Fully qualified domain name
A fully qualified domain name (FQDN), sometimes also referred 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 and the root zone. 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 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 that 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 process a domain name that contains a dot in any position as being fully qualified[note 1] or add the final dot needed for the root of the DNS tree. Resolvers 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, 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).[2]
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[edit] The root of the tree and DNS
As a special case to the FQDN, a single dot should represent the root of the directory tree, which in turn would mean that a hypothetical computer system would be called the root server. There is no such thing as a root server on the global Internet, however, since there is no A record for the "." domain.[3]
There are 13 authoritative root nameservers which contain the DNS records for root name lookups. Each name server knows the IP addresses of the name servers of first or "top level domains" (TLDs). The "com." and "uk." domains are TLDs.
Just like the root domain, most TLDs do not resolve to an IP address, but usually have three or more distinct name servers which answer queries for the TLD. (e.g. There is no server known by the FQDN "net." nor "uk." but there are 13 name servers listed for "net." and 11 name servers "uk.")[4]
An example of a TLD which resolves is "uz.", meaning that the .uz domain is an example of the shortest resolving FQDN with a URL of http://uz/ for web access and is notable because no dot appears in the URL. Due to the scarcity of domains without a dot, not all browsers will permit this to work[citation needed].
[edit] Notes
- ^ Note: On Unix-like systems, this is controlled by the
ndotsoption 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.
[edit] References
- ^ RFC 1035, Domain names: implementation and specification
- ^ Definition of domain names in Simple Mail Transfer Protocol
- ^ Root Zone Text file, retrieved on 4/12/2009
- ^ Root Zone Text file, retrieved on 4/12/2009
[edit] External links
- 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
- RFC 2826: IAB Technical Comment on the Unique DNS Root
- http://uz/, a URL with no dot because it is a top-level domain
