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 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.
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 &
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