Apex domains cannot have CNAME records.
Question and response
Question
dig 6.in-addr.arpa. NS
Answer
6.in-addr.arpa. 5 NS ns02.army.mil.
6.in-addr.arpa. 5 NS ns01.army.mil.
6.in-addr.arpa. 5 NS ns03.army.mil.
Authority
The response has no authority records.
Additional
NS01.army.mil. 5 A 140.153.43.44
NS02.army.mil. 5 A 192.82.113.7
NS03.army.mil. 5 A 130.114.200.6
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0