X-Forwarded-For - Format

Format

The general format of the field is:

X-Forwarded-For: client, proxy1, proxy2

where the value is a comma+space separated list of IP addresses, the left-most being the original client, and each successive proxy that passed the request adding the IP address where it received the request from. In this example, the request passed through proxy1, proxy2, and then proxy3 (not shown in the header). proxy3 appears as remote address of the request.

Since it is easy to forge an X-Forwarded-For field the given information should be used with care. The last IP address is always the IP address that connects to the last proxy, which means it is the most reliable source of information. X-Forwarded-For data can be used in a forward or reverse proxy scenario.

In a forward proxy scenario you can track the real client IP address on your network through an internal proxy chain and log that IP address on a gateway device. For security reasons, your gateway device should strip any X-Forwarded-For before sending the request to the Internet. You should be able to trust X-Forwarded-For information in this scenario as it is all generated within your network.

In a reverse proxy scenario you can track the real IP address of a client on the Internet accessing your web server, even if your web server is not routable from the Internet – i.e. it is behind a layer 7 proxy device. You should NOT trust all X-Forwarded-For information in this scenario as you may have received bogus information from the Internet. As such a trust list should be used to make sure that proxy IP addresses in the X-Forwarded-For field are trusted by you.

Just logging the X-Forwarded-For field is not always enough as the last proxy IP address in a chain is not contained within the X-Forwarded-For field, it is in the actual IP header. A web server should log BOTH the request's source IP address and the X-Forwarded-For field information for completeness.