# find bootstrap server IP $ nslookup bootstrap1.fopnu.com Server: 10.8.8.1 Address: 10.8.8.1#53 Non-authoritative answer: Name: bootstrap1.fopnu.com Address: 91.219.237.221 $ nslookup bootstrap2.fopnu.com ; nslookup bootstrap3.fopnu.com ; nslookup bootstrap4.fopnu.com Server: 10.8.8.1 Address: 10.8.8.1#53 Non-authoritative answer: Name: bootstrap2.fopnu.com Address: 91.219.237.221 Server: 10.8.8.1 Address: 10.8.8.1#53 Non-authoritative answer: Name: bootstrap3.fopnu.com Address: 91.219.237.221 Server: 10.8.8.1 Address: 10.8.8.1#53 ** server can't find bootstrap4.fopnu.com: NXDOMAIN # block fopnu bootstrap IP $ sudo iptables -A OUTPUT -d 91.219.237.221 -j DROP