Lookup for reserved LDH labels
sanz at denic.de
Tue Nov 6 16:43:02 CET 2012
I had this small discussion with Mark and Markus and, despite our treefold
homonymy, we couldn't get to common ground, so I've decided to get a
reading of the standard directly from the IDNA2008 editors.
According to my interpretation (cf. RFC 5891, Section 5) the lookup
protocol relies on the assumption that names that are already present in
the DNS are valid. And, in fact, I have a bunch of domains in my database
with hyphens in the third and fourth position, so-called reserved LDH
labels that are not XN-labels (s. nomenclature in RFC 5890, Section
184.108.40.206). Take for instance "ad--acta.de". My expectation would be that
the protocol doesn't fail on those*. Mark however reminded me of the
restrictions in RFC 5891, Section 220.127.116.11. But those are for the
registration, which I am not interested in at the moment. If at all
relevant, we'd have Section 5.4:
"Putative U-labels with any of the following characteristics MUST be
rejected prior to DNS lookup:
o Labels containing '--' (two consecutive hyphens) in the third and
fourth character positions."
On my side, I claim that that restriction simply does not apply because
"ad--acta.de" is not a "putative U-label", in fact it is no U-label at all
(cf. U-Label definition in RFC 5890, Section 18.104.22.168).
Thus, the protocol should never fail on lookup for "ad--acta.de". Is that
* FWIW idnkit-2.2 works according to my expectations, ICU does not.
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