I Have the Following Network Setup.

Internet ISP 1 = Cable Modem
Internet ISP 2 = ADSL Modem

Linux 7.3 Gateway PC
NIC 1 Internal LAN Static Assigned
NIC 2 ISP 1 DHCP Assigned
NIC 3 ISP 2 Static Assigned.

I am using IPTABLES as the Firewall with NAT

where NIC 1 SNAT > NIC 2 Works OK.

I would like to manual be able to change ISP 2 as Internet Route
but when I change SNAT to use NIC 3 it will not work.

During all testing the IPTABLES Firewall is set to allow all input,output and forward rules.

The following Works OK

iptables -t nat -A POSTROUTING -s ${INET}${IMASK} -o ${OIF} -j SNAT --to-source ${OIP}

The Following fails

iptables -t nat -A POSTROUTING -s ${INET}${IMASK} -o ${OIF2} -j SNAT --to-source ${OIP2}

routing table displays

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.X.0 * 255.255.255.0 U 0 0 0 eth0
207.x.x.0 * 255.255.255.0 U 0 0 0 eth1
68.x.0.0 * 255.255.254.0 U 0 0 0 eth2
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
default 68.x.0.1 0.0.0.0 UG 0 0 0 eth2
default * 0.0.0.0 U 0 0 0 eth1

I do not what to use both at the same time and only manually change if one of the ISP connections is down, or I require to test a service from a difference source IP Address.

Thanks if you have some ideas of where to go.

Is there any tools that displays what route a packet will take ?

There is a root program called "tracepath".

Is the third octet of your dsl's IP address 0? Also, I don't get the last line of your route table.

If you only want to use either Cable or DSL, then shut down the unused interface.

I need both interfaces running since I connect to them from remote systems.

ip route add default via 207.x.1.1 table 1
ip route add 207.x.1.0 dev eth1 src 207.x.1.35 table 1
ip rule add from 207.x.1.35 table 1 priority 100

ip route add 68.x.0.0 dev eth2 src 68.151.1.60 table 2
ip rule add from 68.x.1.60 table 2 priority 800

These are additional route info at startup.

No the DSL IP is 207.x.1.0 is correct.