I am having problems with my ftp, and ircing behind my firewall, ive been goggling and trying to find out why some time these services work, and then out of the blue dont until I flush ip tables. I came to the conclusion that i need to use conntrack, but I really need help with this, I couldnt get it to work
my table looks something like this now

target prot opt source destination
ACCEPT udp -- anywhere anywhere udp dpt:fsp
ACCEPT tcp -- anywhere anywhere tcp dpt:ftp
ACCEPT udp -- anywhere anywhere udp dpt:ssh
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
ACCEPT udp -- anywhere anywhere udp dpt:auth
ACCEPT tcp -- anywhere anywhere tcp dpt:ident
ACCEPT udp -- anywhere anywhere udp dpts:33000:44000
ACCEPT tcp -- anywhere anywhere tcp dpts:33000:44000
ACCEPT udp -- anywhere anywhere udp dpts:6660:6669
ACCEPT tcp -- anywhere anywhere tcp dpts:6660:6669
DROP tcp -- anywhere anywhere
DROP udp -- anywhere anywhere

and i tried replacing the ftp and irc definitions to something like this and didnt go over to well.
iptables -A INPUT --ctproto tcp -s 0/0 -d 0/0 --destination-port 20 --syn -j ACCEPT

has anyone else had any experience with this that could help?

i think ur question is about internet connection sharing.
google about ip masquerade.

no i am not sharing a connection, it's straight through, no nat