Apex domains cannot have CNAME records.
Question and response
Question
dig infra.6level.net. NS
Answer
infra.6level.net. 5 NS ns4.6level.fr.
infra.6level.net. 5 NS ns2.6level.eu.
infra.6level.net. 5 NS ns3.6level.net.
infra.6level.net. 5 NS ns1.6level.net.
infra.6level.net. 5 NS ns5.6level.net.
Authority
The response has no authority records.
Additional
ns1.6level.net. 5 A 45.83.248.61
ns3.6level.net. 5 A 45.83.250.61
ns5.6level.net. 5 A 62.4.1.171
ns1.6level.net. 5 AAAA 2a06:fb00:2:0:0:0:1:61
ns3.6level.net. 5 AAAA 2a06:fb00:2:0:0:0:3:61
ns5.6level.net. 5 AAAA 2a12:8ac1:a0:0:0:0:0:153
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0