How to configure 2 lan cards on ubuntu - lan

Question

I want to configure 2 lan cards on 1 pc such that it seems that both are connected on different networks.
Also i want the gateway of one lan card be the ip address of the other so that the traffic of one passes through the second. I am applying mitmproxy on my pc to monitor my own traffic.
I am new to ubuntu as well as networking. So if anything i have said is wrong, please do correct me.
ifconfig -a
eth0
Link encap:Ethernet HWaddr 00:1c:c0:0d:85:38
inet addr:10.0.0.55 Bcast:10.0.0.255 Mask:255.255.255.0
inet6 addr: fe80::21c:c0ff:fe0d:8538/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:8094 errors:0 dropped:0 overruns:0 frame:0
TX packets:7628 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:6890690 (6.8 MB) TX bytes:1094128 (1.0 MB)
eth1
Link encap:Ethernet HWaddr 10:fe:ed:01:ce:34
inet addr:10.0.0.95 Bcast:10.0.0.255 Mask:255.255.255.0
inet6 addr: fe80::12fe:edff:fe01:ce34/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2338 errors:0 dropped:0 overruns:0 frame:0
TX packets:2322 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1303961 (1.3 MB) TX bytes:256352 (256.3 KB)
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:3849 errors:0 dropped:0 overruns:0 frame:0
TX packets:3849 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:1008514 (1.0 MB) TX bytes:1008514 (1.0 MB)
Thanks

Answer 1

You can try iptables to redirect traffic from 1 interface to other.
iptables -t nat -A PREROUTING -i ppp0 -p tcp --dport 80 -j REDIRECT --to-ports 8080
will redirect all traffic coming on interface ppp0 on port 80 to port 8080 , where you can run an mitm proxy. Your mitm proxy can use eth0 to forward traffic out.
You might get a better response on serverfault or superuser.

Answer 2

In order to put two different network cards on two different networks you need to assign their IP's properly.
Based on the fact that you are using Net Mask of 255.255.255.0 setting one network card to have IP 10.0.0.1 and second to 10.0.1.1 would put them on two seperate networks. Netmask of 255.255.255.0 defines that your network can contain all IP's which have same first three numbers and fourth from 0 to 255. So if you change one of the first three numbers you put your network card on another network.
As for gateway it must be on the same network that your lan card is. So setting two network card to be on two seperate networks and then setting one lans card gateway to be the other network card won't work. Athleast not until you bridge the two networks together, but that won't prevent computers from one network to access computers on another which I belive is what you wanna do.