Internet Sharing configuration

How do I configure how Mac OS X shares it's internet? Currently it seems to setup a NAT between the internet and whatever it is sharing it's connection with. Is there a way to have it just forward everything and have it act as a network hub or something? If not, is there a way to get it to forward certain ports to the computer that it is sharing it's connection with? Where are the config files and things for all of the different sharing options? My brother's internet connection sharing seems to be broken, it makes the computer act as a gateway but it doesn't act as a DHCP server, though it does assign itself an IP address. How can I get detailed information about this?

Specifically I'm looking to forward certain ports so the xbox360 that the mac is sharing it's internet with stops saying that it's connected to a moderate NAT router.

 

Found it. "man InternetSharing"
search term was "internet sharing plist"