CNAME stands for Canonical Name. CNAME records can be used to alias one name to another.
For example, if you have a server where you keep all of your documents online, it might normally be accessed through docs.example.com. You may also want to access it through documents.example.com. One way to make this possible is to add a CNAME record that points documents.example.com to docs.example.com. When someone visits documents.example.com they will see the exact same content as docs.example.com.
Add a CNAME record
To use CNAME records, select CNAME from the Add Record drop down in the advanced editor. Then enter the hostname you would like to alias from and the fully-qualified domain name you would like to alias to. You may also enter @ in the Alias for field to represent the domain itself.
For example, if the domain were example.com and you wanted www.example.com to point to example.com you could put www in the name field and @ in the alias for field.
CNAME records define an alias for canonical name for your server (one defined by an A or AAAA record).
For instance, we could have an A name record defining the "server1" host and then use the "www" as an alias for this host:
server1 IN A 111.111.111.111
www IN CNAME server1
Be aware that these aliases come with some performance losses because they require an additional query to the server. Most of the time, the same result could be achieved by using additional A or AAAA records.
One case when a CNAME is recommended is to provide an alias for a resource outside of the current zone.
Error: “Cannot add a new record where a CNAME exists”
To understand the error, it is important to understand that a CNAME points a whole subdomain to another name in the domain name system. If you have another record on that subdomain you can’t add a CNAME, as that CNAME would render the other records useless. Let’s look at an example:
Let’s assume there is an MX record on email.example.com, and now you try to add a CNAME on that exact subdomain (email.example.com). If you added the CNAME, it would override the subdomain (email.example.com) and render the MX record useless, leading to a lot of potential confusion when email stops. To counter this potential confusion, the domain name system does not allow other records alongside a CNAME.
You can achive a similar behavior as a CNAME with a ALIAS record. Thus, if you want a sub-domain always resolve to the IP address of another domain, you can use a ALIAS record pointing to that domain. You should only do this if you absolutely need it as the ALIAS record does have a small amount of additional overhead when compared to A and CNAME records.
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