Apex domains cannot have CNAME records.
Question and response
Question
dig dns.net.kh. NS
Answer
dns.net.kh. 5 NS ns.camnet.com.kh.
dns.net.kh. 5 NS ns2.camnet.com.kh.
Authority
The response has no authority records.
Additional
ns.camnet.com.kh. 5 A 203.223.32.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0