Apex domains cannot have CNAME records.
Question and response
Question
dig 1und1.net. NS
Answer
1und1.net. 5 NS ns04.versatel.de.
1und1.net. 5 NS ns02.versatel.de.
1und1.net. 5 NS ns01.versatel.de.
Authority
The response has no authority records.
Additional
ns01.versatel.de. 5 A 212.7.128.162
ns02.versatel.de. 5 A 212.7.128.165
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0