dns2.duoservers.com has no NS records.
NS records for this zone are defined at duoservers.com.
NS records for this zone are defined at duoservers.com.
Question and response
Question
dig dns2.duoservers.com. NS
Answer
The response has no answer records.
Authority
duoservers.com. 2560 SOA dns1.duoservers.com. hostmaster.duoservers.com. 1772473505 16384 2048 1048576 2560
Additional
The response has no additional records.