XDM(8)

📦
version 1.1.17 · 6 versions
, in opensuse-leap 16.0

okay so bestie, welcome to XDM(8) — the System Manager's Manual page for xdm, aka the X Display Manager with support for XDMCP and a whole host chooser moment. We're keeping every single detail, no cap. ✨

💖 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, and those displays can be on the local host or on 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. periodt. 💅

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. iykyk. 👑

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. noted, bestie.

  • -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 decides whether xdm rescans the configuration, servers, access control and authentication keys files after a session terminates and the files have changed. Default is true.'' `` ngl it's on by default. You can force xdm to reread these files yourself by sending a SIGHUP to the main process 💅
  • DisplayManager.removeDomainname — okay so when xdm computes the display name for XDMCP clients, the name resolver will typically make a fully qualified host name for the terminal. That's sometimes confusing, so when this variable is set, xdm will chop the domain name part off the host name if it matches the domain name of the local host. Default value: true.'' `` ✨
  • DisplayManager.keyFile — XDM-AUTHENTICATION-1 style XDMCP authentication needs a private key shared between xdm and the terminal. This resource tells xdm which file holds those values. Every entry in that file is a display name plus the shared key 🔑
  • DisplayManager.accessFile — this file exists to prevent unauthorized XDMCP service and to allow forwarding of XDMCP IndirectQuery requests. It's a database of hostnames that are either allowed direct access to this machine, or that have a list of hosts to which queries should be forwarded. Format is spelled out in the XDMCP Access Control section, iykyk 🫶
  • DisplayManager.exportList — a list of extra environment variables, separated by white space, passed on to the Xsetup, Xstartup, Xsession, and Xreset programs.
  • DisplayManager.randomDevice — a file to read 8 bytes from, to generate the seed of authorization keys. The default is /dev/urandom. If this file can't 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.greeterLib — on systems that support a dynamically-loadable greeter library, this is the name of the library. Default: /usr/lib64/X11/xdm/libXdmGreet.so 🌸
  • DisplayManager.choiceTimeout — number of seconds to wait for the display to respond after the user picked a host from the chooser. If the display sends an XDMCP IndirectQuery within this time, the request gets forwarded to the chosen host. Otherwise it's 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. Why? To avoid trying to connect on the wrong interface, which might be down at that moment 😭
  • DisplayManager.willing — specifies a program that's run (as) root when 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 one names the file that xrdb loads as the resource database onto the root window of screen 0 of the display. The Xsetup program, the Login widget, and chooser all use the resources set in this file. It gets loaded just before the authentication procedure starts, so it can control how the login window looks 👑 See the Authentication Widget section — it describes the various resources that are appropriate to put in this file. There's no default value for this resource, but /etc/X11/xdm/Xresources is the conventional name.
  • DisplayManager.DISPLAY.chooser — the program run to offer a host menu for Indirect queries redirected to the special host name CHOOSER. Default is /usr/lib64/X11/xdm/chooser. Peep the XDMCP Access Control and Chooser sections 🦋
  • DisplayManager.DISPLAY.xrdb — the program used to load the resources. xdm uses /usr/bin/xrdb by default.
  • DisplayManager.DISPLAY.setup — a program run (as root) before offering the Login window. Use it to change how the screen around the Login window looks, or to put up other windows (like, you might wanna run xconsole here). By default no program is run. Conventional name for a file used here is Xsetup. See the Setup Program section ✨
  • DisplayManager.DISPLAY.startup — a program run (as root) after the authentication process succeeds. By default no program is run. Conventional name: Xstartup. See the Startup Program section.
  • DisplayManager.DISPLAY.session — the session to be executed (and heads up, this one is not running as root). By default /usr/bin/xterm is run. Conventional name is Xsession. See the Session Program section 🌈
  • DisplayManager.DISPLAY.reset — a program run (as root) after the session terminates. By default no program is run. Conventional name is Xreset. See the Reset Program section.
  • DisplayManager.DISPLAY.openDelay
  • DisplayManager.DISPLAY.openRepeat
  • DisplayManager.DISPLAY.openTimeout
  • DisplayManager.DISPLAY.startAttempts
  • DisplayManager.DISPLAY.reservAttempts

These numeric resources control how xdm behaves when it's attempting to open intransigent servers, fr fr. 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 whole process repeats startAttempts times, at which point the display is declared dead and disabled 💀 Although this behavior may seem arbitrary, it's 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 that's reached, the display is disabled. Default values: 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 is the time (in minutes) between each ping attempt, and pingTimeout is the maximum amount of time (in minutes) to wait for the terminal to respond to the request. If the terminal doesn't respond, the session is declared dead and terminated. By default both are set to 5 minutes. If you frequently use X terminals that can become isolated from the managing host, you might wanna increase this value. The only worry? Sessions continuing to exist after the terminal has been accidentally disabled 🙈 xdm will not ping local displays. And yeah, even though it'd seem harmless, it's genuinely unpleasant when the workstation session gets terminated because the server is hanging for NFS service and not answering the ping 😭

  • DisplayManager.DISPLAY.terminateServer — this boolean resource says whether the X server should be terminated when a session terminates (instead of resetting it). It's handy when the server tends to grow without bound over time, to limit how long the server runs. 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 the full description. Default value: /bin:/usr/bin:/usr/bin:/usr/ucb'' `` 🌟
  • DisplayManager.DISPLAY.systemPath — xdm sets the PATH environment variable for the startup and reset scripts to the value of this resource. Default is /etc:/bin:/usr/bin:/usr/bin:/usr/ucb'' . Note the absence of .'' from this entry — that's 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 this value. It's /bin/sh by default.
  • DisplayManager.DISPLAY.failsafeClient — if the default session fails to execute, xdm falls back to this program. It's executed with no arguments, but it runs using the same environment variables the session would have had (see the Session Program section). By default /usr/bin/xterm is used 🫶
  • DisplayManager.DISPLAY.grabServer
  • DisplayManager.DISPLAY.grabTimeout

For better security, xdm grabs the server and keyboard while it reads the login name and password. The grabServer resource says whether the server should be held for the whole duration of the name/password reading. When false,'' the server is ungrabbed after the keyboard grab succeeds; otherwise the server stays grabbed until just before the session begins. Default is false.'' The grabTimeout resource is 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 network latencies are very high. This resource defaults to 3 seconds. Be cautious raising it — 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 — and this icon's description is literally picking up in the next chunk, so hold that thought bestie 🎀
  • DisplayManager.DISPLAY.authName — okay so authorize is the boolean resource that 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, and 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:         /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 ✨

Okay so The Internet Assigned Numbers Authority has assigned ff0X:0:0:0:0:0:0:12b as the permanently assigned range of multicast addresses for XDMCP. The X in the prefix may be replaced by any valid scope identifier, such as 1 for Interface-Local, 2 for Link-Local, 5 for Site-Local, and so on. (See IETF RFC 4291 or its replacement for further details and scope definitions, bestie.) 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. Iconic.

✨ LOCAL SERVER SPECIFICATION ✨

The resource DisplayManager.servers gives a server specification or, if the value starts with a slash (/), the name of a file containing server specifications, one per line. Lowkey important.

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. Vibe check: empty means XDMCP-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. Slay.

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. Periodt.

✨ 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 — no cap, bestie, root is main character energy and you do not want a security hole ruining the vibe. This is the place to change the root background or bring up other windows that should appear on the screen along with the Login widget.

In addition to any specified by DisplayManager.exportList, the following environment variables are passed:

  • DISPLAY — the associated display name.
  • PATH — the value of DisplayManager.DISPLAY.systemPath.
  • SHELL — the value of DisplayManager.DISPLAY.systemShell.
  • XAUTHORITY — may be set to an authority file.

Note that since xdm grabs the keyboard, any other windows will not be able to receive keyboard input. They will be able to interact with the mouse, however; beware of potential security holes here. If DisplayManager.DISPLAY.grabServer is set, Xsetup will not be able to connect to the display at all. Resources for this program can be put into the file named by DisplayManager.DISPLAY.resources.

Here is a sample Xsetup script:

#!/bin/sh
# Xsetup_0 - setup script for one workstation
xcmsdb < /etc/X11/xdm/monitors/alex.0
xconsole -geometry 480x130-0-0 -notify -verbose -exitOnFail &

💅 AUTHENTICATION WIDGET 👑

Okay bestie, this is the widget that actually asks you who you are — it prompts for the username, password, and/or whatever other authentication data it needs, straight from the keyboard. And like… nearly every imaginable parameter can be controlled with a resource. No cap. All the resources for this widget go into the file named by DisplayManager.DISPLAY.resources. And good news: all of these have totally reasonable default values, so you literally do not have to specify any of them. Slay.

The resource file gets loaded with xrdb(1), so it can use the substitutions that program defines — like 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. Multitasking icon behavior, honestly.

Now, Xdm can be compiled with support for the Xft(3) library for font rendering. If that 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 it's not present, 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). Two vibes, pick your fighter 💖

  • xlogin.Login.width, xlogin.Login.height, xlogin.Login.x, xlogin.Login.y — the geometry of the Login widget is normally computed automatically. But if you wanna position it somewhere else, bestie, you specify each of these resources. All four. Go off.
  • 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. 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. 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. Default is This is an unsecure session 😬
  • xlogin.Login.greetFace — the face used to display the greeting when built with Xft support. 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. Heads up: Xrdb strips trailing white space from resource values, so to add spaces at the end of the prompt (usually a nice thing, fr), add spaces escaped with backslashes. 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. Default is Password:
  • xlogin.Login.promptFace — the face used to display prompts when built with Xft support. 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. 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. Default is Login incorrect 💀
  • xlogin.Login.failFace — the face used to display the failure message when built with Xft support. 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. Default is 10.
  • xlogin.Login.logoFileName — name of an XPM format pixmap to display in the greeter window, if built with XPM support. 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. 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. 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. Default for both is the foreground color, providing a flat appearance. It's giving depth, it's giving dimension ✨
  • 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. Default is true. Note that 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. 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. Default is false.
  • xlogin.Login.echoPasswdChar — character to display if echoPasswd is true. Default is *. If set to an empty value, the cursor will advance for each character input, but no text will be drawn. Sneaky.
  • 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()
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, Bestie

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, she's aware 😭) 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. The moment of truth ngl 💅
  • 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. Iconic little refresh moment 🔥
  • 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 allow-all-access; it might be better to disconnect the machine from the network before doing this. Like, genuinely, no cap — that's a whole security situation. 🚨

Also, 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. Rules are rules, bestie 🫶

🎀 STARTUP PROGRAM 🎀

Okay so the Xstartup program is run as root when the user logs in, bestie. It's usually a shell script. And because it's running as root, Xstartup has to be very careful about security — like, main-character-with-a-secret levels of careful 💅.

This is the spot for commands that 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 anything specified by DisplayManager.exportList, these environment variables get 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. So yeah, this is not a complete example — it's 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 that's run as the user's session. It runs with the permissions of the authorized user. Slay.

In addition to anything specified by DisplayManager.exportList, these environment variables get 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 — so users can 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, bestie 🫶

#! /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. It runs as root, and 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. No cap.

👑 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. It's giving "stay in your lane," honestly.

🍓 FILES 🍓

  • /etc/X11/xdm/xdm-config — the default configuration file
  • $HOME/.Xauthority — user authorization file where xdm stores keys for clients to read
  • /usr/lib64/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(8), fonts.conf(5).

X Display Manager Control Protocol IETF RFC 4291: IP Version 6 Addressing Architecture.

💜 AUTHOR 💜

Keith Packard, MIT X Consortium.

And that's the whole manual, bestie — we ate and left no crumbs 💅✨ The footer credits X Version 11, xdm 1.1.17, and XDM(8); there's no date printed in this version, so if a date was supposed to show up, it ghosted us 😭🫶

📦 where this page comes from