XDM(1) UNIX Programmer's Manual XDM(1)
xdm - X Display Manager with support for XDMCP, host chooser
xdm [ -config configuration_file ] [ -nodaemon ] [ -debug
debug_level ] [ -error error_log_file ] [ -resources
resource_file ] [ -server server_entry ] [ -session
session_program ]
Xdm manages a collection of X displays, which may be on the
local host or remote servers. The design of xdm was guided
by the needs of X terminals as well as The Open Group stan-
dard XDMCP, the X Display Manager Control Protocol. Xdm pro-
vides 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 neces-
sarily have any terminal-like interface with which to con-
nect. When a real session manager is not available, a window
manager or terminal emulator is typically used as the ``ses-
sion manager,'' meaning that termination of this process
terminates the user's session.
When the session is terminated, xdm resets the X server and
(optionally) restarts the whole process.
When xdm receives an Indirect query via XDMCP, it can run a
chooser process to perform an XDMCP BroadcastQuery (or an
XDMCP Query to specified hosts) on behalf of the display and
offer a menu of possible hosts that offer XDMCP display
management. This feature is useful with X terminals that do
not offer a host menu themselves.
Xdm can be configured to ignore BroadcastQuery messages from
selected hosts. This is useful when you don't want the host
to appear in menus produced by chooser or X terminals them-
selves.
Because xdm provides the first interface that users will
see, it is designed to be simple to use and easy to custom-
ize 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
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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 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.
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.
/usr/X11R6/lib/X11/xdm/xdm-config is the default. See
the section Configuration File.
-nodaemon
Specifies ``false'' as the value for the
DisplayManager.daemonMode resource. This suppresses the
normal daemon behavior, which is for xdm to close all
file descriptors, disassociate itself from the control-
ling 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 neces-
sity. 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 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
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.
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 exam-
ple, 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-
independent implementation. This file also contains
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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 authori-
zation files while initializing the session. The
default value is /usr/X11R6/lib/X11/xdm. Can be over-
ridden for specific displays by
DisplayManager.DISPLAY.authFile.
DisplayManager.autoRescan
This boolean controls whether xdm rescans the confi-
guration, 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.greeterLib
On systems that support a dynamically-loadable greeter
library, the name of the library. The default is
/usr/X11R6/lib/X11/xdm/libXdmGreet.so.
DisplayManager.choiceTimeout
Number of seconds to wait for display to respond after
user has selected a host from the chooser. If the
display sends an XDMCP IndirectQuery within this time,
the request is forwarded to the chosen host. Other-
wise, 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.
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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
/usr/X11R6/lib/X11/xdm/Xresources is the conventional
name.
DisplayManager.DISPLAY.chooser
Specifies the program run to offer a host menu for
Indirect queries redirected to the special host name
CHOOSER.
/usr/X11R6/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/X11R6/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.
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DisplayManager.DISPLAY.session
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 Ses-
sion 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
startAttempts is the number of times this entire pro-
cess is done before giving up on the server. After
openRepeat attempts have been made, or if openTimeout
seconds elapse in any particular attempt, xdm ter-
minates 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 sys-
tems. The default values are 5 for openDelay, 5 for
openRepeat, 30 for openTimeout and 4 for startAttempts.
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
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continue to exist after the terminal has been acciden-
tally disabled. xdm will not ping local displays.
Although it would seem harmless, it is unpleasant when
the workstation session is terminated as a result of
the server hanging for NFS service and not responding
to the ping.
DisplayManager.DISPLAY.terminateServer
This boolean resource specifies whether the X server
should be terminated when a session terminates (instead
of resetting it). This option can be used when the
server tends to grow without bound over time, in order
to limit the amount of time the server is run. The
default value is ``false.''
DisplayManager.DISPLAY.userPath
Xdm sets the PATH environment variable for the session
to this value. It should be a colon separated list of
directories; see sh(1) for a full description.
``:/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 with Defaul-
tUserPath.
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 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 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/X11R6/bin/xterm is used.
DisplayManager.DISPLAY.grabServer
DisplayManager.DISPLAY.grabTimeout
To improve security, xdm grabs the server and keyboard
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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
displayed in the login widget. By default, authorize
is ``true.'' authName is ``MIT-MAGIC-COOKIE-1,'' or,
if XDM-AUTHORIZATION-1 is available, ``XDM-
AUTHORIZATION-1 MIT-MAGIC-COOKIE-1.''
DisplayManager.DISPLAY.authFile
This file is used to communicate the authorization data
from xdm to the server, using the -auth server command
line option. It should be kept in a directory which is
not world-writable as it could easily be removed, disa-
bling the authorization mechanism in the server. If not
specified, a name is generated from
DisplayManager.authDir and the name of the display.
DisplayManager.DISPLAY.authComplain
If set to ``false,'' disables the use of the unsecure-
Greeting in the login window. See the section Authenti-
cation 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).
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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
authorization information will be read. The default is
``false,'' which will work for all MIT servers.
DisplayManager.DISPLAY.userAuthDir
When xdm is unable to write to the usual user authori-
zation 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.
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.
Here is a reasonable configuration file, which could be
named xdm-config:
DisplayManager.servers: /usr/X11R6/lib/X11/xdm/Xservers
DisplayManager.errorLogFile: /var/log/xdm.log
DisplayManager*resources: /usr/X11R6/lib/X11/xdm/Xresources
DisplayManager*startup: /usr/X11R6/lib/X11/xdm/Xstartup
DisplayManager*session: /usr/X11R6/lib/X11/xdm/Xsession
DisplayManager.pidFile: /usr/X11R6/lib/X11/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.
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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 com-
parisons 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.
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:
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#
# Xaccess - XDMCP access control file
#
#
# Direct/Broadcast query entries
#
!xtra.lcs.mit.edu # disallow direct/broadcast service for xtra
bambi.ogi.edu # allow access from this particular display
*.lcs.mit.edu # allow access from any display in LCS
*.deshaw.com NOBROADCAST # allow only direct access
*.gw.com # allow direct and broadcast
#
# Indirect query entries
#
%HOSTS expo.lcs.mit.edu xenon.lcs.mit.edu \
excess.lcs.mit.edu kanga.lcs.mit.edu
extract.lcs.mit.edu xenon.lcs.mit.edu #force extract to contact xenon
!xtra.lcs.mit.edu dummy #disallow indirect access
*.lcs.mit.edu %HOSTS #all others get to choose
If compiled with IPv6 support, multicast address groups may
also be included in the list of addresses indirect queries
are set to. Multicast addresses may be followed by an
optional / character and hop count. If no hop count is
specified, the multicast hop count defaults to 1, keeping
the packet on the local network. For IPv4 multicasting, the
hop count is used as the TTL.
Examples:
rincewind.sample.net ff02::1 #IPv6 Multicast to ff02::1
#with a hop count of 1
ponder.sample.net CHOOSER 239.192.1.1/16 #Offer a menu of hosts
#who respond to IPv4 Multicast
# to 239.192.1.1 with a TTL of 16
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
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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 flexibil-
ity 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 ses-
sion on that display.
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.
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
listening on all interfaces is preserved for backwards com-
patibility. 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.
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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 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 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
display name, a display class, a display type, and (for
local servers) a command line to start the server. A typi-
cal 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 por-
tion 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
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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 ter-
minal should document the display class string for your dev-
ice. If it doesn't, you can run xdm in debug mode and look
at the resource strings which it generates for that device,
which will include the class string.
When xdm starts a session, it sets up authorization data for
the server. For local servers, xdm passes ``-auth
filename'' on the server's command line to point it at its
authorization data. For XDMCP servers, xdm passes the
authorization data to the server via the Accept XDMCP
request.
The Xresources file is loaded onto the display as a resource
database using xrdb. As the authentication widget reads this
database before starting up, it usually contains parameters
for that widget:
xlogin*login.translations: #override\
Ctrl<Key>R: abort-display()\n\/&
<Key>F1: set-session-argument(failsafe) finish-field()\n\
<Key>Return: set-session-argument() finish-field()
xlogin*borderWidth: 3
xlogin*greeting: CLIENTHOST
#ifdef COLOR
xlogin*greetColor: CadetBlue
xlogin*failColor: red
#endif
Please note the translations entry; it specifies a few new
translations for the widget which allow users to escape from
the default session (and avoid troubles that may occur in
it). Note that if #override is not specified, the default
translations are removed and replaced by the new value, not
a very useful result as some of the default translations are
quite useful (such as ``<Key>: insert-char ()'' which
responds to normal typing).
This file may also contain resources for the setup program
and chooser.
The Xsetup file is run after the server is reset, but before
the Login window is offered. The file is typically a shell
script. It is run as root, so should be careful about 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.
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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.grabServer is set, Xsetup will not be
able to connect to the display at all. Resources for this
program can be put into the file named by
DisplayManager.DISPLAY.resources.
Here is a sample Xsetup script:
#!/bin/mksh
# 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 con-
trolled with a resource. Resources for this widget should
be put into the file named by
DisplayManager.DISPLAY.resources. All of these have reason-
able default values, so it is not necessary to specify any
of them.
xlogin.Login.y
xlogin.Login.width, xlogin.Login.height, xlogin.Login.x,
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 typed-in user name.
xlogin.Login.font
The font used to display the typed-in user name.
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
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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.greetFont
The font used to display the greeting.
xlogin.Login.greetColor
The color used to display the greeting.
xlogin.Login.namePrompt
The string displayed to prompt for a user name. Xrdb
strips trailing white space from resource values, so to
add spaces at the end of the prompt (usually a nice
thing), add spaces escaped with backslashes. The
default is ``Login: ''
xlogin.Login.passwdPrompt
The string displayed to prompt for a password. The
default is ``Password: ''
xlogin.Login.promptFont
The font used to display both prompts.
xlogin.Login.promptColor
The color used to display both prompts.
xlogin.Login.fail
A message which is displayed when the authentication
fails. The default is ``Login incorrect''
xlogin.Login.failFont
The font used to display the failure message.
xlogin.Login.failColor
The color used to display the failure message.
xlogin.Login.failTimeout
The number of seconds that the failure message is
displayed. The default is 30.
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.
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xlogin.Login.translations
This specifies the translations used for the login
widget. Refer to the X Toolkit documentation for a
complete discussion on translations. The default
translation table is:
Ctrl<Key>H: delete-previous-character() \n\
Ctrl<Key>D: delete-character() \n\
Ctrl<Key>B: move-backward-character() \n\
Ctrl<Key>F: move-forward-character() \n\
Ctrl<Key>A: move-to-begining() \n\
Ctrl<Key>E: move-to-end() \n\
Ctrl<Key>K: erase-to-end-of-line() \n\
Ctrl<Key>U: erase-line() \n\
Ctrl<Key>X: erase-line() \n\
Ctrl<Key>C: restart-session() \n\
Ctrl<Key>\\: abort-session() \n\
<Key>BackSpace:delete-previous-character() \n\
<Key>Delete: delete-previous-character() \n\
<Key>Return: finish-field() \n\
<Key>: insert-char() \
The actions which are supported by the widget are:
delete-previous-character
Erases the character before the cursor.
delete-character
Erases the character after the cursor.
move-backward-character
Moves the cursor backward.
move-forward-character
Moves the cursor forward.
move-to-begining
(Apologies about the spelling error.) Moves the cursor
to the beginning of the editable text.
move-to-end
Moves the cursor to the end of the editable text.
erase-to-end-of-line
Erases all text after the cursor.
erase-line
Erases the entire text.
finish-field
If the cursor is in the name field, proceeds to the
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password field; if the cursor is in the password field,
checks the current name/password pair. If the
name/password pair is valid, xdm starts the session.
Otherwise the failure message is displayed and the user
is prompted again.
abort-session
Terminates and restarts the server.
abort-display
Terminates the server, disabling it. This action is
not accessible in the default configuration. There are
various reasons to stop xdm on a system console, such
as when shutting the system down, when using xdmshell,
to start another type of server, or to generally access
the console. Sending xdm a SIGHUP will restart the
display. See the section Controlling XDM.
restart-session
Resets the X server and starts a new session. This can
be used when the resources have been changed and you
want to test them or when the screen has been overwrit-
ten 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.
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.
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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 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 exam-
ple, but simply a demonstration of the available functional-
ity.
Here is a sample Xstartup script:
#!/bin/mksh
#
# 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 /usr/X11R6/lib/xdm/Xservers $LOGNAME
/usr/X11R6/lib/xdm/GiveConsole
exit 0
The Xsession program is the command which is run as the
user's session. It is run with the permissions of the
authorized user.
In addition to any specified by DisplayManager.exportList,
the following environment variables are passed:
DISPLAY the associated display name
HOME the initial working directory of the user
LOGNAME the user name
USER the user name
PATH the value of DisplayManager.DISPLAY.userPath
SHELL the user's default shell (from getpwnam)
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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 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.
This example recognizes the special ``failsafe'' mode,
specified in the translations in the Xresources file, to
provide an escape from the ordinary session. It also
requires that the .xsession file be executable so we don't
have to guess what shell it wants to use.
#!/bin/mksh
#
# 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 &
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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
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 script:
#!/bin/mksh
#
# Xreset
#
# This program is run as root after the session ends
#
sessreg -d -l $DISPLAY -x /usr/X11R6/lib/xdm/Xservers $LOGNAME
/usr/X11R6/lib/xdm/TakeConsole
exit 0
Xdm controls local servers using POSIX signals. SIGHUP is
expected to reset the server, closing all client connections
and performing other cleanup duties. SIGTERM is expected to
terminate the server. If these signals do not perform the
expected actions, the resources
DisplayManager.DISPLAY.resetSignal and
DisplayManager.DISPLAY.termSignal can specify alternate sig-
nals.
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.
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Xdm responds to two signals: SIGHUP and SIGTERM. When sent
a SIGHUP, xdm rereads the configuration file, the access
control file, and the servers file. For the servers file,
it notices if entries have been added or removed. If a new
entry has been added, xdm starts a session on the associated
display. Entries which have been removed are disabled
immediately, meaning that any session in progress will be
terminated without notice and no new session will be
started.
When sent a SIGTERM, xdm terminates all sessions in progress
and exits. This can be used when shutting down the system.
Xdm attempts to mark its various sub-processes for ps(1) by
editing the command line argument list in place. Because
xdm can't allocate additional space for this task, it is
useful to start xdm with a reasonably long command line
(using the full path name should be enough). Each process
which is servicing a display is marked -display.
To add an additional local display, add a line for it to the
Xservers file. (See the section Local Server Specification.)
Examine the display-specific resources in xdm-config (e.g.,
DisplayManager._0.authorize) and consider which of them
should be copied for the new display. The default xdm-config
has all the appropriate lines for displays :0 and :1.
You can use xdm to run a single session at a time, using the
4.3 init options or other suitable daemon by specifying the
server on the command line:
xdm -server :0 SUN-3/60CG4 local /usr/X11R6/bin/X :0
Or, you might have a file server and a collection of X 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(8).
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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.
/usr/X11R6/lib/X11/xdm/xdm-config
the default configuration file
$HOME/.Xauthority user authorization file where xdm stores
keys for clients to read
/usr/X11R6/lib/X11/xdm/chooser
the default chooser
/usr/X11R6/bin/xrdb the default resource database loader
/usr/X11R6/bin/X the default server
/usr/X11R6/bin/xterm
the default session program and failsafe
client
/usr/X11R6/lib/X11/xdm/authdir/authfiles/A<display>-<suffix>
the default place for authorization
files
/tmp/K5C<display> Kerberos credentials cache
X(7), xinit(1), xauth(1), Xsecurity(7), sessreg(1),
Xserver(1),
X Display Manager Control Protocol
Keith Packard, MIT X Consortium
XFree86 Version 4.5.0 25
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