The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Name Server Concepts and Operation

DNS Root Name Servers 1 2 3 DNS Name Server Load Balancing

Negative Caching

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.

DNS Root Name Servers 1 2 3 DNS Name Server Load Balancing