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TCP/IP Guide by Charles M. Kozierok

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DNS Message Header Format

The header is the most important part of any message, since it is where critical control fields are carried. In DNS messages, the Header section carries several key control flags, and it also indicates which of the other sections are used in the message. Examining the Header section can help you understand several of the nuances of how messaging works in DNS.

The format of the Header section used in all DNS messages is illustrated in Figure 57-2 and described in detail in Tables Table 57-2, Table 57-3, and Table 57-4. Where fields are used differently by the client and server in an exchange, I have mentioned in Table 57-2 how the use is differentiated between the two.

Note that the current lists of valid question types, ...

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