Untangle NG as failover DHCP server for Windows Server 2008

We have an Untangle NG Firewall with DHCP turned off.  And, we have an Active Directory domain with our Windows 2008 server running DHCP.   I'd like to use the Untangle Firewall as a DHCP failover in case the Windows 2008 server is down.  I'm looking for a failover solution which would support reservations.    Want to know if anyone has used Untangle as a failover DHCP server and how you configured Windows 2008 server to do it.   I know we could use a split scope solution, but, looking for full scope failover solution if possible.