Lookup for reserved LDH labels

Vint Cerf vint at google.com
Wed Nov 7 00:52:32 CET 2012

mark is correct. No label with "--" should be allowed except (for the
present) "xn--"

"putative" means that the "--" appears in 3rd and 4th position of the
string - it means the label purports to be the prefix of a U-label.


On Tue, Nov 6, 2012 at 12:01 PM, Mark Davis ☕ <mark at macchiato.com> wrote:

> 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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