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Table Of Contents  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

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DNS Message Header and Question Section Format
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DNS Name Notation and Message Compression Technique
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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.


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