Apex domains cannot have CNAME records.
Question and response
Question
dig admin.grnet.gr. NS
Answer
admin.grnet.gr. 5 NS sns1.grnet.gr.
admin.grnet.gr. 5 NS ns0.admin.grnet.gr.
admin.grnet.gr. 5 NS sns0.grnet.gr.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0