Connected: An Internet Encyclopedia
DNS Protocol Overview

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DNS Protocol Overview

DNS Protocol Overview

Part of the confusion associated with the DNS protocol is that it lacks a special name. Thus DNS can refer either to the entire system, or to the protocol that makes it work. This page documents the protocol, which operates in one of two basic modes - lookups or zone transfers.

DNS Lookups

Normal resource records lookups are done with UDP. An "intelligent retransmission" is to be used, though one is not specified in the protocol, resulting in a mix of poor strategies with good ones. The protocol itself is stateless; all the information needed is contained in a single message, fully documented in RFC 1035 4.1, and having the following format:

    +---------------------+
    |        Header       |
    +---------------------+
    |       Question      | the question for the name server
    +---------------------+
    |        Answer       | RRs answering the question
    +---------------------+
    |      Authority      | RRs pointing toward an authority
    +---------------------+
    |      Additional     | RRs holding additional information
    +---------------------+

Zone Transfers

Sometimes, it is necessary to efficiently transfer the resource records of an entire DNS zone. This is most commonly done by a secondary name server having determined the need to update its database.

The operation of a zone transfer is almost identical to a normal DNS query, except that TCP is used (due to large quantity of reply records) and a special Class exists to trigger a zone transfer. A DNS query with Name=FreeSoft.org, Class=IN, Type=AXFR will trigger a zone transfer for FreeSoft.org. The end of a zone transfer is marked by duplicating the SOA RR that started the zone.

Zone transfers are discussed in more detail in RFC 1034 4.3.5.

Lower-Level Transport

Either TCP or UDP can be used to transport DNS protocol messages, connecting to server port 53 for either. Ordinary DNS requests can be made with TCP, though convention dictates the use of UDP for normal operation. TCP must be used for zone transfers, however, because of the danger of dropping records with an unreliable delivery protocol such as UDP.


Next: Resource Records RRs

Connected: An Internet Encyclopedia
DNS Protocol Overview