dns06.salesforce.com has no NS records.
NS records for this zone are defined at salesforce.com.
NS records for this zone are defined at salesforce.com.
Question and response
Question
dig dns06.salesforce.com. NS
Answer
The response has no answer records.
Authority
salesforce.com. 1800 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2016654145 600 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0