mail4.knockbot.info has no NS records.
NS records for this zone are defined at knockbot.info.
NS records for this zone are defined at knockbot.info.
Question and response
Question
dig mail4.knockbot.info. NS
Answer
The response has no answer records.
Authority
knockbot.info. 300 SOA 01.dnsv.jp. hostmaster.dnsv.jp. 1752887637 3600 900 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0