Apex domains cannot have CNAME records.
Question and response
Question
dig puc.rediris.es. NS
Answer
puc.rediris.es. 1 NS ns02.fccn.pt.
puc.rediris.es. 1 NS chico.rediris.es.
puc.rediris.es. 1 NS scsnms.switch.ch.
puc.rediris.es. 1 NS sun.rediris.es.
Authority
The response has no authority records.
Additional
ns02.fccn.pt. 2 A 193.136.2.228
scsnms.switch.ch. 2 A 130.59.31.26
ns02.fccn.pt. 2 AAAA 2001:690:a80:4001:0:0:0:200
scsnms.switch.ch. 2 AAAA 2001:620:0:ff:0:0:0:a7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0