odd question, but there is a request for X-Forwarded-For to apply to the forward-proxy mode of mod_proxy_http. http://meta.wikimedia.org/wiki/XFF_project illustrates that Apache isn't even a proxy server [supporting this feature].
Does anyone want to take a walk through why we never enabled this in the forward proxy mode? It doesn't even belong in the reverse proxy mode, since most load balancers use X-Remote-IP or similar to record the literal direct client of their services :) See https://issues.apache.org/bugzilla/show_bug.cgi?id=47601
