The TCP/IP Guide - DNS Message Processing and General Message Format

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The TCP/IP Guide

The TCP/IP Guide
9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9 Name Systems and TCP/IP Name Registration and Name Resolution
           9 TCP/IP Name Systems: Host Tables and Domain Name System (DNS)
                9 TCP/IP Domain Name System (DNS)
                     9 DNS Name Servers and Name Resolution
                          9 DNS Messaging and Message, Resource Record and Master File Formats

DNS Message Generation and Transport

DNS Message Header and Question Section Format

DNS Message Processing and General Message Format
(Page 1 of 3)

The previous topic discussed DNS message exchanges and the fact that they are all based on the principle of client/server computing. In a particular exchange, one device acts as a client, initiating the communication by sending a query; the other acts as the server by responding to the query with an answer. This “query/response” behavior is an integral part of DNS, and is reflected in the format used for DNS messages.

A common message format is used for DNS queries and responses. This message format contains five sections that provide a place for the query asked by the client; the answer(s) provided by the server; and header information that controls the entire process. Table 168 describes the DNS general message format, providing a brief summary of each of its sections and how they are used. You can also see a simplified illustration of the message format in Figure 247.

**Table 168: DNS General Message Format**
Section Name	Description
Header	Contains fields that describe the type of message and provide important information about it. Also contains fields that indicate the number of entries in the other sections of the message.
Question	Carries one or more “questions”, that is, queries for information being sent to a DNS name server.
Answer	Carries one or more resource records that answer the question(s) indicated in the Question section above.
Authority	Contains one or more resource records that point to authoritative name servers that can be used to continue the resolution process.
Additional	Conveys one or more resource records that contain additional information related to the query that is not strictly necessary to answer the queries (questions) in the message.

**Figure 247: DNS General Message Format**

DNS Message Generation and Transport

DNS Message Header and Question Section Format

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