Apex domains cannot have CNAME records.
Question and response
Question
dig kr. NS
Answer
kr. 5 NS d.dns.kr.
kr. 5 NS e.dns.kr.
kr. 5 NS f.dns.kr.
kr. 5 NS g.dns.kr.
kr. 5 NS b.dns.kr.
kr. 5 NS c.dns.kr.
Authority
The response has no authority records.
Additional
b.dns.kr. 43729 A 210.101.60.1
c.dns.kr. 43729 A 210.101.61.1
d.dns.kr. 43729 A 203.83.159.1
e.dns.kr. 43729 A 202.30.124.100
f.dns.kr. 43729 A 210.101.62.1
g.dns.kr. 43729 A 202.31.190.1
d.dns.kr. 43729 AAAA 2001:dcc:4:0:0:0:0:1
e.dns.kr. 43729 AAAA 2001:dcc:5:0:0:0:0:100
f.dns.kr. 43729 AAAA 2001:dcc:6:0:0:0:0:1
g.dns.kr. 43729 AAAA 2001:dc5:a:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0