On Fri, Jul 03, 2026 at 01:51:07AM +0200, Gerald Galster via Postfix-users
wrote:
> A common tool for conversion is idn2:
>
> $ idn2 vestskovsløbet.dk
> xn--vestskovslbet-knb.dk
>
> $ idn2 -d xn--vestskovslbet-knb.dk
> vestskovsløbet.dk
If you need an IDNA2008 power-tool, I offer my "idnaparse" utility, it
actually adheres to IDNA2008, adding optional support for mappings, emoji,
mixing in specific forms of non-ldh ASCII labels, debugging malformed
u-labels, ...
https://hackage.haskell.org/package/idnaparse-1.0.0.0
To install "cabal install idnaparse". Sample output:
$ printf "%s\n" "vestskovsløbet.dk" | idnaparse | jq -r .presentation
xn--vestskovslbet-knb.dk
$ printf "%s\n" "xn--vestskovslbet-knb.dk" | idnaparse | jq -r .output.text
vestskovsløbet.dk
For a label-by-label view:
$ ( printf "%s\n" "_domainkey.vestskovsløbet.dk" \
"0----0.xn--vestskovslbet-knb.dk" \
"\\027[H\\027[2J.vestskovsløbet.dk" ) |
idnaparse --forms +attr,rldh | jq -rc '... extract essential details
...'
_domainkey.vestskovsløbet.dk
["ATTRLEAF","ULABEL","LDH"]
_domainkey.xn--vestskovslbet-knb.dk
["ATTRLEAF","ULABEL","LDH"]
_domainkey.vestskovsløbet.dk
0----0.xn--vestskovslbet-knb.dk
["RLDH","ALABEL","LDH"]
0----0.xn--vestskovslbet-knb.dk
["RLDH","ULABEL","LDH"]
0----0.vestskovsløbet.dk
\027[H\027[2J.vestskovsløbet.dk
{"reason":"FormNotAllowed","label":0,"form":"OCTET"}
> Does the receiver's MUA transparently convert domain names?
One thing to be aware of is that DKIM signers are prone to "normalising"
IDNA domains in headers changing explicit A-label forms back to U-label
forms, perhaps in conformance with some RFC text, but as a result making
the mail undeliverable in some cases. :-(
So you can try to bypass SMTPUTF8 issues when mailing "приклад.example"
by using the A-label form "xn--80aikifvh.example", but the DKIM signer
will helpfully change it back to "приклад.example".
--
Viktor. 🇺🇦 Слава Україні!
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