Blocking Mozilla / Google Prefetch

April 6, 2005 by Pete Freitag

Mozilla browsers support a feature called link prefetching, which allows a web page to tell the browser to prefetch a url if it is idle. Google has been using this technique in their search results, telling Mozilla to start loading the first result. I also noticed that MXNA 2.0 is including 3 prefetch tags.

How to you tell the browser to prefetch a url?

By using the following code:

<link rel="prefetch" href="http://url.to.prefetch/" />

How can I detect prefetching on my web site?

When mozilla does a prefetch, it sends a header X-moz: prefetch, you can then block based on that header. Google recommends sending a 404 back to block the prefetch.

How can I block prefetching?

Using mod_rewrite to send a 404:

RewriteEngine On
SetEnvIf X-moz prefetch HAS_X-moz 
RewriteCond %{ENV:HAS_X-moz} prefetch 
RewriteRule .* /prefetch-attempt [L]

This will redirect all prefetch-attempts to /prefetch-attempt as long as that file doesn't exist, the client will get a 404.

You could also block with a 503 Forbidden response:

RewriteEngine On
SetEnvIf X-moz prefetch HAS_X-moz
RewriteCond %{ENV:HAS_X-moz} prefetch 
RewriteRule .* [F,L]

In IIS using isapi rewrite:

RewriteCond X-moz: prefetch
RewriteRule .* /prefetch-attemtp [L]

Or with ColdFusion:

<cfset httpHeaders = GetHttpRequestData().headers>
<cfif StructKeyExists(httpHeaders, "X-moz") AND httpHeaders["X-moz"] IS "prefetch">
	<cfheader statuscode="404" statustext="Not Found">
	<cfabort>
</cfif>

How can I disable prefetching on mozilla browsers?

Add this line to your prefs.js file located in your Mozilla profile directory to disable prefetching in firefox and mozilla browsers.

user_pref("network.prefetch-next",false);

Some interesting things about prefetch:

• Query Strings are ignored - if you have a page with a query string it will not be prefetched.
• HTTP only - https pages are not prefetched by firefox. ftp and other protocols are also ignored
• Cross domain cookie problems - With prefetching enabled, you may end up with cookies and web pages in your web browser's cache from web sites that you did not click on since prefetching happens automatically when you view pages.
• A good way to preload images - prefetching is a good way to preload images, instead of using javascript to do this because your browser can do it while it is idle.