Eu utilizo as seguintes regras para o emule.
# eMule
$IPTABLES -A PREROUTING -t nat -p tcp --dport 4661 -j DNAT --to 192.168.0.11
$IPTABLES -A PREROUTING -t nat -p tcp --dport 4662 -j DNAT --to 192.168.0.11
$IPTABLES -A PREROUTING -t nat -p udp --dport 4665 -j DNAT --to 192.168.0.11
$IPTABLES -A FORWARD -i $ETH0IP -p udp -m udp --dport 4665 -j ACCEPT
$IPTABLES -A FORWARD -i $ETH0IP -p tcp -m tcp --dport 4662 -j ACCEPT
$IPTABLES -A FORWARD -i $ETH0IP -p tcp -m tcp --dport 4661 -j ACCEPT
$IPTABLES -A FORWARD -s 192.168.0.11 -i eth1 -j ACCEPT