Lookup for reserved LDH labels

Mark Davis ☕ mark at macchiato.com
Tue Nov 6 18:01:08 CET 2012

Thanks for bringing this up.

My understanding of the spec was that the intention is to permanently
reserve anything of the form "xy--..." (where x and y are in [a-z0-9]. If
that is not a requirement, we'd be glad to change our implementation.

Mark <https://plus.google.com/114199149796022210033>
*— Il meglio è l’inimico del bene —*

On Tue, Nov 6, 2012 at 7:43 AM, Marcos Sanz <sanz at denic.de> wrote:

> Dear all,
> 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
> 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 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
> Thus, the protocol should never fail on lookup for "ad--acta.de". Is that
> correct?
> Best regards,
> Marcos
> * FWIW idnkit-2.2 works according to my expectations, ICU does not.
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