xdm Command

Purpose

Manages a collection of X Displays with support for XDMCP.

Syntax

xdm-config ConfigurationFile] [  -debug DebugLevel ] [  -nodaemon ] [  -error ErrorLogFile ] [  -resources ResourceFile ] [  -server ServerEntry ] [  -udpPort PortNumber ] [  -session SessionProgram ] [  -xrm ResourceSpecification ]

Description

The xdm (X Display Manager) command manages a collection of X displays, which may be on the local host or remote servers. The design of the xdm command was guided by the needs of X terminals as well as the X Consortium standard XDMCP, the X Display Manager Control Protocol. The xdm command provides services similar to those provided by the init, getty, and login commands 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 ending this process ends the user's session.

When the session is ended, xdm resets the X server and (optionally) restarts the whole process.

When the xdm command receives an Indirect query by way of 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 the xdm command provides the first interface that users see, it is designed to be simple to use and easy to customize to the needs of a particular site.

Typical Usage

The xdm command is designed to operate in a wide variety of environments.

First, set up the xdm configuration file. Make a directory (usually /usr/lib/X11/xdm) to contain all of the relevant files. The following is a reasonable configuration file, which could be named xdm-config:
DisplayManager.servers:       /usr/lib/X11/xdm/Xservers
DisplayManager.errorLogFile:  /usr/lib/X11/xdm/xdm-errors
DisplayManager*resources:     /usr/lib/X11/xdm/Xresources
DisplayManager*startup:       /usr/lib/X11/xdm/Xstartup
DisplayManager*session:       /usr/lib/X11/xdm/Xsession
DisplayManager.pidFile:       /usr/lib/X11/xdm/xdm-pid
DisplayManager._0.authorize:  true
DisplayManager*authorize:     false

This file contains references to other files. Some of the resources are specified with an * (asterisk) separating the components. These resources can be made unique for each display by replacing the * (asterisk) with the display name, but typically this is not useful. See the Resources section on the next page 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 (zero), so the file looks something like this:

:0 Local local /usr/bin/X11/X -force

This keeps /usr/bin/X11/X running on this display and manages a continuous cycle of sessions.

The /usr/lib/X11/xdm/xdm-errors file contains error messages from xdm and anything output to standard error by Xsetup, Xstartup, Xsession or Xreset scripts. If you have trouble starting the xdm command, check the /usr/lib/X11/xdm/xdm-errors file to see if the xdm command has any clues to the trouble.

The next configuration entry, /usr/lib/X11/xdm/Xresources, is loaded onto the display as a resource database using the xrdb command. As the authentication widget reads this database before starting up, it usually contains parameters for that widget.

Flags

All of these options (except -config) specify values that can also be specified in the configuration file as resources.

Item Description
-config ConfigurationFile Names the configuration file, which specifies resources to control the behavior of the xdm command. The /usr/lib/X11/xdm/xdm-config file is the default.
-debug DebugLevel Specifies the numeric value for the DisplayManager.debugLevel resource. A nonzero value causes xdm to print debugging statements to the terminal and disables the DisplayManager.daemonMode resource, forcing xdm to run synchronously. These error messages may be unclear. To interpret them, check the X11R4 source code for the xdm command.
-nodaemon Specifies False as the value for the DisplayManager.daemonMode resource. This suppresses the usual daemon behavior, in which the xdm command closes all file descriptors, disassociates itself from the controlling terminal, and puts itself in the background when it first starts up.
-error ErrorLogFile Specifies the value for the DisplayManager.errorLogFile resource. This file contains errors from xdm as well as anything written to standard error by the various scripts and programs run during the progress of the session.
-resources ResourceFile Specifies the value for the DisplayManager*resources resource. This file is loaded using the xrdb command to specify configuration parameters for the authentication widget.
-server ServerEntry Specifies the value for the DisplayManager.servers resource. See the section Server Specification for a description of this resource.
-udpPort PortNumber Specifies the value for the DisplayManager.requestPort resource. This sets the port number that the xdm command monitors for XDMCP requests. XDMCP uses the registered well-known UDP port 177. Do not change this resource except when debugging.
-session SessionProgram Specifies the value for the DisplayManager*session resource. This indicates the program to run as the session after the user has logged in.
-xrm ResourceSpecification 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. When 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.

Item Description
DisplayManager.servers Specifies either 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 details.
DisplayManager.requestPort Indicates the UDP port number that the xdm command uses to listen for incoming XDMCP requests. Unless you need to debug the system, leave this with its default value of 177.
DisplayManager.errorLogFile Redirects error messages to go to the named file rather than to the console. This file also contains any output directed to standard error 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 0 (zero), the xdm command outputs a large amount of debugging information. It also disables daemon mode, which would discard the information and allow nonroot users to run the xdm command that would typically not be useful.
DisplayManager.daemonMode The xdm command attempts to make itself into a daemon process unassociated with any terminal. This is accomplished by forking and leaving the parent process to exit, and 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 disables this feature.
DisplayManager.pidFile The file name specified is created to contain an ASCII representation of the process ID of the main xdm process. The xdm command also uses file locking on this file to attempt to eliminate multiple daemons running on the same machine, which would have unpredictable results.
DisplayManager.lockPidFile Controls whether the xdm command uses file locking to keep multiple display managers from running simultaneously.
DisplayManager.authDir Names a directory in which the xdm command stores authorization files while initializing the session. The default value is /usr/lib/X11/xdm.
DisplayManager.autoRescan A Boolean value that controls whether the xdm command rescans the configuration, servers, access control, and authentication keys files after a session ends and the files have changed. By default the value is True. You can force the xdm daemon to reread these files by sending a SIGHUP signal to the main process.
DisplayManager.removeDomainname When computing the display name for XDMCP clients, the name resolver typically creates a fully qualified host name for the terminal. As this is sometimes confusing, the xdm command removes 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. The default value is True.
DisplayManager.keyFile XDM-AUTHENTICATION-1 style XDMCP authentication requires that a private key be shared between the xdm daemon 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, the xdm command does not include support for XDM-AUTHENTICATION-1 because it requires the data encryption method (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 host names that are allowed direct access to this machine or have a list of hosts to which queries should be forwarded. The format of this file is described in the XDMCP Access Control section.
DisplayManager.exportList A whitespace-separated list of additional environment variables 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.choiceTimeout Number of seconds to wait for the display to respond after a 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. The default is 15.
DisplayManager.DISPLAY.resources Specifies the name of the file to be loaded by the xrdb command as the resource database onto the root window of screen 0 of the display. The Login widget, Xsetup, and chooser programs 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 Client , 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, the xdm command uses /usr/bin/X11/xrdb.
DisplayManager.DISPLAY.cpp Specifies the name of the C preprocessor that is used by the xrdb command.
DisplayManager.DISPLAY.setup Specifies a program that is run (as root) before offering the login window. This resource 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. See the section Setup Program.
DisplayManager.DISPLAY.startup 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 Xstartup. See the section Startup Program.
DisplayManager.DISPLAY.session Specifies the session to be run (when not running as root). By default, /usr/bin/X11/xterm is run. The conventional name is the Xsession script. See the section Session Program.
DisplayManager.DISPLAY.reset Specifies a program that is run (as root) after the session ends. By default, no program is run. The conventional name is the Xreset script. See the section Reset Program.
DisplayManager.DISPLAY.openDelay Controls the behavior of the xdm command when attempting to open intransigent servers by specifying the length of the pause (in seconds) between successive attempts.
DisplayManager.DISPLAY.openRepeat Controls the behavior of the xdm command when attempting to open intransigent servers by specifying the number of attempts to make.
DisplayManager.DISPLAY.openTimeout Controls the behavior of the xdm command when attempting to open intransigent servers by specifying the number of seconds to wait while actually attempting the open (that is, the maximum time spent in the connect(2) system call).
Item Description
DisplayManager.DISPLAY.startAttempts Controls the behavior of the xdm command when attempting to open intransigent servers by specifying the number of times that the entire process is completed before giving up on the server. After the number of attempts specified by the Display Manager openRepeat resource have been made, or if the number of seconds specified by the Display Manager openTimeout resource elapse in any particular attempt, the xdm command ends and restarts the server, attempting to connect again. This process is repeated startAttempts times, at which point the display is declared inactive and disabled. Although this behavior may seem arbitrary, it has been empirically developed and works well on most systems. The default is a value of 5 for openDelay, a value of 5 for openRepeat, a value of 30 for openTimeout, and a value of 4 for startAttempts.
DisplayManager.DISPLAY.pingInterval To discover when remote displays disappear, the xdm command occasionally pings them, using an X connection and XSync calls. This resource specifies the time (in minutes) between ping attempts. By default, it is set to 5 minutes. If you frequently use X terminals, which can become isolated from the managing host, you may want to increase this value.
Note: AIXwindows sessions may continue to exist after the terminal has been accidentally disabled. The xdm command does not ping local displays. A workstation session can be ended if the server hangs for NFS service and does not respond to the ping.
DisplayManager.DISPLAY.pingTimeout To discover when remote displays disappear, the xdm command occasionally pings them, using an X connection and XSync calls. This resource 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 inactive and ended. By default, it is set to 5 minutes. If you frequently use X terminals, which can become isolated from the managing host, you may want to increase this value.
Note: AIXwindows sessions may continue to exist after the terminal has been accidentally disabled. The xdm command does not ping local displays. A workstation session could be ended if the server hangs for NFS service and does not respond to the ping.
DisplayManager.DISPLAY.terminateServer Specifies whether the X server should be canceled when a session ends (instead of resetting it). This option can be used when the server tends to grow without bound over time, to limit the amount of time the server is run. The default value is False.
DisplayManager.DISPLAY.userPath The xdm command sets the PATH environment variable for the session to this value. It should be a list of directories separated by colons; see the sh command in AIX® Version 7.1 Commands Reference 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 AIXwindows system configuration file with the DefaultUserPath resource.
DisplayManager.DISPLAY.systemPath The xdm command 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 resource entry in the system configuration file; /etc:/bin:/usr/bin:/usr/bin/X11:/usr/ucb is a common choice. Note the absence of . (period) (the current directory) from this entry. This is a good practice to follow for root; it avoids many common "Trojan Horse" system penetration schemes.
DisplayManager.DISPLAY.systemShell The xdm command 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 run, the xdm command returns to this program. This program is run with no arguments, 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, the xdm command 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 set to False, the server is ungrabbed after the keyboard grab succeeds, otherwise the server is grabbed until just before the session begins. The default value is False. The grabTimeout resource specifies the maximum time that the xdm command waits for the grab to succeed. The grab may fail if some other client has the server grabbed, or possibly if the network latencies are high. This resource has a default value of 3 seconds; be cautious when raising it, as a user may be confused by a look-alike window on the display. If the grab fails, the xdm command becomes inactive and restarts the server (if possible) and the session.
DisplayManager.DISPLAY.authorize

DisplayManager.DISPLAY.authName

The authorize is a Boolean resource that controls whether the xdm command generates and uses authorization for the local server connections. If authorization is used, the xdm command uses the authorization mechanisms indicated as a whitespace-separated list as the value of the authName resource. XDMCP connections dynamically specify which authorization mechanisms are supported, so the authName resource is ignored in this case. When the authorize resource is set for a display and authorization is not available, the user is informed by a different message displayed in the Login widget. By default, the authorize resource is True; authName is MIT-MAGIC-COOKIE-1.
DisplayManager.DISPLAY.authFile Indicates the file is used to communicate the authorization data from the xdm command to the server, using the -auth server command-line option. It should be kept in a directory with restricted write permissions as it could easily be removed, disabling the authorization mechanism in the server.
DisplayManager.DISPLAY.authComplain If set to a value of False, this disables the use of the unsecureGreeting in the login window. See the section Authentication Client . The default is a value of True.
DisplayManager.DISPLAY.resetSignal The number of the signal that the xdm command sends to reset the server. See the section Controlling the Server . The default is 1(SIGHUP).
DisplayManager.DISPLAY.termSignal The number of the signal that the xdm command sends to end the server. See the section Controlling the Server . The default is 15(SIGTERM).
DisplayManager.DISPLAY.resetForAuth Causes the xdm command to send SIGHUP to the server after setting up the authorization file, causing an additional server reset to occur, during which time the new authorization information is read. The default is a value of False, which works for all AIXwindows servers.
DisplayManager.DISPLAY.userAuthDir When the xdm command is unable to write to the usual user authorization file ($HOME/.Xauthority), it creates a unique file name in this directory and sets the XAUTHORITY environment variable to the name of the created file. It uses /tmp by default.

XDMCP Access Control

The database file specified by the DisplayManager.accessFile resource provides information that the xdm command uses to control access from displays requesting XDMCP service. This file contains three types of entries:

Direct query entries contain either a host name or a pattern, which is distinguished from a host name by the inclusion of one or more pattern-matching characters. An * (asterisk) matches any sequence of 0 (zero) or more characters, and a ? (question mark) matches any single character. These 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 that any name that converts to the correct network address may be used. For patterns, only actual 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 an ! (exclamation point) causes hosts that 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 host names, macro names start with a % (percent) character. Macros may be nested.

Indirect entries may also specify to have the xdm command run the chooser command to offer a menu of hosts to which to connect. For more information, see Chooser.

When checking access for a particular display host, each entry is scanned in turn and the first matching entry determines the response. For example, a Direct query entry is ignored when scanning for an Indirect entry. A Broadcast query entry is ignored when scanning for a Direct entry.

Blank lines are ignored. The # character is treated as a comment delimiter causing the rest of that line to be ignored, and a \ (backslash) at the end of the line causes the new line to be ignored, allowing indirect host lists to span multiple lines.

The following 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. The chooser program sends a Query request to each of the remaining host names in the list and offers a menu of all the hosts that respond.

The list may consist of the word BROADCAST, in which case chooser sends a Broadcast query instead, again offering a menu of all hosts that respond.

The following is an 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 the DisplayManager.DISPLAY.resources resource.

The chooser has been implemented using a Motif SelectionBoxWidget. Refer to the XmSelectionBoxWidget Class documentation for a description of resources and widget or gadget names.

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 typical entry for local display number 0 would be:

:0 IBM-GT local /usr/bin/X11/X :0

The display types are:

Item Description
local local display: \fIxdm\fP must run the server
foreign remote display: \fIxdm\fP 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 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 does not, you can run the xdm command in debug mode and look at the resource strings that it generates for that device, which will include the class string.

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 be careful about security. This is the place to change the root background or bring up other windows that should be displayed on the screen along with the Login widget. Because xdm grabs the keyboard, other windows will not be able to receive keyboard input. They will be able to interact with the mouse, however; beware of potential security holes here. If DisplayManager.DISPLAY.grabServer is set, Xsetup will not be able to connect to the display at all. Resources for this program can be put into the file named by DisplayManager.DISPLAY.resources.

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

Item Description
DISPLAY Specifies the associated display name.
PATH Specifies the value of DisplayManager.DISPLAY.systemPath.
SHELL Specifies the value of DisplayManager.DISPLAY.systemShell.
XAUTHORITY Specifies that it may be set to an authority file.

Authentication Client

The MIT authentication widget has been replaced by an authentication client composed of standard Motif widgets. The following is a list of the widget names (and their widget class):

outframe(xmFrameWidget)
  inframe(xmFrameWidget)
    main(XmFormWidget)
      tframe(xmFrameWidget)
        greeting(xmLabelGadget)
      logoline(xmFormWidget)
        dpyname(xmLabelWidget)
      userline(xmRowColumnWidget)
        userlabel(xmLabelWidget)
        username(xmTextWidget)
        passlabel(xmLabelWidget)
        password(xmTextWidget)
      failsafeline(xmFormWidget)
        failsafe(xmToggleButtonWidget)
      cancelline(xmFormWidget)
        cancel(xmPushButtonWidget)
      message(xmLabelWidget)

The authentication client reads a name/password pair from the keyboard. Put resources for this client into the file named by DisplayManager.DISPLAY.resources. All of these have reasonable default values, so it is unnecessary to specify any of them. See /usr/lib/X11/xdm/Xresources for more information on default values for authentication client resources as well as the appropriate widget class documentation. The following resources are also supported by the authentication client:

Item Description
Xlogin*foreground Specifies the color used for the foreground.
Xlogin*background Specifies the color used for the background.
Xlogin*greeting Specifies a string that identifies this window. The default is AIXwindows environment.
Xlogin*greetFont Specifies the font used to display the greeting.
Xlogin*frameColor Specifies the background color used to display the greeting.
Xlogin*titleFont Specifies the font used to display the title.
Xlogin*namePrompt Specifies the string displayed to prompt for a user name. The Xrdb program strips trailing white space from resource values. Add spaces escaped with backslashes at the end of the prompt. The default is "login:".
Xlogin*promptFont Specifies the font used to display both prompts.
Xlogin*failFont Specifies the font used for the failsafe button.
Xlogin*cancelFont Specifies the font used for the cancel button.
Xlogin*messageFontlist Specifies the font used to display the failure message.
Xlogin*failColor Specifies the color used to display the failure message.
Xlogin*failTimeout Specifies the number of seconds that the failure message is displayed. The default is thirty seconds.
Xlogin*sessionArgument Specifies the argument to be passed to the session program.
Xlogin*XmText.translations This specifies the translations use for the authentification client. Refer to the X Toolkit documentation for a complete discussion on translations. The default translation table is:
Ctrl<Key>b: backward-character()\n\
Ctrl<Key>a: beginning-of-line()\n\
Ctrl<Key>e: end-of-line()\n\
Ctrl<Key>f: forward-character()\n\
Ctrl<Key>d: kill-next-character()\n\
Ctrl<Key>k: kill-to-end-of-line()\n\
Ctrl<Key>u: kill-to-start-of-line()\n

You may setup XDM to use the standard XDM translations by replacing the XmText translations as defined in Xresources:

Note: Use <Key>osfHelp instead of <Key>F1 due to the Motif default virtual bindings.)
Xlogin*XmText.translations: #override\n\

<Key>osfHelp:    set-session-argument(failsafe) finish-field()\n\
Ctrl<Key>Return: set-session-argument(failsafe) finish-field()\n\
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-beginning() \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\
<Key>Return:     finish-field() \n
<Key>BackSpace:  delete-previous-character() \n\
<Key>Delete:     delete-previous-character() \n

In addition to the typical XmText actions, the following actions are also supported by the client to be compatible with the standard XDM translations:

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-beginning
Moves the cursor to the beginning of the editable text.
 
move-to-end
Moves the cursor to the end of the editable text.
erase-to-end-of-line
Erases all text after the cursor.
erase-line
Erases the entire text.
finish-field
If the cursor is in the name field, proceeds to the password field; if the cursor is in the password field, checks the current name/password pair. If the name/password pair is valid, xdm starts the session. Otherwise the failure message is displayed and the user is prompted again.
insert-char
Inserts the character typed.
set-session-argument
Specifies a single word argument that is passed to the session at startup. See the sections Session Program and Typical Usage.

Startup Program

The Xstartup file is typically a shell script. Because it is run as the root user, be careful about security when it runs. It usually contains commands that add entries to /etc/utmp, mount users' home directories from file servers, display the message of the day, or cancel the session if logins are not allowed.

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

Item Description
DISPLAY Specifies the associated display name.
HOME Specifies the initial working directory of the user.
USER Specifies the user name.
PATH Specifies the value of DisplayManager.DISPLAY.systemPath.
SHELL Specifies the value of DisplayManager.DISPLAY.systemShell.
XAUTHORITY May be set to an authority file.

No arguments are passed to the script. The xdm command waits until this script exits before starting the user session. If the exit value of this script is nonzero, the xdm command discontinues the session and starts another authentication cycle.

Session Program

The Xsession program establishes the style of 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:

Item Description
DISPLAY Specifies the associated display name.
HOME Specifies the initial working directory of the user.
USER Specifies the user name.
PATH Specifies the value of DisplayManager.DISPLAY.userPath.
SHELL Specifies the user's default shell (from getpwnam).
XAUTHORITY May be set to a nonstandard authority file.

At most installations, the Xsession program should look in the user's home directory ($HOME) for a file .xsession, which contains the commands that the user would like to use as a session. The Xsession program 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. Usually, this feature is used 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

The Xreset script is run after the user session has ended. Run as root, it should contain commands that undo the effects of commands in Xstartup by removing entries from /etc/utmp or unmounting directories from file servers. The environment variables that are passed to Xstartup are also passed to Xreset. This program is symmetrical with the Xstartup program.

Controlling the Server

The xdm command controls local servers using POSIX signals. The SIGHUP signal is expected to reset the server, closing all client connections and performing other cleanup duties. The SIGTERM signal is expected to cancel 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 that are not using XDMCP, the xdm command 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 cause all of the clients to become inactive, because only those that have created windows will be noticed. XDMCP provides a more sure mechanism; when the xdm command closes its initial connection, the session is over and the terminal is required to close all other connections.

Controlling XDM

The xdm command 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, the xdm command starts a session on the associated display. Entries that have been removed are disabled immediately, meaning that any session in progress is ended without notice and no new session is started.

When sent a SIGTERM, the xdm command stops all sessions in progress and exits. This can be used when shutting down the system.

The xdm command attempts to mark its various subprocesses for use by the ps command by editing the command-line argument list in place. Because the xdm command cannot allocate additional space for this task, it is useful to start the xdm command 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 the xdm command to run a single session at a time, using the xinit command options or other suitable daemons by specifying the server on the command line:

xdm -server ":0 local /usr/bin/X11/X :0 -force"

It might also run a file server and a collection of X terminals. The configuration for this is identical to the previous sample, except the Xservers file would look like the following:

        extol:0 VISUAL-19 foreign
        exalt:0 NCD-19 foreign
        explode:0 NCR-TOWERVIEW3000 foreign
This directs the xdm command 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.
Note: The xdm command does not coexist well with other window systems. To use multiple window systems on the same hardware, use the xinit command.

Examples

  1. The sample xstartup script that follows prevents login while the file /etc/nologin exists. As there is no provision for displaying any messages here (there is no core X client that displays files), the setup in this example is not recommended because the login would fail without explanation. Thus, this is not a complete example, but a demonstration of the available functionality.
    #!/bin/sh
    #
    # Xstartup
    #
    # This program is run as root after the user is verified
    #
    if [ \-f /etc/nologin ]; then
            exit 1
    fi
    exit 0
  2. This Xsession script recognizes the special failsafe mode, specified in the translations in the preceding Xresources file, to provide an escape from the ordinary session:
    #!/bin/sh
    exec > $HOME/.xsession-errors 2>&1
    case $# in
    1)
            case $1 in failsafe)
                   exec aixterm -geometry 80x24-0-0
                   ;;
            esac
    esac
    startup=$HOME/.xsession
    resources=$HOME/.Xresources
    if [ -f /usr/bin/X11/startx ]; then
           exec /usr/bin/X11/startx -t -wait
    elif [ -f $startup]; then
           exec $startup
    else
           if [ -f $resources ]; then
                   xrdb -load $resources
           fi
           mwm &
           exec aixterm -geometry 80x24+10+10 -ls
    fi
  3. To have xdm come up from system startup, as root type the following:
    /usr/lib/X11/xdm/xdmconf
  4. To disable xdm on reboot, as root type the following:
    /usr/lib/X11/xdm/xdmconf -d
  5. When using xdm to manage your display, an authentication procedure insures that only clients that are allowed can connect to your display. Clients that are built using X11 R4 and X11 R5 libraries understand this protocol. Clients that are built with X11 R3 or earlier libraries do not support this authentication protocol and are not allowed to connect to the Xserver unless xhost permission is granted. You can connect local clients by typing the following:
    xhost =localhost
    or
    xhost =machine
    where machine is the hostname of the local client.

Files

Item Description
/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 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.