Caching Name Server
Caching name servers, also called DNS caches, store DNS query results for a period of time determined in the configuration (time-to-live) of each domain-name record. DNS caches improve the efficiency of the DNS by reducing DNS traffic across the Internet, and by reducing load on authoritative name-servers, particularly root name-servers. Because they can answer questions more quickly, they also increase the performance of end-user applications that use the DNS.
Recursive name servers resolve any query they receive, even if they are not authoritative for the question being asked, by consulting the server or servers that are authoritative for the question.
Caching name servers are often also recursive name servers—they perform every step necessary to answer any DNS query they receive. To do this the name server queries each authoritative name-server in turn, starting from the DNS root zone. It continues until it reaches the authoritative server for the zone that contains the queried domain name. That server provides the answer to the question, or definitively says it can't be answered, and the caching resolver then returns this response to the client that asked the question.
The authority, resolving and caching functions can all be present in a DNS server implementation, but this is not required: a DNS server can implement any one of these functions alone, without implementing the others.
Internet service providers typically provide caching resolvers for their customers. In addition, many home-networking routers implement caching resolvers to improve efficiency in the local network.
Some systems utilize nscd - the name service caching daemon.
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