Domain name alias

A Canonical Name record (abbreviated as CNAME record) is a type of resource record in the Domain Name System (DNS) used to specify that a domain name is an alias for another domain, which is the "canonical" domain.

This can prove convenient when running multiple services (like an FTP server and a webserver; each running on different ports) from a single IP address. One can, for example, point ftp.example.com and www.example.com to the DNS entry for example.com, which in turn has an A record which points to the IP address. Then, if the IP address ever changes, one only has to record the change in one place within the network: in the DNS A record for example.com.

CNAME records must always point to another domain name, never directly to an IP address.

DNS CNAME records are specified in RFC 1034 and clarified in Section 10 of RFC 2181.

CNAME records are handled specially in the domain name system, and have several restrictions on their use. When a DNS resolver encounters a CNAME record while looking for a regular resource record, it will restart the query using the canonical name instead of the original name. (If the resolver is specifically told to look for CNAME records, the canonical name (right-hand side) is returned, rather than restarting the query.) The canonical name that a CNAME record points to can be anywhere in the DNS, whether local or on a remote server in a different DNS zone.

For example, if there is a DNS zone as follows:

when an A record lookup for bar.example.com is carried out, the resolver will see a CNAME record and restart the checking at foo.example.com and will then return 192.0.2.23.

With a CNAME record, one can point a name such as "bar.example.com" to "foo.example.com." Because of this, during casual discussion the "bar.example.com." (left-hand) side of a DNS entry can be incorrectly identified as "the CNAME" or "a CNAME." However, this is inaccurate. The canonical (true) name of "bar.example.com." is "foo.example.com." Because CNAME stands for Canonical Name, the right-hand side is the actual "CNAME."

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