🚀 go-pugleaf

karrer@ife.ee.ethz.ch (Andreas Karrer) wrote:

>I was quite sure that underscores (_), ascii dec(95) were not allowed
>in host and domain names. If memory serves, BIND in its later versions
>enforces this (unless explicitly overwritten). I'm not aware of
>an RFC that relaxed the host name requirements later than RFC 1123
>(which relaxed the rules such that a non-toplevel domain part may
>begin with a digit). But then, there are a lot of RFCs issued these
>days, and I don't read them all :-)

I investigated this one once, because I wrote some 'intelligent'
scripts which should parse such silly beasts. According to RFC 882 (
DOMAIN NAMES - CONCEPTS and FACILITIES) the parts of domain-names have
to be of the following structure (as perl-regexp)
[a..z][a..z\d-]*[0..9a..z]/i.

But if you go and ask in comp.protocols.tcp-ip.domains, you will get
no consensus, since a lot of people use '_' and '/' in their domain
and hostnames.

The relevant section of RFC 882 says the following (BNF included)

<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

   Note that while upper and lower case letters are allowed in domain
   names no significance is attached to the case.  That is, two names
   with the same spelling but different case are to be treated as if
   identical.

   The labels must follow the rules for ARPANET host names.  They must
   start with a letter, end with a letter or digit, and have as
interior
   characters only letters, digits, and hyphen.  There are also some
   restrictions on the length.  Labels must be 63 characters or less.


-daniel

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