Apex domains cannot have CNAME records.
Question and response
Question
dig 112-28.231.225.130.in-addr.arpa. NS
Answer
112-28.231.225.130.in-addr.arpa. 5 NS dns2.dtu.dk.
112-28.231.225.130.in-addr.arpa. 5 NS dns1.dtu.dk.
112-28.231.225.130.in-addr.arpa. 5 NS ns-soa.darenet.dk.
112-28.231.225.130.in-addr.arpa. 5 NS dns3.dtu.dk.
Authority
The response has no authority records.
Additional
ns-soa.darenet.dk. 5 A 130.226.179.50
ns-soa.darenet.dk. 5 AAAA 2001:878:0:e000:82:e2:b3:32
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0