rblcheck is a very basic interface to RBL-style DNS listings such as those operated by the MAPS and ORBS projects.
The general idea behind RBL-style listings is rapid lookup of IP addresses using DNS (for example, for blacklisting IP addresses because of abuse).
Each IP address is reversed and has a domain name attached to it; for example, the IP address 127.0.0.2 would become 2.0.0.127, and then a domain such as "relays.visi.com" would be added to it.
You would then try to resolve the result (ie. 2.0.0.127.relays.visi.com); if you receive a positive reply, then you know that the address is listed. Further information can also be queried, such as text descriptions of why the address was listed.
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