Apex domains cannot have CNAME records.
Question and response
Question
dig usdns.dnsracks.com. NS
Answer
usdns.dnsracks.com. 5 NS ns2.dnsracks.com.
usdns.dnsracks.com. 5 NS ns1.dnsracks.com.
Authority
The response has no authority records.
Additional
ns1.dnsracks.com. 4 A 128.199.140.115
ns2.dnsracks.com. 4 A 45.56.75.88
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0