Fully qualified domain name

Fully qualified domain name

A fully qualified domain name (or FQDN) is an unambiguous domain name that specifies the exact location in the Domain Name System's tree hierarchy through to a top-level domain and finally to the root domain. Technically, a FQDN has a trailing dot (for example: somehost.example.com.), but most DNS resolvers will treat any domain name that already has a dot as being an FQDN [Note: On Unix-like systems, this is controlled by the ndots option in the resolv.conf configuration file. The default is one dot implying a FQDN, but it can be changed to any number. Security issues with not interpreting a domain name with a dot as a FQDN are discussed in RFC 1535.] and add the final dot needed for the root of the DNS tree. Resolvers will treat a domain name without a dot as unqualified and automatically add a 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 can not find the domain. In other cases, such as DNS zone files, the trailing dot is always required to denote a FQDN. An FQDN differs from a regular domain name by its absoluteness; a default domain name will not be added. Note however that valid hostnames in email are not the same as valid DNS hostnames: email hostnames "never" end with a dot. [http://archives.neohapsis.com/archives/postfix/2008-10/0009.html -- the source of this tidbit] [https://tools.ietf.org/html/rfc5321#section-2.3.5 -- the definition of domain names in email]

For example, given a device with a local hostname of "myhost" and a default parent domain name of "example.com", the fully qualified domain name is "myhost.example.com". It therefore uniquely defines the device — whilst there might be many hosts in the world called "myhost", there can only be one "myhost.example.com".

There are situations in specific implementations of the Domain Name System, in which, for technical reasons, a fully qualified domain name must end with a full stop (dot), indicating the DNS root zone. This ensures that the name is intended to be fully qualified. For example myhost.bar.com could be ambiguous, as it may be just a hostname within another higher level domain, and because not all resolvers assume that domain names containing a dot are absolute. myhost.bar.com., with the dot appended, is an unambigous fully qualified domain name. In most practical uses, the trailing dot is not required.


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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