NAME

xdm − X Display Manager with support for XDMCP

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 X Consortium 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. 

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 reference page, picking and choosing the things you want to change.  Pay particular attention to the Session Program section, which describes how to set up the style of session desired. 

In handling a user’s login to the X display, xdm records the login in the /var/adm/utmp file, the same way that a normal, non-X login does.  This allows the finger and who commands to show the user logged in to the X display. 

TYPICAL USAGE

Actually, xdm is designed to operate in such a wide variety of environments that typical is probably a misnomer. 

First, the xdm configuration file should be set up.  Make a directory (usually /usr/lib/X11/xdm) to contain all of the relevant files.  Here is a reasonable configuration file, which could be named xdm-config:

   DisplayManager.errorLogFile:   /usr/lib/X11/xdm/xdm-errors
  DisplayManager.pidFile:        /usr/lib/X11/xdm/xdm-pid
  DisplayManager.keyFile:        /usr/lib/X11/xdm/xdm-keys
  DisplayManager.servers:        /usr/lib/X11/xdm/Xservers
  DisplayManager.accessFile:     /usr/lib/X11/xdm/Xaccess
  DisplayManager._0.authorize:   true
  DisplayManager._0.setup:       /usr/lib/X11/xdm/Xsetup_0
  DisplayManager._0.startup:     /usr/lib/X11/xdm/GiveConsole
  DisplayManager._0.reset:       /usr/lib/X11/xdm/TakeConsole
  DisplayManager*resources:      /usr/lib/X11/xdm/Xresources
  DisplayManager*session:        /usr/lib/X11/xdm/Xsession
  DisplayManager*authComplain:   false
 

 

Note that this file simply contains references to other files.  Note also that some of the resources are specified with “*” separating the components.  These resources can be made unique for each different display, by replacing the “*” with the display-name, but normally this is not very useful.  See the Resources section for a complete discussion. 

The first file, /usr/lib/X11/xdm/Xservers, contains the list of displays to manage that are not using XDMCP.  Most workstations have only one display, numbered 0, so the file will look something like this:

 :0 Local local /usr/bin/X11/X :0
 

This will keep /usr/bin/X11/X running on this display and manage a continuous cycle of sessions. 

The file /usr/lib/X11/xdm/xdm-errors will contain error messages from xdm and anything output to stderr by Xsetup_0, GiveConsole, Xsession or TakeConsole.  When you have trouble getting xdm working, check this file to see if xdm has any clues to the trouble. 

GiveConsole assigns ownership of the console to the user.  Here is an example GiveConsole file:

 #!/bin/sh
# Assign ownership of the console to the invoking user.
#
# By convention, both xconsole and xterm -C check that the
# console is owned by the invoking user and is readable before
# attaching the console output.  This way, a random user can
# invoke xterm -C without causing serious problems.
#
 chown $USER /dev/console
  

TakeConsole assigns ownership back to root.  Here is an example TakeConsole file:

 #!/bin/sh
# Reassign ownership of the console to root -- this
# should disallow assignment of console output to any
# random users’s xterm
#
chmod 622 /dev/console
chown root /dev/console

The next configuration entry, /usr/lib/X11/xdm/Xresources, 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. 

The most interesting script is Xsession.  It establishes the default login session for all users of the workstation.  Here is an example Xsession file:

 #!/bin/sh
 if [ -d $HOME -a -w $HOME ]
then
exec > $HOME/.xsession-errors 2>&1
else
echo "Xsession: $HOME directory not writable by $USER" \
> /dev/console
exec dxterm -geometry 80x40+0+0
# exec xterm -geometry 80x24+0+0
fi
 case $# in
1)
case $1 in
failsafe)
exec dxterm -geometry 80x40+0+0
# exec xterm -geometry 80x24+0+0
;;
esac
esac
 startup=$HOME/.xsession
resources=$HOME/.Xresources
 if [ -f $startup ]; then
if [ -x $startup ]
then
exec $startup
else
exec /bin/sh $startup
fi
else
if [ -f $resources ]; then
xrdb -load -retain $resources
fi
#
# Motif/DECWindows Version
#
dxsession
 #
# MIT/Athena Version
#
# For a MIT/Athena version,
# uncomment the following lines and comment the Motif
# lines above
   # xconsole -geometry 480x130-0-0 -daemon -notify -verbose \
#      -fn fixed -exitOnFail
# twm &
# exec xterm -geometry 80x24+10+10 -ls
fi

The preceding version of the Xsession script recognizes the special “failsafe” mode, specified in the translations in the Xresources file above, to provide an escape from the ordinary session.  Failsafe mode enables you to start a dxterm even when your Xsession or $HOME/.xsession script is faulty.  To enter failsafe mode, enter your username and password at the login prompt and then press either the F1 key or the F2 key, instead of pressing the carriage return key.  This sequence initiates a dxterm session, enabling you to edit the faulty Xsession or $HOME/.xsession file.  Also, if you do not have a login directory or if your login directory is not writable (as in the case of a login directory that belongs to someone else), failsafe mode is invoked and brings up a dxterm session to allow you to make adjustments. 

OPTIONS

All of these options, except −config, 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.  /usr/lib/X11/xdm/xdm-config is the default. 

−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 to specify configuration parameters for the authentication widget. 

−server server_entry
Specifies the value for the DisplayManager.servers resource.  See the section Server Specification for a description of this resource. 

−udpPort port_number
Specifies the value for the DisplayManager.requestPort resource.  This sets the port-number that 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. 

−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 example, DisplayManager.expo_0.startup is the name of the resource that defines the startup shell file on the “expo:0” display.  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. 

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 Server Specification for the details. 

DisplayManager.requestPort
This indicates the UDP port number that 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 that 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_0, GiveConsole, Xsession and TakeConsole 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 that 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 in which xdm stores authorization files while initializing the session.  The default value is /usr/lib/X11/xdm.

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
Not used.

DisplayManager.accessFile
To prevent unauthorized XDMCP service and to allow forwarding of XDMCP IndirectQuery requests, this file contains a database of hostnames that 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 whitespace-separated list of additional environment variables to pass on to the Xsetup_0, GiveConsole, Xsession, and TakeConsole programs. 

DisplayManager.randomFile
Not used.

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_0 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 /usr/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. /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/X11/xrdb. 

DisplayManager.DISPLAY.setup
This specifies a program that 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 (for example, you may want to run xconsole here).  By default, no program is run.  The conventional name for a file used here is Xsetup_0.  See the section Setup Program.

DisplayManager.DISPLAY.startup
This specifies a program that is run (as root) after the authentication process succeeds.  By default, no program is run. The conventional name for a file used here is GiveConsole.  See the section Startup Program. 

DisplayManager.DISPLAY.session
This specifies the session to be executed (not running as root). By default, /usr/bin/X11/xterm is run.  The conventional name is Xsession.  See the section Session Program. 

DisplayManager.DISPLAY.reset
This specifies a program that is run (as root) after the session terminates.  Again, by default no program is run. The conventional name is TakeConsole.  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 (that is, 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 that 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/X11:/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/X11:/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/X11/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 that controls whether xdm generates and uses authorization for the local server connections.  If authorization is used, authName is a whitespace-separated list of authorization mechanisms to use.  XDMCP connections dynamically specify which authorization mechanisms are supported, so authName is ignored in this case.  By default, authorize is “true”; authName is “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 that is not world-writable as it could easily be removed, disabling the authorization mechanism in the server.  If this resource is not specified, unique file names are generated and written into the directory specified by the DisplayManager.authDir resource. 

DisplayManager.DISPLAY.authComplain
Not used.

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. 

DisplayManager.Display.keymaps
Defines the default keymap that the local Xserver uses.  Maps the value of the console’s language variable to a keymap name. This resource applies only to local displays.

XDMCP ACCESS CONTROL

The database file specified by the DisplayManager.accessFile provides information that xdm uses to control access from displays requesting XDMCP service.  This file contains three types of entries:  entries that control the response to Direct and Broadcast queries, entries that 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) that 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 that 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. 

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
 #
# 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.DISPLAY.chooser resource.  Resources for this program can be put into the file named by DisplayManager.DISPLAY.resources. 

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 that should constantly be managed and that is not using XDMCP.  Each 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/X11/X :0
 

The display types are:

 locallocal display: xdm must run the server
foreignremote 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 local /usr/bin/X11/X :0” instead of “localhost:0 local /usr/bin/X11/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 (like 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 that it generates for that device, which will include the class string. 

SETUP PROGRAM

The program named in the DisplayManager.DISPLAY.setup resource 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:

 DISPLAYthe associated display name
PATHthe value of DisplayManager.DISPLAY.systemPath
SHELLthe value of DisplayManager.DISPLAY.systemShell
XAUTHORITYmay 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_0 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.  xlogin.Login.y"

STARTUP PROGRAM

The program specified by the DisplayManager.DISPLAY.startup resource is typically a shell script.  It is run as root and should be very careful about security.  This is the place to put commands that add entries to /etc/utmp, mount users’ home directories from file servers, display the message of the day, or abort the session if logins are not allowed. 

In addition to any specified by DisplayManager.exportList, the following environment variables are passed:

 DISPLAYthe associated display name
HOMEthe initial working directory of the user
USERthe user name
PATHthe value of DisplayManager.DISPLAY.systemPath
SHELLthe value of DisplayManager.DISPLAY.systemShell
XAUTHORITYmay 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. 

SESSION PROGRAM

The Xsession program (specified by the DisplayManager.DISPLAY.session resource) is the command that 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:

 DISPLAYthe associated display name
HOMEthe initial working directory of the user
USERthe user name
PATHthe value of DisplayManager.DISPLAY.userPath
SHELLthe user’s default shell (from getpwnam)
XAUTHORITYmay be set to a non-standard authority file
 

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.  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 requiring administrative intervention.  The section Typical Usage demonstrates this feature. 

RESET PROGRAM

Symmetrical with the startup program, the program specified by the DisplayManager.DISPLAY.startup resource is run after the user session has terminated.  Run as root, it should contain commands that undo the effects of commands in the startup program, removing entries from /etc/utmp or unmounting directories from file servers.  The environment variables that were passed to the startup program are also passed to the program specified by the DisplayManager.DISPLAY.startup resource. 

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 that 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 that is servicing a display is marked −display. 

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. 

FILES

/usr/lib/X11/xdm/xdm-config
the default configuration file

/usr/lib/X11/xdm/Xaccess
the default access file, listing authorized displays

/usr/lib/X11/xdm/Xservers
the default server file, listing non-XDMCP servers to manage

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

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

/usr/bin/X11/xrdb the default resource database loader

/usr/bin/X11/X the default server

/usr/bin/X11/xterm the default session program and failsafe client

/usr/lib/X11/xdm/A<host>−<suffix>
the default place for authorization files

SEE ALSO

X(1X), xauth(1X), Xsecurity(1X), and XDMCP

COPYRIGHT

Copyright 1988, Massachusetts Institute of Technology. 
See X(1X) for a full statement of rights and permissions.

AUTHOR

Keith Packard, MIT X Consortium