Apex domains cannot have CNAME records.
Question and response
Question
dig dfn.de. NS
Answer
dfn.de. 5 NS ns3.dfn.de.
dfn.de. 5 NS ns1.dfn.de.
dfn.de. 5 NS ns2.dfn.de.
Authority
The response has no authority records.
Additional
ns1.dfn.de. 5 A 194.95.236.51
ns2.dfn.de. 5 A 194.95.236.59
ns3.dfn.de. 5 A 194.95.236.67
ns1.dfn.de. 5 AAAA 2001:638:d:c110:0:0:0:51
ns2.dfn.de. 5 AAAA 2001:638:d:c111:0:0:0:59
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0