Apex domains cannot have CNAME records.
Question and response
Question
dig 6level.fr. NS
Answer
6level.fr. 1 NS ns2.openprovider.be.
6level.fr. 1 NS ns3.openprovider.eu.
6level.fr. 1 NS ns1.openprovider.nl.
Authority
The response has no authority records.
Additional
ns1.openprovider.nl. 5 A 162.159.26.10
ns2.openprovider.be. 5 A 162.159.27.96
ns3.openprovider.eu. 5 A 162.159.48.89
ns1.openprovider.nl. 5 AAAA 2400:cb00:2049:1:0:0:a29f:1a0a
ns2.openprovider.be. 5 AAAA 2400:cb00:2049:1:0:0:a29f:1b60
ns3.openprovider.eu. 5 AAAA 2400:cb00:2049:1:0:0:a29f:3059
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0