Apex domains cannot have CNAME records.
Question and response
Question
dig 25.71.27.in-addr.arpa. NS
Answer
25.71.27.in-addr.arpa. 5 NS dns1.vietel.com.vn.
25.71.27.in-addr.arpa. 5 NS dns2.vietel.com.vn.
Authority
The response has no authority records.
Additional
dns1.vietel.com.vn. 2 A 203.113.131.1
dns2.vietel.com.vn. 2 A 203.113.131.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0