XFF: X-Forwarded-For

The X-Forwarded-For (XFF) HTTP header field is a de facto standard for identifying the originating IP address of a client connecting to a web server through an HTTP proxy or load balancer.

PS: 
   de facto standard簡單來說就是不成文的規定，已被大眾所接受的事實。其中de facto源自於拉丁語。

This is an HTTP request header which was introduced by the Squid caching proxy server's developers. An effort has been started at IETF for standardizing the Forwarded HTTP header.^[1]


Reference
http://en.wikipedia.org/wiki/X-Forwarded-For