I have a linux server which is configured as a router connecting my internal network and the internet. using the following iptables rules:

iptables -A FORWARD -i eth0 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
iptables -A FORWARD -j LOG
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

where eth0 is the netword adapter linking to the internet and the eth1 is the adapter linking to the internal network.

It seems the network works quite well, except for the voice connection of MSN messenger 6.2. I can't either initial a voice chat or accept a voice chat.

I've got absolutely no idea how to make it work. Any ideas?