What distro of Linux are you using?
What is you current display manager (xdm, gdm, kdm)? All these display managers have xdmcp capability.
If you boot up in a gui mode you are probably using one of these display managers ... you can figure out which one in several different ways. probably the easiest is:
ps -ef | grep xdm
ps -ef | grep gdm
ps -ef | grep kdmMy ps -ef | grep gdm shows this:
CODE
ps -ef | grep gdm
root 4032 1 0 Jan24 ? 00:00:00 /usr/bin/gdm-binary -nodaemon
root 4085 4032 0 Jan24 ? 00:00:00 /usr/bin/gdm-binary -nodaemon
root 20883 4085 1 Jan28 ? 00:17:19 /usr/X11R6/bin/X :0 -auth /var/gdm/:0.Xauth vt7
root 22640 21758 0 07:53 pts/1 00:00:00 grep gdm
...so I have gdm running