Apex domains cannot have CNAME records.
Question and response
Question
dig hosting.co.kr. NS
Answer
hosting.co.kr. 4 NS ns1.hosting.co.kr.
hosting.co.kr. 4 NS ns3.hosting.co.kr.
hosting.co.kr. 4 NS ns2.hosting.co.kr.
hosting.co.kr. 4 NS ns4.hosting.co.kr.
Authority
The response has no authority records.
Additional
ns1.hosting.co.kr. 4 A 121.254.170.11
ns2.hosting.co.kr. 4 A 43.201.141.93
ns3.hosting.co.kr. 4 A 121.254.170.12
ns4.hosting.co.kr. 4 A 43.201.141.93
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0