Apex domains cannot have CNAME records.
Question and response
Question
dig cru.fr. NS
Answer
cru.fr. 5 NS ns1.u-strasbg.fr.
cru.fr. 5 NS resone.univ-rennes1.fr.
cru.fr. 5 NS ns1.renater.fr.
cru.fr. 5 NS ns2.renater.fr.
cru.fr. 5 NS dns-3.dfn.de.
Authority
The response has no authority records.
Additional
ns1.renater.fr. 5 A 193.49.159.2
ns1.u-strasbg.fr. 5 A 130.79.200.1
ns2.renater.fr. 5 A 193.49.159.9
resone.univ-rennes1.fr. 5 A 129.20.254.1
ns1.renater.fr. 5 AAAA 2001:660:3001:4002:0:0:0:2
ns1.u-strasbg.fr. 5 AAAA 2001:660:2402:0:0:0:0:1
ns2.renater.fr. 5 AAAA 2001:660:3001:4002:0:0:0:9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0