rpaf changes the remote address of the client on incoming proxy 
requests. It is used in backend servers that needs to know the IP of the 
client, and works in conjunction with mod_proxy_add_forward (which sets 
the X-Forwarded-For header). When an X-Host header exists rpaf will take 
the hostname found here, put it into Apache, and update the virtualhost 
settings so that you can serve virtualhosts on the backend.

WWW: http://www.stderr.net/apache/rpaf/