Apex domains cannot have CNAME records.
Question and response
Question
dig kh. NS
Answer
kh. 5 NS ns1.dns.net.kh.
kh. 5 NS dns1.online.com.kh.
kh. 5 NS ns.camnet.com.kh.
kh. 5 NS ns4.apnic.net.
Authority
The response has no authority records.
Additional
ns.camnet.com.kh. 165884 A 203.223.32.3
ns1.dns.net.kh. 124916 A 203.223.32.21
ns4.apnic.net. 1 A 202.12.31.53
dns1.online.com.kh. 172792 A 203.189.128.1
ns4.apnic.net. 1 AAAA 2001:dd8:12:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0