XDM(1)

Contents

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 ter-
       minals  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 man-
       ager 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  (option-
       ally) 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 speci-
       fied 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  them-
       selves 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 pro-
       gram 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  manage-
       ment.

       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 /usr/X11R6/lib/X11/xdm/xdm-errors 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 con-
              trol the behavior of xdm.  <XRoot>/lib/X11/xdm/xdm-config is the
              default.  See the section Configuration File.

       -nodaemon
              Specifies  ``false'' as the value for the DisplayManager.daemon-
              Mode 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  debug-
              ging  statements  to the terminal; it also disables the Display-
              Manager.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  any-
              thing  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 to specify configuration parame-
              ters 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 mod-
       ify its behavior on a single display.  Where actions relate to  a  spe-
       cific  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 debug-
              ging).  Setting this resource to  ``false''  will  disable  this
              feature.

       DisplayManager.pidFile
              The  filename specified will be created to contain an ASCII rep-
              resentation 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 lock-
              ing 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
              <XRoot>/lib/X11/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 for-
              warded 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.greeterLib
              On  systems that support a dynamically-loadable greeter library,
              the name of the library.  Default is <XRoot>/lib/X11/xdm/libXdm-
              Greet.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 multi-
              homed 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 dis-
              played  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 vari-
              ous resources that are appropriate to place in this file.  There
              is    no    default    value    for    this    resource,     but
              <XRoot>/lib/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.
              <XRoot>/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 <XRoot>/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 pro-
              gram 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, <XRoot>/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  ses-
              sion  terminates.   By  default, no program is run.  The conven-
              tional 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 star-
              tAttempts 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 devel-
              oped  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, ping-
              Timeout specifies the maximum amount of  time  (in  minutes)  to
              wait  for the terminal to respond to the request.  If the termi-
              nal does not respond, the session is declared  dead  and  termi-
              nated.   By  default,  both  are  set to 5 minutes.  If you fre-
              quently use X terminals which can become isolated from the  man-
              aging 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 work-
              station  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/X11R6/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/X11R6/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,
              <XRoot>/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 gen-
              erates 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 sup-
              ported, 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 Dis-
              playManager.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 gener-
              ates 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 informa-
              tion  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 <XRoot>/lib/X11/xdm, where <XRoot> refers to the root  of  the
       X11  install  tree) to contain all of the relevant files.  In the exam-
       ples that follow, we use /usr/X11R6 as the value of <XRoot>.

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


            DisplayManager.servers:            /usr/X11R6/lib/X11/xdm/Xservers
            DisplayManager.errorLogFile:       /usr/X11R6/lib/X11/xdm/xdm-errors
            DisplayManager*resources:          /usr/X11R6/lib/X11/xdm/Xresources
            DisplayManager*startup:            /usr/X11R6/lib/X11/xdm/Xstartup
            DisplayManager*session:            /usr/X11R6/lib/X11/xdm/Xsession
            DisplayManager.pidFile:            /usr/X11R6/lib/X11/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 dis-
       play, by replacing the ``*'' with the display-name, but  normally  this
       is  not  very useful.  See the Resources section for a complete discus-
       sion.

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 pat-
       terns, 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 fol-
       lowed 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

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.DIS-
       PLAY.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  Display-
       Manager.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  sec-
       onds)  and forwards the request to the chosen host, which starts a ses-
       sion on that display.

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  man-
       aged  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/X11R6/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/X11R6/bin/X :0'' instead of ``localhost:0 Sun-
       CG3 local /usr/X11R6/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 dis-
       play class, so the manual for your particular X terminal  should  docu-
       ment  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  gener-
       ates 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: #override\
                 Ctrl<Key>R: abort-display()\n\
                 <Key>F1: set-session-argument(failsafe) finish-field()\n\
                 <Key>Return: set-session-argument() finish-field()
            xlogin*borderWidth: 3
            xlogin*greeting: CLIENTHOST
            #ifdef COLOR
            xlogin*greetColor: CadetBlue
            xlogin*failColor: red
            #endif


       Please note the translations entry; it specifies a few new translations
       for the widget which allow users to escape  from  the  default  session
       (and  avoid  troubles that may occur in it).  Note that if #override is
       not specified, the default translations are removed and replaced by the
       new value, not a very useful result as some of the default translations
       are quite useful (such as ``<Key>: insert-char ()'' which  responds  to
       normal typing).

       This file may also contain resources for the setup program and chooser.

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  follow-
       ing 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 Display-
       Manager.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 < /usr/X11R6/lib/monitors/alex.0
            xconsole -geometry 480x130-0-0 -notify -verbose -exitOnFail &

AUTHENTICATION WIDGET

       The authentication widget reads a name/password pair 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 Display-
       Manager.DISPLAY.resources.   All  of these have reasonable default val-
       ues, so it is not necessary to specify any of them.

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

       xlogin.Login.foreground
              The color used to display the typed-in user name.

       xlogin.Login.font
              The font used to display the typed-in user name.

       xlogin.Login.greeting
              A string which identifies this window.  The default is ``X  Win-
              dow 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 ses-
              sion''

       xlogin.Login.greetFont
              The font used to display the greeting.

       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.  The default is
              ``Password:  ''

       xlogin.Login.promptFont
              The font used to display both prompts.

       xlogin.Login.promptColor
              The color used to display both prompts.

       xlogin.Login.fail
              A message which is displayed when the authentication fails.  The
              default is ``Login incorrect''

       xlogin.Login.failFont
              The font used to display the failure message.

       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 30.

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


       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.

       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  cur-
              rent  name/password  pair.   If the name/password pair is valid,
              xdm starts the session.  Otherwise the failure message  is  dis-
              played and the user is prompted again.

       abort-session
              Terminates and restarts the server.

       abort-display
              Terminates  the server, disabling it.  This action is not acces-
              sible in the default configuration.  There are  various  reasons
              to  stop xdm on a system console, such as when shutting the sys-
              tem 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 typi-
       cally 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 follow-
       ing 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


       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 /usr/X11R6/lib/xdm/Xservers $LOGNAME
            /usr/X11R6/lib/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  follow-
       ing 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


       At  most installations, Xsession should look in $HOME for a file .xses-
       sion, 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.  See the section Typical Usage.

       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 requir-
       ing  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  exe-
       cutable 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 ses-
       sion 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 /usr/X11R6/lib/xdm/Xservers $LOGNAME
            /usr/X11R6/lib/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 DisplayMan-
       ager.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 win-
       dows  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 dis-
       abled immediately, 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 addi-
       tional 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., DisplayMan-
       ager._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 com-
       mand line:

            xdm -server ":0 SUN-3/60CG4 local /usr/X11R6/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.

FILES

       <XRoot>/lib/X11/xdm/xdm-config
                           the default configuration file

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

       <XRoot>/lib/X11/xdm/chooser
                           the default chooser

       <XRoot>/bin/xrdb    the default resource database loader

       <XRoot>/bin/X       the default server

       <XRoot>/bin/xterm   the default session program and failsafe client

       <XRoot>/lib/X11/xdm/A<display>-<suffix>
                           the default place for authorization files

       /tmp/K5C<display>   Kerberos credentials cache

       Note: <XRoot> refers to the root of the X11 install tree.

SEE ALSO

       X(7x), xinit(1), xauth(1), Xsecurity(7x), sessreg(1), Xserver(1),
       X Display Manager Control Protocol

AUTHOR

       Keith Packard, MIT X Consortium



X Version 11                      Release 6.6                           XDM(1)

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