xdm - X Display Manager with support for XDMCP, host chooser
xdm [
-config configuration_file ] [
-nodaemon ] [
-debug debug_level ] [
-error error_log_file ] [
-resources resource_file ] [
-server server_entry
] [
-session session_program ]
Xdm manages a collection of X displays, which may be on the local host or
remote servers. The design of
xdm was guided by the needs of X
terminals as well as The Open Group 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.
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.
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.
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.
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.
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
For X terminals that do not offer a host menu for use with Broadcast or Indirect
queries, the
chooser program can do this for them. In the
Xaccess file, specify ``CHOOSER'' as the first entry in the Indirect
host list.
Chooser will send a Query request to each of the remaining
host names in the list and offer a menu of all the hosts that respond.
The list may consist of the word ``BROADCAST,'' in which case
chooser
will send a Broadcast instead, again offering a menu of all hosts that
respond. Note that on some operating systems, UDP packets cannot be broadcast,
so this feature will not work.
Example
Xaccess file using
chooser:
extract.lcs.mit.edu CHOOSER %HOSTS #offer a menu of these hosts
xtra.lcs.mit.edu CHOOSER BROADCAST #offer a menu of all hosts
The program to use for
chooser is specified by the
DisplayManager.DISPLAY.chooser resource. For more
flexibility at this step, the chooser could be a shell script.
Chooser
is the session manager here; it is run instead of a child
xdm to manage
the display.
Resources for this program can be put into the file named by
DisplayManager. DISPLAY.resources.
When the user selects a host,
chooser prints the host chosen, which is
read by the parent
xdm, and exits.
xdm closes its connection to
the X server, and the server resets and sends another
Indirect XDMCP
request.
xdm remembers the user's choice (for
DisplayManager.choiceTimeout seconds) and forwards the request to the
chosen host, which starts a session on that display.
The following configuration directive is also defined for the Xaccess
configuration file:
-
LISTEN interface [list of multicast 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.
The Internet Assigned Numbers Authority has has assigned ff0
X: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.
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.
The
Xresources file is loaded onto the display as a resource database
using
xrdb. As the authentication widget reads this database before
starting up, it usually contains parameters for that widget:
xlogin*login.translations: #override\
Ctrl<Key>R: abort-display()\n\
<Key>F1: set-session-argument(failsafe) finish-field()\n\
<Key>Return: set-session-argument() finish-field()
xlogin*borderWidth: 3
xlogin*greeting: CLIENTHOST
#ifdef COLOR
xlogin*greetColor: CadetBlue
xlogin*failColor: red
#endif
Please note the translations entry; it specifies a few new translations for the
widget which allow users to escape from the default session (and avoid
troubles that may occur in it). Note that if #override is not specified, the
default translations are removed and replaced by the new value, not a very
useful result as some of the default translations are quite useful (such as
``<Key>: insert-char ()'' which responds to normal typing).
This file may also contain resources for the setup program and
chooser.
The
Xsetup file is run after the server is reset, but before the Login
window is offered. The file is typically a shell script. It is run as root, so
should be careful about 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 &
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.
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
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
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
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.
Xdm responds to two signals: SIGHUP and SIGTERM. When sent a SIGHUP,
xdm rereads the configuration file, the access control file, and the
servers file. For the servers file, it notices if entries have been added or
removed. If a new entry has been added,
xdm starts a session on the
associated display. Entries which have been removed are disabled immediately,
meaning that any session in progress will be terminated without notice and no
new session will be started.
When sent a SIGTERM,
xdm terminates all sessions in progress and exits.
This can be used when shutting down the system.
Xdm attempts to mark its various sub-processes for
ps(1) by
editing the command line argument list in place. Because
xdm can't
allocate additional space for this task, it is useful to start
xdm with
a reasonably long command line (using the full path name should be enough).
Each process which is servicing a display is marked
-display.
To add an additional local display, add a line for it to the
Xservers
file. (See the section
Local Server Specification.)
Examine the display-specific resources in
xdm-config (e.g.,
DisplayManager._0.authorize) and consider which of them should be
copied for the new display. The default
xdm-config has all the
appropriate lines for displays
:0 and
:1.
You can use
xdm to run a single session at a time, using the 4.3
init options or other suitable daemon by specifying the server on the
command line:
xdm -server “:0 SUN-3/60CG4 local /usr/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).
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.
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.
- /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
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.
Keith Packard, MIT X Consortium