Apex domains cannot have CNAME records.
Question and response
Question
dig etb.net.co. NS
Answer
etb.net.co. 4 NS ns2-auth.etb.net.co.
etb.net.co. 4 NS ns1-auth.etb.net.co.
Authority
The response has no authority records.
Additional
ns1-auth.etb.net.co. 4 A 200.69.125.10
ns2-auth.etb.net.co. 4 A 201.244.1.170
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0