Packet sniffer/proxy.. kind of

hi ... i wanted to know if it's possible to "sniff" the data and be able to send data to an already enstablished connection between 2 apps (server, client) ...

Client ======================================= Server
||
||
My app that can sniff
the data they're sending to each other
and that's able to send a packet
to the client or to the server

it looks kinda proxy but i can't make a proxy cause the port changes and lot of stuff ... i just want to attach to the already enstablished socket and do what i've wrote above... is that possible? ... using API? or how? .. any help or source would be appreciated...

PS: it's not for illegal purpouse i need it to understand the protocol an app is using... to make another app...

You'll want to look into using the WinPcap library that is used most commonly with Ethereal (win32 of course). There is a great OCX called PacketX that automates all this, but it is a 30 day trial.