DNS - CNAME (Canonical Name) record (Alias)

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.

You can achieve the same alias functionality with the alias record but faster because there is only one query needed.

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:

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

Specification

DNS CNAME records are specified in RFC 1034 and clarified in Section 10 of RFC 2181.

2)


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