Linux ipv4 forwarding not working as expected

Solution 1:

When your Pi gets a package that is not handled directly, it probably issues a ICMP route redirect, telling your PC to use 10.1.1.1 directly. This however doesn't work very well; it's a security risk, so many modern operating systems ignore them.

The proper solution is one of two:

• Have one router per subnet
• Have a route explicitly declared on the client.

One router per subnet

Configure the network between your Pi and your PC to a different subnet than the rest of your LAN, e.g. 10.1.2.0/24. Your Pi will then have a default route with gw 10.1.1.1, and 10.1.4.0/24 with some gateway.

Any machines behind the Pi will simply have 10.1.2.0/24 with Pi as gateway.

Declare route on client

Configure your Pi to have a IP in the 10.1.1.0/24-subnet. Do not set it as default gateway on any PC's; they should all have 10.1.1.1.

Add a route to 10.1.4.0/24 via the Pi. On windows this can be done with

route ADD 10.1.4.0 MASK 255.255.255.0 10.1.1.42 

Use route -p ADD... to persist the route across reboots.