Apex domains cannot have CNAME records.
Question and response
Question
dig dsl.telepac.pt. NS
Answer
dsl.telepac.pt. 5 NS dnsbho1aut01.local.
dsl.telepac.pt. 5 NS dnscoi1aut02.local.
dsl.telepac.pt. 5 NS ns.telepac.pt.
dsl.telepac.pt. 5 NS dnsbho1aut02.local.
dsl.telepac.pt. 5 NS dnscoi1aut01.local.
Authority
The response has no authority records.
Additional
ns.telepac.pt. 5 A 194.65.3.20
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0