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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 1 2 3 DNS Message Header and Question Section Format

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.

Figure 247: DNS General Message Format

DNS Message Generation and Transport 1 2 3 DNS Message Header and Question Section Format

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