Q:

How to create a generic network proxy using Python or C++?

I have an application which communicates over the local area network. However, I want to instead make it communicate over the internet. To do this I propose making an intermediate program which will read the network traffic generated from the application on one computer and send it to the application on another computer.
This involves:

Reading the outgoing network traffic of the application
Sending a copy of this traffic over the internet to another computer
Giving this copy to the application on the other computer

Instead of this:
Application on computer A <-LAN-> Application on computer B
I want to achieve this:
Application on A <--> My Program on A <-INTERNET-> My program on B <--> Application on B
I can accomplish (2), but with (1) and (3) my problem is that I have very little experience with networking and I do not know where to start. I can program in python but would be willing to use c++ to accomplish this.
(Hamachi does not work for this application, I do not know why.)
In response to comments
I do not intend to manipulate any data unless it is necessary to make the connection work. I have no control over the application itself and it does not provide me with any methods to configure the connection with the exception of a port number.
TCP and UDP are both used on the port 6112. The IP addresses used are first 255.255.255.255 for a generic broadcast used to discover other applications on the LAN (with UDP), then a TCP connection is established.

A:

The term you are missing in your original question is proxy. You specifically need a transparent forwarding proxy.
Here is a link to some source code in Python that will get you started with writing a proxy.
That said, if you search around you should be able to find a transparent forwarding proxy that you don't have to write yourself.
If you want to do this the most robust way, you can do it in hardware and setup a managed router/switch/firewall to route things to where ever you need without having to write anything.

