iptables -t nat -A PREROUTING -i INT_IF -p tcp --dport SERVER_LISTEN_PORT -j DNAT --to SERVER_IP_IN_LOCAL_NETWORK:SERVER_LISTEN_PORT
iptables -t nat -A POSTROUTING -o EXT_IF -s SERVER_IP_IN_LOCAL_NETWORK -p tcp --sport SERVER_LISTEN_PORT -j SNAT --to-source EXT_ROUTER_IP_ADDRESS

iptables -A FORWARD -i EXT_IF -p tcp --dport SERVER_LISTEN_PORT -j ACCEPT
iptables -A FORWARD -i INT_IF -p tcp --sport SERVER_LISTEN_PORT -j ACCEPT