This is an excellent article that describes in detail how P2P applications can bypass the firewall and accept incoming data without opening any ports in the firewall configuration. This trick is used by the majority of VoIP software. In this article, the author not only describes how the popular internet telephony software Skype manages to establish a connection between two peers which are behind a firewall, but also provides an example about how to perform this trick with tools like netcat and the packet generator hping2. A must read!
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The The hole trick by George Notaras, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.