Apex domains cannot have CNAME records.
Question and response
Question
dig core.xifos.net. NS
Answer
core.xifos.net. 5 NS ns2.xifos.net.
core.xifos.net. 5 NS ns1.xifos.net.
core.xifos.net. 5 NS ns3.xifos.net.
Authority
The response has no authority records.
Additional
ns2.xifos.net. 5 A 80.77.240.32
ns3.xifos.net. 5 A 77.246.22.99
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0