One of the approaches (the easiest from my point of view) is filtering Server(WAN) interface substituting the original IP (if it is from LAN) in the outgoing packet with Server WAN card IP (port substitution also should be done in order to perform backward mapping). For incoming packets on the Server (WAN) interface you should match against NAT table and perform the reversed operation. Please note that packet forwarding should be enabled on the Server. You don’t need to route packets youself, TCP/IP will do it for you.
