|XDM(1)||General Commands Manual||XDM(1)|
xdm - X Display Manager with support for XDMCP, host chooser
xdm [ -config configuration_file ] [ -nodaemon ] [ -debug debug_level ] [ -error error_log_file ] [ -resources resource_file ] [ -server server_entry ] [ -session session_program ]
Xdm manages a collection of X displays, which may be on the local host or remote servers. The design of xdm was guided by the needs of X terminals as well as The Open Group standard XDMCP, the X Display Manager Control Protocol. Xdm provides services similar to those provided by init, getty and login on character terminals: prompting for login name and password, authenticating the user, and running a ``session.''
A ``session'' is defined by the lifetime of a particular process; in the traditional character-based terminal world, it is the user's login shell. In the xdm context, it is an arbitrary session manager. This is because in a windowing environment, a user's login shell process does not necessarily have any terminal-like interface with which to connect. When a real session manager is not available, a window manager or terminal emulator is typically used as the ``session manager,'' meaning that termination of this process terminates the user's session.
When the session is terminated, xdm resets the X server and (optionally) restarts the whole process.
When xdm receives an Indirect query via XDMCP, it can run a chooser process to perform an XDMCP BroadcastQuery (or an XDMCP Query to specified hosts) on behalf of the display and offer a menu of possible hosts that offer XDMCP display management. This feature is useful with X terminals that do not offer a host menu themselves.
Xdm can be configured to ignore BroadcastQuery messages from selected hosts. This is useful when you don't want the host to appear in menus produced by chooser or X terminals themselves.
Because xdm provides the first interface that users will see, it is designed to be simple to use and easy to customize to the needs of a particular site. Xdm has many options, most of which have reasonable defaults. Browse through the various sections of this manual, picking and choosing the things you want to change. Pay particular attention to the Session Program section, which will describe how to set up the style of session desired.
xdm is highly configurable, and most of its behavior can be controlled by resource files and shell scripts. The names of these files themselves are resources read from the file xdm-config or the file named by the -config option.
xdm offers display management two different ways. It can manage X servers running on the local machine and specified in Xservers, and it can manage remote X servers (typically X terminals) using XDMCP (the XDM Control Protocol) as specified in the Xaccess file.
The resources of the X clients run by xdm outside the user's session, including xdm's own login window, can be affected by setting resources in the Xresources file.
For X terminals that do not offer a menu of hosts to get display management from, xdm can collect willing hosts and run the chooser program to offer the user a menu. For X displays attached to a host, this step is typically not used, as the local host does the display management.
After resetting the X server, xdm runs the Xsetup script to assist in setting up the screen the user sees along with the xlogin widget.
The xlogin widget, which xdm presents, offers the familiar login and password prompts.
After the user logs in, xdm runs the Xstartup script as root.
Then xdm runs the Xsession script as the user. This system session file may do some additional startup and typically runs the .xsession script in the user's home directory. When the Xsession script exits, the session is over.
At the end of the session, the Xreset script is run to clean up, the X server is reset, and the cycle starts over.
The file /var/log/xdm.log will contain error messages from xdm and anything output to stderr by Xsetup, Xstartup, Xsession or Xreset. When you have trouble getting xdm working, check this file to see if xdm has any clues to the trouble.
All of these options, except -config itself, specify values that can also be specified in the configuration file as 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.
First, the xdm configuration file should be set up. Make a directory (usually /etc/X11/xdm) to contain all of the relevant files.
Here is a reasonable configuration file, which could be named xdm-config:
DisplayManager.servers: /etc/X11/xdm/Xservers DisplayManager.errorLogFile: /var/log/xdm.log DisplayManager*resources: /etc/X11/xdm/Xresources DisplayManager*startup: /etc/X11/xdm/Xstartup DisplayManager*session: /etc/X11/xdm/Xsession DisplayManager.pidFile: /var/run/xdm-pid DisplayManager._0.authorize: true DisplayManager*authorize: false
Note that this file mostly contains references to other files. Note also that some of the resources are specified with ``*'' separating the components. These resources can be made unique for each different display, by replacing the ``*'' with the display-name, but normally this is not very useful. See the Resources section for a complete discussion.
The database file specified by the DisplayManager.accessFile provides information which xdm uses to control access from displays requesting XDMCP service. This file contains three types of entries: entries which control the response to Direct and Broadcast queries, entries which control the response to Indirect queries, and macro definitions.
The format of the Direct entries is simple, either a host name or a pattern, which is distinguished from a host name by the inclusion of one or more meta characters (`*' matches any sequence of 0 or more characters, and `?' matches any single character) which are compared against the host name of the display device. If the entry is a host name, all comparisons are done using network addresses, so any name which converts to the correct network address may be used. For patterns, only canonical host names are used in the comparison, so ensure that you do not attempt to match aliases. Preceding either a host name or a pattern with a `!' character causes hosts which match that entry to be excluded.
To only respond to Direct queries for a host or pattern, it can be followed by the optional ``NOBROADCAST'' keyword. This can be used to prevent an xdm server from appearing on menus based on Broadcast queries.
An Indirect entry also contains a host name or pattern, but follows it with a list of host names or macros to which indirect queries should be sent.
A macro definition contains a macro name and a list of host names and other macros that the macro expands to. To distinguish macros from hostnames, macro names start with a `%' character. Macros may be nested.
Indirect entries may also specify to have xdm run chooser to offer a menu of hosts to connect to. See the section Chooser.
When checking access for a particular display host, each entry is scanned in turn and the first matching entry determines the response. Direct and Broadcast entries are ignored when scanning for an Indirect entry and vice-versa.
Blank lines are ignored, `#' is treated as a comment delimiter causing the rest of that line to be ignored, and `\newline' causes the newline to be ignored, allowing indirect host lists to span multiple lines.
Here is an example Xaccess file:
# # Xaccess - XDMCP access control file # # # Direct/Broadcast query entries # !xtra.lcs.mit.edu # disallow direct/broadcast service for xtra bambi.ogi.edu # allow access from this particular display *.lcs.mit.edu # allow access from any display in LCS *.deshaw.com NOBROADCAST # allow only direct access *.gw.com # allow direct and broadcast # # Indirect query entries # %HOSTS expo.lcs.mit.edu xenon.lcs.mit.edu \ excess.lcs.mit.edu kanga.lcs.mit.edu extract.lcs.mit.edu xenon.lcs.mit.edu #force extract to contact xenon !xtra.lcs.mit.edu dummy #disallow indirect access *.lcs.mit.edu %HOSTS #all others get to choose
If compiled with IPv6 support, multicast address groups may also be included in the list of addresses indirect queries are set to. Multicast addresses may be followed by an optional / character and hop count. If no hop count is specified, the multicast hop count defaults to 1, keeping the packet on the local network. For IPv4 multicasting, the hop count is used as the TTL.
rincewind.sample.net ff02::1 #IPv6 Multicast to ff02::1 #with a hop count of 1 ponder.sample.net CHOOSER 126.96.36.199/16 #Offer a menu of hosts #who respond to IPv4 Multicast # to 188.8.131.52 with a TTL of 16
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.
The following configuration directive is also defined for the Xaccess configuration file:
If one or more LISTEN lines are specified, xdm only listens for XDMCP connections on the specified interfaces. If multicast group addresses are listed on a listen line, xdm joins the multicast groups on the given interface.
If no LISTEN lines are given, the original behavior of listening on all interfaces is preserved for backwards compatibility. Additionally, if no LISTEN is specified, xdm joins the default XDMCP IPv6 multicast group, when compiled with IPv6 support.
To disable listening for XDMCP connections altogther, a line of LISTEN with no addresses may be specified, or the previously supported method of setting DisplayManager.requestPort to 0 may be used.
LISTEN * ff02::1 # Listen on all interfaces and to the # ff02::1 IPv6 multicast group. LISTEN 10.11.12.13 # Listen only on this interface, as long # as no other listen directives appear in # file.
The Internet Assigned Numbers Authority has has assigned ff0X:0:0:0:0:0:0:12b as the permanently assigned range of multicast addresses for XDMCP. The X in the prefix may be replaced by any valid scope identifier, such as 1 for Interface-Local, 2 for Link-Local, 5 for Site-Local, and so on. (See IETF RFC 4291 or its replacement for further details and scope definitions.) xdm defaults to listening on the Link-Local scope address ff02:0:0:0:0:0:0:12b to most closely match the old IPv4 subnet broadcast behavior.
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:
The display types are:
:0 Digital-QV local /usr/X11R6/bin/X :0
local local display: xdm must run the server foreign remote display: xdm opens an X connection to a running server
The display name must be something that can be passed in the -display option to an X program. This string is used to generate the display-specific resource names, so be careful to match the names (e.g., use ``:0 Sun-CG3 local /usr/X11R6/bin/X :0'' instead of ``localhost:0 Sun-CG3 local /usr/X11R6/bin/X :0'' if your other resources are specified as ``DisplayManager._0.session''). The display class portion is also used in the display-specific resources, as the class of the resource. This is useful if you have a large collection of similar displays (such as a corral of X terminals) and would like to set resources for groups of them. When using XDMCP, the display is required to specify the display class, so the manual for your particular X terminal should document the display class string for your device. If it doesn't, you can run xdm in debug mode and look at the resource strings which it generates for that device, which will include the class string.
When xdm starts a session, it sets up authorization data for the server. For local servers, xdm passes ``-auth filename'' on the server's command line to point it at its authorization data. For XDMCP servers, xdm passes the authorization data to the server via the Accept XDMCP request.
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.
The Xsetup file is run after the server is reset, but before the Login window is offered. The file is typically a shell script. It is run as root, so should be careful about security. This is the place to change the root background or bring up other windows that should appear on the screen along with the Login widget.
In addition to any specified by DisplayManager.exportList, the following environment variables are passed:
DISPLAY the associated display name PATH the value of DisplayManager.DISPLAY.systemPath SHELL the value of DisplayManager.DISPLAY.systemShell XAUTHORITY may be set to an authority file
Note that since xdm grabs the keyboard, any other windows will not be able to receive keyboard input. They will be able to interact with the mouse, however; beware of potential security holes here. If DisplayManager.DISPLAY.grabServer is set, Xsetup will not be able to connect to the display at all. Resources for this program can be put into the file named by DisplayManager.DISPLAY.resources.
Here is a sample Xsetup script:
#!/bin/sh # Xsetup_0 - setup script for one workstation xcmsdb < /etc/X11/xdm/monitors/alex.0 xconsole -geometry 480x130-0-0 -notify -verbose -exitOnFail &
The authentication widget prompts the user for the username, password, and/or other required authentication data from the keyboard. Nearly every imaginable parameter can be controlled with a resource. Resources for this widget should be put into the file named by DisplayManager.DISPLAY.resources. All of these have reasonable default values, so it is not necessary to specify any of them.
The resource file is loaded with xrdb(1) so it may use the substitutions defined by that program such as CLIENTHOST for the client hostname in the login message, or C pre-processor #ifdef statements to produce different displays depending on color depth or other variables.
Xdm can be compiled with support for the Xft(3) library for font rendering. If this support is present, font faces are specified using the resources with names ending in ``face'' in the fontconfig face format described in the Font Names section of fonts.conf(5). If not, then fonts are specified using the resources with names ending in ``font'' in the traditional X Logical Font Description format described in the Font Names section of X(7).
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:
On some systems (OpenBSD) the user's shell must be listed in /etc/shells to allow login through xdm. The normal password and account expiration dates are enforced too.
The Xstartup program is run as root when the user logs in. It is typically a shell script. Since it is run as root, Xstartup should be very careful about security. This is the place to put commands which add entries to utmp or wtmp files, (the sessreg program may be useful here), mount users' home directories from file servers, or abort the session if logins are not allowed.
In addition to any specified by DisplayManager.exportList, the following environment variables are passed:
DISPLAY the associated display name HOME the initial working directory of the user LOGNAME the user name USER the user name PATH the value of DisplayManager.DISPLAY.systemPath SHELL the value of DisplayManager.DISPLAY.systemShell XAUTHORITY may be set to an authority file WINDOWPATH may be set to the "window path" leading to the X server
No arguments are passed to the script. Xdm waits until this script exits before starting the user session. If the exit value of this script is non-zero, xdm discontinues the session and starts another authentication cycle.
The sample Xstartup file shown here prevents login while the file /etc/nologin exists. Thus this is not a complete example, but simply a demonstration of the available functionality.
Here is a sample Xstartup script:
#!/bin/sh # # Xstartup # # This program is run as root after the user is verified # if [ -f /etc/nologin ]; then xmessage -file /etc/nologin -timeout 30 -center exit 1 fi sessreg -a -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME /etc/X11/xdm/GiveConsole exit 0
The Xsession program is the command which is run as the user's session. It is run with the permissions of the authorized user.
In addition to any specified by DisplayManager.exportList, the following environment variables are passed:
DISPLAY the associated display name HOME the initial working directory of the user LOGNAME the user name USER the user name PATH the value of DisplayManager.DISPLAY.userPath SHELL the user's default shell (from getpwnam) XAUTHORITY may be set to a non-standard authority file KRB5CCNAME may be set to a Kerberos credentials cache name WINDOWPATH may be set to the "window path" leading to the X server
At most installations, Xsession should look in $HOME for a file .xsession, which contains commands that each user would like to use as a session. Xsession should also implement a system default session if no user-specified session exists.
An argument may be passed to this program from the authentication widget using the `set-session-argument' action. This can be used to select different styles of session. One good use of this feature is to allow the user to escape from the ordinary session when it fails. This allows users to repair their own .xsession if it fails, without requiring administrative intervention. The example following demonstrates this feature.
This example recognizes the special ``failsafe'' mode, specified in the translations in the Xresources file, to provide an escape from the ordinary session. It also requires that the .xsession file be executable so we don't have to guess what shell it wants to use.
#!/bin/sh # # Xsession # # This is the program that is run as the client # for the display manager. case $# in 1) case $1 in failsafe) exec xterm -geometry 80x24-0-0 ;; esac esac startup=$HOME/.xsession resources=$HOME/.Xresources if [ -f "$startup" ]; then exec "$startup" else if [ -f "$resources" ]; then xrdb -load "$resources" fi twm & xman -geometry +10-10 & exec xterm -geometry 80x24+10+10 -ls fi
The user's .xsession file might look something like this example. Don't forget that the file must have execute permission.
#! /bin/csh # no -f in the previous line so .cshrc gets run to set $PATH twm & xrdb -merge "$HOME/.Xresources" emacs -geometry +0+50 & xbiff -geometry -430+5 & xterm -geometry -0+50 -ls
Symmetrical with Xstartup, the Xreset script is run after the user session has terminated. Run as root, it should contain commands that undo the effects of commands in Xstartup, updating entries in utmp or wtmp files, or unmounting directories from file servers. The environment variables that were passed to Xstartup are also passed to Xreset.
A sample Xreset script:
#!/bin/sh # # Xreset # # This program is run as root after the session ends # sessreg -d -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME /etc/X11/xdm/TakeConsole exit 0
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.
Xdm responds to two signals: SIGHUP and SIGTERM. When sent a SIGHUP, xdm rereads the configuration file, the access control file, and the servers file. For the servers file, it notices if entries have been added or removed. If a new entry has been added, xdm starts a session on the associated display. Entries which have been removed are disabled immediately, meaning that any session in progress will be terminated without notice and no new session will be started.
When sent a SIGTERM, xdm terminates all sessions in progress and exits. This can be used when shutting down the system.
Xdm attempts to mark its various sub-processes for ps(1) by editing the command line argument list in place. Because xdm can't allocate additional space for this task, it is useful to start xdm with a reasonably long command line (using the full path name should be enough). Each process which is servicing a display is marked -display.
To add an additional local display, add a line for it to the Xservers file. (See the section Local Server Specification.)
Examine the display-specific resources in xdm-config (e.g., DisplayManager._0.authorize) and consider which of them should be copied for the new display. The default xdm-config has all the appropriate lines for displays :0 and :1.
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”
Or, you might have a file server and a collection of X terminals. The configuration for this is identical to the sample above, except the Xservers file would look like
extol:0 VISUAL-19 foreign exalt:0 NCD-19 foreign explode:0 NCR-TOWERVIEW3000 foreign
This directs xdm to manage sessions on all three of these terminals. See the section Controlling Xdm for a description of using signals to enable and disable these terminals in a manner reminiscent of init(8).
One thing that xdm isn't very good at doing is coexisting with other window systems. To use multiple window systems on the same hardware, you'll probably be more interested in xinit.
X(7), xinit(1), xauth(1), xrdb(1),
Xsecurity(7), sessreg(1), Xserver(1),
X Display Manager Control Protocol
IETF RFC 4291: IP Version 6 Addressing Architecture.
Keith Packard, MIT X Consortium
|xdm 1.1.11||X Version 11|