Provided by: xdm_1.1.11-3ubuntu1_amd64 
NAME
xdm - X Display Manager with support for XDMCP, host chooser
SYNOPSIS
xdm [ -config configuration_file ] [ -nodaemon ] [ -debug debug_level ] [ -error error_log_file ] [
-resources resource_file ] [ -server server_entry ] [ -session session_program ]
DESCRIPTION
Xdm manages a collection of X displays, which may be on the local host or remote servers. The design of
xdm was guided by the needs of X 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.
OVERVIEW
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.
OPTIONS
All of these options, except -config itself, specify values that can also be specified in the
configuration file as resources.
-config configuration_file
Names the configuration file, which specifies resources to control the behavior of xdm.
/etc/X11/xdm/xdm-config is the default. See the section Configuration File.
-nodaemon
Specifies ``false'' as the value for the DisplayManager.daemonMode resource. This suppresses the
normal daemon behavior, which is for xdm to close all file descriptors, disassociate itself from
the controlling terminal, and put itself in the background when it first starts up.
-debug debug_level
Specifies the numeric value for the DisplayManager.debugLevel resource. A non-zero value causes
xdm to print lots of debugging statements to the terminal; it also disables the
DisplayManager.daemonMode resource, forcing xdm to run synchronously. To interpret these
debugging messages, a copy of the source code for xdm is almost a necessity. No attempt has been
made to rationalize or standardize the output.
-error error_log_file
Specifies the value for the DisplayManager.errorLogFile resource. This file contains errors from
xdm as well as anything written to stderr by the various scripts and programs run during the
progress of the session.
-resources resource_file
Specifies the value for the DisplayManager*resources resource. This file is loaded using xrdb(1)
to specify configuration parameters for the authentication widget.
-server server_entry
Specifies the value for the DisplayManager.servers resource. See the section Local Server
Specification for a description of this resource.
-udpPort port_number
Specifies the value for the DisplayManager.requestPort resource. This sets the port-number which
xdm will monitor for XDMCP requests. If set to 0, xdm will not listen for XDMCP or Chooser
requests. As XDMCP uses the registered well-known UDP port 177, this resource should not be
changed to a value other than 0, except for debugging.
-session session_program
Specifies the value for the DisplayManager*session resource. This indicates the program to run as
the session after the user has logged in.
-xrm resource_specification
Allows an arbitrary resource to be specified, as in most X Toolkit applications.
RESOURCES
At many stages the actions of xdm can be controlled through the use of its configuration file, which is
in the X resource format. Some resources modify the behavior of xdm on all displays, while others modify
its behavior on a single display. Where actions relate to a specific display, the display name is
inserted into the resource name between ``DisplayManager'' and the final resource name segment.
For local displays, the resource name and class are as read from the Xservers file.
For remote displays, the resource name is what the network address of the display resolves to. See the
removeDomain resource. The name must match exactly; xdm is not aware of all the network aliases that
might reach a given display. If the name resolve fails, the address is used. The resource class is as
sent by the display in the XDMCP Manage request.
Because the resource manager uses colons to separate the name of the resource from its value and dots to
separate resource name parts, xdm substitutes underscores for both dots and colons when generating the
resource name. For example, DisplayManager.expo_x_org_0.startup is the name of the resource which
defines the startup shell file for the ``expo.x.org:0'' display.
DisplayManager.servers
This resource either specifies a file name full of server entries, one per line (if the value
starts with a slash), or a single server entry. See the section Local Server Specification for
the details.
DisplayManager.requestPort
This indicates the UDP port number which xdm uses to listen for incoming XDMCP requests. Unless
you need to debug the system, leave this with its default value of 177.
DisplayManager.errorLogFile
Error output is normally directed at the system console. To redirect it, set this resource to a
file name. A method to send these messages to syslog should be developed for systems which
support it; however, the wide variety of interfaces precludes any system-independent
implementation. This file also contains any output directed to stderr by the Xsetup, Xstartup,
Xsession and Xreset files, so it will contain descriptions of problems in those scripts as well.
DisplayManager.debugLevel
If the integer value of this resource is greater than zero, reams of debugging information will be
printed. It also disables daemon mode, which would redirect the information into the bit-bucket,
and allows non-root users to run xdm, which would normally not be useful.
DisplayManager.daemonMode
Normally, xdm attempts to make itself into a daemon process unassociated with any terminal. This
is accomplished by forking and leaving the parent process to exit, then closing file descriptors
and releasing the controlling terminal. In some environments this is not desired (in particular,
when debugging). Setting this resource to ``false'' will disable this feature.
DisplayManager.pidFile
The filename specified will be created to contain an ASCII representation of the process-id of the
main xdm process. Xdm also uses file locking on this file to attempt to eliminate multiple
daemons running on the same machine, which would cause quite a bit of havoc.
DisplayManager.lockPidFile
This is the resource 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.
DisplayManager.authDir
This names a directory under which xdm stores authorization files while initializing the session.
The default value is /var/lib/xdm. Can be overridden for specific displays by
DisplayManager.DISPLAY.authFile.
DisplayManager.autoRescan
This boolean controls whether xdm rescans the configuration, servers, access control and
authentication keys files after a session terminates and the files have changed. By default it is
``true.'' You can force xdm to reread these files by sending a SIGHUP to the main process.
DisplayManager.removeDomainname
When computing the display name for XDMCP clients, the name resolver will typically create a fully
qualified host name for the terminal. As this is sometimes confusing, xdm will remove the domain
name portion of the host name if it is the same as the domain name of the local host when this
variable is set. By default the value is ``true.''
DisplayManager.keyFile
XDM-AUTHENTICATION-1 style XDMCP authentication requires that a private key be shared between xdm
and the terminal. This resource specifies the file containing those values. Each entry in the
file consists of a display name and the shared key. By default, xdm does not include support for
XDM-AUTHENTICATION-1, as it requires DES which is not generally distributable because of United
States export restrictions.
DisplayManager.accessFile
To prevent unauthorized XDMCP service and to allow forwarding of XDMCP IndirectQuery requests,
this file contains a database of hostnames which are either allowed direct access to this machine,
or have a list of hosts to which queries should be forwarded to. The format of this file is
described in the section XDMCP Access Control.
DisplayManager.exportList
A list of additional environment variables, separated by white space, to pass on to the Xsetup,
Xstartup, Xsession, and Xreset programs.
DisplayManager.randomFile
A file to checksum to generate the seed of authorization keys. This should be a file that changes
frequently. The default is /dev/mem.
DisplayManager.randomDevice
A file to read 8 bytes from to generate the seed of authorization keys. The default is
/dev/urandom . If this file cannot be read, or if a read blocks for more than 5 seconds, xdm falls
back to using a checksum of DisplayManager.randomFile to generate the seed.
DisplayManager.prngdSocket
DisplayManager.prngPort
A UNIX domain socket name or a TCP socket port number on local host on which a Pseudo-Random
Number Generator Daemon, like EGD (http://egd.sourceforge.net) is listening, in order to generate
the autorization keys. Either a non null port or a valid socket name must be specified. The
default is to use the Unix-domain socket /tmp/entropy.
On systems that don't have such a daemon, a fall-back entropy gathering system, based on various log file
contents hashed by the MD5 algorithm is used instead.
DisplayManager.greeterLib
On systems that support a dynamically-loadable greeter library, the name of the library. The
default is
/usr/lib/X11/xdm/libXdmGreet.so.
DisplayManager.choiceTimeout
Number of seconds to wait for display to respond after user has selected a host from the chooser.
If the display sends an XDMCP IndirectQuery within this time, the request is forwarded to the
chosen host. Otherwise, it is assumed to be from a new session and the chooser is offered again.
Default is 15.
DisplayManager.sourceAddress
Use the numeric IP address of the incoming connection on multihomed hosts instead of the host
name. This is to avoid trying to connect on the wrong interface which might be down at this time.
DisplayManager.willing
This specifies a program which is run (as) root when an an XDMCP BroadcastQuery is received and
this host is configured to offer XDMCP display management. The output of this program may be
displayed on a chooser window. If no program is specified, the string Willing to manage is sent.
DisplayManager.DISPLAY.resources
This resource specifies the name of the file to be loaded by xrdb as the resource database onto
the root window of screen 0 of the display. The Xsetup program, the Login widget, and chooser
will use the resources set in this file. This resource data base is loaded just before the
authentication procedure is started, so it can control the appearance of the login window. See
the section Authentication Widget, which describes the various resources that are appropriate to
place in this file. There is no default value for this resource, but
/etc/X11/xdm/Xresources is the conventional name.
DisplayManager.DISPLAY.chooser
Specifies the program run to offer a host menu for Indirect queries redirected to the special host
name CHOOSER.
/usr/lib/X11/xdm/chooser is the default. See the sections XDMCP Access Control and Chooser.
DisplayManager.DISPLAY.xrdb
Specifies the program used to load the resources. By default, xdm uses /usr/bin/xrdb.
DisplayManager.DISPLAY.cpp
This specifies the name of the C preprocessor which is used by xrdb.
DisplayManager.DISPLAY.setup
This specifies a program which is run (as root) before offering the Login window. This may be
used to change the appearance of the screen around the Login window or to put up other windows
(e.g., you may want to run xconsole here). By default, no program is run. The conventional name
for a file used here is Xsetup. See the section Setup Program.
DisplayManager.DISPLAY.startup
This specifies a program which is run (as root) after the authentication process succeeds. By
default, no program is run. The conventional name for a file used here is Xstartup. See the
section Startup Program.
DisplayManager.DISPLAY.session
This specifies the session to be executed (not running as root). By default, /usr/bin/xterm is
run. The conventional name is Xsession. See the section Session Program.
DisplayManager.DISPLAY.reset
This specifies a program which is run (as root) after the session terminates. By default, no
program is run. The conventional name is Xreset. See the section Reset Program.
DisplayManager.DISPLAY.openDelay
DisplayManager.DISPLAY.openRepeat
DisplayManager.DISPLAY.openTimeout
DisplayManager.DISPLAY.startAttempts
DisplayManager.DISPLAY.reservAttempts
These numeric resources control the behavior of xdm when attempting to open intransigent servers.
openDelay is the length of the pause in seconds between successive attempts, openRepeat is the
number of attempts to make, openTimeout is the amount of time to wait while actually attempting
the open (i.e., the maximum time spent in the connect(2) system call) and startAttempts is the
number of times this entire process is done before giving up on the server. After openRepeat
attempts have been made, or if openTimeout seconds elapse in any particular attempt, xdm
terminates and restarts the server, attempting to connect again. This process is repeated
startAttempts times, at which point the display is declared dead and disabled. Although this
behavior may seem arbitrary, it has been empirically developed and works quite well on most
systems. The bound reservAttempts is the number of times a successful connect is allowed to be
followed by a fatal error. When reached, the display is disabled. The default values are
openDelay: 15, openRepeat: 5, openTimeout: 120, startAttempts: 4 and reservAttempts: 2.
DisplayManager.DISPLAY.pingInterval
DisplayManager.DISPLAY.pingTimeout
To discover when remote displays disappear, xdm occasionally pings them, using an X connection and
XSync calls. pingInterval specifies the time (in minutes) between each ping attempt, pingTimeout
specifies the maximum amount of time (in minutes) to wait for the terminal to respond to the
request. If the terminal does not respond, the session is declared dead and terminated. By
default, both are set to 5 minutes. If you frequently use X terminals which can become isolated
from the managing host, you may wish to increase this value. The only worry is that sessions will
continue to exist after the terminal has been accidentally disabled. xdm will not ping local
displays. Although it would seem harmless, it is unpleasant when the workstation session is
terminated as a result of the server hanging for NFS service and not responding to the ping.
DisplayManager.DISPLAY.terminateServer
This boolean resource specifies whether the X server should be terminated when a session
terminates (instead of resetting it). This option can be used when the server tends to grow
without bound over time, in order to limit the amount of time the server is run. The default
value is ``false.''
DisplayManager.DISPLAY.userPath
Xdm sets the PATH environment variable for the session to this value. It should be a colon
separated list of directories; see sh(1) for a full description. The default value is
``/usr/local/bin:/usr/bin:/bin:/usr/games''.
DisplayManager.DISPLAY.systemPath
Xdm sets the PATH environment variable for the startup and reset scripts to the value of this
resource. The default for this resource is
``/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin''. Note the absence of ``.'' from
this entry. This is a good practice to follow for root; it avoids many common Trojan Horse system
penetration schemes.
DisplayManager.DISPLAY.systemShell
Xdm sets the SHELL environment variable for the startup and reset scripts to the value of this
resource. It is /bin/sh by default.
DisplayManager.DISPLAY.failsafeClient
If the default session fails to execute, xdm will fall back to this program. This program is
executed with no arguments, but executes using the same environment variables as the session would
have had (see the section Session Program). By default, /usr/bin/xterm is used.
DisplayManager.DISPLAY.grabServer
DisplayManager.DISPLAY.grabTimeout
To improve security, xdm grabs the server and keyboard while reading the login name and password.
The grabServer resource specifies if the server should be held for the duration of the
name/password reading. When ``false,'' the server is ungrabbed after the keyboard grab succeeds,
otherwise the server is grabbed until just before the session begins. The default is ``false.''
The grabTimeout resource specifies the maximum time xdm will wait for the grab to succeed. The
grab may fail if some other client has the server grabbed, or possibly if the network latencies
are very high. This resource has a default value of 3 seconds; you should be cautious when
raising it, as a user can be spoofed by a look-alike window on the display. If the grab fails,
xdm kills and restarts the server (if possible) and the session.
DisplayManager.DISPLAY.authorize
DisplayManager.DISPLAY.authName
authorize is a boolean resource which controls whether xdm 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. When authorize is
set for a display and authorization is not available, the user is informed by having a different
message displayed in the login widget. By default, authorize is ``true,'' authName is ``MIT-
MAGIC-COOKIE-1,'' or, if XDM-AUTHORIZATION-1 is available, ``XDM-AUTHORIZATION-1 MIT-MAGIC-
COOKIE-1.''
DisplayManager.DISPLAY.authFile
This file is used to communicate the authorization data from xdm to the server, using the -auth
server command line option. It should be kept in a directory which is not world-writable as it
could easily be removed, disabling the authorization mechanism in the server. If not specified, a
name is generated from DisplayManager.authDir and the name of the display.
DisplayManager.DISPLAY.authComplain
If set to ``false,'' disables the use of the unsecureGreeting in the login window. See the
section Authentication Widget. The default is ``true.''
DisplayManager.DISPLAY.resetSignal
The number of the signal xdm sends to reset the server. See the section Controlling the Server.
The default is 1 (SIGHUP).
DisplayManager.DISPLAY.termSignal
The number of the signal xdm sends to terminate the server. See the section Controlling the
Server. The default is 15 (SIGTERM).
DisplayManager.DISPLAY.resetForAuth
The original implementation of authorization in the sample server reread the authorization file at
server reset time, instead of when checking the initial connection. As xdm generates the
authorization information just before connecting to the display, an old server would not get up-
to-date authorization information. This resource causes xdm to send SIGHUP to the server after
setting up the file, causing an additional server reset to occur, during which time the new
authorization information will be read. The default is ``false,'' which will work for all MIT
servers.
DisplayManager.DISPLAY.userAuthDir
When xdm is unable to write to the usual user authorization file ($HOME/.Xauthority), it creates a
unique file name in this directory and points the environment variable XAUTHORITY at the created
file. It uses /tmp by default.
CONFIGURATION FILE
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.
XDMCP ACCESS CONTROL
The database file specified by the DisplayManager.accessFile provides information which xdm uses to
control access from displays requesting XDMCP service. This file contains three types of entries:
entries which control the response to Direct and Broadcast queries, entries which control the response to
Indirect queries, and macro definitions.
The format of the Direct entries is simple, either a host name or a pattern, which is distinguished from
a host name by the inclusion of one or more meta characters (`*' matches any sequence of 0 or more
characters, and `?' matches any single character) which are compared against the host name of the display
device. If the entry is a host name, all comparisons are done using network addresses, so any name which
converts to the correct network address may be used. For 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.
Examples:
rincewind.sample.net ff02::1 #IPv6 Multicast to ff02::1
#with a hop count of 1
ponder.sample.net CHOOSER 239.192.1.1/16 #Offer a menu of hosts
#who respond to IPv4 Multicast
# to 239.192.1.1 with a TTL of 16
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.
LISTEN
The following configuration directive is also defined for the Xaccess configuration file:
LISTEN interface [list of multicast group addresses]
interface may be a hostname or IP address representing a network interface on this machine, or the
wildcard * to represent all available network interfaces.
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.
Examples:
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.
IPv6 MULTICAST ADDRESS SPECIFICATION
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.
LOCAL SERVER SPECIFICATION
The resource DisplayManager.servers gives a server specification or, if the values starts with a slash
(/), the name of a file containing server specifications, one per line.
Each specification indicates a display which should constantly be 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 Digital-QV local /usr/bin/X :0
The display types are:
local local display: xdm must run the server
foreign remote display: xdm opens an X connection to a running server
The display name must be something that can be passed in the -display option to an X program. This
string is used to generate the display-specific resource names, so be careful to match the names (e.g.,
use ``:0 Sun-CG3 local /usr/bin/X :0'' instead of ``localhost:0 Sun-CG3 local /usr/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.
RESOURCES FILE
The Xresources file is loaded onto the display as a resource database using xrdb. As the authentication
widget reads this database before starting up, it usually contains parameters for that widget:
xlogin*login.translations: #override\
Ctrl<Key>R: abort-display()\n\
<Key>F1: set-session-argument(failsafe) finish-field()\n\
<Key>Return: set-session-argument() finish-field()
xlogin*borderWidth: 3
xlogin*greeting: CLIENTHOST
#ifdef COLOR
xlogin*greetColor: CadetBlue
xlogin*failColor: red
#endif
Please note the translations entry; it specifies a few new translations for the widget which allow users
to escape from the default session (and avoid troubles that may occur in it). Note that if #override is
not specified, the default translations are removed and replaced by the new value, not a very useful
result as some of the default translations are quite useful (such as ``<Key>: insert-char ()'' which
responds to normal typing).
This file may also contain resources for the setup program and chooser.
SETUP PROGRAM
The Xsetup file is run after the server is reset, but before the Login window is offered. The file is
typically a shell script. It is run as root, so should be careful about security. This is the place to
change the root background or bring up other windows that should appear on the screen along with the
Login widget.
In addition to any specified by DisplayManager.exportList, the 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 &
AUTHENTICATION WIDGET
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).
xlogin.Login.width, xlogin.Login.height, xlogin.Login.x, xlogin.Login.y
The geometry of the Login widget is normally computed automatically. If you wish to position it
elsewhere, specify each of these resources.
xlogin.Login.foreground
The color used to display the input typed by the user.
xlogin.Login.face
The face used to display the input typed by the user when built with Xft support. The default is
``Serif-18''.
xlogin.Login.font
The font used to display the input typed by the user when not built with Xft support.
xlogin.Login.greeting
A string which identifies this window. The default is ``X Window System.''
xlogin.Login.unsecureGreeting
When X authorization is requested in the configuration file for this display and none is in use,
this greeting replaces the standard greeting. The default is ``This is an unsecure session''
xlogin.Login.greetFace
The face used to display the greeting when built with Xft support. The default is
``Serif-24:italic''.
xlogin.Login.greetFont
The font used to display the greeting when not built with Xft support.
xlogin.Login.greetColor
The color used to display the greeting.
xlogin.Login.namePrompt
The string displayed to prompt for a user name. Xrdb strips trailing white space from resource
values, so to add spaces at the end of the prompt (usually a nice thing), add spaces escaped with
backslashes. The default is ``Login: ''
xlogin.Login.passwdPrompt
The string displayed to prompt for a password, when not using an authentication system such as PAM
that provides its own prompts. The default is ``Password: ''
xlogin.Login.promptFace
The face used to display prompts when built with Xft support. The default is ``Serif-18:bold''.
xlogin.Login.promptFont
The font used to display prompts when not built with Xft support.
xlogin.Login.promptColor
The color used to display prompts.
xlogin.Login.changePasswdMessage
A message which is displayed when the users password has expired. The default is ``Password
Change Required''
xlogin.Login.fail
A message which is displayed when the authentication fails, when not using an authentication
system such as PAM that provides its own prompts. The default is ``Login incorrect''
xlogin.Login.failFace
The face used to display the failure message when built with Xft support. The default is
``Serif-18:bold''.
xlogin.Login.failFont
The font used to display the failure message when not built with Xft support.
xlogin.Login.failColor
The color used to display the failure message.
xlogin.Login.failTimeout
The number of seconds that the failure message is displayed. The default is 10.
xlogin.Login.logoFileName
Name of an XPM format pixmap to display in the greeter window, if built with XPM support. The
default is no pixmap.
xlogin.Login.logoPadding
Number of pixels of space between the logo pixmap and other elements of the greeter window, if the
pixmap is displayed. The default is 5.
xlogin.Login.useShape
If set to ``true'', when built with XPM support, attempt to use the X Non-Rectangular Window Shape
Extension to set the window shape. The default is ``true''.
xlogin.Login.hiColor, xlogin.Login.shdColor
Raised appearance bezels may be drawn around the greeter frame and text input boxes by setting
these resources. hiColor is the highlight color, used on the top and left sides of the frame, and
the bottom and right sides of text input areas. shdColor is the shadow color, used on the bottom
and right sides of the frame, and the top and left sides of text input areas. The default for
both is the foreground color, providing a flat appearance.
xlogin.Login.frameWidth
frameWidth is the width in pixels of the area around the greeter frame drawn in hiColor and
shdColor.
xlogin.Login.innerFramesWidth
innerFramesWidth is the width in pixels of the area around text input areas drawn in hiColor and
shdColor.
xlogin.Login.sepWidth
sepWidth is the width in pixels of the bezeled line between the greeting and input areas drawn in
hiColor and shdColor.
xlogin.Login.allowRootLogin
If set to ``false'', don't allow root (and any other user with uid = 0) to log in directly. The
default is ``true''. This setting is only checked by some of the authentication backends at this
time.
xlogin.Login.allowNullPasswd
If set to ``true'', allow an otherwise failing password match to succeed if the account does not
require a password at all. The default is ``false'', so only users that have passwords assigned
can log in.
xlogin.Login.echoPasswd
If set to ``true'', a placeholder character (echoPasswdChar) will be shown for fields normally set
to not echo, such as password input. The default is ``false''.
xlogin.Login.echoPasswdChar
Character to display if echoPasswd is true. The default is ``*''. If set to an empty value, the
cursor will advance for each character input, but no text will be drawn.
xlogin.Login.translations
This specifies the translations used for the login widget. Refer to the X Toolkit documentation
for a complete discussion on translations. The default translation table is:
Ctrl<Key>H: delete-previous-character() \n\
Ctrl<Key>D: delete-character() \n\
Ctrl<Key>B: move-backward-character() \n\
Ctrl<Key>F: move-forward-character() \n\
Ctrl<Key>A: move-to-begining() \n\
Ctrl<Key>E: move-to-end() \n\
Ctrl<Key>K: erase-to-end-of-line() \n\
Ctrl<Key>U: erase-line() \n\
Ctrl<Key>X: erase-line() \n\
Ctrl<Key>C: restart-session() \n\
Ctrl<Key>\\: abort-session() \n\
<Key>BackSpace:delete-previous-character() \n\
<Key>Delete: delete-previous-character() \n\
<Key>Return: finish-field() \n\
<Key>: insert-char() \
The actions which are supported by the widget are:
delete-previous-character
Erases the character before the cursor.
delete-character
Erases the character after the cursor.
move-backward-character
Moves the cursor backward.
move-forward-character
Moves the cursor forward.
move-to-begining
(Apologies about the spelling error.) Moves the cursor to the beginning of the editable text.
move-to-end
Moves the cursor to the end of the editable text.
erase-to-end-of-line
Erases all text after the cursor.
erase-line
Erases the entire text.
finish-field
If the cursor is in the name field, proceeds to the password field; if the cursor is in the
password field, checks the 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.
abort-session
Terminates and restarts the server.
abort-display
Terminates the server, disabling it. This action is not accessible in the default configuration.
There are various reasons to stop xdm on a system console, such as when shutting the system down,
when using xdmshell, to start another type of server, or to generally access the console. Sending
xdm a SIGHUP will restart the display. See the section Controlling XDM.
restart-session
Resets the X server and starts a new session. This can be used when the resources have been
changed and you want to test them or when the screen has been overwritten with system messages.
insert-char
Inserts the character typed.
set-session-argument
Specifies a single word argument which is passed to the session at startup. See the section
Session Program.
allow-all-access
Disables access control in the server. This can be used when the .Xauthority file cannot be
created by xdm. Be very careful using this; it might be better to disconnect the machine from the
network before doing this.
On some systems (OpenBSD) the user's shell must be listed in /etc/shells to allow login through xdm. The
normal password and account expiration dates are enforced too.
STARTUP PROGRAM
The Xstartup program is run as root when the user logs in. It is 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
SESSION PROGRAM
The Xsession program is the command which is run as the user's session. It is run with the permissions
of the authorized user.
In addition to any specified by DisplayManager.exportList, the 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
RESET PROGRAM
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
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 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.
ADDITIONAL LOCAL DISPLAYS
To add an additional local display, add a line for it to the Xservers file. (See the section Local
Server Specification.)
Examine the display-specific resources in xdm-config (e.g., 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.
OTHER POSSIBILITIES
You can use xdm to run a single session at a time, using the 4.3 init options or other suitable daemon by
specifying the server on the command line:
xdm -server “:0 SUN-3/60CG4 local /usr/bin/X :0”
Or, you might have a file server and a collection of X terminals. The configuration for this is
identical to the sample above, except the Xservers file would look like
extol:0 VISUAL-19 foreign
exalt:0 NCD-19 foreign
explode:0 NCR-TOWERVIEW3000 foreign
This directs xdm to manage sessions on all three of these terminals. See the section Controlling Xdm for
a description of using signals to enable and disable these terminals in a manner reminiscent of init(8).
LIMITATIONS
One thing that xdm isn't very good at doing is coexisting with other window systems. To use multiple
window systems on the same hardware, you'll probably be more interested in xinit.
ASYNCHRONOUS EVENTS
xdm uses SIGALRM and SIGUSR1 for its own inter-process communication purposes, managing the relationship
between the parent xdm process and its children. Sending these signals to any xdm process may result in
unexpected behavior.
SIGHUP causes xdm to rescan its configuration files and reopen its log file.
SIGTERM
causes xdm to terminate its children and shut down.
SIGUSR2
causes xdm to reopen its log file. This is useful if log rotation is desired, but SIGHUP is too
disruptive.
FILES
/etc/X11/xdm/xdm-config
the default configuration file
$HOME/.Xauthority user authorization file where xdm stores keys for clients to read
/usr/lib/X11/xdm/chooser
the default chooser
/usr/bin/xrdb the default resource database loader
/usr/bin/X the default server
/usr/bin/xterm the default session program and failsafe client
/var/lib/xdm/A<display>-<suffix>
the default place for authorization files
/tmp/K5C<display> Kerberos credentials cache
SEE ALSO
X(7), xinit(1), xauth(1), xrdb(1), Xsecurity(7), sessreg(1), Xserver(1), xdmshell(1), fonts.conf(5),
xdm.options(5).
X Display Manager Control Protocol
IETF RFC 4291: IP Version 6 Addressing Architecture.
AUTHOR
Keith Packard, MIT X Consortium