X-Forwarded-For (XFF)

When connecting to a web service via a load balancer acting as a proxy server, or via a simple proxy server, the IP address of the originating client is often hidden from the web service. In many situations it is beneficial or required that the web server knows the client IP address so it can communicate back directly, or record the access request in a log file. The HTTP protocol defines header fields that can be used for passing information between clients and servers. One of these header fields is the X-Forwarded-For (XFF) request and is populated by the load balancer or proxy server with the client IP address of the originating client. This allows the web server to use this IP address if required.