Two wired connection at the same time - linux

Question

I am struggling with a network problem.
My computer needs to be linked to two differents networks. one via PCI the other one via a USB adapter. The pci is the "usual" network, the usb is to use for specific address.
I have tried differents solutions, with dns, multiple wired connection, modifiying /etc/network/interfaces, ...
But I can't manage to have the 2 networking working at the same time.
Do you have any solution. I am working with Debian - jessie.
Cheers

Answer 1

Since you haven't specified any networks, IP addresses or device names, I will use my machine as an example.
I have an IOGear ethernet USB dongle which shows up as device enx0050b6d341bb, and an RTL811 PCI ethernet device which shows up as eth0. eth0 is plugged into the "main" network which has a DHCP server and enx0050b6d341bb is connected to a private switch on my workbench.
If I want to use eth0 to connect to the internet, but use enx0050b6d341bb to connect to anything on network 192.168.168.0/24, /etc/network/interfaces will look like this:
auto lo
iface lo inet loopback
# Obtain DHCP address from server
auto eth0
iface eth0 inet dhcp
# Connect to 192.168.168.0 network
auto enx0050b6d341bb
iface enx0050b6d341bb inet static
address 192.168.168.3
network 192.168.168.0
netmask 255.255.255.0
Since I only have one device using DHCP, my default route will automatically go through that device, which happens to be exactly what I want :-)
solargy#GEPY633007AX:~$ ip route
default via 192.168.10.1 dev eth0
192.168.10.0/24 dev eth0 proto kernel scope link src 192.168.10.67
192.168.168.0/24 dev enx0050b6d341bb proto kernel scope link src 192.168.168.3
The above shows that my default traffic will go through eth0 and that any traffic for addresses in network 192.168.168.0/24 will go through enx0050b6d341bb. To verify that, you can find out which device will be used to communicate with address 192.168.168.2:
solargy#GEPY633007AX:~$ ip route get 192.168.168.2
192.168.168.2 dev enx0050b6d341bb src 192.168.168.3
cache
As you can see, any traffic for 192.168.168.2 will go through enx0050b6d341bb.