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DNS on Windows 2000, Second Edition by Cricket Liu, Matt Larson

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DNS Messages

To write programs that parse DNS messages, you need to understand the message format. DNS queries and responses are most often contained within UDP datagrams. Each message is fully contained within a UDP datagram. If the query and response are sent over TCP, they are prefixed with a 2-byte value indicating the length of the query or response, excluding the 2-byte length. The format and content of the DNS message are as follows.

Message Format

(From RFC 1035, page 25)

All communications inside of the domain protocol are carried in a single format called a message. The top level format of the message is divided into 5 sections (some of which are empty in certain cases), shown below:

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

The Header section is always present. The header includes fields that specify which of the remaining sections are present, and also specify whether the message is a query or a response, a standard query or some other opcode, etc.

The names of the sections after the header are derived from their use in standard queries. The Question section contains fields that describe a question to a name server. These fields are a query type (QTYPE), a query class (QCLASS), ...

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