For XDMCP connection to Red Hat 5 EL
1. XDM Configuration
1. Change runlevel to 5
Open/etc/inittab and set the initial runlevel to 5 as following:
id:5:initdefault:

2. Enable XDMCP

For GDM:

Open /etc/gdm/custom.conf and set the Enable entry to 1 in the [xdmcp] section as following.
[xdmcp]
Enable=1

For KDM:

Open /usr/share/config/kdm/Xaccess and remove the initial "#" character for the following line:
#* #any host can get a login window ==>
* #any host can get a login window
Open /usr/share/config/kdm/kdmrc and set the Enable entry to true in the [xdmcp] section as following.
[xdmcp]
Enable=true

2. Firewall (TCP/UDP Ports) Configuration
Open UDP port 177 from the PC to the remote host direction.
Open incoming TCP ports 6000~6010 from the remote host to your PC.
3. Reboot the remote host and start Xmanager
# init 3; init 5