Apex domains cannot have CNAME records.
Question and response
Question
dig greentelecom.net.br. NS
Answer
greentelecom.net.br. 1 NS ns344.hostgator.com.br.
greentelecom.net.br. 1 NS ns345.hostgator.com.br.
Authority
The response has no authority records.
Additional
ns344.hostgator.com.br. 3 A 108.167.132.72
ns345.hostgator.com.br. 3 A 108.167.169.45
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0