Apex domains cannot have CNAME records.
Question and response
Question
dig pavietnam.net. NS
Answer
pavietnam.net. 5 NS ns2.pavietnam.net.
pavietnam.net. 5 NS nsbak.pavietnam.net.
pavietnam.net. 5 NS ns1.pavietnam.net.
Authority
The response has no authority records.
Additional
ns1.pavietnam.net. 5 A 112.213.89.3
ns2.pavietnam.net. 5 A 222.255.121.247
nsbak.pavietnam.net. 5 A 112.213.89.22
ns1.pavietnam.net. 5 AAAA 2406:9c80:0:0:0:0:0:66
ns2.pavietnam.net. 5 AAAA 2406:9c80:0:0:0:0:9000:111
nsbak.pavietnam.net. 5 AAAA 2406:9c80:0:0:0:0:9000:302
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0