I quote RFC 1123 Section 2.1 [1] in full: <quote> 2.1 Host Names and Numbers
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.
Whenever a user inputs the identity of an Internet host, it SHOULD
be possible to enter either (1) a host domain name or (2) an IP
address in dotted-decimal ("#.#.#.#") form. The host SHOULD check
the string syntactically for a dotted-decimal number before
looking it up in the Domain Name System.
DISCUSSION:
This last requirement is not intended to specify the complete
syntactic form for entering a dotted-decimal host number;
that is considered to be a user-interface issue. For
example, a dotted-decimal number must be enclosed within
"[ ]" brackets for SMTP mail (see Section 5.2.17). This
notation could be made universal within a host system,
simplifying the syntactic checking for a dotted-decimal
number.
If a dotted-decimal number can be entered without such
identifying delimiters, then a full syntactic check must be
made, because a segment of a host domain name is now allowed
to begin with a digit and could legally be entirely numeric
(see Section 6.1.2.4). However, a valid host name can never
have the dotted-decimal form #.#.#.#, since at least the
highest-level component label will be alphabetic.
</quote>
In summary,
if (4==sscanf(user_input, "%d.%d.%d.%d", a,b,c,d)) {
// Its an dotted quad IP address, do further checks to
// make sure its valid, eg. if (a<256 && b<256 ...)
} else {
// Its a DNS name, and it should supposedly contain some
// alphabet character [a-z].
}
Its not entirely clear to me what this means if its not a FQDN. Eg,
what happens in the example, Debian bug #65041? Since hostname(1) is
getting a local hostname (short format), I guess it should accept any
alphanumeric character sequence (potentially multiple components,
separated by dots).
The only check needed for DNS names, then, is this:
If there are exactly 4 '.' characters in user_input, ensure that
the last component is not entirely numeric.
I don't actually understand where this condition comes from. Seems
bogus to me: who says 4 levels of DNS names can't all be numeric?
--
Justin
aptitude install iraf saods9 eclipse xpa sextractor x11iraf wcstools pyraf
http://www.justinpryzby.com/debian/
References
[1] http://www.faqs.org/rfcs/rfc1123.html
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