Apex domains cannot have CNAME records.
Question and response
Question
dig rackhosting.dk. NS
Answer
rackhosting.dk. 5 NS dns2.rackhosting.com.
rackhosting.dk. 5 NS dns1.rackhosting.com.
rackhosting.dk. 5 NS dns3.rackhosting.com.
Authority
The response has no authority records.
Additional
dns1.rackhosting.com. 5 A 77.247.69.212
dns2.rackhosting.com. 5 A 45.32.187.211
dns3.rackhosting.com. 5 A 77.247.69.213
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0