DNS CNAME Records

DNS CNAME records, also known as aliases, indicate a hostname whose IP address is the same as another hostname. The response to a CNAME query is another hostname requiring a second DNS query for that new hostname to determine the IP.

In plain English, the DNS server is telling you "Hey! The IP address of this host is the same as this other host so you'll need to go look that host up."

E.g.
Here we can see www.dnsmonks.com is a CNAME of dnsmonks.com.

% dig www.dnsmonks.com CNAME +short
dnsmonks.com.
%
If we further queried for the IP address of dnsmonks.com:

% dig dnsmonks.com +short
18.188.64.96
%
We can see that the final IP address of www.dnsmonks.com is 18.188.64.96.

Unlike A records, CNAMEs cannot have more than one record associated with a particular hostname or owner. I could not for example have an A record for www.dnsmonks.com and a CNAME for www.dnsmonks.com. This introduces interesting hurdles for the apex of a zone (e.g. dnsmonks.com). Because the apex of a zone always contains at least one NS record and one SOA record, you cannot have (without some behind the scenes magic) a CNAME at the apex of a zone.

In practice, DNS is typically smart enough to return a CNAME even if you request an A record where only a CNAME exists.

% dig www.dnsmonks.com +short
dnsmonks.com.
18.188.64.96
%
Here, we didn't specify a CNAME query in our dig command. We didn't actually specify which kind of record we wanted which means dig defaulted to querying for an A record. But you can see it figured out that there was a CNAME there and even went ahead and looked up that new hostname's A record to get the final IP.

It is possible to chain CNAMEs. You could for example:

dnsmonks.com A 18.188.64.96
www.dnsmonks.com CNAME dnsmonks.com.
www2.dnsmonks.com CNAME www.dnsmonks.com.

This would essentially generate three different DNS lookups. Assuming your DNS query response times are quick, this wouldn't be the end of the world. However generally speaking, you want to avoid CNAME chaining if possible.