Lookup for reserved LDH labels

Marcos Sanz sanz at denic.de
Tue Nov 6 16:43:02 CET 2012

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 

Best regards,

* FWIW idnkit-2.2 works according to my expectations, ICU does not.

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