BLD stands for "blacklist daemon" and is intended to serve a
blacklist.  The blacklist is built by simply inserting IP addresses
or by using submission rate limits based on a maximum number of
submissions of the same IP address within a minimum time interval.
You can build a BLD cluster by configuring the daemon to notify
other similar daemon(s) every time an IP address is added to the
blacklist.  BLD was primarily designed to fight against dictionnary-based
spams (by making the MTA report to BLD any host that tries to send
a mail to an unknown user) but can be used by any program.

See homepage for more infos:

WWW: http://www.online.redhate.org/bld/

Olivier Beyssac <obld@r14.freenix.org>