Pre-Resolve DNS
Objective
Reduce DNS lookup time by pre-resolving at the browser.
Configuration
The 'Pre-Resolve DNS' filter is enabled by specifying:
- Apache:
ModPagespeedEnableFilters insert_dns_prefetch
- Nginx:
pagespeed EnableFilters insert_dns_prefetch;
in the configuration file.
Description
DNS resolution time varies from <1ms for locally cached results, to hundreds of milliseconds due to the cascading nature of DNS. This can contribute significantly towards total page load time. This filter reduces DNS lookup time by providing hints to the browser at the beginning of the HTML, which allows the browser to pre-resolve DNS for resources on the page.
Operation
This filter inserts the tag <link rel="dns-prefetch"> (or
<link rel="prefetch"> for IE9) in the HEAD section, for all
domains present in the HTML document.
Example
The example below shows the HTML before rewriting:
<html>
<head>
</head>
<body>
<img src="www.domain1.com/image1.jpeg">
<script src="www.domain2.com/script1.js">
</body>
</html>
and after rewriting:
<html>
<head>
<link rel="dns-prefetch" href="//www.domain1.com">
<link rel="dns-prefetch" href="//www.domain2.com">
</head>
<body>
<img src="www.domain1.com/image1.jpeg">
<script src="www.domain2.com/script1.js">
</body>
</html>
Limitations
This filter will be applied only on Firefox 3.5+, Chrome, Safari 5+ and IE 9+.
Risks
This filter is considered minimal risk.