I've had a bit of a strange situation on my network - Googling never did come up with anything for me.
I have a private network. All my servers sit behind the firewall on the private LAN. A few of those servers provide services to the Internet on specific ports.
Sharing those ports to the outside using iptables is trivial. The problem arises when you are trying to access the internal server using the external DNS / IP from inside the network. What would happen is that iptables would redirect your packets to the correct server, but the return address would be your local internal IP, so the packets return to you as if coming from the private address instead of the public address, and your local computer won't know what to do with them.
There's actually a very simple solution, assuming you have static IP's on the Internet and on the internal server. Using a combination of source and destination masquerading, you can rewrite those IP addresses to match. The downside is you double the traffic on your internal network, but it enables you to provide services that are harder to NAT such as SIP traffic.
Read more for an example fix: