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DNS on Windows 2000, Second Edition by Cricket Liu, Matt Larson

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Advanced Resolver Features

The Windows 2000 resolver has some advanced features that are worth describing here.

Caching

The Windows 2000 resolver stores every record it receives in a shared cache available to all programs on the system. The Windows NT 4.0 resolver caches, but only on a per-process basis. For example, if you have two different web browsers running (say, Internet Explorer and Netscape Navigator), each has its own copy of the resolver with a separate cache. Windows 98, 95, and 3.1 resolvers don’t do any caching.

The Windows 2000 resolver obeys the TTL (time to live) field on resource records it caches, up to a maximum of 24 hours by default. So if a record specifies a TTL longer than that, the resolver rounds down to 24 hours. This maximum TTL is configurable with a Registry setting:

MaxCacheEntryTtlLimit
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNSCache\Parameters
Data type: REG_DWORD
Range: 0x0 - 0xFFFFFFFF seconds
Default value: 0x15180 (86,400 seconds = 24 hours)

The Windows 2000 resolver also supports negative caching. It caches negative responses for five minutes by default. This negative caching timeout is also configurable with a Registry setting:

NegativeCacheTime
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNSCache\Parameters
Data type: REG_DWORD
Range: 0x0 - 0xFFFFFFFF seconds
Default value: 0x12C (300 seconds = 5 minutes)

To disable negative caching altogether, set this value to zero.

To view the resolver’s cache, use ipconfig /displaydns ...

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