Apex domains cannot have CNAME records.
Question and response
Question
dig cdnsun.com. NS
Answer
cdnsun.com. 1 NS gns3.cloudns.net.
cdnsun.com. 1 NS gns2.cloudns.net.
cdnsun.com. 1 NS gns1.cloudns.net.
cdnsun.com. 1 NS gns4.cloudns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0