Today's word in the Networking ABC's is the letter X.  There really aren't that many words that start with X so my choices for today was limited.  But, luckily for you all, there is one word that stands out as a necessity for application servers hosted behind HTTP proxies.  The "X-Forwarded-For" HTTP header is used to allow a proxy server to inject the true originating IP address of a client connection into the HTTP request allowing the application server to know the callers true identity.

"X" is for X-Forwarded-For

X-Forwarded-For

Pronounced: Eks-Fôr'wərd-ed-Fawr

The X-Forwarded-For (XFF) HTTP header is a de facto standard for identifying the originating IP address of a client connecting to a web server through an HTTP proxy.  In this context, the caching servers are most often those of large ISPs who either encourage or force their users to use proxy servers for access to the Internet.  Without the use of XFF, any connection through the proxy would reveal only the originating IP address of the proxy server, effectively turning the proxy server into an anonymizing service, thus making detection and prevention of abusive accesses significantly harder than if the originating IP address was available.