You need to set up an alternate routing table that only contains your default route, mark packets that you want to go out your default route, and then divert marked packets to the alternate routing table.
1) Create an alternate routing table:
echo '1 DEFGW' >> /etc/iproute2/rt_tables
ip route add default via 192.168.0.1 dev eth1 table DEFGW
This creates an routing tabled called
DEFGW which contains only a default route. Adjust the IP and interface as appropriate for your configuration.
2) Arrange for marked packets to take the alternate table:
ip rule add fwmark 1 table DEFGW
3) Mark packets that should be subject to our rule.
iptables -t mangle -A PREROUTING -d x.x.x.0/255.255.255.0 -j MARK --set-mark 1
This takes packets bound for
x.x.x.0/24 addresses and puts mark 1 on them, the mark that diverts them to the
DEFGW table. Adjust the IP as appropriate for your configuration. We mark the packets before routing them so that the routing engine will know to use the alternate table.