NTALG - TCP NAT traversal with application-level gateways

Consumer computers or home communication devices are usually connected to the Internet via a Network Address Translation (NAT) router. This imposes restrictions for networking applications that require inbound connections. Existing solutions for NAT traversal can remedy the restrictions, but still there is a fraction of home users which lack support of it, especially when it comes to TCP. We present a framework for traversing NAT routers by exploiting their built-in FTP and IRC application-level gateways (ALG) for arbitrary TCP-based applications. While this does not work in every scenario, it significantly improves the success chance without requiring any user interaction at all. To demonstrate the framework, we show a small test setup with laptop computers and home NAT routers.