NAT gateway traversal utility

 nat-traverse establishes connections between nodes which are
 behind NAT gateways, i.e. hosts which do not have public IP
 addresses. Additionally, you can setup a small VPN by using pppd
 on top of nat-traverse. nat-traverse does not need an external
 server on the Internet, and it isn't necessary to reconfigure
 the involved NAT gateways, either. nat-traverse works
 out-of-the-box.