[dns-operations] xn--

[Andrew Sullivan]

Hi,

On Thu, Jun 26, 2025 at 08:37:56AM -0500, 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?

I'm not sure whether this strictly answers the question you were trying to ask, but https://mailarchive.ietf.org/arch/msg/idn/E75mJULaEHFGH8_V8qJN-glLcfg/ gives you the rules for how the selection was to be performed and what the result was from IANA.

I dimly recalled an idea to create a protocol parameters registry for [a-z][a-z]-- label prefixes in the DNS when we were doing IDNABIS or PRECIS (I forget which); but I did a quick scan of the IANA index for this stuff and didn't find one.  That doesn't mean it's not there, but I suspect the reservation of those characters is exclusively in the RFCs.  I suspect to get a similar kind of reservation, you'd need a pretty compelling case.  In-band signalling like this for applications is in general a fairly crummy option (though maybe the only one available, as in the case of IDNA), and I think mutliplying the occasions on which the A-label form of stuff will inevitably be exposed to users is bound to attract some opposition.

Best regards,

A

-- 
Andrew Sullivan
ajs at anvilwalrusden.com