Apex domains cannot have CNAME records.
Question and response
Question
dig RETN.RU. NS
Answer
RETN.RU. 4 NS nsb.retn.net.
RETN.RU. 4 NS nsa.retn.net.
Authority
The response has no authority records.
Additional
nsa.retn.net. 4 A 87.245.224.254
nsb.retn.net. 4 A 87.245.226.254
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0