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DNS Message Processing and General Message Format
(Page 3 of 3)
Resource Record Sections: Answer, Authority and Additional
The server is also responsible for filling in the other three sections of the message: Answer, Authority and Additional. These sections share the same basic format, each carrying one or more resource records that use a common record format. The number of records in each section is indicated using the “count” fields in the message header. The sections differ only in terms of the types of records they carry. Answer records are directly related to the question asked, while Authority records carry resource records that identify other name servers. Authority records are thus the means by which name servers are hierarchically “linked” when the server doesn't have the information the client requested.
The Additional section exists for the specific purpose of improving DNS efficiency. There are cases where a server supplies an answer to a query that it has reason to believe will lead to a subsequent question that the server can also answer. For example, suppose a server provides the name of another name server in the Authority section (an NS resource record). The client may not have the address for that server, which would mean it has to perform an extra name resolution to contact the referenced server. If the server providing the NS record already knows the IP address for this name server, it can include it in the Additional section. The same goes for a server providing an MX record as I explained in the topic on DNS mail support.
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Key Concept: DNS uses a general message format for all messages. It consists of a fixed 12-byte Header, a Question section that contains a query, and then three additional sections that can carry resource records of different types. The Answer section usually contains records that directly answer the Question of the message; the Authority section holds the names of name servers being sent back to the client, and the Additional section holds extra information that may be of value to the client, such as the IP address of a name server mentioned in the Authority section.
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Note: The special Notify and Update messages use a different format than the regular DNS query/response messages. These special messages (whose use is described in
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Version 3.0 - Version Date: September 20, 2005
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