What is a DNS CNAME (Canonical Name) ?

About

A CNAME (Canonical Name) is a type of DNS record that maps one domain name to another name.

It brings a Domain Name Alias functionality (synonym).

It's also known as an alias record.

When a DNS resolver encounters a CNAME record:

• it will query the server again using the result of the CNAME as input
• until it found an IP.

Unicity

CNAMEs are, according to DNS standards, meant to be the only record in a host's DNS records. Therefore, no other record type can be set with the same name.

It means that you shouldn't create a CNAME for your apex domain because the apex domain is generally also set:

• in the MX record in order to receive email
• or other validation TXT records (SPF, …)

Example

On the below domain data, we see:

• a CNAME record
• and a A record

NAME                    TYPE   VALUE
--------------------------------------------------
bar.example.com.        CNAME  foo.example.com.
foo.example.com.        A      192.0.2.23

When a DNS query is performed against bar.example.com, the resolver:

• will get a CNAME record with the value foo.example.com
• will query the domain table again with the value foo.example.com and get an A record
• and will return the IP 192.0.2.23

SEO

To redirect to a name to a server (known as a custom domain), you may use:

• a CNAME
• or a A or AAAA record , (directly linked to the IP addresses)

By using a CNAME, the server is not considered as a 1st party server by search engine ¹⁾. The script and cookies could be considered as third-party.

Usage

• Custom Tracking Domain

Specification

DNS CNAME records are specified in search/rfc_search_detail.php and clarified in Section 10 of RFC 2181.

²⁾