DHCP Scopes with Cisco dhcp-relay

I am currently looking into a replacement for some Cisco CNR servers
and would like to use dhcpd under Linux but have a few things to work
out first and was hoping I can get some help on the list.

Basically I have an interface configured as follows:

interface GigabitEthernet0/1
ip address 192.168.1.1 255.255.255.0
ip address 172.16.0.1 255.255.0.0 secondary

I have dhcp-relay configured to forward dhcp requests to my dhcp
server. Because of the nature of DHCP/BOOTP all of the requests look
as though they are coming from the 192.168.1.0 subnet and thus thats
what my dhcp server replies back with.

Using Cisco CNR you can configure secondary pools. For example in this
case I would configure the 172.16.0.0 network as a secondary pool of
192.168.1.0. The end result is when all of the 192.16.1.0 addresses
get assigned it will start handing out 172.16.0.0 addresses.

Does anybody know of a way to configure this using dhcpd (dhcpd.conf)?

I would really like to figure this out but have not found much in the
man page for dhcpd.

Thanks,
Matt