In 2010 Samy Kamkar discovered a method that he called "NAT Pinning." The idea was, an attacker lures a victim to a web page and that web page forces the victim's router or firewall to forward any port number back to the user's machine. The router, firewall, NAT-device must support connection tracking. Samy Kamkar has successfully tested this on a Belkin N1 Vision Wireless Router.
Now, IRC NAT Pinning is integrated in BeEF as a module. It requires the victim to use Firefox due to blocked port number 6667 in most other browsers. In this example, iptables is used to demonstrate how it actually works.