Iptables

iptables -n -L -v -t nat -

iptables -n -L -v -t filter -

Port mapping

echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A PREROUTING -i ethX -p tcp --dport 21 -j DNAT --to-destination WIN_IP:3389
iptables -t nat -A POSTROUTING -o ethX -j MASQUERADE