[6bone] Re: reverse 6dns painful (was Re: reverse DNSconsideredpointless)

[Ben Winslow]

On Sat, 2004-02-14 at 07:48, Gert Doering wrote:
> Good point.  Now the interesting question is - are the authors of RFC2782
> aware that they violate DNS host name requirements, or are they even
> doing it *on purpose*, so that...

A while ago, while researching something else, I came upon the following
tidbits:

RFC 1033 § NAMES:
   The domain system allows a label to contain any 8-bit character.
   Although the domain system has no restrictions, other protocols such
   as SMTP do have name restrictions.  Because of other protocol
   restrictions, only the following characters are recommended for use
   in a host name (besides the dot separator):

           "A-Z", "a-z", "0-9", dash and underscore

RFC 1034 § 3.1:
When you receive a domain name or label, you should preserve its case. 
The rationale for this choice is that we may someday need to add full
binary domain names for new services; existing services would not be
changed.

RFC 1034 § 3.5 suggests the [A-Za-z0-9] recommendation again.

Basically, DNS itself has no standardized restrictions on label names
(that I know about), although it's standard practice (and a good one) to
use a restricted character set, because we know there are broken
resolvers in use, and protocols which had previous requirements for
hostnames a lá HOSTS.TXT.

In the present-day situation, I would wager that most applications will
deal gracefully with any 8-bit character in DNS labels, but there are
definitely applications and resolvers still in use that do not.  I have
not seen an application that outright *breaks* if an non-recommended
printable char (like / or _) is used in a hostname in quite some time
(the closest contender being Squid, which has (had?) a configuration
option to make it fail on such hostnames.

-- 
Ben Winslow <rain at bluecherry.net>
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