Apex domains cannot have CNAME records.
Question and response
Question
dig exmedia.co.kr. NS
Answer
exmedia.co.kr. 1 NS ns4.exmedia.co.kr.
exmedia.co.kr. 1 NS ns.exmedia.co.kr.
exmedia.co.kr. 1 NS ns2.exmedia.co.kr.
exmedia.co.kr. 1 NS ns3.exmedia.co.kr.
Authority
The response has no authority records.
Additional
ns.exmedia.co.kr. 1 A 174.35.55.22
ns2.exmedia.co.kr. 1 A 163.171.252.10
ns3.exmedia.co.kr. 1 A 174.35.55.22
ns4.exmedia.co.kr. 1 A 163.171.252.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0