xdm
(1)
Name
xdm - X Display Manager with support for XDMCP, host chooser
Synopsis
/usr/sbin/xdm [ -config configuration_file ] [ -nodaemon ] [
-debug debug_level ] [ -error error_log_file ] [ -resources
resource_file ] [ -server server_entry ] [ -session ses-
sion_program ]
Description
User Commands XDM(1)
NAME
xdm - X Display Manager with support for XDMCP, host chooser
SYNOPSIS
/usr/sbin/xdm [ -config configuration_file ] [ -nodaemon ] [
-debug debug_level ] [ -error error_log_file ] [ -resources
resource_file ] [ -server server_entry ] [ -session ses-
sion_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 stan-
dard 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 ``ses-
sion.''
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 necessar-
ily have any terminal-like interface with which to connect.
When a real session manager is not available, a window man-
ager or terminal emulator is typically used as the ``session
manager,'' meaning that termination of this process termi-
nates 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 man-
agement. 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 them-
selves.
Because xdm provides the first interface that users will
see, it is designed to be simple to use and easy to cus-
tomize 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
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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 speci-
fied 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, Xses-
sion 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.
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-config configuration_file
Names the configuration file, which specifies resources
to control the behavior of xdm. /etc/X11/xdm/xdm-con-
fig is the default. See the section Configuration
File.
-nodaemon
Specifies ``false'' as the value for the DisplayMan-
ager.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 DisplayMan-
ager.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 inter-
pret 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
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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 sin-
gle 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 con-
sole. 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-
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independent implementation. This file also contains
any output directed to stderr by the Xsetup, Xstartup,
Xsession and Xreset files, so it will contain descrip-
tions 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 accom-
plished 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 autho-
rization files while initializing the session. The
default value is /var/run/xdm. Can be overridden for
specific displays by DisplayManager.DISPLAY.authFile.
DisplayManager.autoRescan
This boolean controls whether xdm rescans the configu-
ration, 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
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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 con-
taining 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 for-
warding of XDMCP IndirectQuery requests, this file con-
tains 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 Con-
trol.
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 authoriza-
tion keys. This should be a file that changes fre-
quently. 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 lis-
tening, in order to generate the autorization keys.
Either a non null port or a valid socket name must be
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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
/etc/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 dis-
play 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 out-
put of this program may be displayed on a chooser win-
dow. 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.
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/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
These numeric resources control the behavior of xdm
when attempting to open intransigent servers. openDe-
lay 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 max-
imum time spent in the connect(2) system call) and
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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 termi-
nates and restarts the server, attempting to connect
again. This process is repeated startAttempts times,
at which point the display is declared dead and dis-
abled. 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 startAt-
tempts.
DisplayManager.DISPLAY.pingInterval
DisplayManager.DISPLAY.pingTimeout
To discover when remote displays disappear, xdm occa-
sionally 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 ter-
minated. By default, both are set to 5 minutes. If
you frequently use X terminals which can become iso-
lated from the managing host, you may wish to increase
this value. The only worry is that sessions will con-
tinue 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 work-
station 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/bin:/usr/sbin''.
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/bin:/usr/sbin''.
Note the absence of ``.'' from this entry. This is a
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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 sec-
tion 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 grab-
Server 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 max-
imum 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 auth-
Name 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 dis-
played in the login widget. By default, authorize is
``true,'' authName is ``MIT-MAGIC-COOKIE-1,'' or, if
XDM-AUTHORIZATION-1 is available, ``XDM-AUTHORIZA-
TION-1 MIT-MAGIC-COOKIE-1.''
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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, dis-
abling the authorization mechanism in the server. If
not specified, a name is generated from DisplayMan-
ager.authDir and the name of the display.
DisplayManager.DISPLAY.authComplain
If set to ``false,'' disables the use of the unsecure-
Greeting in the login window. See the section Authen-
tication 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 con-
nection. As xdm generates the authorization informa-
tion just before connecting to the display, an old
server would not get up-to-date authorization informa-
tion. 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 autho-
rization 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 autho-
rization 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 rel-
evant files.
Here is a reasonable configuration file, which could be
named xdm-config:
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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/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 discus-
sion.
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 com-
parison, so ensure that you do not attempt to match aliases.
Preceding either a host name or a pattern with a `!' charac-
ter 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 dis-
tinguish macros from hostnames, macro names start with a `%'
character. Macros may be nested.
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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 deter-
mines 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 delim-
iter causing the rest of that line to be ignored, and `\new-
line' 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 spec-
ified, 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:
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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 Display-
Manager.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 cho-
sen, 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 ses-
sion 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 addresss representing
a network interface on this machine, or the wildcard *
to represent all available network interfaces.
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User Commands XDM(1)
If one or more LISTEN lines are specified, xdm only listens
for XDMCP connections on the specified interfaces. If multi-
cast 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 lis-
tening on all interfaces is preserved for backwards compati-
bility. 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 previ-
ously 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 Inter-
face-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 specifi-
cation 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 con-
stantly 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 dis-
play 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
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User Commands XDM(1)
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 ``DisplayMan-
ager._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 file-
name'' 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 param-
eters for that widget:
xlogin*login.translations: #overrideCtrl<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
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User Commands XDM(1)
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 secu-
rity. 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 poten-
tial security holes here. If DisplayManager.DISPLAY.grab-
Server 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 con-
trolled with a resource. Resources for this widget should
be put into the file named by DisplayManager.DIS-
PLAY.resources. All of these have reasonable default val-
ues, 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
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User Commands XDM(1)
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(4). 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(5).
gin.Login.y
xlogin.Login.width, xlogin.Login.height, xlogin.Login.x,
xlo-
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.
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User Commands XDM(1)
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 pro-
vides 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 dis-
played. 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
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User Commands XDM(1)
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''.
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.translations
This specifies the translations used for the login wid-
get. Refer to the X Toolkit documentation for a com-
plete discussion on translations. The default
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User Commands XDM(1)
translation table is:
Ctrl<Key>H: delete-previous-character() \nCtrl<Key>D:delete-character() \nCtrl<Key>B:move-backward-character() \nCtrl<Key>F:move-forward-character() \nCtrl<Key>A:move-to-begining() \nCtrl<Key>E:move-to-end() \nCtrl<Key>K:erase-to-end-of-line() \nCtrl<Key>U:erase-line() \nCtrl<Key>X:erase-line() \nCtrl<Key>C:restart-session() \nCtrl<Key>\\:abort-session() \n<Key>BackSpace:delete-previous-character() \n<Key>Delete:delete-previous-character() \n<Key>Return:finish-field() \n<Key>:insert-char() .fi
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
X Version 11 Last change: xdm 1.1.9 21
User Commands XDM(1)
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 ses-
sion 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 ses-
sion 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
X Version 11 Last change: xdm 1.1.9 22
User Commands XDM(1)
example, but simply a demonstration of the available func-
tionality.
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 authenti-
cation 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 adminis-
trative intervention. The example following demonstrates
this feature.
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User Commands XDM(1)
This example recognizes the special ``failsafe'' mode, spec-
ified 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 per-
mission.
#! /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
X Version 11 Last change: xdm 1.1.9 24
User Commands XDM(1)
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.DIS-
PLAY.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 imme-
diately, meaning that any session in progress will be termi-
nated 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
X Version 11 Last change: xdm 1.1.9 25
User Commands XDM(1)
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 Specifica-
tion.)
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-con-
fig 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 ter-
minals. The configuration for this is identical to the sam-
ple 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 descrip-
tion of using signals to enable and disable these terminals
in a manner reminiscent of init(1m).
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.
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
X Version 11 Last change: xdm 1.1.9 26
User Commands XDM(1)
/usr/bin/X the default server
/usr/bin/xterm the default session program and failsafe
client
/var/run/xdm/A<display>-<suffix>
the default place for authorization
files
/tmp/K5C<display> Kerberos credentials cache
SEE ALSO
X(5), xinit(1), xauth(1), xrdb(1), Xsecurity(5), sessreg(1),
Xserver(1), fonts.conf(4).
X Display Manager Control Protocol
IETF RFC 4291: IP Version 6 Addressing Architecture.
AUTHOR
Keith Packard, MIT X Consortium
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Availability |system/display-manager/xdm |
+-----------------------------+-----------------------------+
|Interface Stability |Committed |
+-----------------------------+-----------------------------+
X Version 11 Last change: xdm 1.1.9 27