Github Pages does not work properly with cyrillic custom domain

Question

So, I am trying to setup custom domain on github pages and my domain contains cyrillic letters, and I can't add custom domain:

But with english letters all ok:

Answer 1

You need to follow the IDNA convention:

To prevent non-international domain names containing hyphens from being accidentally interpreted as Punycode, international domain name Punycode sequences have a so-called ASCII Compatible Encoding (ACE) prefix, "xn--", prepended.[2] Thus the domain name "bücher.tld" would be represented in ASCII as "xn--bcher-kva.tld".

Using python you could try encode('idna') to find what characters to use, for example:

$ python
>>> x = u'bücher.tld'
>>> x.encode('idna')
b'xn--bcher-kva.tld'
>>> x.encode('idna').decode('idna')
'bücher.tld'