Question about forwarders...

[Brian Thomas]

Standard disclaimer: If this topic has been covered recently, I apologize.

In testing the configurationg of forwarding-only servers, my experience is
that when a lookup is done, the first server in the list of forwarders
is always chosen -- It does not round-robin or randomly choose a server to
try.

While this isn't critical, it does cause problems. For example, if the
first forwarder in the list is unavailable, only then will lookups use the
next one. Meaning I now have a spankin' new cache on that second
forwarder, requiring every single name be recursively looked up since
it's bereft of entries. Second, I can't load-balance the requests across
multiple forwarders, I'm always going to have one that gets more and
more pegged and memory-starved as it's cache grows.

For the record, the situation is I have is a large farm of MTA's 
delivering mail to many recipients. I want to run a caching-only
server on each of those MTA's for speedy lookups, and have two (Or more)
dedicated servers to act as forwarders for outbound lookups. That should
limit the Internet traffic (Since the caching servers will never talk
over the 'net link), and insure the dedicated forwarders have large
caches for multiple lookups on the same domain.

No, this isn't for spamming. :)

Here's a test named.conf I was using:

acl internals { 192.168.0.0/24; 127.0.0.0/24; };

options {
        forward only;
        forwarders { 192.168.0.5; 192.168.0.25; };
        allow-transfer { internals; };
        allow-query { internals; };
        allow-recursion { internals; };
};

Snoops of lookups through this server demonstrate shows traffic only
between the caching server and 192.168.0.5. The .25 nameserver is not
quieried unless the .5 server is not responding.

Brian