The TCP/IP Guide - DNS Name Server Data Storage: Resource Records and Classes

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The main DNS standards, RFC 1034 and 1035, defined a number of resource record types. Over time, the list has changed, with new RR types being created in subsequent standards and the use of others changed. Like other Internet parameters, the list of DNS RR types is maintained in a file at IANA.Also like other Internet parameters, there are in fact several dozen defined RRs in DNS, but only a few are commonly used; others are now obsolete, used for special purposes or “experimental” in nature.

Table 166 summarizes the most important resource record types. For each, I have shown the numeric Type value for the record, which is used to identify the resource record type in message exchanges, and also the text code used for the RR in master files. I have also provided a brief description of each.

**Table 166: Summary Of Common DNS Resource Records**
RR Type Value	RR Text Code	RR Type	Description
1	A	Address	Contains a 32-bit IP address. This is the “meat and potatoes” of DNS, since it is where the address of a node is stored for name resolution purposes.
2	NS	Name Server	Specifies the name of a DNS name server that is authoritative for the zone. Each zone must have at least one NS record that points to its primary name server, and that name must also have a valid A (Address) record.
5	CNAME	Canonical Name	This resource record is used to allow aliases to be defined that point to the real name of a node. The CNAME record provides a mapping between this alias and the “canonical” (real) name of the node. The CNAME resource record is commonly used to hide changes in the internal DNS structure from outside users, by letting them use an unchanging alias while the internal names are modified based on the needs of the organization. See the topic on name resolution for an example.
6	SOA	Start Of Authority	The SOA resource record is used to mark the start of a DNS zone and provide important information about it. Every zone must have exactly one SOA record, which contains details such as the name of the zone, its primary (master) authoritative server name, and technical details such as the e-mail address of its administrator and parameters for how often slave (secondary) name servers are updated.
12	PTR	Pointer	Provides a pointer to another location in the name space. These records are best known for their use in reverse resolution through the IN-ADDR.ARPA domain.
15	MX	Mail Exchange	Specifies the location (device name) that is responsible for handling e-mail sent to the domain.
16	TXT	Text String	Allows arbitrary additional text associated with the domain to be stored.

All of these resource records are used in different ways to define zones and devices within them, and then permit name resolution and other functions to take place. We'll see how they are used in more detail in the section on name resolution. You can also find a more lengthy description of some of them in the topic devoted to resource record field formats.