A Canonical Name record (abbreviated as CNAME record) is a type of resource record in the Domain Name System (DNS) which maps one domain name (an alias) to another (the Canonical Name.)
This can prove convenient when running multiple services (like an FTP server and a web server, 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.
NAME TYPE VALUE -------------------------------------------------- bar.example.com. CNAME foo.example.com. foo.example.com. A 192.0.2.23
- CNAME records must always be pointed to another domain name, never to an IP address.
- An alias defined in a CNAME record must have no other resource records of other types (MX, A, etc.). (RFC 1034 section 3.6.2, RFC 1912 section 2.4) The exception is when DNSSEC is being used, in which case there can be DNSSEC related records such as RRSIG, NSEC, etc. (RFC 2181 section 10.1)
- CNAME records that point to other CNAME records should be avoided due to their lack of efficiency, but are not an error. It is possible, then, to create unresolvable loops with CNAME records.