAY.authComplain
- If set to ``false,'' disables the use of the unsecureGreeting in
      the login window. See the section Authentication Widget. The
      default is ``true.''
- DisplayManager.DISPLAY.resetSignal
- The number of the signal xdm sends to reset the server. See the
      section Controlling the Server. The default is 1 (SIGHUP).
- DisplayManager.DISPLAY.termSignal
- The number of the signal xdm sends to terminate the server. See the
      section Controlling the Server. The default is 15 (SIGTERM).
- DisplayManager.DISPLAY.resetForAuth
- The original implementation of authorization in the sample server reread
      the authorization file at server reset time, instead of when checking the
      initial connection. As xdm generates the authorization information
      just before connecting to the display, an old server would not get
      up-to-date authorization information. This resource causes xdm to
      send SIGHUP to the server after setting up the file, causing an additional
      server reset to occur, during which time the new authorization information
      will be read. The default is ``false,'' which will work for all MIT
      servers.
- DisplayManager.DISPLAY.userAuthDir
- When xdm is unable to write to the usual user authorization file
      ($HOME/.Xauthority), it creates a unique file name in this directory and
      points the environment variable XAUTHORITY at the created file. It uses
      /tmp by default.
First, the xdm configuration file should be set up. Make a directory
  (usually  /etc/X11/xdm) to contain all of the relevant files.Here is a reasonable configuration file, which could be named
    xdm-config:
DisplayManager.servers:         /etc/X11/xdm/Xservers
DisplayManager.errorLogFile:    /var/log/xdm.log
DisplayManager*resources:       /etc/X11/xdm/Xresources
DisplayManager*startup:         /etc/X11/xdm/Xstartup
DisplayManager*session:         /etc/X11/xdm/Xsession
DisplayManager.pidFile:         /var/run/xdm-pid
DisplayManager._0.authorize:    true
DisplayManager*authorize:       false
Note that this file mostly contains references to other files.
    Note also that some of the resources are specified with ``*'' separating the
    components. These resources can be made unique for each different display,
    by replacing the ``*'' with the display-name, but normally this is not very
    useful. See the Resources section for a complete discussion.
The database file specified by the DisplayManager.accessFile provides
  information which xdm uses to control access from displays requesting
  XDMCP service. This file contains three types of entries: entries which
  control the response to Direct and Broadcast queries, entries which control
  the response to Indirect queries, and macro definitions.
The format of the Direct entries is simple, either a host name or
    a pattern, which is distinguished from a host name by the inclusion of one
    or more meta characters (`*' matches any sequence of 0 or more characters,
    and `?' matches any single character) which are compared against the host
    name of the display device. If the entry is a host name, all comparisons are
    done using network addresses, so any name which converts to the correct
    network address may be used. For patterns, only canonical host names are
    used in the comparison, so ensure that you do not attempt to match aliases.
    Preceding either a host name or a pattern with a `!' character causes hosts
    which match that entry to be excluded.
To only respond to Direct queries for a host or pattern, it can be
    followed by the optional ``NOBROADCAST'' keyword. This can be used to
    prevent an xdm server from appearing on menus based on Broadcast
  queries.
An Indirect entry also contains a host name or pattern, but
    follows it with a list of host names or macros to which indirect queries
    should be sent.
A macro definition contains a macro name and a list of host names
    and other macros that the macro expands to. To distinguish macros from
    hostnames, macro names start with a `%' character. Macros may be nested.
Indirect entries may also specify to have xdm run
    chooser to offer a menu of hosts to connect to. See the section
    Chooser.
When checking access for a particular display host, each entry is
    scanned in turn and the first matching entry determines the response. Direct
    and Broadcast entries are ignored when scanning for an Indirect entry and
    vice-versa.
Blank lines are ignored, `#' is treated as a comment delimiter
    causing the rest of that line to be ignored, and `\newline' causes
    the newline to be ignored, allowing indirect host lists to span multiple
    lines.
Here is an example Xaccess file:
#
# Xaccess - XDMCP access control file
#
#
# Direct/Broadcast query entries
#
!xtra.lcs.mit.edu       # disallow direct/broadcast service for xtra
bambi.ogi.edu           # allow access from this particular display
*.lcs.mit.edu           # allow access from any display in LCS
*.deshaw.com    NOBROADCAST     # allow only direct access
*.gw.com                        # allow direct and broadcast
#
# Indirect query entries
#
%HOSTS  expo.lcs.mit.edu xenon.lcs.mit.edu \
        excess.lcs.mit.edu kanga.lcs.mit.edu
extract.lcs.mit.edu     xenon.lcs.mit.edu  #force extract to contact xenon
!xtra.lcs.mit.edu       dummy              #disallow indirect access
*.lcs.mit.edu           %HOSTS             #all others get to choose
If compiled with IPv6 support, multicast address groups may also
    be included in the list of addresses indirect queries are set to. Multicast
    addresses may be followed by an optional / character and hop count. If no
    hop count is specified, the multicast hop count defaults to 1, keeping the
    packet on the local network. For IPv4 multicasting, the hop count is used as
    the TTL.
Examples:
rincewind.sample.net  ff02::1 #IPv6 Multicast to ff02::1
                              #with a hop count of 1
ponder.sample.net     CHOOSER 239.192.1.1/16  #Offer a menu of hosts
                                              #who respond to IPv4 Multicast
                                              #to 239.192.1.1 with a
                                              #TTL of 16
For X terminals that do not offer a host menu for use with Broadcast or Indirect
  queries, the chooser program can do this for them. In the
  Xaccess file, specify ``CHOOSER'' as the first entry in the Indirect
  host list. Chooser will send a Query request to each of the remaining
  host names in the list and offer a menu of all the hosts that respond.
The list may consist of the word ``BROADCAST,'' in which case
    chooser will send a Broadcast instead, again offering a menu of all
    hosts that respond. Note that on some operating systems, UDP packets cannot
    be broadcast, so this feature will not work.
Example Xaccess file using chooser:
extract.lcs.mit.edu CHOOSER %HOSTS      #offer a menu of these hosts
xtra.lcs.mit.edu    CHOOSER BROADCAST   #offer a menu of all hosts
The program to use for chooser is specified by the
    DisplayManager.DISPLAY.chooser resource. For more
    flexibility at this step, the chooser could be a shell script.
    Chooser is the session manager here; it is run instead of a child
    xdm to manage the display.
Resources for this program can be put into the file named by
    DisplayManager.DISPLAY.resources.
When the user selects a host, chooser prints the host
    chosen, which is read by the parent xdm, and exits. xdm closes
    its connection to the X server, and the server resets and sends another
    Indirect XDMCP request. xdm remembers the user's choice (for
    DisplayManager.choiceTimeout seconds) and forwards the request to the
    chosen host, which starts a session on that display.
The following configuration directive is also defined for the Xaccess
  configuration file:
  - LISTEN interface [list of multicast group
    addresses]
- interface may be a hostname or IP address representing a network
      interface on this machine, or the wildcard * to represent all available
      network interfaces.
If one or more LISTEN lines are specified, xdm only listens for
    XDMCP connections on the specified interfaces. If multicast group addresses
    are listed on a listen line, xdm joins the multicast groups on the given
    interface.
If no LISTEN lines are given, the original behavior of listening
    on all interfaces is preserved for backwards compatibility. Additionally, if
    no LISTEN is specified, xdm joins the default XDMCP IPv6 multicast group,
    when compiled with IPv6 support.
To disable listening for XDMCP connections altogther, a line of
    LISTEN with no addresses may be specified, or the previously supported
    method of setting DisplayManager.requestPort to 0 may be used.
Examples:
LISTEN * ff02::1        # Listen on all interfaces and to the
                        # ff02::1 IPv6 multicast group.
LISTEN 10.11.12.13      # Listen only on this interface, as long
                        # as no other listen directives appear in
                        # file.
The Internet Assigned Numbers Authority has has assigned
  ff0X:0:0:0:0:0:0:12b as the permanently assigned range of multicast
  addresses for XDMCP. The X in the prefix may be replaced by any valid
  scope identifier, such as 1 for Interface-Local, 2 for Link-Local, 5 for
  Site-Local, and so on. (See IETF RFC 4291 or its replacement for further
  details and scope definitions.) xdm defaults to listening on the Link-Local
  scope address ff02:0:0:0:0:0:0:12b to most closely match the old IPv4 subnet
  broadcast behavior.
The resource DisplayManager.servers gives a server specification or, if
  the values starts with a slash (/), the name of a file containing server
  specifications, one per line.
Each specification indicates a display which should constantly be
    managed and which is not using XDMCP. This method is used typically for
    local servers only. If the resource or the file named by the resource is
    empty, xdm will offer XDMCP service only.
Each specification consists of at least three parts: a display
    name, a display class, a display type, and (for local servers) a command
    line to start the server. A typical entry for local display number 0 would
    be:
 :0 Digital-QV local /usr/X11R7/bin/X :0
The display types are:
  
    | local | local display: xdm must run the server | 
  
    | foreign | remote display: xdm opens an X connection to a running
      server | 
The display name must be something that can be passed in the
    -display option to an X program. This string is used to generate the
    display-specific resource names, so be careful to match the names (e.g., use
    ``:0 Sun-CG3 local /usr/X11R7/bin/X :0'' instead of ``localhost:0 Sun-CG3
    local /usr/X11R7/bin/X :0'' if your other resources are specified as
    ``DisplayManager._0.session''). The display class portion is also used in
    the display-specific resources, as the class of the resource. This is useful
    if you have a large collection of similar displays (such as a corral of X
    terminals) and would like to set resources for groups of them. When using
    XDMCP, the display is required to specify the display class, so the manual
    for your particular X terminal should document the display class string for
    your device. If it doesn't, you can run xdm in debug mode and look at
    the resource strings which it generates for that device, which will include
    the class string.
When xdm starts a session, it sets up authorization data
    for the server. For local servers, xdm passes ``-auth
    filename'' on the server's command line to point it at its
    authorization data. For XDMCP servers, xdm passes the authorization
    data to the server via the Accept XDMCP request.
The Xresources file is loaded onto the display as a resource database
  using xrdb. As the authentication widget reads this database before
  starting up, it usually contains parameters for that widget:
        xlogin*login.translations: #override\
                Ctrl<Key>R: abort-display()\n\
                <Key>F1: set-session-argument(failsafe) finish-field()\n\
                <Key>Return: set-session-argument() finish-field()
        xlogin*borderWidth: 3
        xlogin*greeting: CLIENTHOST
      #ifdef COLOR
        xlogin*greetColor: CadetBlue
        xlogin*failColor: red
      #endif
Please note the translations entry; it specifies a few new
    translations for the widget which allow users to escape from the default
    session (and avoid troubles that may occur in it). Note that if #override is
    not specified, the default translations are removed and replaced by the new
    value, not a very useful result as some of the default translations are
    quite useful (such as ``<Key>: insert-char ()'' which responds to
    normal typing).
This file may also contain resources for the setup program and
    chooser.
The Xsetup file is run after the server is reset, but before the Login
  window is offered. The file is typically a shell script. It is run as root, so
  should be careful about security. This is the place to change the root
  background or bring up other windows that should appear on the screen along
  with the Login widget.
In addition to any specified by DisplayManager.exportList,
    the following environment variables are passed:
  
    | DISPLAY | the associated display name | 
  
    | PATH | the value of DisplayManager.DISPLAY.systemPath | 
  
    | SHELL | the value of
      DisplayManager.DISPLAY.systemShell | 
  
    | XAUTHORITY | may be set to an authority file | 
Note that since xdm grabs the keyboard, any other windows
    will not be able to receive keyboard input. They will be able to interact
    with the mouse, however; beware of potential security holes here. If
    DisplayManager.DISPLAY.grabServer is set, Xsetup
    will not be able to connect to the display at all. Resources for this
    program can be put into the file named by
    DisplayManager.DISPLAY.resources.
Here is a sample Xsetup script:
#!/bin/sh
# Xsetup_0 - setup script for one workstation
xcmsdb < /etc/X11/xdm/monitors/alex.0
xconsole -geometry 480x130-0-0 -notify -verbose -exitOnFail &
 
The authentication widget prompts the user for the username, password, and/or
  other required authentication data from the keyboard. Nearly every imaginable
  parameter can be controlled with a resource. Resources for this widget should
  be put into the file named by
  DisplayManager.DISPLAY.resources. All of these have
  reasonable default values, so it is not necessary to specify any of them.
The resource file is loaded with xrdb(1) so it may use the
    substitutions defined by that program such as CLIENTHOST for the client
    hostname in the login message, or C pre-processor #ifdef statements to
    produce different displays depending on color depth or other variables.
Xdm can be compiled with support for the Xft(3)
    library for font rendering. If this support is present, font faces are
    specified using the resources with names ending in ``face'' in the
    fontconfig face format described in the Font Names section of
    fonts.conf(5). If not, then fonts are specified using the resources
    with names ending in ``font'' in the traditional X Logical Font
    Description format described in the Font Names section of
    X(7).
  - xlogin.Login.width, xlogin.Login.height, xlogin.Login.x,
    xlogin.Login.y
- The geometry of the Login widget is normally computed automatically. If
      you wish to position it elsewhere, specify each of these resources.
- xlogin.Login.foreground
- The color used to display the input typed by the user.
- xlogin.Login.face
- The face used to display the input typed by the user when built with Xft
      support. The default is ``Serif-18''.
- xlogin.Login.font
- The font used to display the input typed by the user when not built with
      Xft support.
- xlogin.Login.greeting
- A string which identifies this window. The default is ``X Window
    System.''
- xlogin.Login.unsecureGreeting
- When X authorization is requested in the configuration file for this
      display and none is in use, this greeting replaces the standard greeting.
      The default is ``This is an unsecure session''
- xlogin.Login.greetFace
- The face used to display the greeting when built with Xft support. The
      default is ``Serif-24:italic''.
- xlogin.Login.greetFont
- The font used to display the greeting when not built with Xft
    support.
- xlogin.Login.greetColor
- The color used to display the greeting.
- xlogin.Login.namePrompt
- The string displayed to prompt for a user name. Xrdb strips
      trailing white space from resource values, so to add spaces at the end of
      the prompt (usually a nice thing), add spaces escaped with backslashes.
      The default is ``Login: ''
- xlogin.Login.passwdPrompt
- The string displayed to prompt for a password, when not using an
      authentication system such as PAM that provides its own prompts. The
      default is ``Password: ''
- xlogin.Login.promptFace
- The face used to display prompts when built with Xft support. The default
      is ``Serif-18:bold''.
- xlogin.Login.promptFont
- The font used to display prompts when not built with Xft support.
- xlogin.Login.promptColor
- The color used to display prompts.
- xlogin.Login.changePasswdMessage
- A message which is displayed when the users password has expired. The
      default is ``Password Change Required''
- xlogin.Login.fail
- A message which is displayed when the authentication fails, when not using
      an authentication system such as PAM that provides its own prompts. The
      default is ``Login incorrect''
- xlogin.Login.failFace
- The face used to display the failure message when built with Xft support.
      The default is ``Serif-18:bold''.
- xlogin.Login.failFont
- The font used to display the failure message when not built with Xft
      support.
- xlogin.Login.failColor
- The color used to display the failure message.
- xlogin.Login.failTimeout
- The number of seconds that the failure message is displayed. The default
      is 10.
- xlogin.Login.logoFileName
- Name of an XPM format pixmap to display in the greeter window, if built
      with XPM support. The default is no pixmap.
- xlogin.Login.logoPadding
- Number of pixels of space between the logo pixmap and other elements of
      the greeter window, if the pixmap is displayed. The default is 5.
- xlogin.Login.useShape
- If set to ``true'', when built with XPM support, attempt to use the X
      Non-Rectangular Window Shape Extension to set the window shape. The
      default is ``true''.
- xlogin.Login.hiColor, xlogin.Login.shdColor
- Raised appearance bezels may be drawn around the greeter frame and text
      input boxes by setting these resources. hiColor is the highlight color,
      used on the top and left sides of the frame, and the bottom and right
      sides of text input areas. shdColor is the shadow color, used on the
      bottom and right sides of the frame, and the top and left sides of text
      input areas. The default for both is the foreground color, providing a
      flat appearance.
- xlogin.Login.frameWidth
- frameWidth is the width in pixels of the area around the greeter frame
      drawn in hiColor and shdColor.
- xlogin.Login.innerFramesWidth
- innerFramesWidth is the width in pixels of the area around text input
      areas drawn in hiColor and shdColor.
- xlogin.Login.sepWidth
- sepWidth is the width in pixels of the bezeled line between the greeting
      and input areas drawn in hiColor and shdColor.
- xlogin.Login.allowRootLogin
- If set to ``false'', don't allow root (and any other user with uid = 0) to
      log in directly. The default is ``true''. This setting is only checked by
      some of the authentication backends at this time.
- xlogin.Login.allowNullPasswd
- If set to ``true'', allow an otherwise failing password match to succeed
      if the account does not require a password at all. The default is
      ``false'', so only users that have passwords assigned can log in.
- xlogin.Login.echoPasswd
- If set to ``true'', a placeholder character (echoPasswdChar) will be shown
      for fields normally set to not echo, such as password input. The default
      is ``false''.
- xlogin.Login.echoPasswdChar
- Character to display if echoPasswd is true. The default is ``*''. If set
      to an empty value, the cursor will advance for each character input, but
      no text will be drawn.
- xlogin.Login.translations
- This specifies the translations used for the login widget. Refer to the X
      Toolkit documentation for a complete discussion on translations. The
      default translation table is:
    
      
        | Ctrl<Key>H: | delete-previous-character() |  
        | Ctrl<Key>D: | delete-character() |  
        | Ctrl<Key>B: | move-backward-character() |  
        | Ctrl<Key>F: | move-forward-character() |  
        | Ctrl<Key>A: | move-to-begining() |  
        | Ctrl<Key>E: | move-to-end() |  
        | Ctrl<Key>K: | erase-to-end-of-line() |  
        | Ctrl<Key>U: | erase-line() |  
        | Ctrl<Key>X: | erase-line() |  
        | Ctrl<Key>C: | restart-session() |  
        | Ctrl<Key>\\: | abort-session() |  
        | <Key>BackSpace: | delete-previous-character() |  
        | <Key>Delete: | delete-previous-character() |  
        | <Key>Return: | finish-field() |  
        | <Key>: | insert-char() |  
 
The actions which are supported by the widget are:
  - delete-previous-character
- Erases the character before the cursor.
- delete-character
- Erases the character after the cursor.
- move-backward-character
- Moves the cursor backward.
- move-forward-character
- Moves the cursor forward.
- move-to-begining
- (Apologies about the spelling error.) Moves the cursor to the beginning of
      the editable text.
- move-to-end
- Moves the cursor to the end of the editable text.
- erase-to-end-of-line
- Erases all text after the cursor.
- erase-line
- Erases the entire text.
- finish-field
- If the cursor is in the name field, proceeds to the password field; if the
      cursor is in the password field, checks the current name/password pair. If
      the name/password pair is valid, xdm starts the session. Otherwise
      the failure message is displayed and the user is prompted again.
- abort-session
- Terminates and restarts the server.
- abort-display
- Terminates the server, disabling it. This action is not accessible in the
      default configuration. There are various reasons to stop xdm on a
      system console, such as when shutting the system down, when using
      xdmshell, to start another type of server, or to generally access
      the console. Sending xdm a SIGHUP will restart the display. See the
      section Controlling XDM.
- restart-session
- Resets the X server and starts a new session. This can be used when the
      resources have been changed and you want to test them or when the screen
      has been overwritten with system messages.
- insert-char
- Inserts the character typed.
- set-session-argument
- Specifies a single word argument which is passed to the session at
      startup. See the section Session Program.
- allow-all-access
- Disables access control in the server. This can be used when the
      .Xauthority file cannot be created by xdm. Be very careful using
      this; it might be better to disconnect the machine from the network before
      doing this.
On some systems (OpenBSD) the user's shell must be listed in
    /etc/shells to allow login through xdm. The normal password and
    account expiration dates are enforced too.
The Xstartup program is run as root when the user logs in. It is
  typically a shell script. Since it is run as root, Xstartup should be
  very careful about security. This is the place to put commands which add
  entries to utmp or wtmp files, (the sessreg program may
  be useful here), mount users' home directories from file servers, or abort the
  session if logins are not allowed.
In addition to any specified by DisplayManager.exportList,
    the following environment variables are passed:
  
    | DISPLAY | the associated display name | 
  
    | HOME | the initial working directory of the user | 
  
    | LOGNAME | the user name | 
  
    | USER | the user name | 
  
    | PATH | the value of DisplayManager.DISPLAY.systemPath | 
  
    | SHELL | the value of
      DisplayManager.DISPLAY.systemShell | 
  
    | XAUTHORITY | may be set to an authority file | 
  
    | WINDOWPATH | may be set to the "window path" leading to the X server | 
No arguments are passed to the script. Xdm waits until this
    script exits before starting the user session. If the exit value of this
    script is non-zero, xdm discontinues the session and starts another
    authentication cycle.
The sample Xstartup file shown here prevents login while
    the file /etc/nologin exists. Thus this is not a complete example,
    but simply a demonstration of the available functionality.
Here is a sample Xstartup script:
#!/bin/sh
#
# Xstartup
#
# This program is run as root after the user is verified
#
if [ -f /etc/nologin ]; then
	xmessage -file /etc/nologin -timeout 30 -center
	exit 1
fi
sessreg -a -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME
/etc/X11/xdm/GiveConsole
exit 0
 
The Xsession program is the command which is run as the user's session.
  It is run with the permissions of the authorized user.
In addition to any specified by DisplayManager.exportList,
    the following environment variables are passed:
  
    | DISPLAY | the associated display name | 
  
    | HOME | the initial working directory of the user | 
  
    | LOGNAME | the user name | 
  
    | USER | the user name | 
  
    | PATH | the value of DisplayManager.DISPLAY.userPath | 
  
    | SHELL | the user's default shell (from getpwnam) | 
  
    | XAUTHORITY | may be set to a non-standard authority file | 
  
    | KRB5CCNAME | may be set to a Kerberos credentials cache name | 
  
    | WINDOWPATH | may be set to the "window path" leading to the X server | 
At most installations, Xsession should look in $HOME for a
    file .xsession, which contains commands that each user would like to
    use as a session. Xsession should also implement a system default
    session if no user-specified session exists.
An argument may be passed to this program from the authentication
    widget using the `set-session-argument' action. This can be used to select
    different styles of session. One good use of this feature is to allow the
    user to escape from the ordinary session when it fails. This allows users to
    repair their own .xsession if it fails, without requiring
    administrative intervention. The example following demonstrates this
    feature.
This example recognizes the special ``failsafe'' mode, specified
    in the translations in the Xresources file, to provide an escape from
    the ordinary session. It also requires that the .xsession file be executable
    so we don't have to guess what shell it wants to use.
#!/bin/sh
#
# Xsession
#
# This is the program that is run as the client
# for the display manager.
case $# in
1)
	case $1 in
	failsafe)
		exec xterm -geometry 80x24-0-0
		;;
	esac
esac
startup=$HOME/.xsession
resources=$HOME/.Xresources
if [ -f "$startup" ]; then
	exec "$startup"
else
	if [ -f "$resources" ]; then
		xrdb -load "$resources"
	fi
	twm &
	xman -geometry +10-10 &
	exec xterm -geometry 80x24+10+10 -ls
fi
 
The user's .xsession file might look something like this
    example. Don't forget that the file must have execute permission.
      #! /bin/csh
      # no -f in the previous line so .cshrc gets run to set $PATH
        twm &
        xrdb -merge "$HOME/.Xresources"
        emacs -geometry +0+50 &
        xbiff -geometry -430+5 &
        xterm -geometry -0+50 -ls
 
Symmetrical with Xstartup, the Xreset script is run after the user
  session has terminated. Run as root, it should contain commands that undo the
  effects of commands in Xstartup, updating entries in utmp or
  wtmp files, or unmounting directories from file servers. The
  environment variables that were passed to Xstartup are also passed to
  Xreset.
A sample Xreset script:
#!/bin/sh
#
# Xreset
#
# This program is run as root after the session ends
#
sessreg -d -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME
/etc/X11/xdm/TakeConsole
exit 0
 
Xdm controls local servers using POSIX signals. SIGHUP is expected to
  reset the server, closing all client connections and performing other cleanup
  duties. SIGTERM is expected to terminate the server. If these signals do not
  perform the expected actions, the resources
  DisplayManager.DISPLAY.resetSignal and
  DisplayManager.DISPLAY.termSignal can specify alternate
  signals.
To control remote terminals not using XDMCP, xdm searches
    the window hierarchy on the display and uses the protocol request KillClient
    in an attempt to clean up the terminal for the next session. This may not
    actually kill all of the clients, as only those which have created windows
    will be noticed. XDMCP provides a more sure mechanism; when xdm
    closes its initial connection, the session is over and the terminal is
    required to close all other connections.
Xdm responds to two signals: SIGHUP and SIGTERM. When sent a SIGHUP,
  xdm rereads the configuration file, the access control file, and the
  servers file. For the servers file, it notices if entries have been added or
  removed. If a new entry has been added, xdm starts a session on the
  associated display. Entries which have been removed are disabled immediately,
  meaning that any session in progress will be terminated without notice and no
  new session will be started.
When sent a SIGTERM, xdm terminates all sessions in
    progress and exits. This can be used when shutting down the system.
Xdm attempts to mark its various sub-processes for
    ps(1) by editing the command line argument list in place. Because
    xdm can't allocate additional space for this task, it is useful to
    start xdm with a reasonably long command line (using the full path
    name should be enough). Each process which is servicing a display is marked
    -display.
To add an additional local display, add a line for it to the Xservers
  file. (See the section Local Server Specification.)
Examine the display-specific resources in xdm-config (e.g.,
    DisplayManager._0.authorize) and consider which of them should be
    copied for the new display. The default xdm-config has all the
    appropriate lines for displays :0 and :1.
You can use xdm to run a single session at a time, using the 4.3
  init options or other suitable daemon by specifying the server on the
  command line:
xdm -server “:0 SUN-3/60CG4 local /usr/X11R7/bin/X :0”
Or, you might have a file server and a collection of X terminals.
    The configuration for this is identical to the sample above, except the
    Xservers file would look like
extol:0 VISUAL-19 foreign
exalt:0 NCD-19 foreign
explode:0 NCR-TOWERVIEW3000 foreign
This directs xdm to manage sessions on all three of these
    terminals. See the section Controlling Xdm for a description of using
    signals to enable and disable these terminals in a manner reminiscent of
    init(8).
One thing that xdm isn't very good at doing is coexisting with other
  window systems. To use multiple window systems on the same hardware, you'll
  probably be more interested in xinit.
  - /etc/X11/xdm/xdm-config
- the default configuration file
- $HOME/.Xauthority
- user authorization file where xdm stores keys for clients to
    read
- /usr/X11R7/libexec/chooser
- the default chooser
- /usr/X11R7/bin/xrdb
- the default resource database loader
- /usr/X11R7/bin/X
- the default server
- /usr/X11R7/bin/xterm
- the default session program and failsafe client
- /var/db/xdm/A<display>-<suffix>
- the default place for authorization files
- /tmp/K5C<display>
- Kerberos credentials cache
X(7), xinit(1), xauth(1), xrdb(1),
  Xsecurity(7), sessreg(1), Xserver(1), xdmshell(8),
  fonts.conf(5).
X Display Manager Control Protocol
IETF RFC 4291: IP Version 6 Addressing Architecture.
Keith Packard, MIT X Consortium