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

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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 5 of 5)

PTR / Pointer Resource Record (Type Value 12)

This record carries a pointer to a resource record; used for reverse address lookups. It contains one data field, shown in Table 176.

**Table 176: DNS *Pointer* Resource Record Data Format**
Subfield Name	Size (bytes)	Description
*PTRDName*	Variable	*Pointer Domain Name:* A variable-length domain name. This is a name “pointed to” by the resource record; see the description of reverse resolution for the most common way that this record type is used.

MX / Mail Exchange Resource Record (Type Value 15)

This special record contains information about the mail server(s) to be used for sending e-mail to the domain. Each record contains two fields as Table 177 illustrates.

**Table 177: DNS *Mail Exchange* Resource Record Data Format**
Subfield Name	Size (bytes)	Description
*Preference*	2	*Preference Value:* The preference level for this mail exchange. Lower values signify higher preference.
*Exchange*	Variable	*Exchange Domain Name:* The domain name, encoded using standard DNS name notation, of a host willing to provide mail exchange services for this named object.

TXT / Text Resource Record (Type Value 16)

This descriptive record contains additional descriptive information about the named object (Table 178).

**Table 178: DNS *Text* Resource Record Data Format**
Subfield Name	Size (bytes)	Description
*TXT-Data*	Variable	*Text Data:* Variable-length descriptive text.

DNS Message Header and Question Section Format

DNS Name Notation and Message Compression Technique

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