Apex domains cannot have CNAME records.
Question and response
Question
dig dk08u40.hostrush.com. NS
Answer
dk08u40.hostrush.com. 5 NS ns1.hostrush.com.
dk08u40.hostrush.com. 5 NS ns2.hostrush.com.
dk08u40.hostrush.com. 5 NS ns3.hostrush.com.
Authority
The response has no authority records.
Additional
ns1.hostrush.com. 5 A 207.231.107.250
ns2.hostrush.com. 5 A 207.231.109.26
ns3.hostrush.com. 5 A 199.241.137.154
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0