Re: [idn] Which names are valid? (was How should labels be encoded?)

["Eric A. Hall" <ehall@ehsco.com>]

> This raises the question:  Is _ldap._tcp.example.com a domain?

It is a domain name associated with one or more resource records.

> Or is it a creature like the mailbox field of SOA records, a sequence
> of labels that does not name a domain?

Email addresses are stored as data in a resource record. They use the
domain name syntax so they can be compressed in the DNS message, but do
not otherwise fall under the purview of any domain name rules (an email
address can exist as DNS data, be a completely invalid domain name, and
still exist -- just not as a piece of data that is probably useful).

> Perhaps nameprep is not the right layer for the non-LDH ASCII
> prohibition.  Maybe that belongs at a higher layer, the same layer
> that checks for beginning/ending hyphens.

That is the application layer.

Hostnames have naming prohibitions due to the legacy of RFCs 972/1123 (the
old HOSTS.TXT database naming spec). Domain names in particular do not
have these prohibitions, but instead, applications apply (or ignore) the
hostname prohibitions when they have to use the hostname for a lookup
operation. EG, since the hostname could exist in /etc/hosts or in DNS, the
"hostname rules" apply regardless of where the data is stored or the
mechanism used to retrieve it.

Applications will continue to have these rules until they choose to extend
their specific ruleset.

-- 
Eric A. Hall                                        http://www.ehsco.com/
Internet Core Protocols          http://www.oreilly.com/catalog/coreprot/