Apex domains cannot have CNAME records.
Question and response
Question
dig lib.ntua.gr. NS
Answer
lib.ntua.gr. 5 NS achilles.noc.ntua.gr.
lib.ntua.gr. 5 NS ulysses.noc.ntua.gr.
lib.ntua.gr. 5 NS diomedes.noc.ntua.gr.
Authority
The response has no authority records.
Additional
ulysses.noc.ntua.gr. 5 A 147.102.222.230
achilles.noc.ntua.gr. 5 A 147.102.222.210
diomedes.noc.ntua.gr. 5 A 147.102.222.220
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0