Apex domains cannot have CNAME records.
Question and response
Question
dig 192.173.207.in-addr.arpa. NS
Answer
192.173.207.in-addr.arpa. 5 NS ns2.eli.net.
192.173.207.in-addr.arpa. 5 NS ns.eli.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0