Apex domains cannot have CNAME records.
Question and response
Question
dig NSK.RU. NS
Answer
nsk.ru. 5 NS ns5.nsk.ru.
nsk.ru. 5 NS ns.nsk.ru.
nsk.ru. 5 NS ns3.nsk.ru.
Authority
The response has no authority records.
Additional
ns.NSK.RU. 5 A 89.251.96.75
ns5.NSK.RU. 5 A 95.131.13.182
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0