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htmlperformancednsprefetch

DNS prefetching of subdomains

Do I have to prefetch a subdomain separately?

E.g. when I have <link rel="dns-prefetch" href="//example.com"> do I need an additional tag for //static.example.com as well?

Solution

  • I've made the following test: first created simple HTML page

    <html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <link rel="dns-prefetch" href="//example.com/">     
  </head>
  <body>
    <a href="http://example.com">Test link</a>
    <a href="http://sub.example.com">Test link 2</a>
  </body>
</html>

    For the domain and subdomain for which I own dns nameserver. Then I cleaned dns cache and opened this page in firefox private window. I observed in the logs of my dns nameserver that only request for "example.com" was made and no requests for subdomains.

    Then I changed the page as follows:

    <html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <link rel="dns-prefetch" href="//example.com/">     
    <link rel="dns-prefetch" href="//sub.example.com/">
  </head>
  <body>
    <a href="http://example.com">Test link</a>
    <a href="http://sub.example.com">Test link 2</a>
  </body>
</html>

    Again cleared dns cache and opened this page in firefox private window. Now I observed that dns requests we made for both domain and it's subdomain.

    So I can conclude that yes - you have to prefetch subdomains separately.