Apex domains cannot have CNAME records.
Question and response
Question
dig 220.212.125.in-addr.arpa. NS
Answer
220.212.125.in-addr.arpa. 3 NS dns2.vietel.com.vn.
220.212.125.in-addr.arpa. 3 NS dns1.vietel.com.vn.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0