Apex domains cannot have CNAME records.
Question and response
Question
dig gslb.net. NS
Answer
gslb.net. 5 NS kerns4.gslb.net.
gslb.net. 5 NS kerns3.gslb.net.
gslb.net. 5 NS kerns1.gslb.net.
gslb.net. 5 NS kerns2.gslb.net.
Authority
The response has no authority records.
Additional
kerns1.gslb.net. 5 A 185.136.96.79
kerns2.gslb.net. 5 A 185.136.97.79
kerns3.gslb.net. 5 A 185.136.98.79
kerns4.gslb.net. 5 A 185.136.99.79
kerns1.gslb.net. 5 AAAA 2a06:fb00:1:0:0:0:1:79
kerns2.gslb.net. 5 AAAA 2a06:fb00:1:0:0:0:2:79
kerns3.gslb.net. 5 AAAA 2a06:fb00:1:0:0:0:3:79
kerns4.gslb.net. 5 AAAA 2a06:fb00:1:0:0:0:4:79
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0