How to specify multiple DNS Servers for a particular domain?

[benze]

Hi,

I've been using the DNS forwarder for my internal domain with plenty of success so far.  I have pointed all my internal machines to pfSense as the DNS resolver, and within pfSense have pointed my internal DNS server for my domain.

My problem now is that I want to make the system a little more robust.  Currently, if my internal DNS server dies, then most communication between my servers will all die, as they all rely on the DNS server.  So I created a secondary DNS slave server as a backup.

However, I cannot figure out how to configure pfSense to use multiple DNS servers for a particular domain.  If I try to seperate the server addresses with a space or a comma, it tells me to use a valid IP address.

If I try to create a second entry for the same domain pointing to a different server, it tells me an entry for that domain already exists.

Is there no way to accomplish this?  Is the pfSense forwarder unable to use multiple DNS servers?  How should I go about making this more robust if that is the case?

Thanks!

Eric