ip route flush table link0
ip route flush table link1

ip rule add from $LINK0_IP table link0
ip route add $LINK0_NET/20 dev eth0 src $LINK0_IP table link0
ip route add default via $LINK0_GW dev eth0 table link0
rule add fwmark 1 table link0 prio 10

ip rule add from 201.64.7.59 table link1
ip route add 201.63.7.0/26 dev eth1 src 201.63.7.59 table link1
ip route add default via 201.63.7.57 dev eth1 table link1
ip rule add fwmark 2 table link1 prio 20

ip route flush cache

Agora é so marcar os pacotes

tipo

iptables -A PREROUTING -t mangle -s <ip interno> -d ! <rede interna> -j MARK --set-mark 1
iptables -t nat -A POSTROUTING -o <eth0> -s <ip interno> -d ! <rede interna> -j MASQUERADE