xdm(1)
NAME
xdm - X Display Manager with support for XDMCP, host chooser
SYNOPSIS
xdm [ -config configuration_file ] [ -nodaemon ] [ -debug debug_level ] [ -error error_log_file ] [ -resources resource_file ] [ -server server_entry ] [ -session session_program ]
DESCRIPTION
Xdm manages a collection of X displays, which may be on the local host or remote servers. The design of xdm was guided by the needs of X terminals as well as The Open Group standard XDMCP, the X Display Manager Control Protocol. Xdm provides services similar to those provided by init, getty and login on character terminals: prompting for login name and password, authenticating the user, and running a ``session.'' A ``session'' is defined by the lifetime of a particular process; in the traditional character-based terminal world, it is the user's login shell. In the xdm context, it is an arbitrary session manager. This is because in a windowing environment, a user's login shell process does not necessarily have any terminal-like interface with which to connect. When a real session manager is not available, a window manager or terminal emulator is typically used as the ``session manager,'' meaning that termination of this process terminates the user's session. When the session is terminated, xdm resets the X server and (optionally) restarts the whole process. When xdm receives an Indirect query via XDMCP, it can run a chooser process to perform an XDMCP BroadcastQuery (or an XDMCP Query to specified hosts) on behalf of the display and offer a menu of possible hosts that offer XDMCP display management. This feature is useful with X terminals that do not offer a host menu themselves. Xdm can be configured to ignore BroadcastQuery messages from selected hosts. This is useful when you don't want the host to appear in menus produced by chooser or X terminals themselves. Because xdm provides the first interface that users will see, it is designed to be simple to use and easy to customize to the needs of a particular site. Xdm has many options, most of which have reasonable defaults. Browse through the various sections of this manual, picking and choosing the things you want to change. Pay particular attention to the Session Program section, which will describe how to set up the style of session desired.
OVERVIEW
xdm is highly configurable, and most of its behavior can be controlled by resource files and shell scripts. The names of these files themselves are resources read from the file xdm-config or the file named by the -config option. xdm offers display management two different ways. It can manage X servers running on the local machine and specified in Xservers, and it can manage remote X servers (typically X terminals) using XDMCP (the XDM Control Protocol) as specified in the Xaccess file. The resources of the X clients run by xdm outside the user's session, including xdm's own login window, can be affected by setting resources in the Xresources file. For X terminals that do not offer a menu of hosts to get display management from, xdm can collect willing hosts and run the chooser program to offer the user a menu. For X displays attached to a host, this step is typically not used, as the local host does the display management. After resetting the X server, xdm runs the Xsetup script to assist in setting up the screen the user sees along with the xlogin widget. The xlogin widget, which xdm presents, offers the familiar login and password prompts. After the user logs in, xdm runs the Xstartup script as root. Then xdm runs the Xsession script as the user. This system session file may do some additional startup and typically runs the .xsession script in the user's home directory. When the Xsession script exits, the session is over. At the end of the session, the Xreset script is run to clean up, the X server is reset, and the cycle starts over. The file /var/log/xdm.log will contain error messages from xdm and anything output to stderr by Xsetup, Xstartup, Xsession or Xreset. When you have trouble getting xdm working, check this file to see if xdm has any clues to the trouble.
OPTIONS
All of these options, except -config itself, specify values that can
also be specified in the configuration file as resources.
-config configuration_file
Names the configuration file, which specifies resources to
control the behavior of xdm. /etc/X11/xdm/xdm-config is the
default. See the section Configuration File.
-nodaemon
Specifies ``false'' as the value for the
DisplayManager.daemonMode resource. This suppresses the normal
daemon behavior, which is for xdm to close all file descriptors,
disassociate itself from the controlling terminal, and put
itself in the background when it first starts up.
-debug debug_level
Specifies the numeric value for the DisplayManager.debugLevel
resource. A non-zero value causes xdm to print lots of
debugging statements to the terminal; it also disables the
DisplayManager.daemonMode resource, forcing xdm to run
synchronously. To interpret these debugging messages, a copy of
the source code for xdm is almost a necessity. No attempt has
been made to rationalize or standardize the output.
-error error_log_file
Specifies the value for the DisplayManager.errorLogFile
resource. This file contains errors from xdm as well as
anything written to stderr by the various scripts and programs
run during the progress of the session.
-resources resource_file
Specifies the value for the DisplayManager*resources resource.
This file is loaded using xrdb(1) to specify configuration
parameters for the authentication widget.
-server server_entry
Specifies the value for the DisplayManager.servers resource.
See the section Local Server Specification for a description of
this resource.
-udpPort port_number
Specifies the value for the DisplayManager.requestPort resource.
This sets the port-number which xdm will monitor for XDMCP
requests. If set to 0, xdm will not listen for XDMCP or Chooser
requests. As XDMCP uses the registered well-known UDP port 177,
this resource should not be changed to a value other than 0,
except for debugging.
-session session_program
Specifies the value for the DisplayManager*session resource.
This indicates the program to run as the session after the user
has logged in.
-xrm resource_specification
Allows an arbitrary resource to be specified, as in most X
Toolkit applications.
RESOURCES
At many stages the actions of xdm can be controlled through the use of
its configuration file, which is in the X resource format. Some
resources modify the behavior of xdm on all displays, while others
modify its behavior on a single display. Where actions relate to a
specific display, the display name is inserted into the resource name
between ``DisplayManager'' and the final resource name segment.
For local displays, the resource name and class are as read from the
Xservers file.
For remote displays, the resource name is what the network address of
the display resolves to. See the removeDomain resource. The name must
match exactly; xdm is not aware of all the network aliases that might
reach a given display. If the name resolve fails, the address is used.
The resource class is as sent by the display in the XDMCP Manage
request.
Because the resource manager uses colons to separate the name of the
resource from its value and dots to separate resource name parts, xdm
substitutes underscores for both dots and colons when generating the
resource name. For example, DisplayManager.expo_x_org_0.startup is the
name of the resource which defines the startup shell file for the
``expo.x.org:0'' display.
DisplayManager.servers
This resource either specifies a file name full of server
entries, one per line (if the value starts with a slash), or a
single server entry. See the section Local Server Specification
for the details.
DisplayManager.requestPort
This indicates the UDP port number which xdm uses to listen for
incoming XDMCP requests. Unless you need to debug the system,
leave this with its default value of 177.
DisplayManager.errorLogFile
Error output is normally directed at the system console. To
redirect it, set this resource to a file name. A method to send
these messages to syslog should be developed for systems which
support it; however, the wide variety of interfaces precludes
any system-independent implementation. This file also contains
any output directed to stderr by the Xsetup, Xstartup, Xsession
and Xreset files, so it will contain descriptions of problems in
those scripts as well.
DisplayManager.debugLevel
If the integer value of this resource is greater than zero,
reams of debugging information will be printed. It also
disables daemon mode, which would redirect the information into
the bit-bucket, and allows non-root users to run xdm, which
would normally not be useful.
DisplayManager.daemonMode
Normally, xdm attempts to make itself into a daemon process
unassociated with any terminal. This is accomplished by forking
and leaving the parent process to exit, then closing file
descriptors and releasing the controlling terminal. In some
environments this is not desired (in particular, when
debugging). Setting this resource to ``false'' will disable
this feature.
DisplayManager.pidFile
The filename specified will be created to contain an ASCII
representation of the process-id of the main xdm process. Xdm
also uses file locking on this file to attempt to eliminate
multiple daemons running on the same machine, which would cause
quite a bit of havoc.
DisplayManager.lockPidFile
This is the resource which controls whether xdm uses file
locking to keep multiple display managers from running amok. On
System V, this uses the lockf library call, while on BSD it uses
flock.
DisplayManager.authDir
This names a directory under which xdm stores authorization
files while initializing the session. The default value is
/var/lib/xdm. Can be overridden for specific displays by
DisplayManager.DISPLAY.authFile.
DisplayManager.autoRescan
This boolean controls whether xdm rescans the configuration,
servers, access control and authentication keys files after a
session terminates and the files have changed. By default it is
``true.'' You can force xdm to reread these files by sending a
SIGHUP to the main process.
DisplayManager.removeDomainname
When computing the display name for XDMCP clients, the name
resolver will typically create a fully qualified host name for
the terminal. As this is sometimes confusing, xdm will remove
the domain name portion of the host name if it is the same as
the domain name of the local host when this variable is set. By
default the value is ``true.''
DisplayManager.keyFile
XDM-AUTHENTICATION-1 style XDMCP authentication requires that a
private key be shared between xdm and the terminal. This
resource specifies the file containing those values. Each entry
in the file consists of a display name and the shared key. By
default, xdm does not include support for XDM-AUTHENTICATION-1,
as it requires DES which is not generally distributable because
of United States export restrictions.
DisplayManager.accessFile
To prevent unauthorized XDMCP service and to allow forwarding of
XDMCP IndirectQuery requests, this file contains a database of
hostnames which are either allowed direct access to this
machine, or have a list of hosts to which queries should be
forwarded to. The format of this file is described in the
section XDMCP Access Control.
DisplayManager.exportList
A list of additional environment variables, separated by white
space, to pass on to the Xsetup, Xstartup, Xsession, and Xreset
programs.
DisplayManager.randomFile
A file to checksum to generate the seed of authorization keys.
This should be a file that changes frequently. The default is
/dev/mem.
DisplayManager.randomDevice
A file to read 8 bytes from to generate the seed of
authorization keys. The default is /dev/urandom . If this file
cannot be read, or if a read blocks for more than 5 seconds, xdm
falls back to using a checksum of DisplayManager.randomFile to
generate the seed.
DisplayManager.prngdSocket
DisplayManager.prngPort
A UNIX domain socket name or a TCP socket port number on local
host on which a Pseudo-Random Number Generator Daemon, like EGD
(http://egd.sourceforge.net) is listening, in order to generate
the autorization keys. Either a non null port or a valid socket
name must be specified. The default is to use the Unix-domain
socket /tmp/entropy.
On systems that don't have such a daemon, a fall-back entropy gathering
system, based on various log file contents hashed by the MD5 algorithm
is used instead.
DisplayManager.greeterLib
On systems that support a dynamically-loadable greeter library,
the name of the library. The default is
/usr/lib/X11/xdm/libXdmGreet.so.
DisplayManager.choiceTimeout
Number of seconds to wait for display to respond after user has
selected a host from the chooser. If the display sends an XDMCP
IndirectQuery within this time, the request is forwarded to the
chosen host. Otherwise, it is assumed to be from a new session
and the chooser is offered again. Default is 15.
DisplayManager.sourceAddress
Use the numeric IP address of the incoming connection on
multihomed hosts instead of the host name. This is to avoid
trying to connect on the wrong interface which might be down at
this time.
DisplayManager.willing
This specifies a program which is run (as) root when an an XDMCP
BroadcastQuery is received and this host is configured to offer
XDMCP display management. The output of this program may be
displayed on a chooser window. If no program is specified, the
string Willing to manage is sent.
DisplayManager.DISPLAY.resources
This resource specifies the name of the file to be loaded by
xrdb as the resource database onto the root window of screen 0
of the display. The Xsetup program, the Login widget, and
chooser will use the resources set in this file. This resource
data base is loaded just before the authentication procedure is
started, so it can control the appearance of the login window.
See the section Authentication Widget, which describes the
various resources that are appropriate to place in this file.
There is no default value for this resource, but
/etc/X11/xdm/Xresources is the conventional name.
DisplayManager.DISPLAY.chooser
Specifies the program run to offer a host menu for Indirect
queries redirected to the special host name CHOOSER.
/usr/lib/X11/xdm/chooser is the default. See the sections
XDMCP Access Control and Chooser.
DisplayManager.DISPLAY.xrdb
Specifies the program used to load the resources. By default,
xdm uses /usr/bin/xrdb.
DisplayManager.DISPLAY.cpp
This specifies the name of the C preprocessor which is used by
xrdb.
DisplayManager.DISPLAY.setup
This specifies a program which is run (as root) before offering
the Login window. This may be used to change the appearance of
the screen around the Login window or to put up other windows
(e.g., you may want to run xconsole here). By default, no
program is run. The conventional name for a file used here is
Xsetup. See the section Setup Program.
DisplayManager.DISPLAY.startup
This specifies a program which is run (as root) after the
authentication process succeeds. By default, no program is run.
The conventional name for a file used here is Xstartup. See the
section Startup Program.
DisplayManager.DISPLAY.session
This specifies the session to be executed (not running as root).
By default, /usr/bin/xterm is run. The conventional name is
Xsession. See the section Session Program.
DisplayManager.DISPLAY.reset
This specifies a program which is run (as root) after the
session terminates. By default, no program is run. The
conventional name is Xreset. See the section Reset Program.
DisplayManager.DISPLAY.openDelay
DisplayManager.DISPLAY.openRepeat
DisplayManager.DISPLAY.openTimeout
DisplayManager.DISPLAY.startAttempts
DisplayManager.DISPLAY.reservAttempts
These numeric resources control the behavior of xdm when
attempting to open intransigent servers. openDelay is the
length of the pause in seconds between successive attempts,
openRepeat is the number of attempts to make, openTimeout is the
amount of time to wait while actually attempting the open (i.e.,
the maximum time spent in the connect(2) system call) and
startAttempts is the number of times this entire process is done
before giving up on the server. After openRepeat attempts have
been made, or if openTimeout seconds elapse in any particular
attempt, xdm terminates and restarts the server, attempting to
connect again. This process is repeated startAttempts times, at
which point the display is declared dead and disabled. Although
this behavior may seem arbitrary, it has been empirically
developed and works quite well on most systems. The bound
reservAttempts is the number of times a successful connect is
allowed to be followed by a fatal error. When reached, the
display is disabled. The default values are openDelay: 15,
openRepeat: 5, openTimeout: 120, startAttempts: 4 and
reservAttempts: 2.
DisplayManager.DISPLAY.pingInterval
DisplayManager.DISPLAY.pingTimeout
To discover when remote displays disappear, xdm occasionally
pings them, using an X connection and XSync calls. pingInterval
specifies the time (in minutes) between each ping attempt,
pingTimeout specifies the maximum amount of time (in minutes) to
wait for the terminal to respond to the request. If the
terminal does not respond, the session is declared dead and
terminated. By default, both are set to 5 minutes. If you
frequently use X terminals which can become isolated from the
managing host, you may wish to increase this value. The only
worry is that sessions will continue to exist after the terminal
has been accidentally disabled. xdm will not ping local
displays. Although it would seem harmless, it is unpleasant
when the workstation session is terminated as a result of the
server hanging for NFS service and not responding to the ping.
DisplayManager.DISPLAY.terminateServer
This boolean resource specifies whether the X server should be
terminated when a session terminates (instead of resetting it).
This option can be used when the server tends to grow without
bound over time, in order to limit the amount of time the server
is run. The default value is ``false.''
DisplayManager.DISPLAY.userPath
Xdm sets the PATH environment variable for the session to this
value. It should be a colon separated list of directories; see
sh(1) for a full description. The default value is
``/usr/local/bin:/usr/bin:/bin:/usr/games''.
DisplayManager.DISPLAY.systemPath
Xdm sets the PATH environment variable for the startup and reset
scripts to the value of this resource. The default for this
resource is
``/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin''.
Note the absence of ``.'' from this entry. This is a good
practice to follow for root; it avoids many common Trojan Horse
system penetration schemes.
DisplayManager.DISPLAY.systemShell
Xdm sets the SHELL environment variable for the startup and
reset scripts to the value of this resource. It is /bin/sh by
default.
DisplayManager.DISPLAY.failsafeClient
If the default session fails to execute, xdm will fall back to
this program. This program is executed with no arguments, but
executes using the same environment variables as the session
would have had (see the section Session Program). By default,
/usr/bin/xterm is used.
DisplayManager.DISPLAY.grabServer
DisplayManager.DISPLAY.grabTimeout
To improve security, xdm grabs the server and keyboard while
reading the login name and password. The grabServer resource
specifies if the server should be held for the duration of the
name/password reading. When ``false,'' the server is ungrabbed
after the keyboard grab succeeds, otherwise the server is
grabbed until just before the session begins. The default is
``false.'' The grabTimeout resource specifies the maximum time
xdm will wait for the grab to succeed. The grab may fail if
some other client has the server grabbed, or possibly if the
network latencies are very high. This resource has a default
value of 3 seconds; you should be cautious when raising it, as a
user can be spoofed by a look-alike window on the display. If
the grab fails, xdm kills and restarts the server (if possible)
and the session.
DisplayManager.DISPLAY.authorize
DisplayManager.DISPLAY.authName
authorize is a boolean resource which controls whether xdm
generates and uses authorization for the local server
connections. If authorization is used, authName is a list of
authorization mechanisms to use, separated by white space.
XDMCP connections dynamically specify which authorization
mechanisms are supported, so authName is ignored in this case.
When authorize is set for a display and authorization is not
available, the user is informed by having a different message
displayed in the login widget. By default, authorize is
``true,'' authName is ``MIT-MAGIC-COOKIE-1,'' or, if XDM-
AUTHORIZATION-1 is available, ``XDM-AUTHORIZATION-1 MIT-MAGIC-
COOKIE-1.''
DisplayManager.DISPLAY.authFile
This file is used to communicate the authorization data from xdm
to the server, using the -auth server command line option. It
should be kept in a directory which is not world-writable as it
could easily be removed, disabling the authorization mechanism
in the server. If not specified, a name is generated from
DisplayManager.authDir and the name of the display.
DisplayManager.DISPLAY.authComplain
If set to ``false,'' disables the use of the unsecureGreeting in
the login window. See the section Authentication Widget. The
default is ``true.''
DisplayManager.DISPLAY.resetSignal
The number of the signal xdm sends to reset the server. See the
section Controlling the Server. The default is 1 (SIGHUP).
DisplayManager.DISPLAY.termSignal
The number of the signal xdm sends to terminate the server. See
the section Controlling the Server. The default is 15
(SIGTERM).
DisplayManager.DISPLAY.resetForAuth
The original implementation of authorization in the sample
server reread the authorization file at server reset time,
instead of when checking the initial connection. As xdm
generates the authorization information just before connecting
to the display, an old server would not get up-to-date
authorization information. This resource causes xdm to send
SIGHUP to the server after setting up the file, causing an
additional server reset to occur, during which time the new
authorization information will be read. The default is
``false,'' which will work for all MIT servers.
DisplayManager.DISPLAY.userAuthDir
When xdm is unable to write to the usual user authorization file
($HOME/.Xauthority), it creates a unique file name in this
directory and points the environment variable XAUTHORITY at the
created file. It uses /tmp by default.
CONFIGURATION FILE
First, the xdm configuration file should be set up. Make a directory
(usually /etc/X11/xdm) to contain all of the relevant files.
Here is a reasonable configuration file, which could be named xdm-
config:
DisplayManager.servers: /etc/X11/xdm/Xservers
DisplayManager.errorLogFile: /var/log/xdm.log
DisplayManager*resources: /etc/X11/xdm/Xresources
DisplayManager*startup: /etc/X11/xdm/Xstartup
DisplayManager*session: /etc/X11/xdm/Xsession
DisplayManager.pidFile: /var/run/xdm-pid
DisplayManager._0.authorize: true
DisplayManager*authorize: false
Note that this file mostly contains references to other files. Note
also that some of the resources are specified with ``*'' separating the
components. These resources can be made unique for each different
display, by replacing the ``*'' with the display-name, but normally
this is not very useful. See the Resources section for a complete
discussion.
XDMCP ACCESS CONTROL
The database file specified by the DisplayManager.accessFile provides
information which xdm uses to control access from displays requesting
XDMCP service. This file contains three types of entries: entries
which control the response to Direct and Broadcast queries, entries
which control the response to Indirect queries, and macro definitions.
The format of the Direct entries is simple, either a host name or a
pattern, which is distinguished from a host name by the inclusion of
one or more meta characters (`*' matches any sequence of 0 or more
characters, and `?' matches any single character) which are compared
against the host name of the display device. If the entry is a host
name, all comparisons are done using network addresses, so any name
which converts to the correct network address may be used. For
patterns, only canonical host names are used in the comparison, so
ensure that you do not attempt to match aliases. Preceding either a
host name or a pattern with a `!' character causes hosts which match
that entry to be excluded.
To only respond to Direct queries for a host or pattern, it can be
followed by the optional ``NOBROADCAST'' keyword. This can be used to
prevent an xdm server from appearing on menus based on Broadcast
queries.
An Indirect entry also contains a host name or pattern, but follows it
with a list of host names or macros to which indirect queries should be
sent.
A macro definition contains a macro name and a list of host names and
other macros that the macro expands to. To distinguish macros from
hostnames, macro names start with a `%' character. Macros may be
nested.
Indirect entries may also specify to have xdm run chooser to offer a
menu of hosts to connect to. See the section Chooser.
When checking access for a particular display host, each entry is
scanned in turn and the first matching entry determines the response.
Direct and Broadcast entries are ignored when scanning for an Indirect
entry and vice-versa.
Blank lines are ignored, `#' is treated as a comment delimiter causing
the rest of that line to be ignored, and `\newline' causes the newline
to be ignored, allowing indirect host lists to span multiple lines.
Here is an example Xaccess file:
#
# Xaccess - XDMCP access control file
#
#
# Direct/Broadcast query entries
#
!xtra.lcs.mit.edu # disallow direct/broadcast service for xtra
bambi.ogi.edu # allow access from this particular display
*.lcs.mit.edu # allow access from any display in LCS
*.deshaw.com NOBROADCAST # allow only direct access
*.gw.com # allow direct and broadcast
#
# Indirect query entries
#
%HOSTS expo.lcs.mit.edu xenon.lcs.mit.edu \
excess.lcs.mit.edu kanga.lcs.mit.edu
extract.lcs.mit.edu xenon.lcs.mit.edu #force extract to contact xenon
!xtra.lcs.mit.edu dummy #disallow indirect access
*.lcs.mit.edu %HOSTS #all others get to choose
If compiled with IPv6 support, multicast address groups may also be
included in the list of addresses indirect queries are set to.
Multicast addresses may be followed by an optional / character and hop
count. If no hop count is specified, the multicast hop count defaults
to 1, keeping the packet on the local network. For IPv4 multicasting,
the hop count is used as the TTL.
Examples:
rincewind.sample.net ff02::1 #IPv6 Multicast to ff02::1
#with a hop count of 1
ponder.sample.net CHOOSER 239.192.1.1/16 #Offer a menu of hosts
#who respond to IPv4 Multicast
# to 239.192.1.1 with a TTL of 16
CHOOSER
For X terminals that do not offer a host menu for use with Broadcast or Indirect queries, the chooser program can do this for them. In the Xaccess file, specify ``CHOOSER'' as the first entry in the Indirect host list. Chooser will send a Query request to each of the remaining host names in the list and offer a menu of all the hosts that respond. The list may consist of the word ``BROADCAST,'' in which case chooser will send a Broadcast instead, again offering a menu of all hosts that respond. Note that on some operating systems, UDP packets cannot be broadcast, so this feature will not work. Example Xaccess file using chooser: extract.lcs.mit.edu CHOOSER %HOSTS #offer a menu of these hosts xtra.lcs.mit.edu CHOOSER BROADCAST #offer a menu of all hosts The program to use for chooser is specified by the DisplayManager.DISPLAY.chooser resource. For more flexibility at this step, the chooser could be a shell script. Chooser is the session manager here; it is run instead of a child xdm to manage the display. Resources for this program can be put into the file named by DisplayManager.DISPLAY.resources. When the user selects a host, chooser prints the host chosen, which is read by the parent xdm, and exits. xdm closes its connection to the X server, and the server resets and sends another Indirect XDMCP request. xdm remembers the user's choice (for DisplayManager.choiceTimeout seconds) and forwards the request to the chosen host, which starts a session on that display.
LISTEN
The following configuration directive is also defined for the Xaccess
configuration file:
LISTEN interface [list of multicast group addresses]
interface may be a hostname or IP address representing a network
interface on this machine, or the wildcard * to represent all
available network interfaces.
If one or more LISTEN lines are specified, xdm only listens for XDMCP
connections on the specified interfaces. If multicast group addresses
are listed on a listen line, xdm joins the multicast groups on the
given interface.
If no LISTEN lines are given, the original behavior of listening on all
interfaces is preserved for backwards compatibility. Additionally, if
no LISTEN is specified, xdm joins the default XDMCP IPv6 multicast
group, when compiled with IPv6 support.
To disable listening for XDMCP connections altogther, a line of LISTEN
with no addresses may be specified, or the previously supported method
of setting DisplayManager.requestPort to 0 may be used.
Examples:
LISTEN * ff02::1 # Listen on all interfaces and to the
# ff02::1 IPv6 multicast group.
LISTEN 10.11.12.13 # Listen only on this interface, as long
# as no other listen directives appear in
# file.
IPv6 MULTICAST ADDRESS SPECIFICATION
The Internet Assigned Numbers Authority has has assigned ff0X:0:0:0:0:0:0:12b as the permanently assigned range of multicast addresses for XDMCP. The X in the prefix may be replaced by any valid scope identifier, such as 1 for Interface-Local, 2 for Link-Local, 5 for Site-Local, and so on. (See IETF RFC 4291 or its replacement for further details and scope definitions.) xdm defaults to listening on the Link-Local scope address ff02:0:0:0:0:0:0:12b to most closely match the old IPv4 subnet broadcast behavior.
LOCAL SERVER SPECIFICATION
The resource DisplayManager.servers gives a server specification or, if
the values starts with a slash (/), the name of a file containing
server specifications, one per line.
Each specification indicates a display which should constantly be
managed and which is not using XDMCP. This method is used typically
for local servers only. If the resource or the file named by the
resource is empty, xdm will offer XDMCP service only.
Each specification consists of at least three parts: a display name, a
display class, a display type, and (for local servers) a command line
to start the server. A typical entry for local display number 0 would
be:
:0 Digital-QV local /usr/bin/X :0
The display types are:
local local display: xdm must run the server
foreign remote display: xdm opens an X connection to a running server
The display name must be something that can be passed in the -display
option to an X program. This string is used to generate the display-
specific resource names, so be careful to match the names (e.g., use
``:0 Sun-CG3 local /usr/bin/X :0'' instead of ``localhost:0 Sun-CG3
local /usr/bin/X :0'' if your other resources are specified as
``DisplayManager._0.session''). The display class portion is also used
in the display-specific resources, as the class of the resource. This
is useful if you have a large collection of similar displays (such as a
corral of X terminals) and would like to set resources for groups of
them. When using XDMCP, the display is required to specify the display
class, so the manual for your particular X terminal should document the
display class string for your device. If it doesn't, you can run xdm
in debug mode and look at the resource strings which it generates for
that device, which will include the class string.
When xdm starts a session, it sets up authorization data for the
server. For local servers, xdm passes ``-auth filename'' on the
server's command line to point it at its authorization data. For XDMCP
servers, xdm passes the authorization data to the server via the Accept
XDMCP request.
RESOURCES FILE
The Xresources file is loaded onto the display as a resource database
using xrdb. As the authentication widget reads this database before
starting up, it usually contains parameters for that widget:
xlogin*login.translations: #override\
Ctrl<Key>R: abort-display()\n\
<Key>F1: set-session-argument(failsafe) finish-field()\n\
<Key>Return: set-session-argument() finish-field()
xlogin*borderWidth: 3
xlogin*greeting: CLIENTHOST
#ifdef COLOR
xlogin*greetColor: CadetBlue
xlogin*failColor: red
#endif
Please note the translations entry; it specifies a few new translations
for the widget which allow users to escape from the default session
(and avoid troubles that may occur in it). Note that if #override is
not specified, the default translations are removed and replaced by the
new value, not a very useful result as some of the default translations
are quite useful (such as ``<Key>: insert-char ()'' which responds to
normal typing).
This file may also contain resources for the setup program and chooser.
SETUP PROGRAM
The Xsetup file is run after the server is reset, but before the Login
window is offered. The file is typically a shell script. It is run as
root, so should be careful about security. This is the place to change
the root background or bring up other windows that should appear on the
screen along with the Login widget.
In addition to any specified by DisplayManager.exportList, the
following environment variables are passed:
DISPLAY the associated display name
PATH the value of DisplayManager.DISPLAY.systemPath
SHELL the value of DisplayManager.DISPLAY.systemShell
XAUTHORITY may be set to an authority file
Note that since xdm grabs the keyboard, any other windows will not be
able to receive keyboard input. They will be able to interact with the
mouse, however; beware of potential security holes here. If
DisplayManager.DISPLAY.grabServer is set, Xsetup will not be able to
connect to the display at all. Resources for this program can be put
into the file named by DisplayManager.DISPLAY.resources.
Here is a sample Xsetup script:
#!/bin/sh
# Xsetup_0 - setup script for one workstation
xcmsdb < /etc/X11/xdm/monitors/alex.0
xconsole -geometry 480x130-0-0 -notify -verbose -exitOnFail &
AUTHENTICATION WIDGET
The authentication widget prompts the user for the username, password, and/or other required authentication data from the keyboard. Nearly every imaginable parameter can be controlled with a resource. Resources for this widget should be put into the file named by DisplayManager.DISPLAY.resources. All of these have reasonable default values, so it is not necessary to specify any of them. The resource file is loaded with xrdb(1) so it may use the substitutions defined by that program such as CLIENTHOST for the client hostname in the login message, or C pre-processor #ifdef statements to produce different displays depending on color depth or other variables. Xdm can be compiled with support for the Xft(3) library for font rendering. If this support is present, font faces are specified using the resources with names ending in ``face'' in the fontconfig face format described in the Font Names section of fonts.conf(5). If not, then fonts are specified using the resources with names ending in ``font'' in the traditional X Logical Font Description format described in the Font Names section of X(7). xlogin.Login.width, xlogin.Login.height, xlogin.Login.x, xlogin.Login.y The geometry of the Login widget is normally computed automatically. If you wish to position it elsewhere, specify each of these resources. xlogin.Login.foreground The color used to display the input typed by the user. xlogin.Login.face The face used to display the input typed by the user when built with Xft support. The default is ``Serif-18''. xlogin.Login.font The font used to display the input typed by the user when not built with Xft support. xlogin.Login.greeting A string which identifies this window. The default is ``X Window System.'' xlogin.Login.unsecureGreeting When X authorization is requested in the configuration file for this display and none is in use, this greeting replaces the standard greeting. The default is ``This is an unsecure session'' xlogin.Login.greetFace The face used to display the greeting when built with Xft support. The default is ``Serif-24:italic''. xlogin.Login.greetFont The font used to display the greeting when not built with Xft support. xlogin.Login.greetColor The color used to display the greeting. xlogin.Login.namePrompt The string displayed to prompt for a user name. Xrdb strips trailing white space from resource values, so to add spaces at the end of the prompt (usually a nice thing), add spaces escaped with backslashes. The default is ``Login: '' xlogin.Login.passwdPrompt The string displayed to prompt for a password, when not using an authentication system such as PAM that provides its own prompts. The default is ``Password: '' xlogin.Login.promptFace The face used to display prompts when built with Xft support. The default is ``Serif-18:bold''. xlogin.Login.promptFont The font used to display prompts when not built with Xft support. xlogin.Login.promptColor The color used to display prompts. xlogin.Login.changePasswdMessage A message which is displayed when the users password has expired. The default is ``Password Change Required'' xlogin.Login.fail A message which is displayed when the authentication fails, when not using an authentication system such as PAM that provides its own prompts. The default is ``Login incorrect'' xlogin.Login.failFace The face used to display the failure message when built with Xft support. The default is ``Serif-18:bold''. xlogin.Login.failFont The font used to display the failure message when not built with Xft support. xlogin.Login.failColor The color used to display the failure message. xlogin.Login.failTimeout The number of seconds that the failure message is displayed. The default is 10. xlogin.Login.logoFileName Name of an XPM format pixmap to display in the greeter window, if built with XPM support. The default is no pixmap. xlogin.Login.logoPadding Number of pixels of space between the logo pixmap and other elements of the greeter window, if the pixmap is displayed. The default is 5. xlogin.Login.useShape If set to ``true'', when built with XPM support, attempt to use the X Non-Rectangular Window Shape Extension to set the window shape. The default is ``true''. xlogin.Login.hiColor, xlogin.Login.shdColor Raised appearance bezels may be drawn around the greeter frame and text input boxes by setting these resources. hiColor is the highlight color, used on the top and left sides of the frame, and the bottom and right sides of text input areas. shdColor is the shadow color, used on the bottom and right sides of the frame, and the top and left sides of text input areas. The default for both is the foreground color, providing a flat appearance. xlogin.Login.frameWidth frameWidth is the width in pixels of the area around the greeter frame drawn in hiColor and shdColor. xlogin.Login.innerFramesWidth innerFramesWidth is the width in pixels of the area around text input areas drawn in hiColor and shdColor. xlogin.Login.sepWidth sepWidth is the width in pixels of the bezeled line between the greeting and input areas drawn in hiColor and shdColor. xlogin.Login.allowRootLogin If set to ``false'', don't allow root (and any other user with uid = 0) to log in directly. The default is ``true''. This setting is only checked by some of the authentication backends at this time. xlogin.Login.allowNullPasswd If set to ``true'', allow an otherwise failing password match to succeed if the account does not require a password at all. The default is ``false'', so only users that have passwords assigned can log in. xlogin.Login.echoPasswd If set to ``true'', a placeholder character (echoPasswdChar) will be shown for fields normally set to not echo, such as password input. The default is ``false''. xlogin.Login.echoPasswdChar Character to display if echoPasswd is true. The default is ``*''. If set to an empty value, the cursor will advance for each character input, but no text will be drawn. xlogin.Login.translations This specifies the translations used for the login widget. Refer to the X Toolkit documentation for a complete discussion on translations. The default translation table is: Ctrl<Key>H: delete-previous-character() \n\ Ctrl<Key>D: delete-character() \n\ Ctrl<Key>B: move-backward-character() \n\ Ctrl<Key>F: move-forward-character() \n\ Ctrl<Key>A: move-to-begining() \n\ Ctrl<Key>E: move-to-end() \n\ Ctrl<Key>K: erase-to-end-of-line() \n\ Ctrl<Key>U: erase-line() \n\ Ctrl<Key>X: erase-line() \n\ Ctrl<Key>C: restart-session() \n\ Ctrl<Key>\\: abort-session() \n\ <Key>BackSpace:delete-previous-character() \n\ <Key>Delete: delete-previous-character() \n\ <Key>Return: finish-field() \n\ <Key>: insert-char() \ The actions which are supported by the widget are: delete-previous-character Erases the character before the cursor. delete-character Erases the character after the cursor. move-backward-character Moves the cursor backward. move-forward-character Moves the cursor forward. move-to-begining (Apologies about the spelling error.) Moves the cursor to the beginning of the editable text. move-to-end Moves the cursor to the end of the editable text. erase-to-end-of-line Erases all text after the cursor. erase-line Erases the entire text. finish-field If the cursor is in the name field, proceeds to the password field; if the cursor is in the password field, checks the current name/password pair. If the name/password pair is valid, xdm starts the session. Otherwise the failure message is displayed and the user is prompted again. abort-session Terminates and restarts the server. abort-display Terminates the server, disabling it. This action is not accessible in the default configuration. There are various reasons to stop xdm on a system console, such as when shutting the system down, when using xdmshell, to start another type of server, or to generally access the console. Sending xdm a SIGHUP will restart the display. See the section Controlling XDM. restart-session Resets the X server and starts a new session. This can be used when the resources have been changed and you want to test them or when the screen has been overwritten with system messages. insert-char Inserts the character typed. set-session-argument Specifies a single word argument which is passed to the session at startup. See the section Session Program. allow-all-access Disables access control in the server. This can be used when the .Xauthority file cannot be created by xdm. Be very careful using this; it might be better to disconnect the machine from the network before doing this. On some systems (OpenBSD) the user's shell must be listed in /etc/shells to allow login through xdm. The normal password and account expiration dates are enforced too.
STARTUP PROGRAM
The Xstartup program is run as root when the user logs in. It is
typically a shell script. Since it is run as root, Xstartup should be
very careful about security. This is the place to put commands which
add entries to utmp or wtmp files, (the sessreg program may be useful
here), mount users' home directories from file servers, or abort the
session if logins are not allowed.
In addition to any specified by DisplayManager.exportList, the
following environment variables are passed:
DISPLAY the associated display name
HOME the initial working directory of the user
LOGNAME the user name
USER the user name
PATH the value of DisplayManager.DISPLAY.systemPath
SHELL the value of DisplayManager.DISPLAY.systemShell
XAUTHORITY may be set to an authority file
WINDOWPATH may be set to the "window path" leading to the X server
No arguments are passed to the script. Xdm waits until this script
exits before starting the user session. If the exit value of this
script is non-zero, xdm discontinues the session and starts another
authentication cycle.
The sample Xstartup file shown here prevents login while the file
/etc/nologin exists. Thus this is not a complete example, but simply a
demonstration of the available functionality.
Here is a sample Xstartup script:
#!/bin/sh
#
# Xstartup
#
# This program is run as root after the user is verified
#
if [ -f /etc/nologin ]; then
xmessage -file /etc/nologin -timeout 30 -center
exit 1
fi
sessreg -a -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME
/etc/X11/xdm/GiveConsole
exit 0
SESSION PROGRAM
The Xsession program is the command which is run as the user's session.
It is run with the permissions of the authorized user.
In addition to any specified by DisplayManager.exportList, the
following environment variables are passed:
DISPLAY the associated display name
HOME the initial working directory of the user
LOGNAME the user name
USER the user name
PATH the value of DisplayManager.DISPLAY.userPath
SHELL the user's default shell (from getpwnam)
XAUTHORITY may be set to a non-standard authority file
KRB5CCNAME may be set to a Kerberos credentials cache name
WINDOWPATH may be set to the "window path" leading to the X server
At most installations, Xsession should look in $HOME for a file
.xsession, which contains commands that each user would like to use as
a session. Xsession should also implement a system default session if
no user-specified session exists.
An argument may be passed to this program from the authentication
widget using the `set-session-argument' action. This can be used to
select different styles of session. One good use of this feature is to
allow the user to escape from the ordinary session when it fails. This
allows users to repair their own .xsession if it fails, without
requiring administrative intervention. The example following
demonstrates this feature.
This example recognizes the special ``failsafe'' mode, specified in the
translations in the Xresources file, to provide an escape from the
ordinary session. It also requires that the .xsession file be
executable so we don't have to guess what shell it wants to use.
#!/bin/sh
#
# Xsession
#
# This is the program that is run as the client
# for the display manager.
case $# in
1)
case $1 in
failsafe)
exec xterm -geometry 80x24-0-0
;;
esac
esac
startup=$HOME/.xsession
resources=$HOME/.Xresources
if [ -f "$startup" ]; then
exec "$startup"
else
if [ -f "$resources" ]; then
xrdb -load "$resources"
fi
twm &
xman -geometry +10-10 &
exec xterm -geometry 80x24+10+10 -ls
fi
The user's .xsession file might look something like this example.
Don't forget that the file must have execute permission.
#! /bin/csh
# no -f in the previous line so .cshrc gets run to set $PATH
twm &
xrdb -merge "$HOME/.Xresources"
emacs -geometry +0+50 &
xbiff -geometry -430+5 &
xterm -geometry -0+50 -ls
RESET PROGRAM
Symmetrical with Xstartup, the Xreset script is run after the user
session has terminated. Run as root, it should contain commands that
undo the effects of commands in Xstartup, updating entries in utmp or
wtmp files, or unmounting directories from file servers. The
environment variables that were passed to Xstartup are also passed to
Xreset.
A sample Xreset script:
#!/bin/sh
#
# Xreset
#
# This program is run as root after the session ends
#
sessreg -d -l $DISPLAY -x /etc/X11/xdm/Xservers $LOGNAME
/etc/X11/xdm/TakeConsole
exit 0
CONTROLLING THE SERVER
Xdm controls local servers using POSIX signals. SIGHUP is expected to reset the server, closing all client connections and performing other cleanup duties. SIGTERM is expected to terminate the server. If these signals do not perform the expected actions, the resources DisplayManager.DISPLAY.resetSignal and DisplayManager.DISPLAY.termSignal can specify alternate signals. To control remote terminals not using XDMCP, xdm searches the window hierarchy on the display and uses the protocol request KillClient in an attempt to clean up the terminal for the next session. This may not actually kill all of the clients, as only those which have created windows will be noticed. XDMCP provides a more sure mechanism; when xdm closes its initial connection, the session is over and the terminal is required to close all other connections.
CONTROLLING XDM
Xdm responds to two signals: SIGHUP and SIGTERM. When sent a SIGHUP, xdm rereads the configuration file, the access control file, and the servers file. For the servers file, it notices if entries have been added or removed. If a new entry has been added, xdm starts a session on the associated display. Entries which have been removed are disabled immediately, meaning that any session in progress will be terminated without notice and no new session will be started. When sent a SIGTERM, xdm terminates all sessions in progress and exits. This can be used when shutting down the system. Xdm attempts to mark its various sub-processes for ps(1) by editing the command line argument list in place. Because xdm can't allocate additional space for this task, it is useful to start xdm with a reasonably long command line (using the full path name should be enough). Each process which is servicing a display is marked -display.
ADDITIONAL LOCAL DISPLAYS
To add an additional local display, add a line for it to the Xservers file. (See the section Local Server Specification.) Examine the display-specific resources in xdm-config (e.g., DisplayManager._0.authorize) and consider which of them should be copied for the new display. The default xdm-config has all the appropriate lines for displays :0 and :1.
OTHER POSSIBILITIES
You can use xdm to run a single session at a time, using the 4.3 init
options or other suitable daemon by specifying the server on the
command line:
xdm -server ":0 SUN-3/60CG4 local /usr/bin/X :0"
Or, you might have a file server and a collection of X terminals. The
configuration for this is identical to the sample above, except the
Xservers file would look like
extol:0 VISUAL-19 foreign
exalt:0 NCD-19 foreign
explode:0 NCR-TOWERVIEW3000 foreign
This directs xdm to manage sessions on all three of these terminals.
See the section Controlling Xdm for a description of using signals to
enable and disable these terminals in a manner reminiscent of init(8).
LIMITATIONS
One thing that xdm isn't very good at doing is coexisting with other window systems. To use multiple window systems on the same hardware, you'll probably be more interested in xinit.
ASYNCHRONOUS EVENTS
xdm uses SIGALRM and SIGUSR1 for its own inter-process communication
purposes, managing the relationship between the parent xdm process and
its children. Sending these signals to any xdm process may result in
unexpected behavior.
SIGHUP causes xdm to rescan its configuration files and reopen its log
file.
SIGTERM
causes xdm to terminate its children and shut down.
SIGUSR2
causes xdm to reopen its log file. This is useful if log
rotation is desired, but SIGHUP is too disruptive.
FILES
/etc/X11/xdm/xdm-config
the default configuration file
$HOME/.Xauthority user authorization file where xdm stores keys for
clients to read
/usr/lib/X11/xdm/chooser
the default chooser
/usr/bin/xrdb the default resource database loader
/usr/bin/X the default server
/usr/bin/xterm the default session program and failsafe client
/var/lib/xdm/A<display>-<suffix>
the default place for authorization files
/tmp/K5C<display> Kerberos credentials cache
SEE ALSO
X(7), xinit(1), xauth(1), xrdb(1), Xsecurity(7), sessreg(1), Xserver(1), xdmshell(1), fonts.conf(5), xdm.options(5). X Display Manager Control Protocol IETF RFC 4291: IP Version 6 Addressing Architecture.
AUTHOR
Keith Packard, MIT X Consortium
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