IP Forwarding Through Tunnels?

fialar asks: "I have a machine (machine A) that provides an encrypted, private channel (VPN via vtun). It connects to Machine B and gives Machine B an IP address on Machine A's subnet (which is unused). I've added the proper arp command line (arp -i eth0 -sD eth0 pub), however Machine B cannot ping outside of Machine A's subnet. After a certain period of time, it can, but I am not sure why. Is there a way to set it up so that when the connection between Machine A and B happens, the proper rules are set up so that Machine B will be able to get out? IP Masquerading is not an option. I need Machine B to show up on Machine A's subnet."