Apex domains cannot have CNAME records.
Question and response
Question
dig csi.forth.gr. NS
Answer
csi.forth.gr. 3 NS dns1.ics.forth.gr.
csi.forth.gr. 3 NS estia.ics.forth.gr.
Authority
The response has no authority records.
Additional
estia.ics.forth.gr. 168810 A 139.91.191.3
estia.ics.forth.gr. 168810 AAAA 2001:648:2c30:0:0:0:191:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0