Background

The 'A' Resource Record (RR) is standarized in RFC 1035 and defines the IPv4 address of a particular host in the domain or zone. Hosts that have multiple Internet addresses will have multiple 'A' records that provides a round-robin style of load balancing. The equivlent RR for IPv6 is the AAAA RR (aka: quad A record). Use 'PTR' RRs for reverse mapping IP addresses to names.


Details

RR Type: A
ID: 1 (0x0001)
Defining RFC: RFC 1035,  Section 3.4.1,  Page 20
Description: IPv4 Address Record
Function: Returns a 32 bit IPv4 Internet address, most commonly used to map hostnames to an IPv4 address of the host.
Status: Active

'A' RR Syntax

name ttl class type rdata {address}

RR Field Example Description
name ns1 The name is unqualified, causing $ORIGIN substitution. You can also write this as a fully qualified domain name (FQDN) such as ns1.yourdomain.com.

Blank Name - A record that has nothing in the 'name' field gets used for all requests being made to the base domain such as yourdomain.com. (Same as Parent)
ttl This is the records time to live value (in seconds). If there is no TTL specified, the zone's default $TTL Directive will be used.
type A Specifies the RDATA field will contain data in the A RDATA format.
class IN Specifies the class to be 'Internet'.
rdata address 65.22.6.79 Specifies that the host 'ns1' has the phyiscal IPv4 address of [65.22.6.79].

Examples

Snippets from a fictitious forward lookup 'yourdomain.com' zone file

Typical 'A' Record Entries

;
;   Zone records
;
ns1 IN A 65.22.6.79
ns2 IN A 4.31.198.44
 
mail IN A 4.31.198.44

Round-Robin Load-Balancing

Round-robin DNS is a technique of load distribution, load balancing, or fault-tolerance provisioning multiple, redundant Internet Protocol service hosts, e.g., Web server, FTP servers, by managing the Domain Name System's (DNS) responses to address requests from client computers according to an appropriate statistical model.

;
;   Zone records
;
www 3600 IN A 104.16.45.99
www 3600 IN A 104.16.44.99

Wildcard DNS record

A wildcard DNS record is a record in a DNS zone that will match requests for non-existent domain names. A wildcard DNS record is specified by using a * as the leftmost label (part) of a domain name, e.g. *.yourdomain.com. The exact rules for when a wild card will match are specified in RFC 1034, but the rules are neither intuitive nor clearly specified. This has resulted in incompatible implementations and unexpected results when they are used.

;
;   Zone records
;
* IN A 104.16.45.99
*.yourdomain.com. IN A 104.16.44.99


See Also


External References




This content was last updated on December 9, 2020
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