I have 2 Linux boxes one acting as a router with a direct connection to the internet, second as a server using the first box as a gateway to the internet.

I need to forward requests that I get from the outside to port 8400 to my internal server box at 192.168.0.7:8400

Router IP 192.168.0.5
Server IP 192.168.0.7

iptables -t nat -A PREROUTING -p tcp -d [internet ip] --dport 8400 -j DNAT --to-destination 192.168.0.7:8400 < Won't work
iptables -t nat -A PREROUTING -p tcp -d [internet ip] --dport 8400 -j DNAT --to-destination 192.168.0.5:8400 < Will work

These rules are on the router (192.168.0.5)

I've been trying to find a solution for hours with no success.

Basically the problem is I can forward ports on the same box but not to a different ip..

Any help would be greatly appreciated.

Well after HOURS of trying I finally figured it out,

iptables -t nat -A POSTROUTING -d 192.168.0.7 -j MASQUERADE
iptables -t nat -A PREROUTING -p tcp -d [internet ip] --dport 8400 -j DNAT --to-destination 192.168.0.7:8400

Adding iptables -t nat -A POSTROUTING -d 192.168.0.7 -j MASQUERADE
did the trick, I'll leave this here in case someone else has the same problem.