dnsmasq

show DHCP leases cat /var/lib/misc/dnsmasq.leases

different DHCP options to advertise routers to different subnets

Simply prefix the interface, in this case eth0 or eth1:

dhcp-option=eth0,3,10.1.0.254
dhcp-option=eth1,3,10.1.20.254

This also works with tags:

dhcp-range=set:net1,10.1.0.50,10.1.0.250,255.255.255.0
dhcp-range=set:net2,10.1.20.50,10.1.20.250,255.255.255.0  

dhcp-option=tag:net1,3,10.1.0.254  
dhcp-option=tag:net2,3,10.1.20.254