I have an ISP that issues a DHCP address to m0n0wall.  For DNS lookups I 
need to use the DNS servers that the ISP gives out through DHCP. 
Everything there is working just fine.

What I need is for m0n0wall to keep using the ISP's DNS servers but to 
send a Dynamic DNS update to a totally different DNS Server to update 
the m0n0wall's IP address so that it resolves to the new IP/hostname.

Looking at the gui for dyndns it appears that it has to use the DNS 
server that is setup for normal DNS queries.  Is there a way to override 
that and use a specific DNS server just for the RFC 2136 Dynamic DNS 
updates?

My goal is to use x.x.x.x DNS server for normal DNS queries and y.y.y.y 
DNS server for RFC 2136 Dynamic DNS for my firewall hostname name.