Apex domains cannot have CNAME records.
Question and response
Question
dig sirio-is.it. NS
Answer
sirio-is.it. 5 NS ns1.leonet.it.
sirio-is.it. 5 NS ns3.leonet.it.
sirio-is.it. 5 NS ns5.leonet.it.
Authority
The response has no authority records.
Additional
ns1.leonet.it. 5 A 212.77.93.110
ns3.leonet.it. 5 A 212.77.89.33
ns5.leonet.it. 5 A 52.169.27.156
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0