Hey - I didnt ask for that!

Hey - I didnt ask for that! 06 Feb 2008 16:14

So I noticed something new today while doing some packet sniffing; When I perform a search in Google, my browser makes another request.. to the first search result that is returned.

What the...?

Well a little bit of digging shows that the browser I am using (Mozilla Firefox) is acting on a block of HTML embedded in the search results:

<link rel="prefetch" href="...">

The use of this is described in the Mozilla documentation:

"Link prefetching is a browser mechanism, which utilizes browser idle time to download or prefetch documents that the user might visit in the near future. A web page provides a set of prefetching hints to the browser, and after the browser is finished loading the page, it begins silently prefetching specified documents and stores them in its cache. When the user visits one of the prefetched documents, it can be served up quickly out of the browser's cache."

So that sounds somewhat useful, and in Googles case, they mark the first search result as being prefetchable since presumably that will be the one clicked on most often.

In fact this is a technique you can apply to your sites as well by using the <link rel="prefetch" href="..."/> tag.

Some things to note here:

The browser will only prefetch when it is idle
The browser will only prefetch http:// links with no querystring arguments
The browser will only prefetch the HTML component
Firefox will add a header X-moz: prefetch to distinguish a prefetch from a real fetch

Now if you want to disable this behavior in Firefox, browse to about:config, and filter on prefetch. For the option network.prefetch-next, set it to false. However its probably only the case where you are on a very low bandwidth connection or you are paying for your (mobile) data.

Lastly, one other thing I wonder about is how many sites believe they are getting referrals, but are really only getting prefetches? :) Curious..

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