On 2/22/13, Jay Ashworth <jra@baylink.com> wrote: RFC103 5.1 is correct in the context of a DNS zonefile. In other contexts, however, a domain is absolute without a trailing dot. One example, would be in the case of the SMTP protocol, where hostnames are required to _always_ be absolute. In various common contexts, a domain is always either fully qualified, or not valid. Sometimes a trailing dot is allowed, and in some protocols, a trailing dot is not allowed; however the domain used is still called a FQDN; it's just different syntax, for a fqdn, with minor variations.. A trailing dot is not included in the domain portion of an e-mail address, however within the context of nobody@example.com; example.com is understood to be a fully qualified domain. Nothing else really makes sense; "example.com" is absolute and not relative in this context.. It is also true in the context of a http URL scheme http://www.example.com/ In that context, the www.example.com is a fully qualified domain; although some browsers might try appending other suffixes, as an aid to the user, if the domain cannot be found. No trailing dot allowed; "each domain label starting and ending with an alphanumerical character"; The URL is the most common context where a fully qualified domain would be encountered, e-mail addresses and URLs are the most common case where the average network user will encounter a domain name. For the sake of consistency, if something is considered a FQDN in a URL and in a SMTP hostname or e-mail address, then it ought to be made to be considered a fully qualified domain, everywhere. " Berners-Lee, Masinter & McCahill [Page 5] RFC 1738 Uniform Resource Locators (URL) December 1994 host The fully qualified domain name of a network host, or its IP address as a set of four decimal digit groups separated by ".". Fully qualified domain names take the form as described in Section 3.5 of RFC 1034 [13] and Section 2.1 of RFC 1123 [5]: a sequence of domain labels separated by ".", each domain label starting and ending with an alphanumerical character and possibly also containing "-" characters. The rightmost domain label will never start with a digit, though, which syntactically distinguishes all domain names from the IP addresses. "
The authoritative document here is, as Joe Abley noted earlier, RFC 1035, which says, in section 5.1:
""" 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; the actual domain name is the concatenation of the relative part with an origin specified in a $ORIGIN, $INCLUDE, or as an argument to the master file loading routine. A relative name is an error when no origin is available. """
Jay R. Ashworth Baylink -- -JH