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Types of DNS records

June 18, 2008 Written by nicolae

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How DNS works

Important categories of data stored in DNS include the following:

An A record or address record maps a hostname to a 32-bit IPv4 address.
An AAAA record or IPv6 address record maps a hostname to a 128-bit IPv6 address.
A CNAME record or canonical name record is an alias of one name to another. The A record to which the alias points can be either local or remote - on a foreign name server. This is useful when running multiple services (like an FTP and a webserver) from a single IP address. Each service can then have its own entry in DNS (like ftp.example.com. and www.example.com.)
An MX record or mail exchange record maps a domain name to a list of mail exchange servers for that domain.
A PTR record or pointer record maps an IPv4 address to the canonical name for that host. Setting up a PTR record for a hostname in the in-addr.arpa. domain that corresponds to an IP address implements reverse DNS lookup for that address. For example (at the time of writing), www.icann.net has the IP address 192.0.34.164, but a PTR record maps 164.34.0.192.in-addr.arpa to its canonical name, referrals.icann.org.
An NS record or name server record maps a domain name to a list of DNS servers authoritative for that domain. Delegations depend on NS records.
An SOA record or start of authority record specifies the DNS server providing authoritative information about an Internet domain, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.
An SRV record is a generalized service location record.
A TXT record allows an administrator to insert arbitrary text into a DNS record. For example, this record is used to implement the Sender Policy Framework and DomainKeys specifications.
NAPTR records ("Naming Authority Pointer") are a newer type of DNS record that support regular expression based rewriting.

Other types of records simply provide information (for example, a LOC record gives the physical location of a host), or experimental data (for example, a WKS record gives a list of servers offering some well known service such as HTTP or POP3 for a domain).

When sent over the internet, all records use the common format specified in RFC 1035 shown below.

**RR (Resource Record) Fields**
Field	Description	Length (Octets)
NAME	Name of the node to which this record pertains.	(variable)
TYPE	Type of RR. For example, MX is type 15.	2
CLASS	Class code.	2
TTL	Signed time in seconds that RR stays valid.	4
RDLENGTH	Length of RDATA field.	2
RDATA	Additional RR-specific data.	(variable)