DNS on Windows Server 2003, 3rd Edition

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 five sections (some of which are empty in certain cases), which are shown here:

    +---------------------+
    |        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), and a query domain name (QNAME). The last three sections have the same format: a possibly empty list of concatenated resource records (RRs). The Answer section contains RRs that answer the question; the Authority section contains RRs that point toward an authoritative name server; and the Additional records section contains RRs which relate to the query, but are not strictly answers for the question.