# Route to 192.168.1.0/24
iptables -A POSTROUTING -t nat --dst 192.168.1.0/255.255.255.0 -j SNAT --to-source 192.168.1.1
# Route to 192.168.2.0/24
iptables -A POSTROUTING -t nat --dst 192.168.2.0/255.255.255.0 -j SNAT --to-source 192.168.2.1
# Accept forwarding
iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT