Apex domains cannot have CNAME records.
Question and response
Question
dig ptn.ru. NS
Answer
ptn.ru. 5 NS ns1.ptn.ru.
ptn.ru. 5 NS ns2.ptn.ru.
Authority
The response has no authority records.
Additional
ns1.PTN.RU. 5 A 213.59.199.163
ns2.PTN.RU. 5 A 188.128.82.235
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0