I'm not quite sure what the question is exactly, but the X-Forwarded-For field can be use for configurations where you have cascading proxies or a content filter before the proxy and you want the upstream proxy to do IP ACL filtering; if the X-Forwarded-For feature isn't configured (or availible) on the first proxy/content filter, the second proxy will only have the first's proxy/filter's IP to filter on, which isn't much use! If the first proxy/filter does add a valid X-Forwarded-For field to the http request, the second proxy will (squid can, but with a patch) be able to filter on that...

That's one use for the X-Forwarded-For field...

Ammo