Intercepting/Modifying/Viewing Packets?

[tackyjan]

Hello all!

I am new to network programming so forgive my ignorance.

I have a question regarding packet intercepting/viewing, etc.

I have a process that runs on my PC. This process communicates to a server using a fixed IP address (I cannot modify it).

What I want to do is write a program that intercepts the packets going out to the server and allows me to view and or modify the packets. And then the reverse, I want to intercept the returning packets then forward them back to the process.

If I cannot change the IP address / port that the PC process uses how can I "redirect" the packets into my interceptor application? I know there is such a thing as port forwarding that you can setup with your router but I don't think what I am doing is port forwarding. (Or maybe it is?)

Basically here is what I need to do.

[Current]
PROCESS -> SERVER
SERVER->PROCESS

[NEW]
PROCESS->INTERCEPTOR->SERVER
SERVER->INTERCEPTOR->PROCESS

If I could modify the IP address that the process uses I could just change it to connect to my "interceptor." I believe that the fact that I cannot change the IP address is the clutch of the problem here.

Also note that I don't want to just "sniff" the packets. Sniffing would allow me to see the packets but I need to modify them as well.

I hope I was able to describe what I am looking to do well enough for anyone to point me in the right direction.

Perhaps an image will help explain what I need:



Thanks in advance!

Jan