Apex domains cannot have CNAME records.
Question and response
Question
dig dc1.ru.itoolabs.net. NS
Answer
dc1.ru.itoolabs.net. 5 NS dns2.dc1.ru.itoolabs.net.
dc1.ru.itoolabs.net. 5 NS dns3.dc1.ru.itoolabs.net.
dc1.ru.itoolabs.net. 5 NS dns1.dc1.ru.itoolabs.net.
Authority
The response has no authority records.
Additional
dns1.dc1.ru.itoolabs.net. 5 A 109.69.176.1
dns2.dc1.ru.itoolabs.net. 5 A 109.69.177.1
dns3.dc1.ru.itoolabs.net. 5 A 213.248.35.141
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0