udns4.salesforce.com has no CNAME records.
Question and response
Question
dig udns4.salesforce.com. NS
Answer
The response has no answer records.
Authority
salesforce.com. 600 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2016654344 600 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0