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Which statement correctly describes how recursive and non-recursive (iterative) queries are used in DNS name resolution?

  1. AA host (stub resolver) sends a recursive query to a full-service resolver asking for the final answer, and the full-service resolver sends non-recursive queries to the root servers and authoritative DNS servers, following the referrals to the next server to ask
  2. BA host itself repeats non-recursive queries in turn to a root server, a TLD server and the authoritative DNS server to get the answer
  3. CRoot servers accept recursive queries and, on behalf of the host, query down to the authoritative DNS server and return the final answer
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Answer: A. A host (stub resolver) sends a recursive query to a full-service resolver asking for the final answer, and the full-service resolver sends non-recursive queries to the root servers and authoritative DNS servers, following the referrals to the next server to ask

A recursive query is a request to "return the final answer," and a host normally sends this kind of query to a full-service resolver (caching DNS server) run by its ISP or organization. If the answer is not in its cache, the full-service resolver sends non-recursive queries to a root server, a TLD server and the domain's authoritative DNS server. A server that receives such a query returns the answer if it knows it, and otherwise returns a referral (such as NS records) saying "ask this server next." Root and TLD servers do not accept recursive queries, for reasons of load and security.

  • A. Correct. Queries between the host and the resolver are recursive, and those between the resolver and each server are non-recursive.
  • B. Walking down the hierarchy is the full-service resolver's job. A host's stub resolver normally sends just one query to the resolver.
  • C. Root servers do not accept recursive queries; they only refer the resolver to TLD servers.
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