What is a DNS CNAME record?

A "canonical name" (CNAME) record points from an alias domain to a "canonical" domain. A CNAME record is used in lieu of an A record, when a domain or subdomain is an alias of another domain. All CNAME records must point to a domain, never to an IP address. Imagine a scavenger hunt where each clue points to another clue, and the final clue points to the treasure. A domain with a CNAME record is like a clue that can point you to another clue (another domain with a CNAME record) or to the treasure (a domain with an A record).

For example, suppose blog.example.com has a CNAME record with a value of "example.com" (without the "blog"). This means when a DNS server hits the DNS records for blog.example.com, it actually triggers another DNS lookup to example.com, returning example.com’s IP address via its A record. In this case we would say that example.com is the canonical name (or true name) of blog.example.com.

Oftentimes, when sites have subdomains such as blog.example.com or shop.example.com, those subdomains will have CNAME records that point to a root domain (example.com). This way if the IP address of the host changes, only the DNS A record for the root domain needs to be updated and all the CNAME records will follow along with whatever changes are made to the root.

A frequent misconception is that a CNAME record must always resolve to the same website as the domain it points to, but this is not the case. The CNAME record only points the client to the same IP address as the root domain. Once the client hits that IP address, the web server will still handle the URL accordingly. So for instance, blog.example.com might have a CNAME that points to example.com, directing the client to example.com’s IP address. But when the client actually connects to that IP address, the web server will look at the URL, see that it is blog.example.com, and deliver the blog page rather than the home page.

Example of a CNAME record:

In this example you can see that blog.example.com points to example.com, and assuming it is based on our example A record we know that it will eventually resolve to the IP address 192.0.2.1. 

Can a CNAME record point to another CNAME record?

Pointing a CNAME record to another CNAME record is inefficient because it requires multiple DNS lookups before the domain can be loaded — which slows down the user experience — but it is possible. For example, blog.example.com could have a CNAME record that pointed to www.example.com's CNAME record, which then pointed to example.com's A record.

CNAME for blog.example.com:

Which points to a CNAME for www.example.com:

This configuration adds an extra step to the DNS lookup process and should be avoided if possible. Instead, the CNAME records for both blog.example.com and www.example.com should point directly to example.com.

What restrictions are there on using CNAME records?

MX and NS records cannot point to a CNAME record; they have to point to an A record (for IPv4) or an AAAA record (for IPv6). An MX record is a mail exchange record that directs email to a mail server. An NS record is a "name server" record and indicates which DNS server is authoritative for that domain.