Provided by: xdm_1.1.8-5_i386 bug

NAME

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

SYNOPSIS

       xdm [ -config configuration_file ] [ -nodaemon ] [ -debug debug_level ]
       [ -error error_log_file  ]  [  -resources  resource_file  ]  [  -server
       server_entry ] [ -session session_program ]

DESCRIPTION

       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.

OVERVIEW

       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.

OPTIONS

       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.  As XDMCP uses the registered well-known UDP port 177,
              this resource should not be changed except for debugging. If set
              to 0 xdm will not listen for XDMCP or Chooser requests.

       -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.

RESOURCES

       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/lib/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.   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.

       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.randomFile
              A file to checksum to generate the seed of  authorization  keys.
              This  should  be a file that changes frequently.  The default is
              /dev/mem.

       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.prngdSocket

       DisplayManager.prngPort
              A UNIX domain socket name or a TCP socket port number  on  local
              host  on which a Pseudo-Random Number Generator Daemon, like EGD
              (http://egd.sourceforge.net) is listening, in order to  generate
              the  autorization keys. Either a non null port or a valid socket
              name must be specified. The default is to  use  the  Unix-domain
              socket /tmp/entropy.

       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.

       DisplayManager.greeterLib
              On  systems that support a dynamically-loadable greeter library,
              the name of the library.  The default is
               /etc/X11/xdm/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/lib/X11/xdm/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/bin/xrdb.

       DisplayManager.DISPLAY.cpp
              This specifies the name of the C preprocessor which is  used  by
              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/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
              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  default
              values are 5 for openDelay, 5 for openRepeat, 30 for openTimeout
              and 4 for startAttempts.

       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.
              ‘‘:/bin:/usr/bin:/usr/bin:/usr/ucb’’ is a common  setting.   The
              default  value  can  be  specified at build time in the X system
              configuration file with DefaultUserPath.

       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 specified at build  time  by  the  DefaultSystemPath
              entry      in      the      system      configuration      file;
              ‘‘/etc:/bin:/usr/bin:/usr/bin:/usr/ucb’’  is  a  common  choice.
              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/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.

CONFIGURATION FILE

       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.

XDMCP ACCESS CONTROL

       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

CHOOSER

       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.

LISTEN

       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.

       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.

IPv6 MULTICAST ADDRESS SPECIFICATION

       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 Node-Local, 2 for Link-Local, 5 for
       Site-Local, and so on.  (See IETF  RFC  2373  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.

LOCAL SERVER SPECIFICATION

       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/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/bin/X :0’’  instead  of  ‘‘localhost:0  Sun-CG3
       local  /usr/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.

RESOURCES FILE

       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: #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
            #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.

SETUP PROGRAM

       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 &

AUTHENTICATION WIDGET

       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’’.

       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.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() \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:

       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.

STARTUP PROGRAM

       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  /etc/utmp  (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

SESSION PROGRAM

       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

RESET PROGRAM

       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,  removing  entries  from
       /etc/utmp 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

CONTROLLING THE SERVER

       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.

CONTROLLING XDM

       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.

ADDITIONAL LOCAL DISPLAYS

       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.

OTHER POSSIBILITIES

       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 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).

LIMITATIONS

       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.

ASYNCHRONOUS EVENTS

       xdm uses SIGALRM and SIGUSR1 for its  own  inter-process  communication
       purposes,  managing the relationship between the parent xdm process and
       its children.  Sending these signals to any xdm process may  result  in
       unexpected behavior.

       SIGHUP causes  xdm to rescan its configuration files and reopen its log
              file.

       SIGTERM
              causes xdm to terminate its children and shut down.

       SIGUSR2
              causes xdm to reopen its  log  file.   This  is  useful  if  log
              rotation is desired, but SIGHUP is too disruptive.

FILES

       /etc/X11/xdm/xdm-config
                           the default configuration file

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

       /usr/lib/X11/xdm/chooser
                           the default chooser

       /usr/bin/xrdb       the default resource database loader

       /usr/bin/X          the default server

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

       /var/lib/xdm/authdir/authfiles/A<display>-<suffix>
                           the default place for authorization files

       /tmp/K5C<display>   Kerberos credentials cache

SEE ALSO

       X(7),   xinit(1),   xauth(1),   xrdb(1),   Xsecurity(7),    sessreg(1),
       Xserver(1), fonts.conf(5).
       X Display Manager Control Protocol

AUTHOR

       Keith Packard, MIT X Consortium