Unconditional punycode conversion

Simon Josefsson simon at josefsson.org
Wed Mar 9 18:13:57 CET 2011

Andrew Sullivan <ajs at shinkuro.com> writes:

> On Wed, Mar 09, 2011 at 05:36:10PM +0100, Simon Josefsson wrote:
>> To verify my understanding: the label "ab--cd" is permitted by IDNA2008
>> despite it having "--" in the third and fourth characater positions?
>> That would be because section 5.4 only applies to non-ascii labels.
> No.  See section 2.3.1 of RFC 5890.

I don't see any MUST/SHOULD language there.  RFC 5891 says:

   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.

Is "ab--cd" a putative U-label?


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