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Resource Record Format

Resource records have the following syntax:

Owner TTL Class Type RDATA

Table 5.2 describes each of these fields.

Table   5.2 Typical Resource Record Fields

Name

Description

Owner

The name of the host or the DNS domain to which this resource record belongs.

Time to Live

A 32-bit integer that represents, in seconds, the length of time that a DNS server or resolver should cache this entry before it is discarded. This field is optional, and if it is not specified, the client uses the Minimum TTL in the SOA record.

Class

Defines the protocol family in use. It is almost always IN for the Internet system. The other value defined in RFC 1034 is CH for the Chaos system, which was used experimentally at the Massachusetts Institute of Technology.

Type

Identifies the type of resource record.

RDATA

The resource record data. It is a variable type that represents the information being described by the type. For example, in an A record, this is the 32-bit IP address that represents the host defined by the resource record.

Resource records are represented in binary form in packets when lookups and responses are made using DNS. In the database files, however, resource records are represented as text entries. Most resource records are represented as single-line text entries. If an entry is going to span more than one line, you can use parentheses to encapsulate the information. In many implementations of DNS, only the Start of Authority (SOA) record can be multiple lines. For readability, blank lines and comments are often inserted in the zone files, and are ignored by the DNS server. Comments always start with a semicolon (;) and end with a carriage return.

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