XDM(1) MachTen Programmer’s Manual XDM(1)

xdm - X Display Manager with support for XDMCP, host

xdm [ -config configuration_file ] [ -nodaemon ] [ -debug
debug_level ] [ -error error_log_file ] [ -resources
resource_file ] [ -server server_entry ] [ -session ses-
sion_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 X Con-
sortium standard XDMCP, the X Display Manager Control Pro-
tocol. 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 typi-
cally used as the ‘‘session manager,’’ meaning that termi-
nation 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 dis-
play and offer a menu of possible hosts that offer XDMCP
display management. This feature is useful with X termi-
nals that do not offer a host menu themselves.

Because xdm provides the first interface that users will
see, it is designed to be simple to use and easy to cus-
tomize 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

xdm offers display management two different ways. It can
manage X servers running on the local machine and speci-
fied in Xservers, and it can manage remote X servers
(typically X terminals) using XDMCP (the XDM Control Pro-
tocol) 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.

When the user logs in, xdm runs the Xstartup script as

Then xdm runs the Xsession script as the user. This sys-
tem session file may do some additional startup and typi-
cally runs a 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

The file /usr/X11R6/lib/X11/xdm/xdm-errors will contain
error messages from xdm and anything output to stderr by
Xsetup, Xstartup, Xsession or Xreset. When you have trou-
ble getting xdm working, check this file to see if xdm has
any clues to the trouble.

All of these options, except -config itself, specify val-
ues 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.
<XRoot>/lib/X11/xdm/xdm-config is the default. See
the section Configuration File.

Specifies ‘‘false’’ as the value for the Display-
Manager.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 back-
ground when it first starts up.

-debug debug_level
Specifies the numeric value for the DisplayMan-
ager.debugLevel resource. A non-zero value causes
xdm to print lots of debugging statements to the
terminal; it also disables the DisplayMan-
ager.daemonMode resource, forcing xdm to run syn-
chronously. 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 DisplayMan-
ager.errorLogFile resource. This file contains
errors from xdm as well as anything written to
stderr by the various scripts and programs run dur-
ing the progress of the session.

-resources resource_file
Specifies the value for the DisplayMan-
ager*resources resource. This file is loaded using
xrdb to specify configuration parameters for the
authentication widget.

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

-udpPort port_number
Specifies the value for the DisplayMan-
ager.requestPort resource. This sets the port-
number which xdm will monitor for XDMCP requests.
As XDMCP uses the registered well-known UDP port
177, this resource should not be changed except for

-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 dis-
play, the display name is inserted into the resource name
between ‘‘DisplayManager’’ and the final resource name

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 dis-
play. 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.

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.

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.

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 sys-
log should be developed for systems which support
it; however, the wide variety of interfaces pre-
cludes 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.

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 nor-
mally not be useful.

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

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.

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.

This names a directory under which xdm stores
authorization files while initializing the session.
The default value is <XRoot>/lib/X11/xdm. Can be
overridden for specific displays by DisplayMan-

This boolean controls whether xdm rescans the con-
figuration, servers, access control and authentica-
tion 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.

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 vari-
able is set. By default the value is ‘‘true.’’

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.

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.

A list of additional environment variables, sepa-
rated by white space, to pass on to the Xsetup,
Xstartup, Xsession, and Xreset programs.

A file to checksum to generate the seed of autho-
rization keys. This should be a file that changes
frequently. The default is /dev/mem.

On systems that support a dynamically-loadable
greeter library, the name of the library. Default
is <XRoot>/lib/X11/xdm/libXdmGreet.so.

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.

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 pro-
cedure is started, so it can control the appearance
of the login window. See the section Authentica-
tion Widget, which describes the various resources
that are appropriate to place in this file. There
is no default value for this resource, but
<XRoot>/lib/X11/xdm/Xresources is the conventional

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

Specifies the program used to load the resources.
By default, xdm uses <XRoot>/bin/xrdb.

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

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 Pro-

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.

This specifies the session to be executed (not run-
ning as root). By default, <XRoot>/bin/xterm is
run. The conventional name is Xsession. See the
section Session Program.

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




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 num-
ber 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 par-
ticular 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 default values are 5 for openDelay, 5
for openRepeat, 30 for openTimeout and 4 for star-


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 termi-
nals 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 respond-
ing to the ping.

This boolean resource specifies whether the X
server should be terminated when a session termi-
nates (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.’’

Xdm sets the PATH environment variable for the ses-
sion to this value. It should be a colon separated
list of directories; see sh(1) for a full descrip-
tion. ‘‘:/bin:/usr/bin:/usr/X11R6/bin:/usr/ucb’’
is a common setting. The default value can be
specified at build time in the X system configura-
tion file with DefaultUserPath.

Xdm sets the PATH environment variable for the
startup and reset scripts to the value of this
resource. The default for this resource is speci-
fied at build time by the DefaultSystemPath entry
in the system configuration file;
‘‘/etc:/bin:/usr/bin:/usr/X11R6/bin:/usr/ucb’’ is a
common choice. Note the absence of ‘‘.’’ from this
entry. This is a good practice to follow for root;
it avoids many common Trojan Horse system penetra-
tion schemes.

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

If the default session fails to execute, xdm will
fall back to this program. This program is exe-
cuted with no arguments, but executes using the
same environment variables as the session would
have had (see the section Session Program). By
default, <XRoot>/bin/xterm is used.


To improve security, xdm grabs the server and key-
board 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, oth-
erwise 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.


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 mecha-
nisms to use, separated by white space. XDMCP con-
nections 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,

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 direc-
tory which is not world-writable as it could easily
be removed, disabling the authorization mechanism
in the server. If not specified, a name is gener-
ated from DisplayManager.authDir and the name of
the display.

If set to ‘‘false,’’ disables the use of the unse-
cureGreeting in the login window. See the section
Authentication Widget. The default is ‘‘true.’’

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

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

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 autho-
rization 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.

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 <XRoot>/lib/X11/xdm, where <XRoot>
refers to the root of the X11 install tree) to contain all
of the relevant files. In the examples that follow, we
use /usr/X11R6 as the value of <XRoot>.

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

DisplayManager.servers: /usr/X11R6/lib/X11/xdm/Xservers
DisplayManager.errorLogFile: /usr/X11R6/lib/X11/xdm/xdm-errors
DisplayManager*resources: /usr/X11R6/lib/X11/xdm/Xresources
DisplayManager*startup: /usr/X11R6/lib/X11/xdm/Xstartup
DisplayManager*session: /usr/X11R6/lib/X11/xdm/Xsession
DisplayManager.pidFile: /usr/X11R6/lib/X11/xdm/xdm-pid
DisplayManager._0.authorize: true
DisplayManager*authorize: false

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

The database file specified by the DisplayMan-
ager.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 net-
work 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.

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

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-

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

# 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

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

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 oper-
ating 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 Dis-
playManager.DISPLAY.chooser resource. For more flexibil-
ity at this step, the chooser could be a shell script.
Chooser is the session manager here; it is run instead of
a child xdm to manage the display.

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

When the user selects a host, chooser prints the host cho-
sen, 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 DisplayMan-
ager.choiceTimeout seconds) and forwards the request to
the chosen host, which starts a session on that display.

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

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

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

:0 Digital-QV local /usr/X11R6/bin/X :0

The display types are:

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

The display name must be something that can be passed in
the -display option to an X program. This string is used
to generate the display-specific resource names, so be
careful to match the names (e.g., use ‘‘:0 Sun-CG3 local
/usr/X11R6/bin/X :0’’ instead of ‘‘localhost:0 Sun-CG3
local /usr/X11R6/bin/X :0’’ if your other resources are
specified as ‘‘DisplayManager._0.session’’). The display
class 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 dis-
play is required to specify the display class, so the man-
ual 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

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

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

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 poten-
tial security holes here. If DisplayMan-
ager.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 DisplayMan-

Here is a sample Xsetup script:

# Xsetup_0 - setup script for one workstation
xcmsdb < /usr/X11R6/lib/monitors/alex.0
xconsole -geometry 480x130-0-0 -notify -verbose -exitOnFail &

The authentication widget reads a name/password pair from
the keyboard. Nearly every imaginable parameter can be
controlled with a resource. Resources for this widget
should be put into the file named by DisplayMan-
ager.DISPLAY.resources. All of these have reasonable
default values, so it is not necessary to specify any of

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

The color used to display the typed-in user name.

The font used to display the typed-in user name.

A string which identifies this window. The default
is ‘‘X Window System.’’

When X authorization is requested in the configura-
tion file for this display and none is in use, this
greeting replaces the standard greeting. The
default is ‘‘This is an unsecure session’’

The font used to display the greeting.

The color used to display the greeting.

The string displayed to prompt for a user name.
Xrdb strips trailing white space from resource val-
ues, so to add spaces at the end of the prompt
(usually a nice thing), add spaces escaped with
backslashes. The default is ‘‘Login: ’’

The string displayed to prompt for a password. The
default is ‘‘Password: ’’

The font used to display both prompts.

The color used to display both prompts.

A message which is displayed when the authentica-
tion fails. The default is ‘‘Login incorrect’’

The font used to display the failure message.

The color used to display the failure message.

The number of seconds that the failure message is
displayed. The default is 30.

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

The actions which are supported by the widget are:

Erases the character before the cursor.

Erases the character after the cursor.

Moves the cursor backward.

Moves the cursor forward.

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

Moves the cursor to the end of the editable text.

Erases all text after the cursor.

Erases the entire text.

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 dis-
played and the user is prompted again.

Terminates and restarts the server.

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. Send-
ing xdm a SIGHUP will restart the display. See the
section Controlling XDM.

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.

Inserts the character typed.

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

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 bet-
ter to disconnect the machine from the network
before doing this.

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

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

No arguments are passed to the script. Xdm waits until
this script exits before starting the user session. If
the exit value of this script is non-zero, xdm discontin-
ues 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 func-

Here is a sample Xstartup script:

# 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
sessreg -a -l $DISPLAY -x /usr/X11R6/lib/xdm/Xservers $USER
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
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

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

An argument may be passed to this program from the authen-
tication 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 exam-
ple 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.

# Xsession
# This is the program that is run as the client
# for the display manager.

case $# in
case $1 in
exec xterm -geometry 80x24-0-0


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

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

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

A sample Xreset script:
# Xreset
# This program is run as root after the session ends
sessreg -d -l $DISPLAY -x /usr/X11R6/lib/xdm/Xservers $USER
exit 0

Xdm controls local servers using POSIX signals. SIGHUP is
expected to reset the server, closing all client connec-
tions and performing other cleanup duties. SIGTERM is
expected to terminate the server. If these signals do not
perform the expected actions, the resources DisplayMan-
ager.DISPLAY.resetSignal and DisplayMan-
ager.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 ses-
sion 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.

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

xdm -server ":0 SUN-3/60CG4 local /usr/X11R6/bin/X :0"

Or, you might have a file server and a collection of X
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 descrip-
tion of using signals to enable and disable these termi-
nals 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.

the default configuration file

$HOME/.Xauthority user authorization file where xdm
stores keys for clients to read

the default chooser

<XRoot>/bin/xrdb the default resource database loader

<XRoot>/bin/X the default server

<XRoot>/bin/xterm the default session program and fail-
safe client

the default place for authorization

/tmp/K5C<display> Kerberos credentials cache

Note: <XRoot> refers to the root of the X11 install tree.

X(3), xinit(1), xauth(1), Xsecurity(3), sessreg(1),
X Display Manager Control Protocol

Keith Packard, MIT X Consortium

X Version 11 Release 6 19