DNS Name Server Caching, Negative Caching and Data Persistence
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Classical DNS caching stores only the results of successful name resolutions. It is also possible for DNS servers to cache the results of unsuccessful name resolution attempts; this is called negative caching. To extend the example above, suppose you mistakenly thought the name of the company's web site was www.xyz-industries.com and typed that into your browser. Your local DNS server would be unable to resolve the name, and would mark that name as unresolvable in its cache; a negative cache entry.
Suppose you typed the name in incorrectly because someone mis-typed it on an internal memo. If a colleague later tried the same name, the DNS server would say I already know this is a bogus name and not try again to resolve it. Since there is no resource record for an invalid name the server itself must decide how long to cache this negative information. Negative caching improves performance because resolving a name that doesn't exist takes resources, just as resolving an existent one does. Note that regular caching is sometimes called positive caching to contrast it to negative caching.
The value to be used for negative caching in a zone is now specified by the Minimum field in the Start Of Authority resource record for each zone. As mentioned above, this was formerly used to specify the default TTL for a zone.
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