Apex domains cannot have CNAME records.
Question and response
Question
dig ibs-bw.de. NS
Answer
ibs-bw.de. 5 NS dns3.belwue.de.
ibs-bw.de. 5 NS dns3.uni-tuebingen.de.
ibs-bw.de. 5 NS dns4.uni-tuebingen.de.
ibs-bw.de. 5 NS dns1.belwue.de.
Authority
The response has no authority records.
Additional
dns1.belwue.de. 5 A 129.143.2.10
dns3.belwue.de. 5 A 129.143.253.133
dns1.belwue.de. 5 AAAA 2001:7c0:0:0:0:0:53:10
dns3.belwue.de. 5 AAAA 2001:7c0:0:253:0:0:0:133
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0