rblcheck - Command-line interface to DNSBL-style filters.
Copyright (C) 1997, 1998, 1999, 2000, 2001 Edward S. Marshall <esm@logic.net>
Copyright (C) 2019 Marco d'Itri <md@linux.it>

This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your option)
any later version.

This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License along
with this program; if not, write to the Free Software Foundation, Inc.,
675 Mass Ave, Cambridge, MA 02139, USA.

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This program is a very basic interface to DNS listings such as the DNSBL
filter operated by the MAPS project. The basic idea of the filter is that
when someone is listed (for example, a blacklist for email abusers), a new
domain name is resolved of the form "2.0.0.127.domain.name.com", where
2.0.0.127 is the abusive IP address in reverse (for example, 2.0.0.127
would be the IP address 127.0.0.2), and "domain.name.com" is the base
domain name of the filtering service (such as "blackholes.mail-abuse.org",
for the MAPS project DNSBL filter).

To learn how to build this program, please read docs/INSTALL. Seriously.
Make sure to check the default lists of DNSBLs in the "sites.h" file.

The official home for rblcheck is https://github.com/rfc1036/rblcheck .