Apex domains cannot have CNAME records.
Question and response
Question
dig serv-dns.ru. NS
Answer
serv-dns.ru. 2 NS ns2.serv-dns.ru.
serv-dns.ru. 2 NS ns1.serv-dns.ru.
Authority
The response has no authority records.
Additional
ns1.serv-dns.ru. 2 A 185.64.76.14
ns2.serv-dns.ru. 2 A 45.155.164.14
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0