ns.rackspace.com has no CNAME records.
Question and response
Question
dig ns.rackspace.com. NS
Answer
The response has no answer records.
Authority
rackspace.com. 120 SOA ns.rackspace.com. hostmaster.rackspace.com. 1770412730 300 300 1814400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0