supdns

CNAME Records

Aliasing one domain name to another for easier management and flexibility.

What is a CNAME Record?

A CNAME record (Canonical Name record) is a type of DNS record that maps an alias name to a true or canonical domain name. CNAME records are typically used to map a subdomain such as www or mail to the domain hosting that subdomain's content.

Structure of a CNAME Record

www.example.com. 3600 IN CNAME example.com.
  • Host: The alias (e.g., www).
  • TTL (Time to Live): How long (in seconds) the record should be cached.
  • Type: CNAME.
  • Value: The canonical domain name it points to.

Crucial Restriction: The Apex Limitation

According to DNS RFCs, a CNAME record cannot coexist with any other record for the same name. Because the root (apex) of a domain (e.g., example.com) must have SOA and NS records, you cannot have a CNAME record at the root of a domain.

Solution: Use an ALIAS or ANAME record if your DNS provider supports it.

How CNAME Resolution Works

When a DNS resolver finds a CNAME record, it restarts the lookup process using the canonical name instead of the original name.

  1. User requests www.mysite.com.
  2. Resolver finds www.mysite.com CNAME mysite.com.
  3. Resolver then looks up mysite.com (the canonical name).
  4. Resolver finds mysite.com A 1.2.3.4 and returns the IP to the user.

Common Use Cases

  • Pointing www to Root: Mapping www.example.com to example.com.
  • Cloud Services: Pointing your domain to a service like Heroku, Shopify, or a CDN (e.g., mysite.github.io).
  • Traffic Management: Moving services between different hostnames without changing client configurations.