Apex domains cannot have CNAME records.
Question and response
Question
dig cdnetworks.co.kr. NS
Answer
cdnetworks.co.kr. 5 NS ns2.cdnetdns.net.
cdnetworks.co.kr. 5 NS ns1.cdnetdns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0