A fully qualified domain name (FQDN), sometimes referred to as an absolute domain name[1], 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 this absoluteness in the name space.
For example, given a device with a local hostname myhost and a parent domain name example.com, the fully qualified domain name is written as myhost.example.com. This fully qualified domain name therefore uniquely identifies the host — while there may be many resources in the world called myhost, there is only one myhost.example.com.
In the Domain Name System, and most notably, in DNS zone files, a fully qualified domain name is specified with a trailing dot. For example,
somehost.example.com.
specifies an absolute domain name which ends with an empty top level domain label.
The DNS root domain is unnamed, which is expressed by an empty label, resulting in a domain name ending with the dot separator. However, many DNS resolvers will process a domain name that contains a dot in any position as being fully qualified[2] 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 resolve the domain name part of a Uniform Resource Locator (URL) if the resolver cannot find the specified domain or if it is clearly not fully qualified by appending frequently used top-level domains and testing the result. Some applications, however, never use trailing dots to indicate absoluteness, because the underlying protocols require the use of FQDNs, such as Simple Mail Transfer Protocol (e-mail).[3]
References
- ^ From RFC 1035. "Domain names that end in a dot are called absolute, and are taken as complete. Domain names which do not end in a dot are called relative […] A relative name is an error when no origin is available."
- ^ 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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