Ok, maybe you have no problems while dealing with web caches but I (and my workmates as well :P ) do, so here it goes this post.

Let imagine you are behind a classical transparent proxy on port 80 and you’re visiting a web site running an internal web cache (so, another proxy). If you inspect your HTTP headers looking for some info, you can find two lines that look like this, given domain.tld as the local website and proxy.local as your internal transparent proxy.

What does this mean? That this is the first time you visit that website (MISS from proxy.local) and that their proxy has a valid copy of the page in its cache (X-Cache HIT proxy.domain.tld). I’ll explain X-Cache-Lookup meaning later

Now, we’ve just refreshed the page (F5, Ctrl+R, you name it) but wait… what’s happening? It seems both proxies are not serving any page, and we’ve got two mysterious HITs in Cache-Lookup. Well, it’s very simple. We are not counting another level of cache. The browser web cache. So, the page now is not pulled at all from the net, instead Firefox (or your web browser of choice) is using it’s own cache to show the page, so we’ve got two MISSes in X-Cache but nonetheless both proxies are telling us that they would send the cache copy if asked. So, if you’re debugging your proxy system, it means it’s working correctly.

Now, what if we empty Firefox’s cache ??

Here it is:

Our transparent proxy has got the page we need so it sends it to us (HIT from proxy.local), the remote proxy doesn’t need to do anything and both could send the page in case we want.

Although it could seem complicated, once you get it it’s very very simple, and you can easily nest more and more cache levels.