Apex domains cannot have CNAME records.
Question and response
Question
dig dooray.co.kr. NS
Answer
dooray.co.kr. 5 NS ns3.toastoven.net.
dooray.co.kr. 5 NS ns1.toastoven.net.
dooray.co.kr. 5 NS ns2.toastoven.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0