Apex domains cannot have CNAME records.
Question and response
Question
dig DEN.RU. NS
Answer
den.ru. 5 NS ns2.den.ru.
den.ru. 5 NS ve.nu.
Authority
The response has no authority records.
Additional
ns2.den.ru. 5 A 87.239.144.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0