DNS on Windows Server 2003, 3rd Edition

Recursion

You may have noticed a big difference in the amount of work done by the name servers in the previous example. Four of the name servers simply returned the best answer they already had—mostly referrals to other name servers—to the queries they received. They didn’t have to send their own queries to find the data requested. But one name server—the one queried by the resolver—had to follow successive referrals until it received an answer.

Why couldn’t the local name server simply have referred the resolver to another name server? Because a stub resolver wouldn’t have had the intelligence to follow a referral. And how did the name server know not to answer with a referral? Because the resolver issued a recursive query.

Queries come in two flavors, recursive and iterative, also called nonrecursive. Recursive queries place most of the burden of resolution on a single name server. Recursion, or recursive resolution, is just a name for the resolution process used by a name server when it receives recursive queries. As with recursive algorithms in programming, the name server repeats the same basic process (querying a remote name server and following any referrals) until it receives an answer.

Iteration, or iterative resolution, on the other hand, refers to the resolution process used by a name server when it receives iterative queries.

In recursion, a resolver sends a recursive query to a name server for information about a particular domain name. The queried name server ...