The X Display Manager (XDM) is an optional part of the X Window System that is used for login session management. This is useful for several types of situations, including minimal “X Terminals”, desktops, and large network display servers. Since the X Window System is network and protocol independent, there are a wide variety of possible configurations for running X clients and servers on different machines connected by a network. XDM provides a graphical interface for choosing which display server to connect to, and entering authorization information such as a login and password combination.
Think of XDM as providing the same functionality to the user as the getty(8) utility (see Section 22.3.2 for details). That is, it performs system logins to the display being connected to and then runs a session manager on behalf of the user (usually an X window manager). XDM then waits for this program to exit, signaling that the user is done and should be logged out of the display. At this point, XDM can display the login and display chooser screens for the next user to login.
The XDM daemon program is located in /usr/X11R6/bin/xdm. This program can be run at any time as root and it will start managing the X display on the local machine. If XDM is to be run every time the machine boots up, a convenient way to do this is by adding an entry to /etc/ttys. For more information about the format and usage of this file, see Section 22.3.2.1. There is a line in the default /etc/ttys file for running the XDM daemon on a virtual terminal:
ttyv8 "/usr/X11R6/bin/xdm -nodaemon" xterm off secure
By default this entry is disabled; in order to enable it change field 5 from off to on and restart init(8) using the directions in Section 22.3.2.2. The first field, the name of the terminal this program will manage, is ttyv8. This means that XDM will start running on the 9th virtual terminal.
The XDM configuration directory is located in /usr/X11R6/lib/X11/xdm. In this directory there are several files used to change the behavior and appearance of XDM. Typically these files will be found:
| File | Description |
|---|---|
| Xaccess | Client authorization ruleset. |
| Xresources | Default X resource values. |
| Xservers | List of remote and local displays to manage. |
| Xsession | Default session script for logins. |
| Xsetup_* | Script to launch applications before the login interface. |
| xdm-config | Global configuration for all displays running on this machine. |
| xdm-errors | Errors generated by the server program. |
| xdm-pid | The process ID of the currently running XDM. |
Also in this directory are a few scripts and programs used to set up the desktop when XDM is running. The purpose of each of these files will be briefly described. The exact syntax and usage of all of these files is described in xdm(1).
The default configuration is a simple rectangular login window with the hostname of the machine displayed at the top in a large font and “Login:” and “Password:” prompts below. This is a good starting point for changing the look and feel of XDM screens.
The protocol for connecting to XDM controlled displays is called the X Display Manager Connection Protocol (XDMCP). This file is a ruleset for controlling XDMCP connections from remote machines. It is ignored unless the xdm-config is changed to listen for remote connections. By default, it does not allow any clients to connect.
This is an application-defaults file for the display chooser and the login screens. This is where the appearance of the login program can be modified. The format is identical to the app-defaults file described in the X11 documentation.
This is a list of the remote displays the chooser should provide as choices.
This is the default session script for XDM to run after a user has logged in. Normally each user will have a customized session script in ~/.xsession that overrides this script.
These will be run automatically before displaying the chooser or login interfaces. There is a script for each display being used, named Xsetup_ followed by the local display number (for instance Xsetup_0). Typically these scripts will run one or two programs in the background such as xconsole.
This contains settings in the form of app-defaults that are applicable to every display that this installation manages.
This contains the output of the X servers that XDM is trying to run. If a display that XDM is trying to start hangs for some reason, this is a good place to look for error messages. These messages are also written to the user's ~/.xsession-errors file on a per-session basis.
In order for other clients to connect to the display server, edit the access control rules, and enable the connection listener. By default these are set to conservative values. To make XDM listen for connections, first comment out a line in the xdm-config file:
! SECURITY: do not listen for XDMCP or Chooser requests ! Comment out this line if you want to manage X terminals with xdm DisplayManager.requestPort: 0
and then restart XDM. Remember that comments in app-defaults files begin with a “!” character, not the usual “#”. More strict access controls may be desired. Look at the example entries in Xaccess, and refer to the xdm(1) manual page.
Several replacements for the default XDM program exist. One of them, kdm (bundled with KDE) is described later in this chapter. The kdm display manager offers many visual improvements and cosmetic frills, as well as the functionality to allow users to choose their window manager of choice at login time.
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