Apex domains cannot have CNAME records.
Question and response
Question
dig teipir.gr. NS
Answer
teipir.gr. 5 NS sns0.grnet.gr.
teipir.gr. 5 NS gea.teipir.gr.
teipir.gr. 5 NS scrat.teipir.gr.
teipir.gr. 5 NS sns1.grnet.gr.
Authority
The response has no authority records.
Additional
gea.teipir.gr. 5 A 195.251.70.246
scrat.teipir.gr. 5 A 195.251.93.78
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0