On 2/21/13, Mark Andrews <marka@isc.org> wrote:
RFC 952 as modified by RFC 1123 describe the legal syntax of a hostname. There is no trailing period.
A hostname is not a domain name, the hostname is just a label, and has stricter syntax than is allowed in a DNS label; however: When hostnames are represented in DNS, they have corresponding domain names. eg for A.example.com The domain name is unqualified if it contains just the hostname "A". It is partially qualified, if a subset of the labels are provided "A.example" The names are called fully qualified, when the domain name shown is the complete DNS name, with all labels; "A.example.com" In the DNS, the implicit trailing dot is understood to be part of the domain name. Technically "A.example.com" without a trailing dot is unqualified, for purposes of DNS resolution; if a DNS resolver receives NXDOMAIN for A.example.com; some resolvers will normally search for A.example.com.suffix next However, it is nevertheless commonly referred to as fully-qualified, with or without the trailing dot, even though syntactically it could be unqualified; because ".COM" is such a well-known TLD . In this case, it doesn't actually matter what the RFCs call a FQDN; it's overridden by common usage of the phrase/acronym (It is commonly understood that no trailing dot is required, except in the context of a zone file). There is little understanding about qualification of hostnames, and DNS resolver search, and these concepts should probably just go away / be simplified, so all valid lookup names are FQDNs or local hostnames with no dots.
-- Brian Reichert <reichert@numachi.com> BSD admin/developer at large
-- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: marka@isc.org
-- -JH