ALPHA DIGIT ALPHA DIGIT - ALPHA DIGIT
_ _ を含める困った無料鯖,
_ も名前文字に仲間入りするのかなあ?
1. A "name" (Net, Host, Gateway, or Domain name) is a text string up to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus sign (-), and period (.). Note that periods are only allowed when they serve to delimit components of "domain style names". (See RFC-921, "Domain Name System Implementation Schedule", for background). No blank or space characters are permitted as part of a name. No distinction is made between upper and lower case. The first character must be an alpha character. The last character must not be a minus sign or period. A host which serves as a GATEWAY should have "-GATEWAY" or "-GW" as part of its name. Hosts which do not serve as Internet gateways should not use "-GATEWAY" and "-GW" as part of their names. A host which is a TAC should have "-TAC" as the last part of its host name, if it is a DoD host. Single character names or nicknames are not allowed.
<hname> ::= <name>*["."<name>]
<name> ::= <let>[*[<let-or-digit-or-hyphen>]<let-or-digit>]<domain> ::= <subdomain> | " " <subdomain> ::= <label> | <subdomain> "." <label> <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ] <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str> <let-dig-hyp> ::= <let-dig> | "-" <let-dig> ::= <letter> | <digit> <letter> ::= any one of the 52 alphabetic characters A through Z in upper case and a through z in lower case <digit> ::= any one of the ten digits 0 through 9
The syntax of a legal Internet host name was specified in RFC-952 [DNS:4]. One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit. Host software MUST support this more liberal syntax.
Host software MUST handle host names of up to 63 characters and SHOULD handle host names of up to 255 characters.
label = (ALPHA / DIGIT) [*61(ALPHA / DIGIT / HYPHEN) (ALPHA / DIGIT)]
ALPHA = %x41-5A / %x61-7A ; "A"-"Z" / "a"-"z"
DIGIT = %x30-39 ; "0"-"9"
HYPHEN = %x2D ; hyphen ("-")
The confusion was thus left as to whether the "preferred" name syntax
was a mandatory restriction in DNS, or merely "preferred".
The syntax for domain and host names described in this document MUST
conform to [RFC0952] and [RFC1123]. At the time of this writing, RFC
3490 [RFC3490] describes a standard to use certain ASCII name labels
to represent non-ASCII name labels.
Host name: This term and its equivalent, "hostname", have been
widely used but are not defined in [RFC1034], [RFC1035],
[RFC1123], or [RFC2181]. The DNS was originally deployed into the
Host Tables environment as outlined in [RFC952], and it is likely
that the term followed informally from the definition there. Over
time, the definition seems to have shifted. "Host name" is often
meant to be a domain name that follows the rules in Section 3.5 of
[RFC1034], the "preferred name syntax". Note that any label in a
domain name can contain any octet value; hostnames are generally
considered to be domain names where every label follows the rules
in the "preferred name syntax", with the amendment that labels can
start with ASCII digits (this amendment comes from Section 2.1 of
[RFC1123]).
People also sometimes use the term hostname to refer to just the
first label of an FQDN, such as "printer" in
"printer.admin.example.com". (Sometimes this is formalized in
Hoffman, et al. Informational [Page 4]
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RFC 7719 DNS Terminology December 2015
configuration in operating systems.) In addition, people
sometimes use this term to describe any name that refers to a
machine, and those might include labels that do not conform to the
"preferred name syntax".
Hostnames MUST conform to the Domain Name System (DNS) syntax
defined in [RFC1034] and [RFC1123]. Internationalized Domain
Names (IDNs) must first be transformed to the A-label form
[RFC5890] as per [RFC5891].