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September 19, 2006

How can I disable the caching of failed DNS lookups?

A Web Exclusive from FAQ for Windows
Jerold Schulman
DNS
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FAQ for Windows
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10829 » How can I disable the caching of failed DNS lookups? (19-Sep-06)

Windows 2000, Windows XP, Windows Server 2003, and future Windows Operating Systems added the ability to cache DNS lookup results in the client's cache. This lowers the activity on the LAN and on the Web, but if a failed query is due to incorrect configuration, the client must use ipconfig /flushdns to remove it from the cache once fixed.

NOTE: To see the cache, type ipconfig /displaydns in the CMD.EXE window. You will see a display like:

Windows IP Configuration

         www.scrubtheweb.com
         ----------------------------------------
         Record Name . . . . . : www.scrubtheweb.com
         Record Type . . . . . : 1
         Time To Live  . . . . : 2802
         Data Length . . . . . : 4
         Section . . . . . . . : Answer
         A (Host) Record . . . : 66.93.156.37

         pagead.l.google.com
         ----------------------------------------
         Record Name . . . . . : pagead.l.google.com
         Record Type . . . . . : 1
         Time To Live  . . . . : 121
         Data Length . . . . . : 4
         Section . . . . . . . : Answer
         A (Host) Record . . . : 64.233.169.104

         ns02.penton.com
         ----------------------------------------
         Record Name . . . . . : ns02.penton.com
         Record Type . . . . . : 1
         Time To Live  . . . . : 81145
         Data Length . . . . . : 4
         Section . . . . . . . : Answer
         A (Host) Record . . . : 63.167.165.7

         .... snip ......
If you wish to avoid caching failed DNS lookup results, you can set the NegativeCacheTime Value Name, a REG_DWORD data type, to 0 at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Dnscache\Parameters using PolicyMaker™ Registry Extension freeware, or you can include the following in a Startup script:
REG.EXE ADD HKLM\SYSTEM\CurrentControlSet\Services\Dnscache\Parameters /V NegativeCacheTime /T REG_DWORD /F /D 0
NOTE: REG.EXE is built into Windows XP, Windows Server 2003, and later operating systems, or installed from the Windows 2000 Support Tools.


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