CNAME

A CNAME, or canonical name record, is a type of DNS resource record used to create an alias from one domain name to another. When a DNS resolver looks up the alias, it is instructed to replace the alias name with the target name and continue resolution to obtain the target’s A or AAAA records. In this way, multiple domain names can point to the same canonical host.

Constraints and behavior: A CNAME record must be the only data for its domain name. A name with a CNAME cannot have any other records (such as A, MX, TXT) at that label. As a result, CNAMEs are not used at the zone apex (root) where other records are required by the zone’s authority, and many DNS providers prohibit or restrict apex CNAMEs. A CNAME must point to another domain name, not to an IP address, and DNS resolvers may follow chains of CNAMEs until they reach a non-CNAME target. Circular references are avoided by resolver logic and can result in resolution failures.

Common uses: CNAME records are often employed to alias a domain to a service provider’s host name, such as pointing www.example.com to a cdn provider or cloud-hosted domain. They facilitate seamless changes of the underlying service by updating only the target of the CNAME, not the alias’s own records.

Limitations and considerations: Using CNAMEs can affect DNS caching and TLS configurations, since the final target’s IPs or certificates come into play. Some services require the apex to resolve without a CNAME and instead use ANAME/ALIAS or URL-based redirects.