Apex domains cannot have CNAME records.
Question and response
Question
dig rackspace.com. NS
Answer
rackspace.com. 5 NS ns.rackspace.com.
rackspace.com. 5 NS ns2.rackspace.com.
Authority
The response has no authority records.
Additional
ns.rackspace.com. 5 A 69.20.95.4
ns2.rackspace.com. 5 A 65.61.188.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0