Apex domains cannot have CNAME records.
Question and response
Question
dig vtxnet.net. NS
Answer
vtxnet.net. 5 NS ns01.vtx.ch.
vtxnet.net. 5 NS ns02.vtx.ch.
Authority
The response has no authority records.
Additional
ns01.vtx.ch. 5 A 212.147.10.15
ns02.vtx.ch. 5 A 212.147.10.161
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0