Apex domains cannot have CNAME records.
Question and response
Question
dig plusnet.de. NS
Answer
plusnet.de. 2 NS ns3.as20676.net.
plusnet.de. 2 NS ns1.plusnet.de.
plusnet.de. 2 NS ns2.plusnet.de.
Authority
The response has no authority records.
Additional
ns1.plusnet.de. 2 A 62.206.0.1
ns2.plusnet.de. 2 A 212.5.0.1
ns3.as20676.net. 2 A 91.107.202.41
ns1.plusnet.de. 2 AAAA 2001:1a80:0:0:0:0:0:53
ns2.plusnet.de. 2 AAAA 2a09:7100:0:0:0:0:0:53
ns3.as20676.net. 2 AAAA 2a01:4f8:c17:3a18:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0