Apex domains cannot have CNAME records.
Question and response
Question
dig dns.kr. NS
Answer
dns.kr. 4 NS d.dns.kr.
dns.kr. 4 NS g.dns.kr.
dns.kr. 4 NS b.dns.kr.
dns.kr. 4 NS c.dns.kr.
dns.kr. 4 NS f.dns.kr.
dns.kr. 4 NS e.dns.kr.
Authority
The response has no authority records.
Additional
b.dns.kr. 17508 A 210.101.60.1
c.dns.kr. 17508 A 210.101.61.1
d.dns.kr. 17508 A 203.83.159.1
e.dns.kr. 17508 A 202.30.124.100
f.dns.kr. 17508 A 210.101.62.1
g.dns.kr. 17508 A 202.31.190.1
d.dns.kr. 17508 AAAA 2001:dcc:4:0:0:0:0:1
e.dns.kr. 17508 AAAA 2001:dcc:5:0:0:0:0:100
f.dns.kr. 17508 AAAA 2001:dcc:6:0:0:0:0:1
g.dns.kr. 17508 AAAA 2001:dc5:a:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0