man pages section 1: User Commands

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Updated: July 2014

xdm (1)


xdm - X Display Manager with support for XDMCP, host chooser


/usr/sbin/xdm [ -config configuration_file ] [ -nodaemon ] [
-debug  debug_level ] [ -error error_log_file ] [ -resources
resource_file ] [ -server server_entry  ]  [  -session  ses-
sion_program ]


User Commands                                              XDM(1)

     xdm - X Display Manager with support for XDMCP, host chooser

     /usr/sbin/xdm [ -config configuration_file ] [ -nodaemon ] [
     -debug  debug_level ] [ -error error_log_file ] [ -resources
     resource_file ] [ -server server_entry  ]  [  -session  ses-
     sion_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 stan-
     dard 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 ``ses-

     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 necessar-
     ily  have any terminal-like interface with which to connect.
     When a real session manager is not available, a window  man-
     ager or terminal emulator is typically used as the ``session
     manager,'' meaning that termination of this  process  termi-
     nates 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 man-
     agement.  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 them-

     Because xdm provides the first  interface  that  users  will
     see,  it  is  designed  to be simple to use and easy to cus-
     tomize 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

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     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 speci-
     fied 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

     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, Xses-
     sion 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

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User Commands                                              XDM(1)

     -config configuration_file
          Names the configuration file, which specifies resources
          to  control the behavior of xdm.  /etc/X11/xdm/xdm-con-
          fig is the  default.   See  the  section  Configuration

          Specifies  ``false''  as  the value for the DisplayMan-
          ager.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   DisplayMan-
          ager.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  inter-
          pret  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

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          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 sin-
     gle 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

     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
     ``'' display.

          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.

          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

          Error  output  is  normally directed at the system con-
          sole.  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-

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          independent implementation.  This  file  also  contains
          any  output directed to stderr by the Xsetup, Xstartup,
          Xsession and Xreset files, so it will contain  descrip-
          tions of problems in those scripts as well.

          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.

          Normally,  xdm  attempts  to  make itself into a daemon
          process unassociated with any terminal.  This is accom-
          plished  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.

          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.

          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.

          This names a directory under which  xdm  stores  autho-
          rization  files  while  initializing  the session.  The
          default value is  /var/run/xdm.  Can be overridden  for
          specific displays by DisplayManager.DISPLAY.authFile.

          This  boolean controls whether xdm rescans the configu-
          ration, 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

          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

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          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.''

          XDM-AUTHENTICATION-1   style    XDMCP    authentication
          requires  that  a private key be shared between xdm and
          the terminal.  This resource specifies  the  file  con-
          taining  those values.  Each entry in the file consists
          of a display name and the shared key.  By default,  xdm
          does  not  include support for XDM-AUTHENTICATION-1, as
          it requires DES which is  not  generally  distributable
          because of United States export restrictions.

          To prevent unauthorized XDMCP service and to allow for-
          warding of XDMCP IndirectQuery requests, this file con-
          tains  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 Con-

          A  list  of additional environment variables, separated
          by white space, to pass on  to  the  Xsetup,  Xstartup,
          Xsession, and Xreset programs.

          A  file  to checksum to generate the seed of authoriza-
          tion keys.  This should be a  file  that  changes  fre-
          quently.  The default is /dev/mem.

          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.


          A  UNIX  domain socket name or a TCP socket port number
          on local host on which a Pseudo-Random Number Generator
          Daemon,  like  EGD ( is lis-
          tening, in order to  generate  the  autorization  keys.
          Either  a  non null port or a valid socket name must be

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          specified. The default is to use the Unix-domain socket

     On  systems  that  don't  have  such  a  daemon, a fall-back
     entropy gathering system, based on various log file contents
     hashed by the MD5 algorithm is used instead.

          On  systems that support a dynamically-loadable greeter
          library, the name of the library.  The default is

          Number of seconds to wait for display to respond  after
          user has selected a host from the chooser.  If the dis-
          play 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.

          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.

          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  out-
          put  of this program may be displayed on a chooser win-
          dow.  If no program is specified, the string Willing to
          manage is sent.

          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.

          Specifies  the  program  run  to  offer a host menu for
          Indirect queries redirected to the  special  host  name

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           /usr/lib/X11/xdm/chooser   is  the  default.   See the
          sections XDMCP Access Control and Chooser.

          Specifies the program used to load the  resources.   By
          default, xdm uses  /usr/bin/xrdb.

          This  specifies the name of the C preprocessor which is
          used by xrdb.

          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.

          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.

          This  specifies the session to be executed (not running
          as root).  By default,   /usr/bin/xterm  is  run.   The
          conventional name is Xsession.  See the section Session

          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




          These  numeric  resources  control  the behavior of xdm
          when attempting to open intransigent servers.   openDe-
          lay  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 max-
          imum  time  spent  in  the  connect(2) system call) and

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          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 termi-
          nates and restarts the server,  attempting  to  connect
          again.   This  process is repeated startAttempts times,
          at which point the display is declared  dead  and  dis-
          abled.   Although  this behavior may seem arbitrary, it
          has been empirically developed and works quite well  on
          most  systems.  The default values are 5 for openDelay,
          5 for openRepeat, 30 for openTimeout and 4 for startAt-


          To  discover  when remote displays disappear, xdm occa-
          sionally 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 ter-
          minated.  By default, both are set to  5  minutes.   If
          you  frequently  use  X terminals which can become iso-
          lated from the managing host, you may wish to  increase
          this  value.  The only worry is that sessions will con-
          tinue 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 work-
          station session is terminated as a result of the server
          hanging for NFS service and not responding to the ping.

          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.''

          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 ``/usr/bin:/usr/sbin''.

          Xdm  sets the PATH environment variable for the startup
          and reset scripts to the value of this  resource.   The
          default  for  this  resource is ``/usr/bin:/usr/sbin''.
          Note the absence of ``.'' from this entry.  This  is  a

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          good practice to follow for root; it avoids many common
          Trojan Horse system penetration schemes.

          Xdm sets the SHELL environment variable for the startup
          and reset scripts to the value of this resource.  It is
          /bin/sh by default.

          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 sec-
          tion Session Program).  By default,  /usr/bin/xterm  is


          To  improve security, xdm grabs the server and keyboard
          while reading the login name and password.   The  grab-
          Server  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 max-
          imum  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.


          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 auth-
          Name 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  dis-
          played  in  the login widget.  By default, authorize is
          ``true,''  authName is ``MIT-MAGIC-COOKIE-1,''  or,  if
          XDM-AUTHORIZATION-1   is   available,  ``XDM-AUTHORIZA-
          TION-1 MIT-MAGIC-COOKIE-1.''

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          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, dis-
          abling the authorization mechanism in the  server.   If
          not  specified,  a  name  is generated from DisplayMan-
          ager.authDir and the name of the display.

          If set to ``false,'' disables the use of the  unsecure-
          Greeting  in the login window.  See the section Authen-
          tication Widget.  The default is ``true.''

          The number of the signal xdm sends to reset the server.
          See the section Controlling the Server.  The default is
          1 (SIGHUP).

          The number of the signal xdm  sends  to  terminate  the
          server.   See  the section Controlling the Server.  The
          default is 15 (SIGTERM).

          The original implementation  of  authorization  in  the
          sample  server  reread the authorization file at server
          reset time, instead of when checking the  initial  con-
          nection.   As  xdm generates the authorization informa-
          tion just before connecting  to  the  display,  an  old
          server  would not get up-to-date authorization informa-
          tion.  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 autho-
          rization  information  will  be  read.   The default is
          ``false,'' which will work for all MIT servers.

          When xdm is unable to write to the  usual  user  autho-
          rization  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 rel-
     evant files.

     Here is a reasonable  configuration  file,  which  could  be
     named xdm-config:

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          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/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  discus-

     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 com-
     parison, so ensure that you do not attempt to match aliases.
     Preceding either a host name or a pattern with a `!' charac-
     ter 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 dis-
     tinguish macros from hostnames, macro names start with a `%'
     character.  Macros may be nested.

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     Indirect entries may also specify to have xdm run chooser to
     offer a menu of  hosts  to  connect  to.   See  the  section

     When  checking  access  for  a particular display host, each
     entry is scanned in turn and the first matching entry deter-
     mines  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 delim-
     iter causing the rest of that line to be ignored, and `\new-
     line'  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

     !   # disallow direct/broadcast service for xtra       # allow access from this particular display
     *       # allow access from any display in LCS

     *        NOBROADCAST         # allow only direct access
     *                                # allow direct and broadcast

     # Indirect query entries

     %HOSTS       #force extract to contact xenon
     !   dummy               #disallow indirect access
     *       %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 spec-
     ified, 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.


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User Commands                                              XDM(1) ff02::1                 #IPv6 Multicast to ff02::1
                                                  #with a hop count of 1    CHOOSER  #Offer a menu of hosts
                                                  #who respond to IPv4 Multicast
                                                  # to 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:  CHOOSER %HOSTS          #offer a menu of these hosts     CHOOSER BROADCAST       #offer a menu of all hosts

     The  program to use for chooser is specified by the Display-
     Manager.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

     When the user selects a host, chooser prints the  host  cho-
     sen, 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 ses-
     sion 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 addresss representing
          a network interface on this machine, or the wildcard  *
          to represent all available network interfaces.

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User Commands                                              XDM(1)

     If  one or more LISTEN lines are specified, xdm only listens
     for XDMCP connections on the specified interfaces. If multi-
     cast  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  lis-
     tening on all interfaces is preserved for backwards compati-
     bility.  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 previ-
     ously supported method of setting DisplayManager.requestPort
     to 0 may be used.

     LISTEN * ff02::1    # Listen on all interfaces and to the
                         # ff02::1 IPv6 multicast group.
     LISTEN  # 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 Inter-
     face-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 specifi-
     cation 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 con-
     stantly 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 dis-
     play 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/bin/X :0

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User Commands                                              XDM(1)

     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/bin/X
     :0'' instead of ``localhost:0 Sun-CG3 local /usr/bin/X  :0''
     if  your  other  resources  are  specified  as ``DisplayMan-
     ager._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 file-
     name'' 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

     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 param-
     eters for that widget:

          xlogin*login.translations: #overrideCtrl<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

     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

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User Commands                                              XDM(1)

     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 secu-
     rity.   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  poten-
     tial  security  holes here.  If DisplayManager.DISPLAY.grab-
     Server 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:

          # 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  con-
     trolled  with  a resource.  Resources for this widget should
     be  put  into  the   file   named   by   DisplayManager.DIS-
     PLAY.resources.   All  of these have reasonable default val-
     ues, 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

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User Commands                                              XDM(1)

     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(4).  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(5).

     xlogin.Login.width,   xlogin.Login.height,  xlogin.Login.x,
          The  geometry  of the Login widget is normally computed
          automatically.  If you wish to position  it  elsewhere,
          specify each of these resources.

          The  color used to display the input typed by the user.

          The face used to display the input typed  by  the  user
          when   built   with   Xft   support.   The  default  is

          The font used to display the input typed  by  the  user
          when not built with Xft support.

          A  string which identifies this window.  The default is
          ``X Window System.''

          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''

          The  face  used to display the greeting when built with
          Xft support.  The default is ``Serif-24:italic''.

          The font used to display the greeting  when  not  built
          with Xft support.

          The color used to display the greeting.

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User Commands                                              XDM(1)

          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:  ''

          The string displayed to prompt for a password, when not
          using an authentication system such as  PAM  that  pro-
          vides its own prompts.  The default is ``Password:  ''

          The  face  used  to display prompts when built with Xft
          support.  The default is ``Serif-18:bold''.

          The font used to display prompts when  not  built  with
          Xft support.

          The color used to display prompts.

          A  message  which  is displayed when the users password
          has  expired.   The  default   is   ``Password   Change
          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''

          The face used to display the failure message when built
          with Xft support.  The default is ``Serif-18:bold''.

          The  font  used to display the failure message when not
          built with Xft support.

          The color used to display the failure message.

          The number of seconds that the failure message is  dis-
          played.  The default is 10.

          Name  of an XPM format pixmap to display in the greeter
          window, if built with XPM support.   The default is  no

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User Commands                                              XDM(1)


          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.

          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

     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.

          frameWidth is the width in pixels of  the  area  around
          the greeter frame drawn in hiColor and shdColor.

          innerFramesWidth  is  the  width  in pixels of the area
          around text input areas drawn in hiColor and  shdColor.

          sepWidth  is  the  width  in pixels of the bezeled line
          between the greeting and input areas drawn  in  hiColor
          and shdColor.

          If  set  to  ``false'', don't allow root (and any other
          user with uid = 0) to log in directly.  The default  is

          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.

          This specifies the translations used for the login wid-
          get.   Refer  to the X Toolkit documentation for a com-
          plete  discussion   on   translations.    The   default

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User Commands                                              XDM(1)

          translation table is:

               Ctrl<Key>H:    delete-previous-character() \nCtrl<Key>D:delete-character() \nCtrl<Key>B:move-backward-character() \nCtrl<Key>F:move-forward-character() \nCtrl<Key>A:move-to-begining() \nCtrl<Key>E:move-to-end() \nCtrl<Key>K:erase-to-end-of-line() \nCtrl<Key>U:erase-line() \nCtrl<Key>X:erase-line() \nCtrl<Key>C:restart-session() \nCtrl<Key>\\:abort-session() \n<Key>BackSpace:delete-previous-character() \n<Key>Delete:delete-previous-character() \n<Key>Return:finish-field() \n<Key>:insert-char() .fi

     The actions which are supported by the widget are:

          Erases the character before the cursor.

          Erases the character after the cursor.

          Moves the cursor backward.

          Moves the cursor forward.

          (Apologies about the spelling error.)
          Moves the cursor to the beginning of the editable text.

          Moves the cursor to the end of the editable text.

          Erases all text after the cursor.

          Erases the entire text.

          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.

          Terminates and restarts the server.

          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.

          Resets the X server and starts a new session.  This can be used when

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User Commands                                              XDM(1)

          the resources have been changed and you want to test them or when
          the screen has been overwritten with system messages.

          Inserts the character typed.

          Specifies a single word argument which is passed to the session at startup.
          See the section Session Program.

          Disables access control in the server.  This can be used when
          the .Xauthority file cannot be created by
          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
     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 ses-
     sion 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  ses-
     sion 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

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User Commands                                              XDM(1)

     example,  but  simply a demonstration of the available func-

     Here is a sample Xstartup script:

          # 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
          sessreg -a -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME
          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 authenti-
     cation 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  adminis-
     trative  intervention.   The  example following demonstrates
     this feature.

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User Commands                                              XDM(1)

     This example recognizes the special ``failsafe'' mode, spec-
     ified 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.

          # Xsession
          # This is the program that is run as the client
          # for the display manager.

          case $# in
               case $1 in
                    exec xterm -geometry 80x24-0-0


          if [ -f "$startup" ]; then
               exec "$startup"
               if [ -f "$resources" ]; then
                    xrdb -load "$resources"
               twm &
               xman -geometry +10-10 &
               exec xterm -geometry 80x24+10+10 -ls

     The user's .xsession file might  look  something  like  this
     example.   Don't forget that the file must have execute per-
          #! /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

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User Commands                                              XDM(1)

     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

     A sample Xreset script:
          # Xreset
          # This program is run as root after the session ends
          sessreg -d -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME
          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.DIS-
     PLAY.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 imme-
     diately, meaning that any session in progress will be termi-
     nated 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

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User Commands                                              XDM(1)

     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  Specifica-

     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-con-
     fig 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/bin/X :0"

     Or, you might have a file server and a collection of X  ter-
     minals.  The configuration for this is identical to the sam-
     ple 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 descrip-
     tion of using signals to enable and disable these  terminals
     in a manner reminiscent of init(1m).

     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

                         the default configuration file

     $HOME/.Xauthority   user authorization file where xdm stores
                         keys for clients to read

                         the default chooser

     /usr/bin/xrdb       the default resource database loader

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User Commands                                              XDM(1)

     /usr/bin/X          the default server

     /usr/bin/xterm      the default session program and failsafe

                         the  default  place  for   authorization

     /tmp/K5C<display>   Kerberos credentials cache

     X(5), xinit(1), xauth(1), xrdb(1), Xsecurity(5), sessreg(1),
     Xserver(1), fonts.conf(4).
     X Display Manager Control Protocol
     IETF RFC 4291: IP Version 6 Addressing Architecture.

     Keith Packard, MIT X Consortium

     See  attributes(5)  for  descriptions   of   the   following

     |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
     |Availability                 |system/display-manager/xdm   |
     |Interface Stability          |Committed                    |

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