Tricky DNS Question - Advanced

[Marc Perkel]

I have another tricky DNS question. I'm scream testing my new dnsbl 
server and I'm going to open it up for others to use soon. Just working 
out the final details.

So - here's my tricky question. I have 2 DNS servers. Server A runs 
MyDNS - a MySQL driven server. It's the one that does the work behind 
the scenes. It has no caching and should be considered to be slow.

Server B is a regular DNS server set up for caching and running BIND. 
It's the one that will be the public face for the blacklist providing 
caching for Server A so as not to load down Server A.

So what I want is that Server B secretly knows where Server A is so that 
any request of dnsbl.junkemailfilter.com goes to Server B and Server B 
passes the request on to Server A and caches it reducing the load on 
Server A and keeling it hidden from the world.

So - how do I do that? What do I put in named.conf to tell it to route 
certain requests to a Server A?

Thanks in advance.