NATBLASTER: Establishing TCP Connections Between Hosts Behind NATs ∗

Firewalls and Network Address Translation (NAT) devices are becoming increasingly prevalent, and they pose a significant p roblem for connection establishment for peer-to-peer protocols. When properly configured, these middle-boxes 1 inhibit TCP connections solicited from outside the local network. This paper proposes novel mechanisms to create direct TCP connections between two hosts behind middle-boxes with minimal help from a third-party. We implement two of these solutions on common hardware within a common environment. We are able to create direct TCP connections between two hosts which are both located behind typical NATs designed for small networks. Once this connection is established, the applications can communicate with each other using a standard TCP implementation with no further external help.