A skeletal model for the client-proxy-server system Client The client should do the following things: Read the file that's going to get sent open(), fp.read() etc. Create a socket and establish a TCP connection to the proxy socket.socket(), proxysock.connect() with SOCK_STREAM Send the file data stored into a variable in step 1 to the proxy proxysock.sendall() Wait for an answer proxysock.recv() Close the connection proxysock.close() Proxy The proxy, being basically both a kind of a server and a client, should do the following things: Create a listening socket, wait for incoming connections from the client, accept and store the handle of an incoming connection socket.socket(), proxyserversock.bind(), proxyserversock.listen(), proxyserversock.accept() Read the data from the incoming connection clientsock.recv() Store the received data into a file open(), fp.write() etc. Create a socket and establish a TCP connection to the server socket.socket(), serversock.connect() again with SOCK_STREAM Send the data received from the client onwards to the server serversock.sendall() Wait for an aswer serversock.recv() Pass the answer on to the client clientsock.sendall() Close the connections to the server and the client, but leave the listening socket on serversock.close(), clientsock.close() Server The server should do the following things: Create a listening socket, wait for incoming connections from the proxy, accept and store the handle of an incoming connection socket.socket(), serversock.bind(), serversock.listen(), serversock.accept() Read the data from the incoming connection proxyclientsock.recv() Store the received data into a file open(), fp.write() etc. Send an answer to the proxy proxyclientsock.sendall() Close the connection, but leave the listening socket on proxyclientsock.close() Additional notes After implementing the basic model described above, you can easily start adding more features. For example, you could add a sleep in the proxy between steps 3 and 4 and instead of directly sending the data received from the client to the server, you could read the file and send the contents - this way you can manually change the file before it gets sent to the server. I also suggest you read Beej's Guide to Network Programming if you want to really learn the basics of network programming. The examples are in C, but the basic principles also apply in Python (and other languages as well).