Port forwarding with OpenVPN

Solution 1:

I assume you're using OpenVPN in routed mode and are NATting its clients onto the WAN. If so, you'll want to do the following:

iptables -t nat -A PREROUTING -i eth0 -d 123.45.67.89 -p tcp --dport 4444 -j DNAT --to-destination 192.168.1.10 (change eth0 to whatever interface your WAN is actually on).

If you're not using NAT, then it's just a matter of making sure the host you're trying to connect from knows to route packets destined for 192.168.1.10 through 123.45.67.89 (which obviously won't work to an RFC1918 IP over the Internet). In any case, the only other requirements are that you have IPv4 routing enabled (I'd assume that the VPN wouldn't be working at all if you didn't) and that you don't have any other firewall rules that would block this traffic.