The term firewall has been completely diluted. If the same thing had happened to anti-virus software we'd be calling file extension based blockers anti-virus ag

The term firewall has been completely diluted. If the same thing had happened to anti-virus software we'd be calling file extension based blockers anti-virus agents. Now, Skype's technique is quite interesting and clearly well executed. But it underlines a fundamental problem with trying to glean protocol information from port numbers and SYN packets. Similarly you can "reverse" SSH connections to bypass incoming connection blockers on firewalls. Besides, philosophically speaking, trying to allow one form of communication but not another is a losing battle. Any communication channel can be used for any purpose. People have been hiding small messages in bigger ones through steganography for a long time. As long as I have friendly server on the outside to reroute my traffic, there's very little you can actually do (Tor anyone?). Don't even get me started on NATs. Those things make IPv6 look like god's gift to network engineers.