-A PREROUTING -p tcp -m tcp -s <A> -j DNAT --to-destination <B>
-A PREROUTING -p tcp -m tcp -s <B> -j DNAT --to-destination <A>
-A POSTROUTING -p tcp -m tcp -s <A> -j SNAT --to-source <C>
-A POSTROUTING -p tcp -m tcp -s <B> -j SNAT --to-source <C>