You need to use mangle rules in firewall to mark the incoming packets
from the gateway interface and then using route marking route them back out the way they came.
I'll happily show you how I did mine on request. That is another approach, but you will have to handle outgoing connections as well.
Hugh: I would love an example! Rob: Define 'handle'
In any case, I missed the complete DNS A and PTR records, lemme get that done first.