Networking / Beginners

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Resource Records (RR)

A DNS resource record (RR) contains all the information about a domain name system. It defines all the attributes for a domain name such as an IP address or a mail route.

A DNS RR has six fields:

<NAME>, <TYPE>, <CLASS>, <TTL>, <RD Length>, and <RDATA>

These fields are explained in the following list:

• Name. This field specifies the DNS name, also known as the owner name, to which the RR belongs.
• Type. This field is a 2-byte value that specifies the type of the resource that is defined in the resource record. This field is necessary because a DNS name can have more than one type of RR.
• Class. This defines the protocol family for the RR record. For example, IN, which stands for Internet.
• Time To Live (TTL). This field is the time, in seconds, for which a name server can cache an RR. A zero TTL implies that a server should not cache the RR.
• RD Length. This field is the RDATA field's length in octets.
• RDATA. This field is a resource data field and is the value to which the entity specified in the NAME field maps. It is unique for each type of RR.

Common DNS Resource Records

Record Type	Description	Usage
A	An address record	Maps FQDN into an IP address. For example, Mail IN A 172.100.100.1 Login IN A 172.100.100.2
PTR	A pointer record	Maps an IP address into FQDN. For example, 1.100 IN PTR mail.example.com.
NS	A name server record	Denotes a name server for a zone. For example, IN NS ns1.abc.com. IN NS ns2.abc.com.
SOA	A Start of Authority record	Specifies many attributes concerning the zone, such as the name of the domain (forward or inverse), administrative contact, the serial number of the zone, refresh interval, retry interval, and so on.
CNAME	A canonical name record	Defines an alias name and maps it to the absolute (canonical) name. For example, POP IN CNAME mail
MX	A mail exchange record	Used to redirect e-mail for a given domain or host to another host. For example, xyz.com IN MX 0 mail.abc.com