Picking an Xdmcp Server

If your computer already boots into GUI mode, it's already running an XDMCP server. To find out which one it's using, type the following command:

The result should be a list of any of the standard XDMCP server processes that are running—gdm-binary, kdm, or xdm. (This command may also turn up a few hits on unrelated programs, but these should be obvious.) In most cases, the easiest course of action is to modify the configuration of your default XDMCP server. If necessary, though, you can change which server your system uses. First, of course, you must ensure that it's installed. Search for the files involved and, if you can't find them, install the relevant package for your distribution. How you proceed thereafter depends on your distribution:

Debian Edit the /etc/X11/default-display-manager file, which contains the full path to the XDMCP server you want to run. For instance, enter /usr/bin/X11/xdm in this file to use XDM. This path must match one in the DAEMON variable in the SysV startup script for the server in question.

Mandrake Edit the /etc/sysconfig/desktop file, which holds variable assignments. Locate the DISPLAYMANAGER variable and set it to the name of the XDMCP server you want to use, such as gdm to run GDM. Mandrake supports two versions of KDM. Setting DISPLAYMANAGER to KDE uses the mdkkdm XDMCP server, while setting DISPLAYMANAGER to KDM uses the kdm XDMCP server. The mdkkdm server provides a stripped-down appearance compared to the regular kdm server.

Red Hat Edit the /etc/sysconfig/desktop file, which holds variable assignments. Locate the DISPLAYMANAGER variable and set it to the name of the environment that's associated with the XDMCP server you want to use, such as "GNOME" to use GDM. Use "XDM" to launch XDM.

Slackware The /etc/rc.d/rc.4 file controls starting X, including launching an XDMCP server. This file tests for the presence of the three major servers and launches the first one in the list it finds, using the sequence GDM, KDM, and then XDM. You must remove higher-ranking servers, remove their execute bits, or edit the startup script file to change which one launches.

SuSE The /etc/sysconfig/displaymanager file sets several variables related to XDMCP operation, including the DISPLAYMANAGER variable, which specifies which server to use. Set this variable to the appropriate name, such as "kdm" to use KDM.

In practice, XDM is the simplest of the display managers; it only provides a login prompt. KDM and GDM both support additional user options, such as a choice of which desktop environment to run. They may also provide shutdown options and the like, although these are usually disabled for all but local users. KDM configuration is basically a superset of XDM configuration, but GDM uses its own unique configuration files. GDM lacks a few of XDM's and KDM's IP-based auditing features, so if you use

GDM, you may want to be particularly diligent about setting up packet filter firewall rules (as described in Chapter 20, "Controlling Network Access") to prevent outside computers from reaching the XDMCP port (UDP port 177).

In some cases, particular servers may be very finicky on certain systems or distributions. If you have problems getting one server to work, try another. It may prove more amenable to modification to accept remote logins than is the default server.

In all cases, you must restart the server after you change your XDMCP configuration. In theory, passing a SIGHUP signal to the process should do the job, but in practice this sometimes doesn't work. You may need to log out, switch to a text mode runlevel (typing telinit 3 should do this on all distributions except for Debian), and then return to the GUI login runlevel (5 for most distributions, but 4 for Slackware).

Warning Don't pass a SIGHUP signal to the XDMCP server unless you're prepared to have the X server terminate, shutting down any programs you're running locally.

In all cases, XDMCP normally runs only when the system is configured to start X and present a GUI login screen on the console. If you want the system to accept XDMCP logins from remote users but not run X locally, you can do so; however, you must alter the XDMCP configuration, as described in the following sections.

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