Apex domains cannot have CNAME records.
Question and response
Question
dig eduroam.de. NS
Answer
eduroam.de. 4 NS ns3.dfn.de.
eduroam.de. 4 NS ns2.dfn.de.
eduroam.de. 4 NS ns1.dfn.de.
eduroam.de. 4 NS ns4.dfn.de.
Authority
The response has no authority records.
Additional
ns1.dfn.de. 4 A 194.95.236.51
ns2.dfn.de. 4 A 194.95.236.59
ns3.dfn.de. 4 A 194.95.236.67
ns4.dfn.de. 4 A 202.61.237.185
ns1.dfn.de. 4 AAAA 2001:638:d:c110:0:0:0:51
ns2.dfn.de. 4 AAAA 2001:638:d:c111:0:0:0:59
ns4.dfn.de. 4 AAAA 2a03:4000:5c:da8:0:0:0:185
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0