Table of Contents
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 ]
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.
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
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.
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
The file /usr/X11R6/lib/X11/xdm/xdm-errors 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.
- -config configuration_file
- Names the configuration file,
which specifies resources to control the behavior of xdm. /usr/X11R6/lib/X11/xdm/xdm-config
is the default. See the section Configuration File.
- Specifies ``false''
as the value for the DisplayManager.daemonMode resource. This suppresses
the normal daemon behavior, which is for xdm to close all file descriptors,
disassociate itself from the controlling terminal, and put itself in the
background when it first starts up.
- -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
- -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
- -resources resource_file
- Specifies the value for the DisplayManager*resources
resource. This file is loaded using xrdb to specify configuration parameters
for the authentication widget.
- -server server_entry
- Specifies the value for
the DisplayManager.servers resource. See the section 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. As XDMCP uses the registered well-known
UDP port 177, this resource should not be changed except for debugging.
If set to 0 xdm will not listen for XDMCP or Chooser requests.
- -session session_program
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
an arbitrary resource to be specified, as in most X Toolkit applications.
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
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''
- 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
- 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.
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.
- 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
- Normally, xdm attempts to make itself
into a daemon process unassociated with any terminal. This is accomplished
by forking and leaving the parent process to exit, then closing file descriptors
and releasing the controlling terminal. In some environments this is not
desired (in particular, when debugging). Setting this resource to ``false''
will disable this feature.
- The filename specified will
be created to contain an ASCII representation of the process-id of the main
xdm process. Xdm also uses file locking on this file to attempt to eliminate
multiple daemons running on the same machine, which would cause quite a
bit of havoc.
- This is the resource which controls
whether xdm uses file locking to keep multiple display managers from running
amok. On System V, this uses the lockf library call, while on BSD it uses
- This names a directory under which xdm stores
authorization files while initializing the session. The default value is
/usr/X11R6/lib/X11/xdm. Can be overridden for specific displays by DisplayManager.DISPLAY.authFile.
- 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.
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.''
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.
- 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
- A list of additional environment
variables, separated by white space, to pass on to the Xsetup, Xstartup,
Xsession, and Xreset programs.
- A file to checksum
to generate the seed of authorization keys. This should be a file that changes
frequently. The default is /dev/mem.
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.
systems that support a dynamically-loadable greeter library, the name of
the library. The default is /usr/X11R6/lib/X11/xdm/libXdmGreet.so.
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.
- 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.
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.
First, the xdm configuration file should be set up. Make a directory
(usually /usr/X11R6/lib/X11/xdm) to contain all of the relevant files.
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 /usr/X11R6/lib/X11/xdm/Xresources is the conventional name.
- Specifies the program run to offer a host menu
for Indirect queries redirected to the special host name CHOOSER. /usr/X11R6/lib/X11/xdm/chooser
is the default. See the sections XDMCP Access Control and Chooser.
the program used to load the resources. By default, xdm uses /usr/X11R6/bin/xrdb.
- This specifies the name of the C preprocessor which
is used by xrdb.
- This specifies a program which
is run (as root) before offering the Login window. This may be used to
change the appearance of the screen around the Login window or to put up
other windows (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
- 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.
- This specifies
the session to be executed (not running as root). By default, /usr/X11R6/bin/xterm
is run. The conventional name is Xsession. See the section Session Program.
- 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.
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 default values are 5 for openDelay, 5 for
openRepeat, 30 for openTimeout and 4 for startAttempts.
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.
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.''
sets the PATH environment variable for the session to this value. It should
be a colon separated list of directories; see sh(1)
for a full description.
``:/bin:/usr/bin:/usr/X11R6/bin:/usr/ucb'' is a common setting. The default
value can be specified at build time in the X system configuration file
- Xdm sets the PATH environment
variable for the startup and reset scripts to the value of this resource.
The default for this resource is specified at build time by the DefaultSystemPath
entry in the system configuration file; ``/etc:/bin:/usr/bin:/usr/X11R6/bin:/usr/ucb''
is a common choice. Note the absence of ``.'' from this entry. This is a good
practice to follow for root; it avoids many common Trojan Horse system
- Xdm sets the SHELL
environment variable for the startup and reset scripts to the value of
this resource. It is /bin/sh by default.
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/X11R6/bin/xterm is used.
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.
- 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
- 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.
- If set to
``false,'' disables the use of the unsecureGreeting in the login window. See
the section Authentication Widget. The default is ``true.''
number of the signal xdm sends to reset the server. See the section Controlling
the Server. The default is 1 (SIGHUP).
number of the signal xdm sends to terminate the server. See the section
Controlling the Server. The default is 15 (SIGTERM).
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.
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.
Here is a reasonable configuration file, which could be named xdm-config:
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.
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.
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
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
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 \
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
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:
- LISTEN interface [list of multicast
- interface may be a hostname or IP addresss 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.
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
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 Node-Local, 2 for Link-Local, 5
for Site-Local, and so on. (See IETF RFC 2373 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.
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/X11R6/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/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.
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:
<Key>F1: set-session-argument(failsafe) finish-field()\n\
<Key>Return: set-session-argument() finish-field()
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).
may also contain resources for the setup program and chooser.
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
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:
# Xsetup_0 - setup script for one workstation
xcmsdb < /usr/X11R6/lib/monitors/alex.0
xconsole -geometry 480x130-0-0 -notify -verbose -exitOnFail &
The authentication widget reads a name/password pair
from the keyboard. Nearly every imaginable parameter can be controlled
with a resource. Resources for this widget should be put into the file
named by DisplayManager.DISPLAY.resources. All of these have reasonable default
values, so it is not necessary to specify any of them.
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.
- The color used to
display the typed-in user name.
- The font used to display the
typed-in user name.
- A string which identifies this window.
The default is ``X Window System.''
- When X authorization
is requested in the configuration file for this display and none is in
use, this greeting replaces the standard greeting. The default is ``This
is an unsecure session''
- The font used to display the
- The color used to display the greeting.
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:
- The string displayed to prompt for a password.
The default is ``Password: ''
- The font used to display
- The color used to display both prompts.
- A message which is displayed when the authentication fails.
The default is ``Login incorrect''
- The font used to display
the failure message.
- The color used to display the failure
- The number of seconds that the failure message
is displayed. The default is 30.
- If set to ``false'',
don't allow root (and any other user with uid = 0) to log in directly. The
default is ``true''.
- 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.
- 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:
the character before the cursor.
- Erases the character after
- Moves the cursor backward.
the cursor forward.
- (Apologies about the spelling error.) Moves
the cursor to the beginning of the editable text.
- Moves the cursor
to the end of the editable text.
- Erases all text after the
- Erases the entire text.
- If the cursor is in the
name field, proceeds to the password field; if the cursor is in the password
field, checks the current name/password pair. If the name/password pair
is valid, xdm starts the session. Otherwise the failure message is displayed
and the user is prompted again.
- Terminates and restarts the
- 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
- 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.
- Inserts the character typed.
- Specifies a single
word argument which is passed to the session at startup. See the section
- 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.
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 /etc/utmp (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
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.
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:
# This program is run as root after the user is verified
if [ -f /etc/nologin ]; then
xmessage -file /etc/nologin -timeout 30 -center
sessreg -a -l $DISPLAY -x /usr/X11R6/lib/xdm/Xservers $LOGNAME
The Xsession program is the command which is run as the
user's session. It is run with the permissions of the authorized user.
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
At most installations, Xsession should look in $HOME for a file .xsession,
which contains commands that each user would like to use as a session. Xsession
should also implement a system default session if no user-specified session
exists. See the section Typical Usage.
An argument may be passed to this
program from the authentication widget using the `set-session-argument' action.
This can be used to select different styles of session. One good use of
this feature is to allow the user to escape from the ordinary session when
it fails. This allows users to repair their own .xsession if it fails, without
requiring administrative intervention. The example following demonstrates
This 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.
# This is the program that is run as the client
# for the display manager.
case $# in
case $1 in
exec xterm -geometry 80x24-0-0
if [ -f "$startup" ]; then
if [ -f "$resources" ]; then
xrdb -load "$resources"
xman -geometry +10-10 &
exec xterm -geometry 80x24+10+10 -ls
The user's .xsession file might look something like this example. Don't forget
that the file must have execute permission.
# no -f in the previous line so .cshrc gets run to set $PATH
xrdb -merge "$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, removing entries from /etc/utmp
or unmounting directories from file servers. The environment variables
that were passed to Xstartup are also passed to Xreset.
A sample Xreset
# This program is run as root after the session ends
sessreg -d -l $DISPLAY -x /usr/X11R6/lib/xdm/Xservers $LOGNAME
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.
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
Xdm attempts to mark its various sub-processes for ps(1)
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 lq:0 SUN-3/60CG4 local /usr/X11R6/bin/X :0rq
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
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)
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.
- the default
- user authorization file where xdm stores
keys for clients to read
- the default chooser
- the default resource database loader
- the default server
- the default session program and
- the default place
for authorization files
- Kerberos credentials cache
X Display Manager Control Protocol
Keith Packard, MIT X Consortium
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