First, I know this is not how IP is intended. But as usual, this is how I need to have it working.
I work for a security company. There are at least 2 network interfaces. Both interfaces should have a default gateway. One interface is a default Ethernet interface, the second is a 3g interface. Both connect to the internet. Different configurations are possible as in one interface is the main and the other is the backup. Or the Ethernet interface is local LAN (with different subnets, gateway) and the 3G modem is the internet connection.
My main question is, how can I get the Linux TCP/IP stack to answer back on the same interface as a request came from.
The second question how can I start a connection through the interface of my choice, without playing with different subnets on different interfaces.