xdm - Online in the Cloud

PROGRAM:

NAME

xdm - X Display Manager with support for XDMCP, host chooser

SYNOPSIS

xdm [ -config configuration_file ] [ -nodaemon ] [ -debug debug_level ] [ -error
error_log_file ] [ -resources resource_file ] [ -server server_entry ] [ -session
session_program ]

DESCRIPTION

Xdm manages a collection of X displays, which may be on the local host or remote servers.
The design of xdm was guided by the needs of X terminals as well as The Open Group
standard XDMCP, the X Display Manager Control Protocol. Xdm provides services similar to
those provided by init, getty and login on character terminals: prompting for login name
and password, authenticating the user, and running a ``session.''

A ``session'' is defined by the lifetime of a particular process; in the traditional
character-based terminal world, it is the user's login shell. In the xdm context, it is
an arbitrary session manager. This is because in a windowing environment, a user's login
shell process does not necessarily have any terminal-like interface with which to connect.
When a real session manager is not available, a window manager or terminal emulator is
typically used as the ``session manager,'' meaning that termination of this process
terminates the user's session.

When the session is terminated, xdm resets the X server and (optionally) restarts the
whole process.

When xdm receives an Indirect query via XDMCP, it can run a chooser process to perform an
XDMCP BroadcastQuery (or an XDMCP Query to specified hosts) on behalf of the display and
offer a menu of possible hosts that offer XDMCP display management. This feature is
useful with X terminals that do not offer a host menu themselves.

Xdm can be configured to ignore BroadcastQuery messages from selected hosts. This is
useful when you don't want the host to appear in menus produced by chooser or X terminals
themselves.

Because xdm provides the first interface that users will see, it is designed to be simple
to use and easy to customize to the needs of a particular site. Xdm has many options,
most of which have reasonable defaults. Browse through the various sections of this
manual, picking and choosing the things you want to change. Pay particular attention to
the Session Program section, which will describe how to set up the style of session
desired.

OVERVIEW

xdm is highly configurable, and most of its behavior can be controlled by resource files
and shell scripts. The names of these files themselves are resources read from the file
xdm-config or the file named by the -config option.

xdm offers display management two different ways. It can manage X servers running on the
local machine and specified in Xservers, and it can manage remote X servers (typically X
terminals) using XDMCP (the XDM Control Protocol) as specified in the Xaccess file.

The resources of the X clients run by xdm outside the user's session, including xdm's own
login window, can be affected by setting resources in the Xresources file.

For X terminals that do not offer a menu of hosts to get display management from, xdm can
collect willing hosts and run the chooser program to offer the user a menu. For X
displays attached to a host, this step is typically not used, as the local host does the
display management.

After resetting the X server, xdm runs the Xsetup script to assist in setting up the
screen the user sees along with the xlogin widget.

The xlogin widget, which xdm presents, offers the familiar login and password prompts.

After the user logs in, xdm runs the Xstartup script as root.

Then xdm runs the Xsession script as the user. This system session file may do some
additional startup and typically runs the .xsession script in the user's home directory.
When the Xsession script exits, the session is over.

At the end of the session, the Xreset script is run to clean up, the X server is reset,
and the cycle starts over.

The file /var/log/xdm.log will contain error messages from xdm and anything output to
stderr by Xsetup, Xstartup, Xsession or Xreset. When you have trouble getting xdm
working, check this file to see if xdm has any clues to the trouble.

OPTIONS

All of these options, except -config itself, specify values that can also be specified in
the configuration file as resources.

-config configuration_file
Names the configuration file, which specifies resources to control the behavior of
xdm. /etc/X11/xdm/xdm-config is the default. See the section Configuration File.

-nodaemon
Specifies ``false'' as the value for the DisplayManager.daemonMode resource. This
suppresses the normal daemon behavior, which is for xdm to close all file
descriptors, disassociate itself from the controlling terminal, and put itself in
the background when it first starts up.

-debug debug_level
Specifies the numeric value for the DisplayManager.debugLevel resource. A non-zero
value causes xdm to print lots of debugging statements to the terminal; it also
disables the DisplayManager.daemonMode resource, forcing xdm to run synchronously.
To interpret these debugging messages, a copy of the source code for xdm is almost
a necessity. No attempt has been made to rationalize or standardize the output.

-error error_log_file
Specifies the value for the DisplayManager.errorLogFile resource. This file
contains errors from xdm as well as anything written to stderr by the various
scripts and programs run during the progress of the session.

-resources resource_file
Specifies the value for the DisplayManager*resources resource. This file is loaded
using xrdb(1) to specify configuration parameters for the authentication widget.

-server server_entry
Specifies the value for the DisplayManager.servers resource. See the section Local
Server Specification for a description of this resource.

-udpPort port_number
Specifies the value for the DisplayManager.requestPort resource. This sets the
port-number which xdm will monitor for XDMCP requests. If set to 0, xdm will not
listen for XDMCP or Chooser requests. As XDMCP uses the registered well-known UDP
port 177, this resource should not be changed to a value other than 0, except for
debugging.

-session session_program
Specifies the value for the DisplayManager*session resource. This indicates the
program to run as the session after the user has logged in.

-xrm resource_specification
Allows an arbitrary resource to be specified, as in most X Toolkit applications.

RESOURCES

At many stages the actions of xdm can be controlled through the use of its configuration
file, which is in the X resource format. Some resources modify the behavior of xdm on all
displays, while others modify its behavior on a single display. Where actions relate to a
specific display, the display name is inserted into the resource name between
``DisplayManager'' and the final resource name segment.

For local displays, the resource name and class are as read from the Xservers file.

For remote displays, the resource name is what the network address of the display resolves
to. See the removeDomain resource. The name must match exactly; xdm is not aware of all
the network aliases that might reach a given display. If the name resolve fails, the
address is used. The resource class is as sent by the display in the XDMCP Manage
request.

Because the resource manager uses colons to separate the name of the resource from its
value and dots to separate resource name parts, xdm substitutes underscores for both dots
and colons when generating the resource name. For example,
DisplayManager.expo_x_org_0.startup is the name of the resource which defines the startup
shell file for the ``expo.x.org:0'' display.

DisplayManager.servers
This resource either specifies a file name full of server entries, one per line (if
the value starts with a slash), or a single server entry. See the section Local
Server Specification for the details.

DisplayManager.requestPort
This indicates the UDP port number which xdm uses to listen for incoming XDMCP
requests. Unless you need to debug the system, leave this with its default value
of 177.

DisplayManager.errorLogFile
Error output is normally directed at the system console. To redirect it, set this
resource to a file name. A method to send these messages to syslog should be
developed for systems which support it; however, the wide variety of interfaces
precludes any system-independent implementation. This file also contains any
output directed to stderr by the Xsetup, Xstartup, Xsession and Xreset files, so it
will contain descriptions of problems in those scripts as well.

DisplayManager.debugLevel
If the integer value of this resource is greater than zero, reams of debugging
information will be printed. It also disables daemon mode, which would redirect
the information into the bit-bucket, and allows non-root users to run xdm, which
would normally not be useful.

DisplayManager.daemonMode
Normally, xdm attempts to make itself into a daemon process unassociated with any
terminal. This is accomplished by forking and leaving the parent process to exit,
then closing file descriptors and releasing the controlling terminal. In some
environments this is not desired (in particular, when debugging). Setting this
resource to ``false'' will disable this feature.

DisplayManager.pidFile
The filename specified will be created to contain an ASCII representation of the
process-id of the main xdm process. Xdm also uses file locking on this file to
attempt to eliminate multiple daemons running on the same machine, which would
cause quite a bit of havoc.

DisplayManager.lockPidFile
This is the resource which controls whether xdm uses file locking to keep multiple
display managers from running amok. On System V, this uses the lockf library call,
while on BSD it uses flock.

DisplayManager.authDir
This names a directory under which xdm stores authorization files while
initializing the session. The default value is /var/lib/xdm. Can be overridden
for specific displays by DisplayManager.DISPLAY.authFile.

DisplayManager.autoRescan
This boolean controls whether xdm rescans the configuration, servers, access
control and authentication keys files after a session terminates and the files have
changed. By default it is ``true.'' You can force xdm to reread these files by
sending a SIGHUP to the main process.

DisplayManager.removeDomainname
When computing the display name for XDMCP clients, the name resolver will typically
create a fully qualified host name for the terminal. As this is sometimes
confusing, xdm will remove the domain name portion of the host name if it is the
same as the domain name of the local host when this variable is set. By default
the value is ``true.''

DisplayManager.keyFile
XDM-AUTHENTICATION-1 style XDMCP authentication requires that a private key be
shared between xdm and the terminal. This resource specifies the file containing
those values. Each entry in the file consists of a display name and the shared
key. By default, xdm does not include support for XDM-AUTHENTICATION-1, as it
requires DES which is not generally distributable because of United States export
restrictions.

DisplayManager.accessFile
To prevent unauthorized XDMCP service and to allow forwarding of XDMCP
IndirectQuery requests, this file contains a database of hostnames which are either
allowed direct access to this machine, or have a list of hosts to which queries
should be forwarded to. The format of this file is described in the section XDMCP
Access Control.

DisplayManager.exportList
A list of additional environment variables, separated by white space, to pass on to
the Xsetup, Xstartup, Xsession, and Xreset programs.

DisplayManager.randomFile
A file to checksum to generate the seed of authorization keys. This should be a
file that changes frequently. The default is /dev/mem.

DisplayManager.randomDevice
A file to read 8 bytes from to generate the seed of authorization keys. The
default is /dev/urandom . If this file cannot be read, or if a read blocks for
more than 5 seconds, xdm falls back to using a checksum of
DisplayManager.randomFile to generate the seed.

DisplayManager.prngdSocket

DisplayManager.prngPort
A UNIX domain socket name or a TCP socket port number on local host on which a
Pseudo-Random Number Generator Daemon, like EGD (http://egd.sourceforge.net) is
listening, in order to generate the autorization keys. Either a non null port or a
valid socket name must be specified. The default is to use the Unix-domain socket
/tmp/entropy.

On systems that don't have such a daemon, a fall-back entropy gathering system, based on
various log file contents hashed by the MD5 algorithm is used instead.

DisplayManager.greeterLib
On systems that support a dynamically-loadable greeter library, the name of the
library. The default is
/usr/lib/X11/xdm/libXdmGreet.so.

DisplayManager.choiceTimeout
Number of seconds to wait for display to respond after user has selected a host
from the chooser. If the display sends an XDMCP IndirectQuery within this time,
the request is forwarded to the chosen host. Otherwise, it is assumed to be from a
new session and the chooser is offered again. Default is 15.

DisplayManager.sourceAddress
Use the numeric IP address of the incoming connection on multihomed hosts instead
of the host name. This is to avoid trying to connect on the wrong interface which
might be down at this time.

DisplayManager.willing
This specifies a program which is run (as) root when an an XDMCP BroadcastQuery is
received and this host is configured to offer XDMCP display management. The output
of this program may be displayed on a chooser window. If no program is specified,
the string Willing to manage is sent.

DisplayManager.DISPLAY.resources
This resource specifies the name of the file to be loaded by xrdb as the resource
database onto the root window of screen 0 of the display. The Xsetup program, the
Login widget, and chooser will use the resources set in this file. This resource
data base is loaded just before the authentication procedure is started, so it can
control the appearance of the login window. See the section Authentication Widget,
which describes the various resources that are appropriate to place in this file.
There is no default value for this resource, but
/etc/X11/xdm/Xresources is the conventional name.

DisplayManager.DISPLAY.chooser
Specifies the program run to offer a host menu for Indirect queries redirected to
the special host name CHOOSER.
/usr/lib/X11/xdm/chooser is the default. See the sections XDMCP Access Control
and Chooser.

DisplayManager.DISPLAY.xrdb
Specifies the program used to load the resources. By default, xdm uses
/usr/bin/xrdb.

DisplayManager.DISPLAY.cpp
This specifies the name of the C preprocessor which is used by xrdb.

DisplayManager.DISPLAY.setup
This specifies a program which is run (as root) before offering the Login window.
This may be used to change the appearance of the screen around the Login window or
to put up other windows (e.g., you may want to run xconsole here). By default, no
program is run. The conventional name for a file used here is Xsetup. See the
section Setup Program.

DisplayManager.DISPLAY.startup
This specifies a program which is run (as root) after the authentication process
succeeds. By default, no program is run. The conventional name for a file used
here is Xstartup. See the section Startup Program.

DisplayManager.DISPLAY.session
This specifies the session to be executed (not running as root). By default,
/usr/bin/xterm is run. The conventional name is Xsession. See the section Session
Program.

DisplayManager.DISPLAY.reset
This specifies a program which is run (as root) after the session terminates. By
default, no program is run. The conventional name is Xreset. See the section
Reset Program.

DisplayManager.DISPLAY.openDelay

DisplayManager.DISPLAY.openRepeat

DisplayManager.DISPLAY.openTimeout

DisplayManager.DISPLAY.startAttempts

DisplayManager.DISPLAY.reservAttempts
These numeric resources control the behavior of xdm when attempting to open
intransigent servers. openDelay is the length of the pause in seconds between
successive attempts, openRepeat is the number of attempts to make, openTimeout is
the amount of time to wait while actually attempting the open (i.e., the maximum
time spent in the connect(2) system call) and startAttempts is the number of times
this entire process is done before giving up on the server. After openRepeat
attempts have been made, or if openTimeout seconds elapse in any particular
attempt, xdm terminates and restarts the server, attempting to connect again. This
process is repeated startAttempts times, at which point the display is declared
dead and disabled. Although this behavior may seem arbitrary, it has been
empirically developed and works quite well on most systems. The bound
reservAttempts is the number of times a successful connect is allowed to be
followed by a fatal error. When reached, the display is disabled. The default
values are openDelay: 15, openRepeat: 5, openTimeout: 120, startAttempts: 4 and
reservAttempts: 2.

DisplayManager.DISPLAY.pingInterval

DisplayManager.DISPLAY.pingTimeout
To discover when remote displays disappear, xdm occasionally pings them, using an X
connection and XSync calls. pingInterval specifies the time (in minutes) between
each ping attempt, pingTimeout specifies the maximum amount of time (in minutes) to
wait for the terminal to respond to the request. If the terminal does not respond,
the session is declared dead and terminated. By default, both are set to 5
minutes. If you frequently use X terminals which can become isolated from the
managing host, you may wish to increase this value. The only worry is that
sessions will continue to exist after the terminal has been accidentally disabled.
xdm will not ping local displays. Although it would seem harmless, it is
unpleasant when the workstation session is terminated as a result of the server
hanging for NFS service and not responding to the ping.

DisplayManager.DISPLAY.terminateServer
This boolean resource specifies whether the X server should be terminated when a
session terminates (instead of resetting it). This option can be used when the
server tends to grow without bound over time, in order to limit the amount of time
the server is run. The default value is ``false.''

DisplayManager.DISPLAY.userPath
Xdm sets the PATH environment variable for the session to this value. It should be
a colon separated list of directories; see sh(1) for a full description. The
default value is ``/usr/local/bin:/usr/bin:/bin:/usr/games''.

DisplayManager.DISPLAY.systemPath
Xdm sets the PATH environment variable for the startup and reset scripts to the
value of this resource. The default for this resource is
``/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin''. Note the absence
of ``.'' from this entry. This is a good practice to follow for root; it avoids
many common Trojan Horse system penetration schemes.

DisplayManager.DISPLAY.systemShell
Xdm sets the SHELL environment variable for the startup and reset scripts to the
value of this resource. It is /bin/sh by default.

DisplayManager.DISPLAY.failsafeClient
If the default session fails to execute, xdm will fall back to this program. This
program is executed with no arguments, but executes using the same environment
variables as the session would have had (see the section Session Program). By
default, /usr/bin/xterm is used.

DisplayManager.DISPLAY.grabServer

DisplayManager.DISPLAY.grabTimeout
To improve security, xdm grabs the server and keyboard while reading the login name
and password. The grabServer resource specifies if the server should be held for
the duration of the name/password reading. When ``false,'' the server is ungrabbed
after the keyboard grab succeeds, otherwise the server is grabbed until just before
the session begins. The default is ``false.'' The grabTimeout resource specifies
the maximum time xdm will wait for the grab to succeed. The grab may fail if some
other client has the server grabbed, or possibly if the network latencies are very
high. This resource has a default value of 3 seconds; you should be cautious when
raising it, as a user can be spoofed by a look-alike window on the display. If the
grab fails, xdm kills and restarts the server (if possible) and the session.

DisplayManager.DISPLAY.authorize

DisplayManager.DISPLAY.authName
authorize is a boolean resource which controls whether xdm generates and uses
authorization for the local server connections. If authorization is used, authName
is a list of authorization mechanisms to use, separated by white space. XDMCP
connections dynamically specify which authorization mechanisms are supported, so
authName is ignored in this case. When authorize is set for a display and
authorization is not available, the user is informed by having a different message
displayed in the login widget. By default, authorize is ``true,'' authName is
``MIT-MAGIC-COOKIE-1,'' or, if XDM-AUTHORIZATION-1 is available, ``XDM-
AUTHORIZATION-1 MIT-MAGIC-COOKIE-1.''

DisplayManager.DISPLAY.authFile
This file is used to communicate the authorization data from xdm to the server,
using the -auth server command line option. It should be kept in a directory which
is not world-writable as it could easily be removed, disabling the authorization
mechanism in the server. If not specified, a name is generated from
DisplayManager.authDir and the name of the display.

DisplayManager.DISPLAY.authComplain
If set to ``false,'' disables the use of the unsecureGreeting in the login window.
See the section Authentication Widget. The default is ``true.''

DisplayManager.DISPLAY.resetSignal
The number of the signal xdm sends to reset the server. See the section
Controlling the Server. The default is 1 (SIGHUP).

DisplayManager.DISPLAY.termSignal
The number of the signal xdm sends to terminate the server. See the section
Controlling the Server. The default is 15 (SIGTERM).

DisplayManager.DISPLAY.resetForAuth
The original implementation of authorization in the sample server reread the
authorization file at server reset time, instead of when checking the initial
connection. As xdm generates the authorization information just before connecting
to the display, an old server would not get up-to-date authorization information.
This resource causes xdm to send SIGHUP to the server after setting up the file,
causing an additional server reset to occur, during which time the new
authorization information will be read. The default is ``false,'' which will work
for all MIT servers.

DisplayManager.DISPLAY.userAuthDir
When xdm is unable to write to the usual user authorization file
($HOME/.Xauthority), it creates a unique file name in this directory and points the
environment variable XAUTHORITY at the created file. It uses /tmp by default.

CONFIGURATION FILE

First, the xdm configuration file should be set up. Make a directory (usually
/etc/X11/xdm) to contain all of the relevant files.

Here is a reasonable configuration file, which could be named xdm-config:

DisplayManager.servers: /etc/X11/xdm/Xservers
DisplayManager.errorLogFile: /var/log/xdm.log
DisplayManager*resources: /etc/X11/xdm/Xresources
DisplayManager*startup: /etc/X11/xdm/Xstartup
DisplayManager*session: /etc/X11/xdm/Xsession
DisplayManager.pidFile: /var/run/xdm-pid
DisplayManager._0.authorize: true
DisplayManager*authorize: false

Note that this file mostly contains references to other files. Note also that some of the
resources are specified with ``*'' separating the components. These resources can be made
unique for each different display, by replacing the ``*'' with the display-name, but
normally this is not very useful. See the Resources section for a complete discussion.

XDMCP ACCESS CONTROL

The database file specified by the DisplayManager.accessFile provides information which
xdm uses to control access from displays requesting XDMCP service. This file contains
three types of entries: entries which control the response to Direct and Broadcast
queries, entries which control the response to Indirect queries, and macro definitions.

The format of the Direct entries is simple, either a host name or a pattern, which is
distinguished from a host name by the inclusion of one or more meta characters (`*'
matches any sequence of 0 or more characters, and `?' matches any single character) which
are compared against the host name of the display device. If the entry is a host name,
all comparisons are done using network addresses, so any name which converts to the
correct network address may be used. For patterns, only canonical host names are used in
the comparison, so ensure that you do not attempt to match aliases. Preceding either a
host name or a pattern with a `!' character causes hosts which match that entry to be
excluded.

To only respond to Direct queries for a host or pattern, it can be followed by the
optional ``NOBROADCAST'' keyword. This can be used to prevent an xdm server from
appearing on menus based on Broadcast queries.

An Indirect entry also contains a host name or pattern, but follows it with a list of host
names or macros to which indirect queries should be sent.

A macro definition contains a macro name and a list of host names and other macros that
the macro expands to. To distinguish macros from hostnames, macro names start with a `%'
character. Macros may be nested.

Indirect entries may also specify to have xdm run chooser to offer a menu of hosts to
connect to. See the section Chooser.

When checking access for a particular display host, each entry is scanned in turn and the
first matching entry determines the response. Direct and Broadcast entries are ignored
when scanning for an Indirect entry and vice-versa.

Blank lines are ignored, `#' is treated as a comment delimiter causing the rest of that
line to be ignored, and `\newline' causes the newline to be ignored, allowing indirect
host lists to span multiple lines.

Here is an example Xaccess file:

#
# Xaccess - XDMCP access control file
#

#
# Direct/Broadcast query entries
#

!xtra.lcs.mit.edu # disallow direct/broadcast service for xtra
bambi.ogi.edu # allow access from this particular display
*.lcs.mit.edu # allow access from any display in LCS

*.deshaw.com NOBROADCAST # allow only direct access
*.gw.com # allow direct and broadcast

#
# Indirect query entries
#

%HOSTS expo.lcs.mit.edu xenon.lcs.mit.edu \
excess.lcs.mit.edu kanga.lcs.mit.edu

extract.lcs.mit.edu xenon.lcs.mit.edu #force extract to contact xenon
!xtra.lcs.mit.edu dummy #disallow indirect access
*.lcs.mit.edu %HOSTS #all others get to choose

If compiled with IPv6 support, multicast address groups may also be included in the list
of addresses indirect queries are set to. Multicast addresses may be followed by an
optional / character and hop count. If no hop count is specified, the multicast hop count
defaults to 1, keeping the packet on the local network. For IPv4 multicasting, the hop
count is used as the TTL.

Examples:

rincewind.sample.net ff02::1 #IPv6 Multicast to ff02::1
#with a hop count of 1
ponder.sample.net CHOOSER 239.192.1.1/16 #Offer a menu of hosts
#who respond to IPv4 Multicast
# to 239.192.1.1 with a TTL of 16

CHOOSER

For X terminals that do not offer a host menu for use with Broadcast or Indirect queries,
the chooser program can do this for them. In the Xaccess file, specify ``CHOOSER'' as the
first entry in the Indirect host list. Chooser will send a Query request to each of the
remaining host names in the list and offer a menu of all the hosts that respond.

The list may consist of the word ``BROADCAST,'' in which case chooser will send a
Broadcast instead, again offering a menu of all hosts that respond. Note that on some
operating systems, UDP packets cannot be broadcast, so this feature will not work.

Example Xaccess file using chooser:

extract.lcs.mit.edu CHOOSER %HOSTS #offer a menu of these hosts
xtra.lcs.mit.edu CHOOSER BROADCAST #offer a menu of all hosts

The program to use for chooser is specified by the DisplayManager.DISPLAY.chooser
resource. For more flexibility at this step, the chooser could be a shell script.
Chooser is the session manager here; it is run instead of a child xdm to manage the
display.

Resources for this program can be put into the file named by
DisplayManager.DISPLAY.resources.

When the user selects a host, chooser prints the host chosen, which is read by the parent
xdm, and exits. xdm closes its connection to the X server, and the server resets and
sends another Indirect XDMCP request. xdm remembers the user's choice (for
DisplayManager.choiceTimeout seconds) and forwards the request to the chosen host, which
starts a session on that display.

LISTEN

The following configuration directive is also defined for the Xaccess configuration file:

LISTEN interface [list of multicast group addresses]
interface may be a hostname or IP address representing a network interface on this
machine, or the wildcard * to represent all available network interfaces.

If one or more LISTEN lines are specified, xdm only listens for XDMCP connections on the
specified interfaces. If multicast group addresses are listed on a listen line, xdm joins
the multicast groups on the given interface.

If no LISTEN lines are given, the original behavior of listening on all interfaces is
preserved for backwards compatibility. Additionally, if no LISTEN is specified, xdm joins
the default XDMCP IPv6 multicast group, when compiled with IPv6 support.

To disable listening for XDMCP connections altogther, a line of LISTEN with no addresses
may be specified, or the previously supported method of setting DisplayManager.requestPort
to 0 may be used.

Examples:
LISTEN * ff02::1 # Listen on all interfaces and to the
# ff02::1 IPv6 multicast group.
LISTEN 10.11.12.13 # Listen only on this interface, as long
# as no other listen directives appear in
# file.

IPv6 MULTICAST ADDRESS SPECIFICATION

The Internet Assigned Numbers Authority has has assigned ff0X:0:0:0:0:0:0:12b as the
permanently assigned range of multicast addresses for XDMCP. The X in the prefix may be
replaced by any valid scope identifier, such as 1 for Interface-Local, 2 for Link-Local, 5
for Site-Local, and so on. (See IETF RFC 4291 or its replacement for further details and
scope definitions.) xdm defaults to listening on the Link-Local scope address
ff02:0:0:0:0:0:0:12b to most closely match the old IPv4 subnet broadcast behavior.

LOCAL SERVER SPECIFICATION

The resource DisplayManager.servers gives a server specification or, if the values starts
with a slash (/), the name of a file containing server specifications, one per line.

Each specification indicates a display which should constantly be managed and which is not
using XDMCP. This method is used typically for local servers only. If the resource or
the file named by the resource is empty, xdm will offer XDMCP service only.

Each specification consists of at least three parts: a display name, a display class, a
display type, and (for local servers) a command line to start the server. A typical entry
for local display number 0 would be:

:0 Digital-QV local /usr/bin/X :0

The display types are:

local local display: xdm must run the server
foreign remote display: xdm opens an X connection to a running server

The display name must be something that can be passed in the -display option to an X
program. This string is used to generate the display-specific resource names, so be
careful to match the names (e.g., use ``:0 Sun-CG3 local /usr/bin/X :0'' instead of
``localhost:0 Sun-CG3 local /usr/bin/X :0'' if your other resources are specified as
``DisplayManager._0.session''). The display class portion is also used in the display-
specific resources, as the class of the resource. This is useful if you have a large
collection of similar displays (such as a corral of X terminals) and would like to set
resources for groups of them. When using XDMCP, the display is required to specify the
display class, so the manual for your particular X terminal should document the display
class string for your device. If it doesn't, you can run xdm in debug mode and look at
the resource strings which it generates for that device, which will include the class
string.

When xdm starts a session, it sets up authorization data for the server. For local
servers, xdm passes ``-auth filename'' on the server's command line to point it at its
authorization data. For XDMCP servers, xdm passes the authorization data to the server
via the Accept XDMCP request.

RESOURCES FILE

The Xresources file is loaded onto the display as a resource database using xrdb. As the
authentication widget reads this database before starting up, it usually contains
parameters for that widget:

xlogin*login.translations: #override\
Ctrl<Key>R: abort-display()\n\
<Key>F1: set-session-argument(failsafe) finish-field()\n\
<Key>Return: set-session-argument() finish-field()
xlogin*borderWidth: 3
xlogin*greeting: CLIENTHOST
#ifdef COLOR
xlogin*greetColor: CadetBlue
xlogin*failColor: red
#endif

Please note the translations entry; it specifies a few new translations for the widget
which allow users to escape from the default session (and avoid troubles that may occur in
it). Note that if #override is not specified, the default translations are removed and
replaced by the new value, not a very useful result as some of the default translations
are quite useful (such as ``<Key>: insert-char ()'' which responds to normal typing).

This file may also contain resources for the setup program and chooser.

SETUP PROGRAM

The Xsetup file is run after the server is reset, but before the Login window is offered.
The file is typically a shell script. It is run as root, so should be careful about
security. This is the place to change the root background or bring up other windows that
should appear on the screen along with the Login widget.

In addition to any specified by DisplayManager.exportList, the following environment
variables are passed:

DISPLAY the associated display name
PATH the value of DisplayManager.DISPLAY.systemPath
SHELL the value of DisplayManager.DISPLAY.systemShell
XAUTHORITY may be set to an authority file

Note that since xdm grabs the keyboard, any other windows will not be able to receive
keyboard input. They will be able to interact with the mouse, however; beware of
potential security holes here. If DisplayManager.DISPLAY.grabServer is set, Xsetup will
not be able to connect to the display at all. Resources for this program can be put into
the file named by DisplayManager.DISPLAY.resources.

Here is a sample Xsetup script:

#!/bin/sh
# Xsetup_0 - setup script for one workstation
xcmsdb < /etc/X11/xdm/monitors/alex.0
xconsole -geometry 480x130-0-0 -notify -verbose -exitOnFail &

AUTHENTICATION WIDGET

The authentication widget prompts the user for the username, password, and/or other
required authentication data from the keyboard. Nearly every imaginable parameter can be
controlled with a resource. Resources for this widget should be put into the file named
by DisplayManager.DISPLAY.resources. All of these have reasonable default values, so it
is not necessary to specify any of them.

The resource file is loaded with xrdb(1) so it may use the substitutions defined by that
program such as CLIENTHOST for the client hostname in the login message, or C pre-
processor #ifdef statements to produce different displays depending on color depth or
other variables.

Xdm can be compiled with support for the Xft(3) library for font rendering. If this
support is present, font faces are specified using the resources with names ending in
``face'' in the fontconfig face format described in the Font Names section of
fonts.conf(5). If not, then fonts are specified using the resources with names ending in
``font'' in the traditional X Logical Font Description format described in the Font Names
section of X(7).

xlogin.Login.width, xlogin.Login.height, xlogin.Login.x, xlogin.Login.y
The geometry of the Login widget is normally computed automatically. If you wish
to position it elsewhere, specify each of these resources.

xlogin.Login.foreground
The color used to display the input typed by the user.

xlogin.Login.face
The face used to display the input typed by the user when built with Xft support.
The default is ``Serif-18''.

xlogin.Login.font
The font used to display the input typed by the user when not built with Xft
support.

xlogin.Login.greeting
A string which identifies this window. The default is ``X Window System.''

xlogin.Login.unsecureGreeting
When X authorization is requested in the configuration file for this display and
none is in use, this greeting replaces the standard greeting. The default is
``This is an unsecure session''

xlogin.Login.greetFace
The face used to display the greeting when built with Xft support. The default is
``Serif-24:italic''.

xlogin.Login.greetFont
The font used to display the greeting when not built with Xft support.

xlogin.Login.greetColor
The color used to display the greeting.

xlogin.Login.namePrompt
The string displayed to prompt for a user name. Xrdb strips trailing white space
from resource values, so to add spaces at the end of the prompt (usually a nice
thing), add spaces escaped with backslashes. The default is ``Login: ''

xlogin.Login.passwdPrompt
The string displayed to prompt for a password, when not using an authentication
system such as PAM that provides its own prompts. The default is ``Password: ''

xlogin.Login.promptFace
The face used to display prompts when built with Xft support. The default is
``Serif-18:bold''.

xlogin.Login.promptFont
The font used to display prompts when not built with Xft support.

xlogin.Login.promptColor
The color used to display prompts.

xlogin.Login.changePasswdMessage
A message which is displayed when the users password has expired. The default is
``Password Change Required''

xlogin.Login.fail
A message which is displayed when the authentication fails, when not using an
authentication system such as PAM that provides its own prompts. The default is
``Login incorrect''

xlogin.Login.failFace
The face used to display the failure message when built with Xft support. The
default is ``Serif-18:bold''.

xlogin.Login.failFont
The font used to display the failure message when not built with Xft support.

xlogin.Login.failColor
The color used to display the failure message.

xlogin.Login.failTimeout
The number of seconds that the failure message is displayed. The default is 10.

xlogin.Login.logoFileName
Name of an XPM format pixmap to display in the greeter window, if built with XPM
support. The default is no pixmap.

xlogin.Login.logoPadding
Number of pixels of space between the logo pixmap and other elements of the greeter
window, if the pixmap is displayed. The default is 5.

xlogin.Login.useShape
If set to ``true'', when built with XPM support, attempt to use the X Non-
Rectangular Window Shape Extension to set the window shape. The default is
``true''.

xlogin.Login.hiColor, xlogin.Login.shdColor
Raised appearance bezels may be drawn around the greeter frame and text input boxes
by setting these resources. hiColor is the highlight color, used on the top and
left sides of the frame, and the bottom and right sides of text input areas.
shdColor is the shadow color, used on the bottom and right sides of the frame, and
the top and left sides of text input areas. The default for both is the foreground
color, providing a flat appearance.

xlogin.Login.frameWidth
frameWidth is the width in pixels of the area around the greeter frame drawn in
hiColor and shdColor.

xlogin.Login.innerFramesWidth
innerFramesWidth is the width in pixels of the area around text input areas drawn
in hiColor and shdColor.

xlogin.Login.sepWidth
sepWidth is the width in pixels of the bezeled line between the greeting and input
areas drawn in hiColor and shdColor.

xlogin.Login.allowRootLogin
If set to ``false'', don't allow root (and any other user with uid = 0) to log in
directly. The default is ``true''. This setting is only checked by some of the
authentication backends at this time.

xlogin.Login.allowNullPasswd
If set to ``true'', allow an otherwise failing password match to succeed if the
account does not require a password at all. The default is ``false'', so only
users that have passwords assigned can log in.

xlogin.Login.echoPasswd
If set to ``true'', a placeholder character (echoPasswdChar) will be shown for
fields normally set to not echo, such as password input. The default is ``false''.

xlogin.Login.echoPasswdChar
Character to display if echoPasswd is true. The default is ``*''. If set to an
empty value, the cursor will advance for each character input, but no text will be
drawn.

xlogin.Login.translations
This specifies the translations used for the login widget. Refer to the X Toolkit
documentation for a complete discussion on translations. The default translation
table is:

Ctrl<Key>H: delete-previous-character() \n\
Ctrl<Key>D: delete-character() \n\
Ctrl<Key>B: move-backward-character() \n\
Ctrl<Key>F: move-forward-character() \n\
Ctrl<Key>A: move-to-begining() \n\
Ctrl<Key>E: move-to-end() \n\
Ctrl<Key>K: erase-to-end-of-line() \n\
Ctrl<Key>U: erase-line() \n\
Ctrl<Key>X: erase-line() \n\
Ctrl<Key>C: restart-session() \n\
Ctrl<Key>\\: abort-session() \n\
<Key>BackSpace:delete-previous-character() \n\
<Key>Delete: delete-previous-character() \n\
<Key>Return: finish-field() \n\
<Key>: insert-char() \

The actions which are supported by the widget are:

delete-previous-character
Erases the character before the cursor.

delete-character
Erases the character after the cursor.

move-backward-character
Moves the cursor backward.

move-forward-character
Moves the cursor forward.

move-to-begining
(Apologies about the spelling error.) Moves the cursor to the beginning of the
editable text.

move-to-end
Moves the cursor to the end of the editable text.

erase-to-end-of-line
Erases all text after the cursor.

erase-line
Erases the entire text.

finish-field
If the cursor is in the name field, proceeds to the password field; if the cursor
is in the password field, checks the current name/password pair. If the
name/password pair is valid, xdm starts the session. Otherwise the failure message
is displayed and the user is prompted again.

abort-session
Terminates and restarts the server.

abort-display
Terminates the server, disabling it. This action is not accessible in the default
configuration. There are various reasons to stop xdm on a system console, such as
when shutting the system down, when using xdmshell, to start another type of
server, or to generally access the console. Sending xdm a SIGHUP will restart the
display. See the section Controlling XDM.

restart-session
Resets the X server and starts a new session. This can be used when the resources
have been changed and you want to test them or when the screen has been overwritten
with system messages.

insert-char
Inserts the character typed.

set-session-argument
Specifies a single word argument which is passed to the session at startup. See
the section Session Program.

allow-all-access
Disables access control in the server. This can be used when the .Xauthority file
cannot be created by xdm. Be very careful using this; it might be better to
disconnect the machine from the network before doing this.

On some systems (OpenBSD) the user's shell must be listed in /etc/shells to allow login
through xdm. The normal password and account expiration dates are enforced too.

STARTUP PROGRAM

The Xstartup program is run as root when the user logs in. It is typically a shell
script. Since it is run as root, Xstartup should be very careful about security. This is
the place to put commands which add entries to utmp or wtmp files, (the sessreg program
may be useful here), mount users' home directories from file servers, or abort the session
if logins are not allowed.

In addition to any specified by DisplayManager.exportList, the following environment
variables are passed:

DISPLAY the associated display name
HOME the initial working directory of the user
LOGNAME the user name
USER the user name
PATH the value of DisplayManager.DISPLAY.systemPath
SHELL the value of DisplayManager.DISPLAY.systemShell
XAUTHORITY may be set to an authority file
WINDOWPATH may be set to the "window path" leading to the X server

No arguments are passed to the script. Xdm waits until this script exits before starting
the user session. If the exit value of this script is non-zero, xdm discontinues the
session and starts another authentication cycle.

The sample Xstartup file shown here prevents login while the file /etc/nologin exists.
Thus this is not a complete example, but simply a demonstration of the available
functionality.

Here is a sample Xstartup script:

#!/bin/sh
#
# Xstartup
#
# This program is run as root after the user is verified
#
if [ -f /etc/nologin ]; then
xmessage -file /etc/nologin -timeout 30 -center
exit 1
fi
sessreg -a -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME
/etc/X11/xdm/GiveConsole
exit 0

SESSION PROGRAM

The Xsession program is the command which is run as the user's session. It is run with
the permissions of the authorized user.

In addition to any specified by DisplayManager.exportList, the following environment
variables are passed:

DISPLAY the associated display name
HOME the initial working directory of the user
LOGNAME the user name
USER the user name
PATH the value of DisplayManager.DISPLAY.userPath
SHELL the user's default shell (from getpwnam)
XAUTHORITY may be set to a non-standard authority file
KRB5CCNAME may be set to a Kerberos credentials cache name
WINDOWPATH may be set to the "window path" leading to the X server

At most installations, Xsession should look in $HOME for a file .xsession, which contains
commands that each user would like to use as a session. Xsession should also implement a
system default session if no user-specified session exists.

An argument may be passed to this program from the authentication widget using the `set-
session-argument' action. This can be used to select different styles of session. One
good use of this feature is to allow the user to escape from the ordinary session when it
fails. This allows users to repair their own .xsession if it fails, without requiring
administrative intervention. The example following demonstrates this feature.

This example recognizes the special ``failsafe'' mode, specified in the translations in
the Xresources file, to provide an escape from the ordinary session. It also requires
that the .xsession file be executable so we don't have to guess what shell it wants to
use.

#!/bin/sh
#
# Xsession
#
# This is the program that is run as the client
# for the display manager.

case $# in
1)
case $1 in
failsafe)
exec xterm -geometry 80x24-0-0
;;
esac
esac

startup=$HOME/.xsession
resources=$HOME/.Xresources

if [ -f "$startup" ]; then
exec "$startup"
else
if [ -f "$resources" ]; then
xrdb -load "$resources"
fi
twm &
xman -geometry +10-10 &
exec xterm -geometry 80x24+10+10 -ls
fi

The user's .xsession file might look something like this example. Don't forget that the
file must have execute permission.
#! /bin/csh
# no -f in the previous line so .cshrc gets run to set $PATH
twm &
xrdb -merge "$HOME/.Xresources"
emacs -geometry +0+50 &
xbiff -geometry -430+5 &
xterm -geometry -0+50 -ls

RESET PROGRAM

Symmetrical with Xstartup, the Xreset script is run after the user session has terminated.
Run as root, it should contain commands that undo the effects of commands in Xstartup,
updating entries in utmp or wtmp files, or unmounting directories from file servers. The
environment variables that were passed to Xstartup are also passed to Xreset.

A sample Xreset script:
#!/bin/sh
#
# Xreset
#
# This program is run as root after the session ends
#
sessreg -d -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME
/etc/X11/xdm/TakeConsole
exit 0

CONTROLLING THE SERVER

Xdm controls local servers using POSIX signals. SIGHUP is expected to reset the server,
closing all client connections and performing other cleanup duties. SIGTERM is expected
to terminate the server. If these signals do not perform the expected actions, the
resources DisplayManager.DISPLAY.resetSignal and DisplayManager.DISPLAY.termSignal can
specify alternate signals.

To control remote terminals not using XDMCP, xdm searches the window hierarchy on the
display and uses the protocol request KillClient in an attempt to clean up the terminal
for the next session. This may not actually kill all of the clients, as only those which
have created windows will be noticed. XDMCP provides a more sure mechanism; when xdm
closes its initial connection, the session is over and the terminal is required to close
all other connections.

CONTROLLING XDM

Xdm responds to two signals: SIGHUP and SIGTERM. When sent a SIGHUP, xdm rereads the
configuration file, the access control file, and the servers file. For the servers file,
it notices if entries have been added or removed. If a new entry has been added, xdm
starts a session on the associated display. Entries which have been removed are disabled
immediately, meaning that any session in progress will be terminated without notice and no
new session will be started.

When sent a SIGTERM, xdm terminates all sessions in progress and exits. This can be used
when shutting down the system.

Xdm attempts to mark its various sub-processes for ps(1) by editing the command line
argument list in place. Because xdm can't allocate additional space for this task, it is
useful to start xdm with a reasonably long command line (using the full path name should
be enough). Each process which is servicing a display is marked -display.

ADDITIONAL LOCAL DISPLAYS

To add an additional local display, add a line for it to the Xservers file. (See the
section Local Server Specification.)

Examine the display-specific resources in xdm-config (e.g., DisplayManager._0.authorize)
and consider which of them should be copied for the new display. The default xdm-config
has all the appropriate lines for displays :0 and :1.

OTHER POSSIBILITIES

You can use xdm to run a single session at a time, using the 4.3 init options or other
suitable daemon by specifying the server on the command line:

xdm -server “:0 SUN-3/60CG4 local /usr/bin/X :0”

Or, you might have a file server and a collection of X terminals. The configuration for
this is identical to the sample above, except the Xservers file would look like

extol:0 VISUAL-19 foreign
exalt:0 NCD-19 foreign
explode:0 NCR-TOWERVIEW3000 foreign

This directs xdm to manage sessions on all three of these terminals. See the section
Controlling Xdm for a description of using signals to enable and disable these terminals
in a manner reminiscent of init(8).

LIMITATIONS

One thing that xdm isn't very good at doing is coexisting with other window systems. To
use multiple window systems on the same hardware, you'll probably be more interested in
xinit.

ASYNCHRONOUS EVENTS

xdm uses SIGALRM and SIGUSR1 for its own inter-process communication purposes, managing
the relationship between the parent xdm process and its children. Sending these signals
to any xdm process may result in unexpected behavior.

SIGHUP causes xdm to rescan its configuration files and reopen its log file.

SIGTERM
causes xdm to terminate its children and shut down.

SIGUSR2
causes xdm to reopen its log file. This is useful if log rotation is desired, but
SIGHUP is too disruptive.

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