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High Performance Web Sites by Steve Souders

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The Browser's Perspective

As discussed earlier in the "Factors Affecting DNS Caching" section, a lot of independent variables determine whether a particular browser request for a hostname makes a remote DNS request. There is a DNS specification (http://tools.ietf.org/html/rfc1034), but it gives clients flexibility in how the DNS cache works. I'll focus on Internet Explorer and Firefox on Microsoft Windows, since they are the most popular platforms.

The DNS cache on Microsoft Windows is managed by the DNS Client service. You can view and flush the DNS Client service using the ipconfig command:

ipconfig /displaydns
ipconfig /flushdns

Rebooting also clears the DNS Client service cache. In addition to the DNS Client service, Internet Explorer and Firefox browsers have their own DNS caches. Restarting the browser clears the browser cache, but not the DNS Client service cache.

Internet Explorer

Internet Explorer's DNS cache is controlled by three registry settings: DnsCacheTimeout, KeepAliveTimeout, and ServerInfoTimeOut, which can be created in the following registry key:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\InternetSettings\

There are two Microsoft Support articles describing how these settings affect the DNS cache.[8] These articles report the following default values for these settings:

  • DnsCacheTimeout: 30 minutes

  • KeepAliveTimeout: 1 minute

  • ServerInfoTimeOut: 2 minutes

What's implied (but not explained very well) is that DNS server TTL values less than 30 minutes have ...

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