IPFuck is a firefox addon created to simulate the use of a proxy. With this addon installed and enabled, and if a lot of us use it, there will no longer be any mean to know who is using a real IP, who isn't and who was charged doing something he didn't... Basicaly : we all become anonymous !
This addon is a "proof of           concept" to show anyone who isn't already aware that IP address has become obsolete and that no one should use an IP address             as an evidence anymore. This plugin is just one of           many ways to spoof an IP address and these spoofing could lead           to outrageous accusation of innocents.
How does it work ?
How does it work ?
You can                 imagine that if I could just overwrite any existing                 information about your IP address I would have done so                 (or somebody else would have a while back ago)...
But it's                 actually a little more tricky : when sending a request                 to a server you will provide several information about                 your IP address : three of them come from the                 Application Layer and the last one comes from the                 Transport Layer. This last one I can't modify : you                 wouldn't get the answer to your request if that was                 done. But the three others can be overwritten without                 any consequence to your browsing...
These                 three headers were created to provide information on the                 real IP of a person surfing through a proxy server. So                 when you enable IPFuck, the websites you are visiting                 will believe that your real IP is a proxy server and (if                 the website was done correctly) focus on the false IP                 you are sending...
A lot of                 websites try and figure out who is hidding behind a                 proxy server. And if you don't believe me (I won't                 mind), just check out this google search request : get                   real ip address php. Most of the snipplets given                 here will check HTTP headers (the one we overwrite)                 before the Transport Layer information ('REMOTE_ADDR').
Source :                 http://ipfuck.paulds.fr/