We currently have a PFsense 2.1.5 forwarding DNS requests to an internal DNS server whose default gateway is the PFsense firewall, this works correctly. There also is a ForefrontTMG firewall on a separate external address that forwards DNS requests to the internal DNS server, this works because TMG has the option to make the forwarded DNS packets appear to have come from the TMG internal address rather than the actual originating address.
I'm replacing the TMG firewall with a second PFsense box. On this the DNS forwarding does not work as the default gateway on the DNS server points to the other PFsense box. Is there a way to make forwarded packets to appear to come from the PFsense box rather then the actual originating address? TMG has the option but I can't find the equivalent option in PFSense