Apex domains cannot have CNAME records.
Question and response
Question
dig sdTneT.de. NS
Answer
sdTneT.de. 4 NS ns1.sdt.net.
sdTneT.de. 4 NS ns2.sdTneT.de.
Authority
The response has no authority records.
Additional
ns1.sdt.net. 2 A 194.88.160.11
ns2.sdtnet.de. 4 A 194.88.168.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0