Add a request header on mod_proxy based on the value of another request header

Question

I have a server running mod_proxy and I need to essentially rename a request header. A proxy closer to the origin is stripping off the X-Forwarded-For header, but we need that data to persist, so I want to add the value to a different header to allow the origin to pick it up.

From my reading of the doco there is no way to rename a header and I can't see how to pass in values into RequestHeader add in mod_headers.

Answer 1

RequestHeader can insert values of environment variables into header values, and SetEnvIf can set environment variables based on request header values, so the following configuration should achieve what you need:

SetEnvIf X-Forwarded-For (.*) saved_x_forwarded_for=$1
RequestHeader set X-Custom-Forwarded-For "%{saved_x_forwarded_for}e"

You should use set instead of add, so that even if a client sends a request containing your custom header, the client-specified value will be overwritten (although even this is not 100% reliable, because the X-Forwarded-For header could be spoofed too). Also test what happens if a request without X-Forwarded-For is received.