Apex domains cannot have CNAME records.
Question and response
Question
dig fib.upc.es. NS
Answer
fib.upc.es. 2 NS dns1.fib.upc.es.
fib.upc.es. 2 NS dns2.fib.upc.es.
Authority
The response has no authority records.
Additional
dns1.fib.upc.es. 2 A 147.83.41.104
dns2.fib.upc.es. 2 A 147.83.41.36
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0