Apex domains cannot have CNAME records.
Question and response
Question
dig netserv.chula.ac.th. NS
Answer
netserv.chula.ac.th. 5 NS ns.netserv.chula.ac.th.
netserv.chula.ac.th. 5 NS explorer.netserv.chula.ac.th.
Authority
The response has no authority records.
Additional
ns.netserv.chula.ac.th. 4 A 161.200.192.4
explorer.netserv.chula.ac.th. 4 A 161.200.192.200
ns.netserv.chula.ac.th. 5 AAAA 2001:3c8:1401:5:0:0:0:4
explorer.netserv.chula.ac.th. 5 AAAA 2001:3c8:1401:3:0:0:0:200
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0