5.6 The X Display Manager

5.6.1 Overview

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 17.3.2 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.

5.6.2 Using XDM

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 17.3.2.1. 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 17.3.2.2. 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.

5.6.3 Configuring XDM

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:

Also in this directory are a few scripts and programs used to setup 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.

5.6.4 Running a Network Display Server

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.

5.6.5 Replacements for XDM

Several replacements for the default XDM program exist. One of them, KDM (bundled with KDE) is described later in this chapter. KDM 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.