Table of Contents

Resource Records

Resource records comprise the data within a DNS zone. Although there is no fixed limit to the number of resource records a zone may own, in general, a zone may own one or more resource records of a given type, or none. There are some exceptions depending on the types involved.

All resource records have the following required entries:

• Name—The name (host) that owns the record, such as example.com.
  
  
• Class (not required for all formats)—DNS supports only the IN (Internet) class of record.
  
  
• TTL (Time to Live)—Amount of time the record can be stored in cache. It is expressed in seconds. If you do not include a TTL, Network Registrar uses the zone default TTL, defined in the SOA resource record.
  
  
• Type—The type of the record. There are many types defined by various RFCs, although 10 or fewer are in common use.
  
  
• Record data—Data types whose format and meaning varies with record type.
  
  

Definition of Common Resource Records

The following are descriptions of the most common resource records.

Address (A)

The Address (A) record provides the name-to-address mapping for the zone. It contains an IP address in dotted decimal form. There must be at least one A record for each host address.

The name specified is the host name expressed as an FQDN (ns.example.com.). Note the trailing dot. The data is an IP address.

The presence of one or more A records indicates that the owner's name is the name of a host.

Canonical Name (CNAME)

The Canonical Name (CNAME) record is used for aliases or nicknames. The data portion is the official or canonical name. You cannot have any other resource records associated with a CNAME. Aliases are useful when you want the outside world to know a single, easily remembered name. You can also use aliases when a host changes its name. In that case, make sure that you have a CNAME pointer so that when people use the original name it can be resolved to the newer name.

Host Info (HINFO)

The Host Info (HINFO) record provides information about a particular host. The data contains a description of the hardware and software. The hardware description contains the name of the manufacturer and the model number. The software description contains the name of the operating system.

Mail Exchanger (MX)

The Mail Exchange (MX) record specifies where the mail for a domain name should be delivered. You can have several MX records for a single domain name, and they can be ranked in order of preference.

Name Server (NS)

The Name Server (NS) record lists the name of the machine that provides domain service for the particular domain. Machines that provide name service do not have to reside in the named domain. For each domain you must have at least one NS record. NS records for a domain must exist in both the zone that delegates the domain and in the domain itself. You must also have a corresponding A record mapped to the host name specified in the data field. The name is the name of the zone (for example, myexample.com) and the data is the name of the host machine on which the name server is running.

NS record names must have an equivalent A record (that is, they cannot point to an alias).

Naming Authority Pointer (NAPTR)

The Naming Authority Pointer (NAPTR) record specifies a regular expression based rewrite rule that, when applied to an existing string, produces a new domain label or Universal Resource Identifier (URI). This allows DNS to be used to look up services for a wide variety of resource names that are not in domain name syntax. For more information, see RFC 2915 and Internet draft-ietf-urn-naptr-rr-04.txt.

Pointer (PTR) or Reverse Mapping

The Pointer (PTR) record enables you to point to some other location in the domain tree. They are used for reverse mapping, specifically in the in-addr.arpa zones for translation of addresses to names. PTRs use official names not aliases. The name in a PTR record is the local IP address portion of the reverse name.

Start of Authority (SOA)

The Start of Authority (SOA) record designates the start of a zone. Every zone must have a single SOA record. The SOA record indicates the name of the machine with master data (the primary name server). The record also contains the e-mail address of the zone's administrator. It informs secondary name servers about the timing of zone transfers.

This record contains:

• The name of the zone
  
  
• The name of the primary server
  
  
• The e-mail address of the person responsible for the zone
  
  
• The serial number of the zone file
  
  
• The secondary refresh time
  
  
• The secondary retry time
  
  
• The secondary expire time
  
  
• The minimum Time To Live (TTL)
  
  

Server (SRV)

The Server (SRV) record allows administrators to use several servers for a single domain, to move services from host to host with little difficulty, and to designate some hosts as primary servers for a service and others as backups. Clients ask for a specific service or protocol for a specific domain and receive the names of any available servers. For more information, refer to RFC 2052bis. Also see "Windows 2000 Interoperability" for how this record affects Windows 2000 servers.

Text (TXT)

The Text (TXT) record contains any string of less than or equal to 256 characters that can contain any type of information. The name is the host name (equivalent A record) and data.

Well Known Services (WKS)

The Well Known Services (WKS) record lists the Well Known Services a host provides on a particular IP protocol. The common protocols are TCP and UDP. The common services are TIME, TELNET, FTP, or SMTP.

Resource Record Types

Table A-1 lists all the resource record types Network Registrar supports.