C

DNS Resource Records

• Understanding DNS resource records
  
  
• Types of resource records
  
  
• Additional resource record types

Understanding DNS resource records

A resource record is an entry in a DNS database file, and consists of a name, a TTL, a type, and data that is specific to the type. These resource records, in a hierarchical structure, make up the domain name system (DNS).

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

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

The resource record 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 name of the previous resource record.
  
  
• ttl
  The second field, ttl (time to live), is optional. This field specifies how long the resource record is stored by the LDNS. If this field is left blank, the default TTL value is specified in the start of authority (SOA) 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, NS, or CNAME.
  
  
• 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 not case-sensitive.

Note

Types of resource records

Many types of resource records are currently in use. This section provides an overview of the most common resource record types, and lists other types of resource records. The six most common types of resource records are shown in Table C.1 .

A (Address)

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

Figure C.1 shows an example of an A record.

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.

Figure C.2 shows an example of a CNAME resource record.

MX (Mail Exchange)

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

Figure C.3 shows an example of an MX resource record.

NS (Name Server)

The name server resource record, NS, defines the name servers for a given domain, creating a delegation point and a subzone. The first name field specifies the zone that is served by the name server that is specified in the name servers name field. Every zone needs at least two name servers.

Figure C.4 shows an example of an NS resource record.

PTR (Pointer)

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

The example of a PTR record shown in Figure C.5 is used to set up reverse pointers for the special IN-ADDR.ARPA domain.

SOA (Start of Authority)

The start of authority resource record, SOA, starts every zone file and indicates that a name server is the best source of information for a particular zone. In other words, the SOA record indicates that a name server is authoritative for a zone. There must be exactly one SOA record per zone.

The following is an example of an SOA record.

The specific fields in an SOA record are defined as follows:

• Person in charge
  The email address for the person responsible for the name server, with the at character ( @ ) changed to a dot ( . ). For example, johndoe@berkeley.edu becomes johndoe.berkeley.edu.
  
  
• Serial number
  The version number of the data file; it must be a positive integer. You must increase this number whenever a change is made to the data.
  
  
• Refresh
  The time interval between calls, in seconds, that the secondary name servers make to the primary name server to check 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.
  
  

Additional resource record types

Table C.2 lists less common resource record types. For more information on these, see RFCs 1035, 1183, and 1664.