Configuring Your XDM Server
The XDM configuration files are usually in /usr/lib/X11/xdm, or /etc/X11/xdm.
First edit the Xaccess file. Look for the line that looks like
#* #any host can get a login window
and remove the leading "#". This will let anybody to get an login window as long as they can connect to port 177 on your machine. You can make this more secure by listing specific machine names instead of "*", but names can be spoofed. You're much safer blocking port 177 at the IP level with a good firewall.
Next edit the xdm-config file. Check to see if there is a line that reads:
If so, comment it out with an explanation point or change the 0 to 177.
You do not have to modify Xservers.
Next you should restart your XDM daemon. Check the XDM man page for the best way to do this on your system.
GDM is the Gnome Display Manager, which seems to be the XDM clone-of-choice on Red Hat Linux and perhaps other flavors of Linux as well.
First follow the steps for XDM above. Then edit the file /etc/X11/gdm/gdm.conf. Look for the [xdmcp] section and set Enable=1. Also make sure that Port is set to 177.
If you have troubles getting a login screen, you can enable debugging in the [debug] section. Debugging messages will be logged to wherever your syslog is configured to log them to (/var/log/messages for example).
KDM is an XDM clone that comes with KDE.
Follow the instructions for XDM above.
If you want to customize the login window, the configuration file for that is /opt/kde3/share/config/kdm/kdmrc, or /usr/share/config/kdm/kdmrc. You also need to edit that file and change the Enable = false to Enable = true, or it won't accept remote connections.