dns06.salesforce.com has no CNAME records.
Question and response
Question
dig dns06.salesforce.com. NS
Answer
The response has no answer records.
Authority
salesforce.com. 600 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2016653410 600 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0