Apex domains cannot have CNAME records.
Question and response
Question
dig rdtc.ru. NS
Answer
rdtc.ru. 5 NS nsv6.rdtc.ru.
rdtc.ru. 5 NS ns3.rdtc.ru.
rdtc.ru. 5 NS ns.rdtc.ru.
rdtc.ru. 5 NS ns.zsttk.ru.
Authority
The response has no authority records.
Additional
ns.rdtc.ru. 5 A 62.231.161.9
ns.zsttk.ru. 5 A 80.89.128.8
ns3.rdtc.ru. 5 A 62.231.190.2
nsv6.rdtc.ru. 5 AAAA 2a03:3100:4:0:0:0:0:9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0