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4.3.1. Queries and responses Connected: An Internet Encyclopedia
4.3.1. Queries and responses

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4.3.1. Queries and responses

4.3.1. Queries and responses

The principal activity of name servers is to answer standard queries. Both the query and its response are carried in a standard message format which is described in [RFC-1035]. The query contains a QTYPE, QCLASS, and QNAME, which describe the types and classes of desired information and the name of interest.

The way that the name server answers the query depends upon whether it is operating in recursive mode or not:

  • The simplest mode for the server is non-recursive, since it can answer queries using only local information: the response contains an error, the answer, or a referral to some other server "closer" to the answer. All name servers must implement non-recursive queries.

  • The simplest mode for the client is recursive, since in this mode the name server acts in the role of a resolver and returns either an error or the answer, but never referrals. This service is optional in a name server, and the name server may also choose to restrict the clients which can use recursive mode.

Recursive service is helpful in several situations:

  • a relatively simple requester that lacks the ability to use anything other than a direct answer to the question.

  • a request that needs to cross protocol or other boundaries and can be sent to a server which can act as intermediary.

  • a network where we want to concentrate the cache rather than having a separate cache for each client.

Non-recursive service is appropriate if the requester is capable of pursuing referrals and interested in information which will aid future requests.

The use of recursive mode is limited to cases where both the client and the name server agree to its use. The agreement is negotiated through the use of two bits in query and response messages:

  • The recursion available, or RA bit, is set or cleared by a name server in all responses. The bit is true if the name server is willing to provide recursive service for the client, regardless of whether the client requested recursive service. That is, RA signals availability rather than use.

  • Queries contain a bit called recursion desired or RD. This bit specifies specifies whether the requester wants recursive service for this query. Clients may request recursive service from any name server, though they should depend upon receiving it only from servers which have previously sent an RA, or servers which have agreed to provide service through private agreement or some other means outside of the DNS protocol.

The recursive mode occurs when a query with RD set arrives at a server which is willing to provide recursive service; the client can verify that recursive mode was used by checking that both RA and RD are set in the reply. Note that the name server should never perform recursive service unless asked via RD, since this interferes with trouble shooting of name servers and their databases.

If recursive service is requested and available, the recursive response to a query will be one of the following:

  • The answer to the query, possibly preface by one or more CNAME RRs that specify aliases encountered on the way to an answer.

  • A name error indicating that the name does not exist. This may include CNAME RRs that indicate that the original query name was an alias for a name which does not exist.

  • A temporary error indication.

If recursive service is not requested or is not available, the non- recursive response will be one of the following:

  • An authoritative name error indicating that the name does not exist.

  • A temporary error indication.

  • Some combination of:

    • RRs that answer the question, together with an indication whether the data comes from a zone or is cached.

    • A referral to name servers which have zones which are closer ancestors to the name than the server sending the reply.

    • RRs that the name server thinks will prove useful to the requester.

Next: 4.3.2. Algorithm

Connected: An Internet Encyclopedia
4.3.1. Queries and responses


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