First make sure you have done the following in the setting:
<code>
net.ipv4.ip_forward=1
</code>


Assume we want to listen to port 12345, and forward it to internal ip 192.168.1.100 with port 12345:

<code>
iptables -t nat -A PREROUTING -p tcp --dport 12345 -j DNAT --to-destination 192.168.1.100:12345
iptables -t nat -A POSTROUTING -j MASQUERADE
</code>

For udp protocol, change the first line with udp instead of tcp.

You might want to save the iptables by:
<code>
iptables-save
</code>