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/db/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.greeterLib
On systems that support a dynamically-loadable greeter library, the name of the library. The default is /usr/X11R6/libexec/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/X11R6/libexec/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/X11R6/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/X11R6/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
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 default values are 5 for openDelay, 5 for openRepeat, 30 for openTimeout and 4 for startAttempts.
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. ``:/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 configuration file with DefaultUserPath.
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 specified 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 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/X11R6/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.