mod_rpaf changes the remote address of the client visible to other Apache modules when two conditions are satisfied. First condition is that the remote client is actually a proxy that is defined in httpd.conf. Secondly if there is an incoming X-Forwarded-For header and the proxy is in it's list of known proxies it takes the last IP from the incoming X-Forwarded-For header and changes the remote address of the client in the request structure. It also takes the incoming X-Host header and updates the virtualhost settings accordingly. For Apache2 mod_proxy it takes the X-Forwared-Host header and updates the virtualhosts.

To load the module into Apache, add it to APACHE_MODULES in /etc/sysconfig/apache2.