The TCP/IP Guide - DNS Message Resource Record Field Formats

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The TCP/IP Guide

The TCP/IP Guide
9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9 Name Systems and TCP/IP Name Registration and Name Resolution
           9 TCP/IP Name Systems: Host Tables and Domain Name System (DNS)
                9 TCP/IP Domain Name System (DNS)
                     9 DNS Name Servers and Name Resolution
                          9 DNS Messaging and Message, Resource Record and Master File Formats

DNS Message Header and Question Section Format

DNS Name Notation and Message Compression Technique

DNS Message Resource Record Field Formats
(Page 3 of 5)

RData Field Formats For Common Resource Records

The RData field consists of one or more subfields that carry the actual “payload” for the resource record. Below is a list of the most common resource record types. For each I have indicated the resource record text code, name, and Type value, provided a brief summary of the resource record’s use, and shown the structure of the RData field in a table:

A / Address Resource Record (Type Value 1)

This is the primary resource record type in DNS; it contains a 32-bit IP address associated with a domain name, as shown in Table 172.

**Table 172: DNS *Address* Resource Record Data Format**
Subfield Name	Size (bytes)	Description
*Address*	4	*Address:* The 32-bit IP address corresponding to this record's named object.

NS / Name Server Resource Record (Type Value 2)

In this record, the data field carries the domain Carries the domain name of a name server (Table 173).

**Table 173: DNS *Name Server* Resource Record Data Format**
Subfield Name	Size (bytes)	Description
*NSDName*	Variable	*Name Server Domain Name:* A variable-length name of a name server that should be authoritative for this record's named object. Like all names, this name is encoded using standard DNS name notation. A request for this resource record type normally results in an A record for the name server specified also being returned in the Additional section of the response, if available.

CName / Canonical Name Resource Record (Type Value 5)

The data field contains the real name of an a named object that has been referenced using an alias (Table 174).

**Table 174: DNS *Canonical Name* Resource Record Data Format**
Subfield Name	Size (bytes)	Description
*CName*	Variable	*Canonical Name:* The canonical (real) name of the named object. This name is then resolved using the standard DNS resolution procedure to get the address for the originally-specified name.

DNS Message Header and Question Section Format

DNS Name Notation and Message Compression Technique

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Version 3.0 - Version Date: September 20, 2005

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