How to check if an IP is blacklisted (DNSBL), from raw DNS to one curl

Your emails start bouncing, or a website suddenly challenges every request from your server. Very often the reason is boring: your IP landed on a DNSBL and you did not know.

A DNSBL (DNS-based blackhole list) is a reputation database you query over plain DNS. Mail servers do this on every incoming connection. Spamhaus, SpamCop, Barracuda and a dozen others each run their own list, their own criteria, and their own delisting process.

The raw mechanic (it is just DNS)

To check an IP against a list, you reverse its octets and query them as a subdomain of the list's zone. Checking 127.0.0.2 against SpamCop looks like this:

$ nslookup -type=A 2.0.0.127.bl.spamcop.net