Apex domains cannot have CNAME records.
Question and response
Question
dig sdn.spb.ru. NS
Answer
sdn.spb.ru. 5 NS ns-02.stackdata.net.
sdn.spb.ru. 5 NS ns-01.stackdata.net.
Authority
The response has no authority records.
Additional
ns-01.stackdata.net. 5 A 185.44.14.254
ns-02.stackdata.net. 5 A 185.44.15.254
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0