D

DNS Resource Records

Overview

A resource record (RR) consists of a name, a type, and data that is specific to the type. These resource records, in a hierarchical structure, make up the DNS.

The standard resource record format, specified in RFC 1035, is as follows:

{name} {ttl} addr-class record type record-specific data

The fields are defined as follows:

• name
  The first field, name, is the name of the domain record and it must always start in column 1. For all resource records that are not the first in a file, the name may be left blank. When the name field is left blank, the record takes the previous resource record.
• ttl
  The second field, ttl (time to live), is optional. This field specifies how long this data will be stored in the database. If this field is left blank, the default time to live value is specified in the Start Of Authority resource record (described later in this chapter).
• address class
  The third field is the address class. Currently, only one class is supported: IN, for internet addresses and other internet information. Limited support is included for the HS class, which is for MIT/Athena "Hesiod" information.
• record type
  The fourth field, record type, defines the type of this resource record, such as "A."
• other fields
  Additional fields may be present in a resource record, depending on its type.

  Although case is preserved in names and data fields when loaded into the name server, comparisons and lookups in the name server database are case insensitive.

Types of resource records

There are many types of resource records currently in use. This section provides an overview of the most common resource record types, and lists other types of resource records.

Common types

There are six standard types of resource records:

A (Address)

The Address record, or A name record, lists the address for a given machine. The name field is the machine name, and the address is the network address. There should be one A name record for each address of the machine.

The following is an example of an A name record:

{name} {ttl} addr-class A address

ucbarpa IN A 128.32.0.4

IN A 10.0.0.78

CNAME (Canonical Name)

The Canonical Name resource record, CNAME, specifies an alias or nickname for the official, or canonical, host name. This record must be the only one associated with the alias name. It is usually easier to supply one A record for a given address and use CNAME records to define alias host names for that address.

The following is an example of a CNAME resource record:

alias {ttl} addr-class CNAME Canonical name

ucbmonet IN CNAME monet

MX (Mail Exchange)

The Mail Exchange resource record, MX, defines the mail system(s) for a given domain.

The following is an example of an MX resource record:

name {ttl} addr-class MX pref value mail exchange

Munnari.OZ.AU. IN MX 0 Seismo.CSS.GOV.

*.IL. IN MX 0 RELAY.CS.NET.

NS (Name Server)

The Name Server resource record, NS, defines the name server(s) for a given domain, creating a delegation point and a subzone. The first name field specifies the zone that is serviced by the name server that is specified by the second name. Every zone needs at least two name servers.

The following is an example of an NS resource record:

{name} {ttl} addr-class NS Name servers name

IN NS ucbarpa.Berkeley.Edu.

PTR (Pointer)

A Name Pointer record, PTR, associates a host name with a given IP address. These records are used for reverse name lookups.

The following example of a PTR record is used in setting up reverse pointers for the special IN-ADDR.ARPA domain:

name {ttl} addr-class PTR real name

7.0 IN PTR monet.Berkeley.Edu.

SOA (Start of Authority)

The Start of Authority, SOA, record starts every zone file. There must be exactly one SOA record per zone.

The following is an example of an SOA resource record:

name {ttl} addr-class SOA Origin Person in charge

@ IN SOA ucbvax.Berkeley.Edu. kjd.ucbvax.Berkeley.Edu. (

1995122103 ; Serial

10800 ; Refresh

1800 ; Retry

3600000 ; Expire

259200 ) ; Minimum

The record-specific fields are defined as follows:

• Person in charge
  The email address for the person responsible for the name server, with "@" changed to a "."
• Serial number
  The version number of this data file; it must be a positive integer. This number must be increased whenever a change is made to the data.
• Refresh
  The time interval, in seconds, between calls that the secondary name servers make to the primary name server to see if an update is necessary.
• Retry
  The time interval, in seconds, that a secondary server waits before retrying a failed zone transfer.
• Expire
  The maximum number of seconds that a secondary name server can use the data before it expires for lack of receiving a refresh.
• Minimum
  The default number of seconds to be used for the time to live (TTL) field on resource records which do not specify a TTL in the zone file. It is also an enforced minimum on TTL if it is specified on a resource record in the zone.

Other types

The following is a list of less common resource record types. For more information see RFCs 1035, 1183, and 1664.