Are IDN domain names case-sensitive?

For old-fashioned ASCII-based domain names, Yes, domain names have been and continue to be case-insensitive.

To quote RFC 1035, DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION:

Note that while upper and lower case letters are allowed in domain names, no significance is attached to the case. That is, two names with the same spelling but different case are to be treated as if identical.

For example, all of these represent the same domain:

• example.com
• Example.com
• EXAMPLE.COM
• EXampLE.com

In modern DNS, we now have Internationalized Domain Names (IDN) which allows Unicode characters. The problem is that defining upper- and lowercase can be tricky in some languages and character sets beyond ASCII (Unicode is a superset of US-ASCII).

The intent of domain names is to be case-insensitive, but there may be complications with particular characters in particular scripts of particular human languages. So there is no simple YES or NO answer to your question.

If using non-ASCII domain names, you should read:

• Internationalized domain name on Wikipedia
• Domain Name System (DNS) Case Insensitivity Clarification Official spec (IETF RFC 4343)