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How Caching Works

As DNS servers process client queries using recursion or iteration, they discover and acquire a significant store of information about the DNS namespace. This information is then cached by the server.

Caching provides a way to speed the performance of DNS resolution for subsequent queries of popular names while substantially reducing DNS-related query traffic on the network.

As DNS servers make recursive queries on behalf of clients, they temporarily cache resource records (RRs). Cached RRs contain information obtained from DNS servers that are authoritative for DNS domain names learned while making iterative queries to search and fully answer a recursive query performed on behalf of a client. Later, when other clients place new queries that request RR information matching cached RRs, the DNS server can use the cached RR information to answer them.

When information is cached, a Time-To-Live (TTL) value applies to all cached RRs. As long as the TTL for a cached RR does not expire, a DNS server can continue to cache and use the RR again when answering queries by its clients that match these RRs. Caching TTL values used by RRs in most zone configurations are assigned the minimum (default) TTL, which is set used in the zone's start of authority (SOA) resource record. By default, the minimum TTL is 3,600 seconds (one hour), but it can be adjusted; or, if needed, individual caching TTLs can be set at each RR.


You can install a Windows 2000 DNS server to operate as a caching-only server (without any configured zones).

By default, Windows 2000 DNS servers use a root hints file, Cache.dns, that is stored in the %SystemRoot%\System32\Dns folder on the server computer. The contents of this file are preloaded into server memory when the service is started, and contain pointer information to root servers for the DNS namespace where you are operating DNS servers.

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