rbldnsd is a small and fast DNS daemon which is especially made to serve DNSBL
zones.  This daemon was inspired by Dan J. Bernstein's rbldns program found in
the djbdns package.

rbldnsd is extremely fast - it outperforms both bind and djbdns greatly.  It
has very small memory footprint.

The daemon can serve both IP-based (ordb.org, dsbl.org etc) and name-based
(rfc-ignorant.org) blocklists.  Unlike DJB's rbldns, it has ability to specify
individual values for every entry, can serve as many zones on a single IP
address as you wish, and, finally, it is a real nameserver: it can reply to DNS
metadata requests.  The daemon keeps all zones in memory for faster operations,
but its memory usage is very efficient, especially for repeated TXT values
which are stored only once.