A fully qualified domain name (FQDN), sometimes referred to as an absolute domain name, is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain, relative to the root domain. A fully qualified domain name is distinguished by its unambiguity; it can only be interpreted one way.
For example, given a device with a local hostname myhost and a parent domain name example.com, the fully qualified domain name is myhost.example.com. The FQDN therefore uniquely identifies the device — while there may be many hosts in the world called myhost, there can only be one myhost.example.com.
In the DNS, and most notably, in DNS zone files, a FQDN is specified with a trailing dot, for example, "somehost.example.com.". The trailing dot denotes the root domain. Most DNS resolvers will process a domain name that contains a dot as being an FQDN[1] or add the final dot needed for the root of the DNS tree. Resolvers will process a domain name without a dot as unqualified and automatically append the system's default domain name and the final dot.
Some applications, such as web browsers will try to qualify the domain name part of a Uniform Resource Locator (URL) if the resolver cannot find the specified domain. Some applications, however, never use trailing dots to indicate absoluteness, because the underlying protocols require the use of FQDNs, such as e-mail.[2]
References
- ^ Note: On Unix-like systems, this is controlled by the ndots option in the resolv.conf configuration file, specifying the number of dots (default 1) recognized to imply a FQDN. There are some security issues in connection with this interpretation as discussed in RFC 1535.
- ^ https://tools.ietf.org/html/rfc5321#section-2.3.5 -- Definition of domain names in Simple Mail Transfer Protocol
External links
- RFC 1035: Domain names: implementation and specification
- RFC 1123: Requirements for Internet Hosts - application and support
- RFC 1535: A Security Problem and Proposed Correction With Widely Deployed DNS Software
- RFC 2181: Clarifications to the DNS specification
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