Apex domains cannot have CNAME records.
Question and response
Question
dig netnam.vn. NS
Answer
netnam.vn. 5 NS ns1.netnam.vn.
netnam.vn. 5 NS ns2.netnam.vn.
netnam.vn. 5 NS ns3.netnam.vn.
Authority
The response has no authority records.
Additional
ns1.netnam.vn. 5 A 202.151.160.10
ns2.netnam.vn. 5 A 202.151.175.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0