DNS labels are limited to some ASCII characters. UTF support in DNS is available via punycode which is an encoding from UTF onto that restricted ASCII set accep

DNS labels are limited to some ASCII characters. UTF support in DNS is available via punycode which is an encoding from UTF onto that restricted ASCII set acceptable for DNS. Libraries as the ones discussed here typically perform UTF to punycode conversions before doing any label comparisons to ensure accuracy. In particular this tolower implementation would likely be used against the punycode encoded version of the UTF domain. https://en.wikipedia.org/wiki/Punycode https://en.wikipedia.org/wiki/Punycode