|
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.
 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.
|
 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 the topic on DNS server enhancements) are based on the regular format but with the meanings of certain fields changed. You can find these field formats in RFC 1996 and RFC 2136 respectively.
|
| If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support! |
|
|
Home -
Table Of Contents - Contact Us
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.
|
