Rerouting Application Network Traffic at the Data Link Layer

Question

Consider the following situation:

You have an application you are tesing, but in order to test the networking functionality of said program, you are required to run multiple instances of it and have them communicate with one another.

Possible solutions are: - Run software on individual machines connected by WAN or LAN. - Run the software on virtual machines, all on the same computer.

I do not want to use either of these methods (the reasoning is irrelevant). I want to know if there is a way that I can reroute network transmissions from the test application (ideally in any programmming language) in a way such that I can run multiple instances of the same software on one computer, and have them behave as if they were the only instance running on that computer.

In other words, I want to be able to code the application so that each instance listens on the same "listening" port (since only one instance will be running on each computer when in production). Then, I want to know if I can reroute the network requests at a lower level then the application so that they do not interfere with eachother (clash over the same port number).

Essentially, I want to build a virtual environment which only redirects the network calls (whereas a virtual machine takes far more resources, and has way more involved). Is this possible, and how might I approach this problem?

Thank you!

UPDATE: This is a more accurate idea of what I want to accomplish:

Basically, I want to program another application which TRANSPARENTLY redirects bind requests to available ports, and manages which applications are bound where... So from the applications perspective, all the instances are bound to port 1000, but in reality, this other application is automatically managing which instance is bound where, and avoiding potential conflicts. I feel like this could be accomplished with Windows Hooks, but I'm not sure how you could implement this?

Answer 1

As far as I know, there is no sane way to multiplex the same port on the same network device. At the very minimum, you will need to choose on of the following:

• Run each instance of your program on a different port
• Create multiple virtual network interfaces

The first choice is easy and may be the one I would choose. The second one is more towards what you are looking for but it would be a true PITA to set up - you can look into VirtualBox and its host-only networks for inspiration. If you are writing things on linux you might look into pipes and chrooting but you'll be spending more time setting up this environment than writing your software.