Straight from my website, check the link below, you'll find a diamond in the rough every now and again ... ok, actually, I write this junk up on my website because I have a horrible memory and I need reference to it:
It's always a pain in the ass for me to remember how to do this, I always forget a step and add an extra step that matters not.
So, here's the example of port forwarding using iptables.
I have an internal server on port 10.0.0.2, running apache on port 80. I want to be able to reach it via port 81 from my gateway computer.
/sbin/iptables -t nat -I PREROUTING -p tcp -d 18.104.22.168 --dport 81 -j DNAT --to 10.0.0.2:80
That's the part I always remember.
/sbin/iptables -I FORWARD -p tcp -d 10.0.0.2 --dport 80 -j ACCEPT
That's the part I always forget.
Anyway, port forwarding in two easy steps.
Hopefully you can extrapolate what you need from the above by changing the port numbers, etc.