Apex domains cannot have CNAME records.
Question and response
Question
dig 56.217.in-addr.arpa. NS
Answer
56.217.in-addr.arpa. 3 NS dnsts.interbusiness.it.
56.217.in-addr.arpa. 3 NS dnsti.interbusiness.it.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0