Apex domains cannot have CNAME records.
Question and response
Question
dig 0.231.125.42.in-addr.arpa. NS
Answer
0.231.125.42.in-addr.arpa. 5 NS ns013-cqqiq17q72qddc6r.c.d-53.net.
0.231.125.42.in-addr.arpa. 5 NS ns013-cqqiq17q72qddc6r.c.d-53.jp.
0.231.125.42.in-addr.arpa. 5 NS ns013-cqqiq17q72qddc6r.c.d-53.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0