Apex domains cannot have CNAME records.
Question and response
Question
dig tuxbox.nu. NS
Answer
tuxbox.nu. 5 NS ns.gal.airwire.ie.
tuxbox.nu. 5 NS ns.ger.airwire.ie.
tuxbox.nu. 5 NS spirillen.marlow.dk.
Authority
The response has no authority records.
Additional
ns.gal.airwire.ie. 5 A 195.222.111.222
ns.ger.airwire.ie. 5 A 80.190.248.146
ns.gal.airwire.ie. 5 AAAA 2a02:278:50:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0