Apex domains cannot have CNAME records.
Question and response
Question
dig MAIL.RU. NS
Answer
MAIL.RU. 5 NS ns2.MAIL.RU.
MAIL.RU. 5 NS ns1.MAIL.RU.
Authority
The response has no authority records.
Additional
ns1.mail.ru. 5 A 217.69.139.112
ns2.mail.ru. 5 A 94.100.180.138
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0