Internet Connection Sharing using routes

Question

Situation

I'm trying to share my Desktop's Internet connection to my Laptop.
Here is the layout of my network:

• I only control the green computers.
• The routes on the desktop are automatically and correctly set by DHCP and the OpenVPN client config file.
• I have access to all the orange networks on it.

Here's what I tried:

• using Windows Internet Connection Sharing, sharing tap0 to eth1 :
  Works for all orange networks except 172.16.0.0/12
• using NET32 IP Router, internet from tap0 and private on eth1 :
  Same problem as I have to choose the interface that has internet access.
  I tried selecting both eth0 and eth1, but it forces me to set them to static ip, which breaks everything because I need to be using DHCP on 172.16.0.0/12.

Main question :

• Is there a way, on the desktop computer, to forward the packets from an interface to their destination, using the system's routes to choose the interface and gateway ?
• On a more general note, is there a reason not to do it by default ?

Answer 1

it sounds more like you need a packetshaper and a router with assignable v-lan ports. i suggest eBay for the packetshaper they can be managed from a laptop and once configured they are autonomous. packetshapers i recommend based on ease of deployment

what im used to working with for switching

and this is something more exacting to what your trying to do

hope this gets you pointed in the right direction.