Apex domains cannot have CNAME records.
Question and response
Question
dig cetsi.fr. NS
Answer
cetsi.fr. 5 NS ns3.cetsi.info.
cetsi.fr. 5 NS ns2.cetsi.eu.
cetsi.fr. 5 NS ns1.cetsi.fr.
Authority
The response has no authority records.
Additional
ns1.cetsi.fr. 5 A 91.151.64.234
ns2.cetsi.eu. 5 A 91.151.65.234
ns3.cetsi.info. 5 A 91.151.77.234
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0