Apex domains cannot have CNAME records.
Question and response
Question
dig grnet.gr. NS
Answer
grnet.gr. 5 NS ns1.grnet.gr.
grnet.gr. 5 NS ns0.grnet.gr.
Authority
The response has no authority records.
Additional
ns0.grnet.gr. 5 A 83.212.5.89
ns1.grnet.gr. 5 A 83.212.5.22
ns0.grnet.gr. 5 AAAA 2001:648:2ffc:203:0:0:0:89
ns1.grnet.gr. 5 AAAA 2001:648:2ffc:112:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0