DNS and domains

DNS and domains

Intro

DNS record types

A

Address record, used to associate a name with an IPv4 Address

AAAA

Similar to an A record, except for IPv6

CNAME

Canonical name, used to create domain aliases

MX

Mail Exchange, this is used by MTAs to find out where to deliver the emails for this domain. There’s often multiple entries for MX, with priorities to allow mail to be delivered to the primary server under normal operation, but with secondaries ready in case there’s a problem

NS

Name Server, this is the name servers for the domain

TXT

Plain text records, the meaning depends on the name of the record

SPF

Sender Policy Framework - SPF records are really just TXT records with a specific format, and are used to help reduce spam by containing information and rules on designated senders for the domain. Usually, you’ll set this according to the instructions given by your mail provider.

WKS

no idea.

SRV

Service record, used for SIP and other protocols

LOC

no idea.

PTR

a Reverse lookup record, to do address-to-hostname resolution, and is only created by the owner of the IP address. You might never need to create one of these.

SOA

Start of Authority record,

Zonefile

A sample DNS zone file.

@ 300 IN A 184.168.47.225
www 10800 IN CNAME antonyh.co.uk.
@ 3600 IN TXT "google-site-verification=the-code-that-google-gave-you"

Line one, is the root site IP address, denoted by @. It is an internet A record, with a value of that IP address.

Name

the first value is the name of the record, such as @ or www.

TTL

the second value is the cache time in seconds for this record. 300 is the minimum, and is often used during transitions. Other common values appear to be 3600 (3 hours) and 86400 (1 day).

Record class

Almost all the records you’ll come across are declared as ‘IN’ meaning they are internet DNS records. There are other namespaces that are used, including CHAOS. Mostly though these are of academic uses, and in all likelihood you’ll never see them.

Record type

one of a set of value to denote the type of this DNS record

Value

this is the actual value of the record, and is subject to escaping, quote marks, and other rules.

Comments

anything following a ; on a line is ignored