dns4.vietel.com.vn has no NS records.
NS records for this zone are defined at vietel.com.vn.
NS records for this zone are defined at vietel.com.vn.
Question and response
Question
dig dns4.vietel.com.vn. NS
Answer
The response has no answer records.
Authority
vietel.com.vn. 86400 SOA dns1.vietel.com.vn. tudq3\@vietel.com.vn. 2018112301 10800 3600 604800 86400
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0