dns-external-master.amazon.com has no CNAME records.
Question and response
Question
dig dns-external-master.amazon.com. NS
Answer
The response has no answer records.
Authority
amazon.com. 870 SOA dns-external-master.amazon.com. hostmaster.amazon.com. 2010197829 180 60 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0