Xdmcp
 

I have 3 NIC's in my machine.

If I do a nmap -sU (UDP Scan) on all my IP addresses it shows port 177 (XDMCP) open on all 3 IP addresses.
What I'd like to do is for port 177 to be open on just one of my ip addresses and it closed on the other 2 ip addresses is this possible and how is this acheived?

using tightvnc and gnome (gdm)

thanks in advance

One option is to create some rules on your server like:
# iptables -A INPUT -j DROP -s <ip that can't connect> -d <your ip> -p udp --dport 117