Windows 2000 includes a caching DNS resolver service, which is enabled by default. For troubleshooting purposes, this service can be viewed, stopped, and started like any other Windows service. The caching resolver reduces DNS network traffic and speeds name resolution by providing a local cache for DNS queries. Name query responses are cached for the TTL specified in the response (not to exceed the value specified in the MaxCacheEntryTtlLimit parameter), and future queries are answered from the cache, when possible. One interesting feature of the DNS Resolver Cache Service is that it supports negative caching. For example, if a query is made to a DNS server for a given host name and the response is negative, succeeding queries for the same name are answered (negatively) from the cache for NegativeCacheTime seconds (the default is 300). Another example of negative caching is that if all DNS servers are queried and none are available, for NetFailureCacheTime seconds (the default is 30) all succeeding name queries fail instantly, instead of timing out. This feature can save time for services that query the DNS during the boot process, especially when the client is booted from the network. The DNS Resolver Cache Service has a number of other adjustable registry parameters, which are documented in Appendix C.
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