Lookup for reserved LDH labels

Andrew Sullivan ajs at anvilwalrusden.com
Wed Nov 7 14:34:06 CET 2012

On Wed, Nov 07, 2012 at 02:18:37PM +0100, Marcos Sanz wrote:
> Then it will not be difficult for somebody to point us to the relevant 
> part in the spec that allegedly states that the lookup algorithm must fail 
> for reserved LDH-labels that are not XN-labels (e.g. "ad--acta"). I'd 
> really appreciate if somebody did that.

   Definitions document [RFC5890].  Putative U-labels with any of the
   following characteristics MUST be rejected prior to DNS lookup:

   o  Labels that are not in NFC [Unicode-UAX15].

   o  Labels containing "--" (two consecutive hyphens) in the third and
      fourth character positions.

This is in section 5.4.  Because of the way that section is
structured, those labels are (at the time of evaluation given the
protocol definition) actually putative U-labels.


Andrew Sullivan
ajs at anvilwalrusden.com

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