4.17 Managing Windows’ DNS Caches - BrainDump

Problem

You know that Windows 2000, XP, and 2003 Server include DNS resolver caches by default. Which is a big surprise to most Windows users, who sometimes get stuck with stale data and don’t understand why some addresses are not resolving correctly. Most of the time you don’t even have to think about it, but when you’re making changes, you want to be sure that your clients are receiving fresh DNS information. How do you handle this?

Solution

On Windows clients, open a DOS window and run this command to see the contents of the cache:

  C:\> ipconfig /displaydns | more

This command clears the cache:

  C:\> ipconfig /flushdns

The default TTL is 86,400 seconds, or one day, for positive responses. Answers to negative queries are stored for 300 seconds (5 minutes). You may change these values, or disable caching entirely by editing the Windows Registry. On Windows 2000, open the Registry Editor and change the TTL for positive entries by creating or modifying the DWORD value in:

  HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\Services\Dnscache\ Parameters
  DWORD: MaxCacheEntryTtlLimit
  Value: 14400

14,400 seconds is four hours, which is typical for most ISPs these days. 0 disables all caching. Be sure you enter your values as Decimal Base, not Hexadecimal Base.

Disable negative answers with this key:

  HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\Services\Dnscache\Parameters
  DWORD: NegativeCacheTime
  Value: 0

On Windows XP and 2003, change the TTL for positive entries with a differentDWORD:

  HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\ Services\Dnscache\ Parameters
  DWORD:  MaxCacheTtl
  Value: 14400

Turn off negative caching with this one:

  HKEY_LOCAL_MACHINE\SYSTEM\ CurrentControlSet\Services\Dnscache\ Parameters
  DWORD:  MaxNegativeCacheTtl
  Value: 0

You may disable caching entirely by setting both values to zero. Reboot, as always, to activate the changes.

Discussion

Linux clients do not activate their own DNS caches by default; you have to set these up on purpose. Client-side caching is a nice thing that speeds up lookups. All those caches cause problems only when DNS is changed and the caches get stale.

See Also

• The documentation for your particular flavors of Windows; a quick Google search on “windows dns cache” should get you all the information you need