- https://www.adampalmer.me/iodigitalsec/2014/11/22/dns-black-list-rbl-checking-in-python/
- https://0xbharath.github.io/python-network-programming/protocols/dns/index.html
- http://www.dnspython.org/docs/1.16.0/
- https://www.spamhaus.org/faq/section/DNSBL%20Usage#366
