Apex domains cannot have CNAME records.
Question and response
Question
dig f6.security. NS
Answer
f6.security. 3 NS a.ns.selectel.ru.
f6.security. 3 NS c.ns.selectel.ru.
f6.security. 3 NS b.ns.selectel.ru.
f6.security. 3 NS d.ns.selectel.ru.
Authority
The response has no authority records.
Additional
a.ns.selectel.ru. 1 A 188.68.203.1
b.ns.selectel.ru. 1 A 77.223.114.2
c.ns.selectel.ru. 1 A 188.68.203.3
d.ns.selectel.ru. 1 A 77.223.114.4
a.ns.selectel.ru. 1 AAAA 2a00:ab00:800:0:0:0:2:53
b.ns.selectel.ru. 1 AAAA 2a00:ab00:801:0:0:0:2:53
c.ns.selectel.ru. 1 AAAA 2a00:ab00:802:0:0:0:2:53
d.ns.selectel.ru. 1 AAAA 2a00:ab00:803:0:0:0:2:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0