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xdm

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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]


 

OPTIONS

All of these options, except -config itself, specify values that can also be specified in the configuration file as resources. Names the configuration file, which specifies resources to control the behavior of xdm. <XRoot>/lib/X11/xdm/xdm-config is the default. See the section CONFIGURATION FILE. 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. 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. 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. Specifies the value for the DisplayManager*resources resource. This file is loaded using xrdb to specify configuration parameters for the authentication widget. Specifies the value for the DisplayManager.servers resource. See the section SERVER SPECIFICATION for a description of this resource. 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. Specifies the value for the DisplayManager*session resource. This indicates the program to run as the session after the user has logged in. Allows an arbitrary resource to be specified, as in most X Toolkit applications.
 

DESCRIPTION

The xdm program 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 will describe 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/var/X11/xdmr or /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/var/X11/xdm/xdm-errors DisplayManager.pidFile: /usr/var/X11/xdm/xdm-pid DisplayManager.keyFile: /usr/var/X11/xdm/xdm-keys DisplayManager.servers: /usr/var/X11/xdm/Xservers DisplayManager.accessFile: /usr/var/X11/xdm/Xaccess DisplayManager.greeterLib: /usr/shlib/X11/libXdmDecGreet.so DisplayManager._0.authorize: true DisplayManager._0.authName: \
          XDM-AUTHORIZATION-1 MIT-MAGIC-COOKIE-1 DisplayManager._0.setup: /usr/var/X11/xdm/Xsetup_0 DisplayManager._0.startup: /usr/var/X11/xdm/GiveConsole DisplayManager._0.reset: /usr/var/X11/xdm/TakeConsole DisplayManager.local_0.authorize: true DisplayManager.local_0.authName: \
          XDM-AUTHORIZATION-1 MIT-MAGIC-COOKIE-1 DisplayManager.local_0.setup: /usr/var/X11/xdm/Xsetup_0 DisplayManager.local_0.startup: /usr/var/X11/xdm/GiveConsole DisplayManager.local_0.reset: /usr/var/X11/xdm/TakeConsole DisplayManager*resources: /usr/var/X11/xdm/Xresources DisplayManager*session: /usr/var/X11/xdm/Xsession DisplayManager*authComplain: false DisplayManager*chooser: /usr/bin/X11/chooser DisplayManager*keymaps: /usr/var/X11/xdm/Xkeymaps !DisplayManager*language: C

Note that this file mainly 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/var/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/var/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.
      #
      # However, don't give up ownership of the console if the
      # alternate console is in use, that is, if the graphics
      # display device is not the console.
      #
      case `/usr/sbin/sizer -wc` in
      1)
        ;;
      *)
        chown $USER /dev/console
        ;;
      esac

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. Since 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.

The file /usr/var/X11/xdm/Xkeymaps defines the keymaps that are loaded into the Xserver for the various languages and keyboards. These keymaps are loaded by the Xsetup_0 script using the xmodmap command. The table in the following file defines the correspondence between the value of the console's language variable, the keyboard types, and the default keymaps loaded into the Xserver:

# # This file defines the language-keymap mapping # # # The first line contains the name of the # link to be created to the default keymap. # /usr/var/X11/xdm/keymap_default # # This is the directory where the keymap files are to be found. # /usr/lib/X11/keymaps/ # # The following lines must contain: # <number> <language> <keymap-filename> # # The <number> field is a 2-byte hex value where the first byte # represents the keyboard type and the second byte is the value of the # console's language variable. The values for the keyboard types are: # LK401 0 # PCXAL 1 # LK201 2 # LK421 3 # LK443/4 4 # LK411 5 # # Don't put any 8-bit characters in the language names or the # isspace() function used in parsing this may think they're spaces # causing the lines to be parsed incorrectly. # # If the <keymap-filename> field is blank, this has the special # meaning that no keymap_default link will be created, nor will any # existing keymap_default be modified. # # The keymap specified for the "fallback" lines is used for any # language value missing from the table for the corresponding # keyboard type. # 000 fallback us_lk401aa.keymap 030 Dansk danish_lk401ad_tw.keymap 032 Deutsch austrian_german_lk401ag.keymap 034 Deutsch(Schweiz) swiss_german_lk401al_tw.keymap 036 English(American) us_lk401aa.keymap 038 English(British/Irish) uk_lk401aa.keymap 03a Espanol spanish_lk401as_tw.keymap 03c Francais belgian_french_lk401ap_tw.keymap 03e Francais(Canadien) canadian_french_lk401ac_tw.keymap 040 Francais(SuisseRomande) swiss_french_lk401ak_tw.keymap 042 Italiano italian_lk401ai_tw.keymap 044 Nederlands dutch_us_lk401ah.keymap 046 Norsk norwegian_lk401an_tw.keymap 048 Portugues portuguese_lk401av.keymap 04a Suomi finnish_lk401af_tw.keymap 04c Svenska swedish_lk401am_tw.keymap 04e Vlaams flemish_lk401ab_tw.keymap

100 fallback us_pcxalka.keymap 130 Dansk danish_pcxalkd.keymap 132 Deutsch austrian_german_pcxalkg.keymap
        .
        .
        . 14c Svenska swedish_pcxalma.keymap 14e Vlaams belgian_pcxalkb.keymap

200 fallback us_lk201re.keymap 230 Dansk danish_lk201ld_tw.keymap * 232 Deutsch austrian_german_lk201lg_tw.keymap *
        .
        .
        . 24c Svenska swedish_lk201lm_tw.keymap * 24e Vlaams flemish_lk201lb_tw.keymap

300 fallback us_lk421aa.keymap 336 English(American) us_lk421aa.keymap 338 English(British/Irish) uk_lk421aa.keymap

400 fallback us_lk443aa.keymap 430 Dansk danish_lk444kd.keymap 432 Deutsch austrian_german_lk444kg.keymap
        .
        .
        . 44c Svenska swedish_lk444ma.keymap
        .
        .
        . 500 fallback us_lk411aa.keymap 530 Dansk danish_lk411ad.keymap 532 Deutsch austrian_german_lk411ag.keymap
        .
        .
        . 54c Svenska swedish_lk411am.keymap 54e Vlaams belgian_lk411ab.keymap


 

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. 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. This resource 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. 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, GiveConsole, Xsession and TakeConsole files, so it will contain descriptions of problems in those scripts as well. If the integer value of this resource is greater than zero, reams of debugging information will be printed. It also disables daemon mode, which would redirect the information into the bit-bucket, and allows non-root users to run xdm, which would normally not be useful. Normally, xdm attempts to make itself into a daemon process unassociated with any terminal. This is 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. The filename specified will be created to contain an ASCII representation of the process-id of the main xdm process. xdm also uses file locking on this file to attempt to eliminate multiple daemons running on the same machine, which would cause quite a bit of havoc. This is the resource which controls whether xdm uses file locking to keep multiple display managers from running amok. On System V, this uses the lockf library call, while on BSD it uses flock. This names a directory in which xdm stores authorization files while initializing the session. The default value is <XRoot>/lib/X11/xdm. 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. 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.'' 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. 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. A list of additional environment variables, separated by white space, to pass on to the Xsetup_0, GiveConsole, Xsession, and TakeConsole programs. This resource is the name of the loadable greeter library. The greeter is the component that displays the login box, collects the username and password from the user, and authenticates the user. The default value for this resource is /usr/shlib/X11/libXdmDecGreet.so which is the Motif login interface. The /usr/shlib/X11/libXdmGreet.so library contains the Athena-style login interface. A file to checksum to generate the seed of authorization keys. This should be a file that changes frequently. The default is /dev/mem. 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. 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 <XRoot>/lib/X11/xdm/Xresources is the conventional name. Specifies the program run to offer a host menu for Indirect queries redirected to the special host name CHOOSER. <XRoot>/lib/X11/xdm/chooser is the default. See the sections XDMCP ACCESS CONTROL and CHOOSER. This resource specifies the program used to load the resources. By default, xdm uses <XRoot>/bin/xrdb. This specifies a program which is run (as root) before offering the Login window. This may be used to change the appearance of the screen around the Login window or to put up other windows. 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. This resource 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 GiveConsole. See the section STARTUP PROGRAM. This resource 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 BSESSION PROGRAM. This specifies a program which 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. These numeric resources control the behavior of xdm when attempting to open intransigent servers. The openDelay resource is the length of the pause (in seconds) between successive attempts. The openRepeat resource is the number of attempts to make. The openTimeout resource is the amount of time to wait while actually attempting the open (that is, the maximum time spent in the connect(2) system call). The startAttempts resource 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 VopenTimeout and 4 for startAttempts. 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 drawback 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. This boolean resource specifies whether the X server should be terminated when a session terminates (instead of resetting it). This option can be used when the server tends to grow without bound over time, in order to limit the amount of time the server is run. The default value is ``false.'' xdm sets the PATH environment variable for the session to this value. It should be a colon separated list of directories; see sh(1) for a full description. ``:/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. 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. xdm sets the SHELL environment variable for the startup and reset scripts to the value of this resource. It is /bin/sh by default. If the default session fails to execute, xdm will fall back to this program. This program is executed with no arguments, but executes using the same environment variables as the session would have had (see the section SESSION PROGRAM). By default, <XRoot>/bin/xterm is used. 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 confused by a look-alike window on the display. If the grab fails, xdm kills and restarts the server (if possible) and the session. The authorize resource 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. 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.'' 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 this resource is not specified, unique file names are generated and written into the directory specified by the DisplayManager.authDir resource. Not used. This resource specifies the number of the signal xdm sends to reset the server. See the section CONTROLLING THE SERVER. The default is 1 (SIGHUP). This resource specifies number of the signal xdm sends to terminate the server. See the section CONTROLLING THE SERVER. The default is 15 (SIGTERM). The original implementation of authorization in the sample server reread the authorization file at server reset time, instead of when checking the initial connection. Because 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. 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. This resource defines the default keymap that the local Xserver uses and maps the value of the console's language variable to a keymap name. This resource applies only to local displays. This resource defines the value of the LANG environment variable. If this resource is defined, the LANG variable will be set for the xdm process controlling the display as well as for the user's X session.
 

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 examples that follow, we use /usr/X11R6 as the value of <XRoot>.

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 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 that 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.

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 can be nested.

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

When xdm checks the 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. 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.
 

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 local /usr/bin/X11/X

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 (for example, 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 feature 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. The manual for your particular X terminal should document the display class string for your device. If it does not, you can run xdm in debug mode and look at the resource strings that it generates for that device. One of these strings is the class string.

To use the Shared Memory Transport as the default transport for communication between the X server and local clients, specify the local display as local:0, in which case the entry in the Xservers file might read as follows:

local:0 local /usr/bin/X11/X

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.
 

ATHENA-STYLE AUTHENTICATION WIDGET

This login widget is used when the greeter library, /usr/lib/X11/xdm/libXdmGreet.so, is specified as the value of the DisplayManager.greeterLib resource.

Note that you cannot use the Athena-style greeter if you have enabled enhanced security on your system. The Athena-style greeter does not use the necessary security mechanisms. See secsetup(8).

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 DisplayManager.DISPLAY.resources. All of these resources have reasonable default values, so it is not necessary to specify any of them. The geometry of the Login widget is normally computed automatically. If you wish to position it elsewhere, specify each of these resources. The color used to display the typed-in user name. The font used to display the typed-in user name. A string that identifies this window. The default is ``X Window System''. When X authorization is requested in the configuration file for this display and none is in use, this greeting replaces the standard greeting. The default is ``This is an unsecure session''. The font used to display the greeting. The color used to display the greeting. The string displayed to prompt for a user name. The xrdb utility strips trailing white space from resource values. To add spaces at the end of the prompt to make it more readable, add spaces escaped with backslashes. The default is ``Login: ''. The string displayed to prompt for a password. The default is ``Password: ''. The font used to display both prompts. The color used to display both prompts. A message that is displayed when the authentication fails. The default is ``Login incorrect''. The font used to display the failure message. The color used to display the failure message. The number of seconds that the failure message is displayed. The default is 30. This resource specifies the translations used for the login widget. Refer to the X Toolkit documentation for a complete discussion of 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() \

The actions which are supported by the widget are: Erases the character before the cursor. Erases the character after the cursor. Moves the cursor backward. Moves the cursor forward. (Apologies about the spelling error.) Moves the cursor to the beginning of the editable text. Moves the cursor to the end of the editable text. Erases all text after the cursor. Erases the entire text. If the cursor is in the name field, proceeds to the password field. If the cursor is in the password field, checks the current name/password pair. If the name/password pair is valid, xdm starts the session. Otherwise, the failure message is displayed and the user is prompted again. Terminates and restarts the server. Terminates the server, disabling it. This is a rash action and is not accessible in the default configuration. It can be used to stop xdm when you are shutting the system down or using xdmshell. Resets the X server and starts a new session. This action 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. Inserts the character typed. Specifies a single word argument that is passed to the session at startup. See the sections SESSION PROGRAM and TYPICAL USAGE. Disables access control in the server. This action can be used when the .Xauthority file cannot be created by xdm. Use this action with caution; you should probably disconnect the machine from the network before using this action.
 

MOTIF AUTHENTICATION WIDGET

This login widget is used when the greeter library, /usr/lib/X11/xdm/libXdmDecGreet.so, is specified as the value of the DisplayManager.greeterLib resource.

The authentication widget reads a name/password pair from the keyboard. Many parameters can be controlled with resources. Resources for this widget should be put into the file named by DisplayManager.DISPLAY.resources. All these resources have reasonable default values, so it is not necessary to specify any of them. The coordinates in pixels of the upper left corner of the logo displayed on the login screen. A value of -1 for LogoX or LogoY causes an appropriate default to be calculated. The foreground color of the logo displayed across the top of the login screen. The default color is rgb:8182/0604/2c28. The logo background color. The default is White. The foreground color of the logo on monochrome systems. The default is Black. The logo background color on monochrome systems. The default is White. If set to True, the root window will be painted the specified solid color and, when the login widget is destroyed, the root window will be restored to its default pattern. The default value is True. The root window color in the login screen. The default value is rgb:3030/5050/6060. The name of the file containing a bitmap in X bitmap format that is displayed in place of the default Digital logo. The name of the file containing the shape mask bitmap to use when displaying the logo. The color of the text displayed in a message box on a failed login. The greeting text displayed as a title in the login box. The default value is "Tru64 UNIX on CLIENTHOST". 'CLIENTHOST' is a macro that xrdb replaces with the name of the xdm host. The greeting text displayed as a secondary (smaller) title in the login box. The default value is "formerly \D\E\C OSF/1". 'DEC' must be escaped or else the xrdb cpp will treat it as a macro. The color of the greeting text in the login box. The default is Black. The color of the text of the prompt strings in the login box. The default is Black. The color of the response text in the login box. The default is Black. The font used to display the greeting text in the login box. The default is '*-new century schoolbook-bold-i-normal-*-240-*'. The font used to display the strings in the login box. The default is '*-new century schoolbook-medium-r-normal-*-180-*'. The font used to display the response text in the login box. The default is '*-new century schoolbook-medium-r-normal-*-180-*'.
 

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 you 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: Sets the associated display name. Sets the value of DisplayManager.DISPLAY.systemPath. Sets the value of DisplayManager.DISPLAY.systemShell. 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. However, they will be able to interact with the mouse, so check for 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.
 

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.
 

STARTUP PROGRAM

The program specified by the DisplayManager.DISPLAY.startup resource is typically shell script. It is run as root and needs to be careful about security. This is the place to put commands that add entries to /etc/utmp (the sessreg program may be useful here), 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: Sets the associated display name. Sets the initial working directory of the user. Sets The user name. Sets the value of DisplayManager.DISPLAY.systemPath. Sets the value of DisplayManager.DISPLAY.systemShell. 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
                exit 1
        fi
        sessreg-a-l $DISPLAY-x /usr/X11R6/lib/xdm/Xservers $USER
        /usr/X11R6/lib/xdm/GiveConsole
        exit 0


 

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: Sets the associated display name. Sets the initial working directory of the user. Sets the user name. PATH sets the value of DisplayManager.DISPLAY.userPath. Sets 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 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 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 do not 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. Do not 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 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 Xstartup, 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.

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 $USER
        /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 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 cannot 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.
 

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/X11R6/bin/X :0"

Suppose you might have a file server and a collection of X terminals. The configuration for this is identical to the preceding example, except the Xservers file would be as follows:

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 similar to init(8).
 

LIMITATIONS

The xdm program does not coexist well with other window systems.
 

FILES

Default configuration file Default access file, listing authorized displays Default server file, listing non-XDMCP servers to manage User authorization file where xdm stores keys for clients to read Default chooser Motif loadable greeter Athena-style loadable greeter Default resource database loader Default server Default session program and failsafe client Default location for authorization files Kerberos credentials cache

Note

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


 

SEE ALSO

X(1X), xauth(1X), XSecurity(1X), Xdec(1X), X Display Manager Control Protocol
 

AUTHOR

Keith Packard, MIT X Consortium


 

Index

NAME
SYNOPSIS
OPTIONS
DESCRIPTION
TYPICAL USAGE
RESOURCES
CONFIGURATION FILE
XDMCP ACCESS CONTROL
CHOOSER
SERVER SPECIFICATION
ATHENA-STYLE AUTHENTICATION WIDGET
MOTIF AUTHENTICATION WIDGET
SETUP PROGRAM
RESOURCES FILE
STARTUP PROGRAM
SESSION PROGRAM
RESET PROGRAM
CONTROLLING THE SERVER
CONTROLLING XDM
OTHER POSSIBILITIES
LIMITATIONS
FILES
SEE ALSO
AUTHOR

This document was created by man2html, using the manual pages.
Time: 02:43:03 GMT, October 02, 2010