Apex domains cannot have CNAME records.
Question and response
Question
dig ujf-grenoble.fr. NS
Answer
ujf-grenoble.fr. 5 NS eip1.u-ga.fr.
ujf-grenoble.fr. 5 NS eip2.u-ga.fr.
Authority
The response has no authority records.
Additional
eip1.u-ga.fr. 5 A 195.83.24.250
eip2.u-ga.fr. 5 A 152.77.1.250
eip1.u-ga.fr. 5 AAAA 2001:660:5305:cafe:0:0:0:2
eip2.u-ga.fr. 5 AAAA 2001:660:5301:1e:0:0:0:101
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0