Apex domains cannot have CNAME records.
Question and response
Question
dig gr. NS
Answer
gr. 5 NS gr-c.ics.forth.gr.
gr. 5 NS grdns.ics.forth.gr.
gr. 5 NS gr-m.ics.forth.gr.
gr. 5 NS estia.ics.forth.gr.
gr. 5 NS gr-at.ics.forth.gr.
gr. 5 NS gr-d.ics.forth.gr.
Authority
The response has no authority records.
Additional
gr-c.ics.forth.gr. 162275 A 194.0.1.25
gr-d.ics.forth.gr. 172655 A 194.0.11.102
gr-m.ics.forth.gr. 114315 A 194.0.4.10
estia.ics.forth.gr. 170897 A 139.91.191.3
gr-at.ics.forth.gr. 108613 A 78.104.145.227
grdns.ics.forth.gr. 162275 A 139.91.1.1
gr-c.ics.forth.gr. 162275 AAAA 2001:678:4:0:0:0:0:19
gr-d.ics.forth.gr. 172655 AAAA 2001:678:e:102:0:0:0:53
gr-m.ics.forth.gr. 114315 AAAA 2001:678:7:0:0:0:4:10
estia.ics.forth.gr. 172594 AAAA 2001:648:2c30:0:0:0:191:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0