Apex domains cannot have CNAME records.
Question and response
Question
dig uth.gr. NS
Answer
uth.gr. 5 NS ns2.uth.gr.
uth.gr. 5 NS sns0.grnet.gr.
uth.gr. 5 NS sns1.grnet.gr.
uth.gr. 5 NS ns1.uth.gr.
Authority
The response has no authority records.
Additional
ns1.uth.gr. 5 A 194.177.200.226
ns2.uth.gr. 5 A 194.177.200.130
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0