Apex domains cannot have CNAME records.
Question and response
Question
dig its.isti.cnr.it. NS
Answer
its.isti.cnr.it. 3 NS ns2.iit.cnr.it.
its.isti.cnr.it. 3 NS ns-slave1.isti.cnr.it.
its.isti.cnr.it. 3 NS ns1.isti.cnr.it.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0