[dns-operations] xn--

[Viktor Dukhovni]

On Thu, Jun 26, 2025 at 08:37:56AM +0530, Gihan Dias wrote:

> Could anyone point me at a description of how xn-- was chosen for the prefix
> for idn domains, and how we could register a similar prefix for another
> application?

The text specifying XN-labels as a subset of R-LDH labels can be found in:

    https://datatracker.ietf.org/doc/html/rfc5890#section-2.3.1

I am not aware of any extant registration process for defining additional
similar prefixes, this would probably require starting a new
specification in "dnsop".  It is perhaps worth highlighting one of
the paragraphs in the linked RFC section:

   Labels within the class of R-LDH labels that are not prefixed with
   "xn--" are also not valid IDNA labels.  To allow for future use of
   mechanisms similar to IDNA, those labels MUST NOT be processed as
   ordinary LDH labels by IDNA-conforming programs and SHOULD NOT be
   mixed with IDNA labels in the same zone.

-- 
    Viktor.