TCP proxy

tcpproxy.js

var net = require('net'),
    tls = require('tls'),
    fs = require("fs");

var targetHost = "imap.gmail.com",
    targetPort = "993",
    targetSecure = true,
    logfile = "log.txt",
    proxyPort = 143;

var sessionCounter = 0,
    logStream = fs.createWriteStream(logfile),

    server = net.createServer(function(client) { //'connection' listener
        console.log('Client connected');
        
        var socket, target, session = ++sessionCounter;
        
        client.on('end', function() {
            console.log('Client disconnected');
            if(socket && !socket.destroyed){
                socket.end();
            }
        });
        
        client.on('data', function(chunk) {
            var str = (chunk || "").toString("utf-8").trim();
            if(str){
                console.log("CLIENT ("+session+"): " + str);
                if(logStream){
                    logStream.write("~~~~~~~~~~~~~~~~~= CLIENT ("+session+") =~~~~~~~~~~~~~~~~~~\r\n"+str+"\r\n");
                }
            }
        });
      
        client.on('error', function(err) {
            console.log("Client error");
            console.log(err);
            if(socket && !socket.destroyed){
                socket.end();
            }
        });
    
        var target = (targetSecure?tls:net).connect(targetPort, targetHost, function() {
            
            console.log("Server connected");
            
            socket = targetSecure ? target.socket : target;
            
            socket.setKeepAlive(true);
    
            if(client && !client.destroyed){
                client.pipe(target);
            }else{
                socket.end();
            }
            
        });
            
        target.on('data', function(chunk) {
            var str = (chunk || "").toString("utf-8").trim();
            if(str){
                console.log("SERVER ("+session+"): " + str);
                if(logStream){
                    logStream.write("~~~~~~~~~~~~~~~~~= SERVER ("+session+") =~~~~~~~~~~~~~~~~~~\r\n"+str+"\r\n");
                }
            }
            
            // do not announce compression support on IMAP
            if(str.match(/\bCOMPRESS\=[\w]+/)){
                chunk = new Buffer(str.replace(/COMPRESS\=[\w]+/g, "").trim()+"\r\n", "utf-8");
            }
            
            client.write(chunk);
        });
    
        target.on('end', function() {
            console.log("Server disconnected");
            client.end();
        });
    
        target.on('error', function(err) {
            console.log("Server error");
            console.log(err);
            client.end();
        });
    
    });

server.listen(proxyPort, function() { //'listening' listener
    console.log('Proxy bound');
});

usage.md

Creates a TCP proxy for logging protocols like IMAP or SMTP between a client and a server.

Update param values in the beginning of the file to set the proxy target.

Outputs traffic both to console and file.