Apex domains cannot have CNAME records.
Question and response
Question
dig lip6.fr. NS
Answer
lip6.fr. 5 NS isis.lip6.fr.
lip6.fr. 5 NS soleil.uvsq.fr.
lip6.fr. 5 NS osiris.lip6.fr.
Authority
The response has no authority records.
Additional
isis.lip6.fr. 5 A 132.227.60.2
osiris.lip6.fr. 5 A 132.227.60.30
soleil.uvsq.fr. 5 A 193.51.24.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0