mod_extract_forwarded hooks itself into Apache's header parsing phase and looks
for the X-Forwarded-For header which some (most?) proxies add to the proxied
HTTP requests. It extracts the IP from the X-Forwarded-For and modifies the
connection data so to the rest of Apache the request looks like it came from
that IP rather than the proxy IP.

mod_extract_forwarded can be dangerous for host based access control because
X-Forwarded-For is easily spoofed. Because of this you can configure which
proxies you trust or don't trust.

WWW: http://www.openinfo.co.uk/apache/