Apex domains cannot have CNAME records.
Question and response
Question
dig sdtnet.de. NS
Answer
sdtnet.de. 5 NS ns2.sdtnet.de.
sdtnet.de. 5 NS ns1.sdt.net.
Authority
The response has no authority records.
Additional
ns1.sdt.net. 5 A 194.88.160.11
ns2.sdtnet.de. 5 A 194.88.168.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0