看你用到是什么linux版本，例如下面：

For kdm (which comes with the KDE desktop), it is a replacement of xdm and configures the same way, except its files are in /etc/X11/kdm in Caldera/SCO, /etc/kde/kdm in Red Hat (and Fedora Core) and /usr/share/config/kdm, which is a symbolic link to /etc/kde/kdm, in Mandrake.

The gdm (Gnome Display Manager) is a re-implementation of the well known xdm. gdm has similar functions to xdm and kdm, gdm is the Gnome Display Manager, and its configuration files are found in /etc/X11/gdm/gdm.conf. The gdm.conf file contains sets of variables and many options for gdm, and the Sessions directory contains a script for each session option; each script calls /etc/X11/xdm/Xsession with the appropriate option. gdm has similar functions to xdm and kdm, but was written from scratch and does not contain any original XDM / X Consortium code.

If you plan to use the GDM as default, one benefit of gdm login window is that it allows you to switch between KDE and GNOME. For gdm, edit /etc/X11/gdm/gdm.conf. This activates XDMCP, causing it to listen to the request. For kdm (if you pick KDE as your DM in your installation), edit /usr/share/config/kdm/kdmrc for Mandrake and /etc/kde/kdm/kdmrc for Red Hat or /opt/kde2/share/config/kdm/kdmrc for Slackware version (KDE2). Change this line:

[xdmcp]

Enable=false (may shown as 0 in some distributions)

to:

Enable=true (or 1 in some distributions)

Make sure "Port=177" is at the end of this block, i.e., by commenting out the line "#Port=177".