Apex domains cannot have CNAME records.
Question and response
Question
dig nessus.at. NS
Answer
nessus.at. 5 NS ns3.nessus.at.
nessus.at. 5 NS ns2.nessus.at.
nessus.at. 5 NS ns1.nessus.at.
Authority
The response has no authority records.
Additional
ns1.nessus.at. 5 A 77.244.240.242
ns2.nessus.at. 5 A 91.206.8.43
ns3.nessus.at. 5 A 77.244.244.242
ns1.nessus.at. 5 AAAA 2a02:1b8:0:3:0:0:0:2
ns3.nessus.at. 5 AAAA 2a02:1b8:0:1:0:0:0:22
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0