1. A way of establishing an outbound connection through a firewall in such a way that it is neither blocked or monitored. This isn't a way of breaking through a firewall, but assuming you've compromised a machine on the other side of a firewall (through some other technique), this will allow you to communicate with that machine from the Internet. It is also used by people behind firewalls that use restrictive rulesets: users simply create a tunnel back to their home machine. Example: People have written tunnels over ICMP, DNS, HTTP, e-mail messages, and TCP connections. Tunnels can either by of the "port redirector" style (which run on top of any TCP/IP stack) or of the network interface variety (below the TCP/IP stack requiring kernel mod). From Hacking-Lexicon