HTTP Headers: What is the difference between X-FORWARDED-FOR, X_FORWARDED_FOR and HTTP_X_FORWARDED_FOR?

Question

I see various spellings of the non-RFC \"XFF\" HTTP header used for identifying the IP addresses the request has been forwarded through. Is there a difference between these

Answer 1

The HTTP_ prefix is used by some languages like PHP simply to distinguish HTTP headers from other server variables:

$_SERVER['HTTP_X_FORWARDED_FOR']

The HTTP header name is actually

X-Forwarded-For

The header name itself is case insensitive. However, when you want to query a request header, programming languages are largely case sensitive about it (again, PHP is one of them).

The X- indicates that the Forwarded-For header is non-standard. I don't think there's a difference whether a language uses dashes or underscores to refer to header names.

Essentially, they're all the same header, just referred to differently by various implementations.