I haven't put this through its paces yet. The recursor can forward zones to a nameserver (I still prefer dnscache's syntax) so this can be used as a caching nameserver in front of rbldnsd

http://doc.powerdns.com/built-in-recursor.html

http://wiki.powerdns.com/projects/trac/

The author of PowerDNS Bert Hubert is well-known in Linux circles for his work in explaining the esoteric commands of iproute via LARTC (Linux Advanced Routing and Traffic Control)

http://lartc.org/

Hope this helps, Regards, Yusuf