In DNS you have multiple types of records, each with their own function and structure. In this article we'd like to explain to you what each type does.


Glossary

FQDN (Fully Qualified Domain Name) = The full domain name of a system on the internet. This usually consists of a hostname (www) and a public internet domain (savvii.com). For websites the base domain (ex. savvii.nl) is also a FQDN


Host = A system connected to a network, for example the internet.


A and AAAA Records

A and AAAA records also known as 'host records' are used to make a (sub)domain point to a specific IP address.


A records are used for IPv4 addresses and AAAA records are used for IPv6.


CNAME Records

A CNAME record or Canonical Name Records can be seen as an 'alias' record. With a CNAME record you can point to an existing A or AAAA record.


For example, you've got a website (example.org) on a host with IP address 198.51.100.5 and you've pointed an A record to it:

(A-record) example.org -> 198.51.100.5

But you also want www.example.org to point to the same host. Of course you can just create another A or AAAA record for this but then you'd have to change both records when you migrate your website. With a CNAME record you can simply refer to the example.org A record and make sure it always gives out the same answer as that record:

(CNAME-record) www.example.org -> (A-record) example.org -> 198.51.100.5

For a CNAME record the resolver does two requests, one for the CNAME and one for the A/AAAA it points to. That means the final answer will be the IP address the A/AAAA record points to.


MX Records

MX or Mail eXchanger records are used to determine which hosts are responsible for receiving email for a domain name.


A MX record should always point to an A- or AAAA record and can not point directly at an IP address. In addition to the responsible host you have to indicate a priority. You can set multiple MX records and they can have differing priorities. Lower values have a higher priority. This is useful for backup mail servers for when you main server fails to accept the email.


It is however not possible to use this to deliver your mail to multiple hosts at once. email only gets delivered to 1 host.


TXT Records

TXT or text records can be used to store arbitrary data in DNS. Usually TXT records are used for domain verification for mail services or for example for Google Webmaster Tools.


TXT records are also used for SPF (Sender Policy Framework) data which is used to determine which hosts are allowed to send email for a domain.


More information on SPF can be found here: SPF Records


SRV Records 

SRV or service records are used by services to provide extra information about location and port number for a service. For example Skype for Business uses this information to determine how to set up a SIP (Session Initiation Protocol) connection.