import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;
 
public class TcpProxy {
static class Listener extends Thread {
final ServerSocket server;
 
final String remoteHost;
 
final int remotePort;
 
Listener(ServerSocket server, String remoteHost,
int remotePort) {
this.server = server;
this.remoteHost = remoteHost;
this.remotePort = remotePort;
}
 
public void run() {
try {
while (true) {
try {
Socket client = server.accept();
System.out.println("accepted connection from "
+ client.getInetAddress() + ":"
+ client.getPort());
new Connection(client, remoteHost,
 remotePort).start();
}
catch (Exception e) {
e.printStackTrace();
}
}
}
finally {
System.exit(0);
}
}
}
 
static class Connection extends Thread {
final Socket client;
 
final Socket target;
 
final InputStream clientIn;
 
final InputStream targetIn;
 
final OutputStream clientOut;
 
final OutputStream targetOut;
 
Connection(Socket client, String remoteHost,
int remotePort)throws Exception {
this.client = client;
try {
clientIn = client.getInputStream();
clientOut = client.getOutputStream();
target = new Socket(remoteHost,
remotePort);
try {
targetIn = target.getInputStream();
targetOut = target.getOutputStream();
}
catch (Exception e) {
try {
target.close();
}
catch (Throwable t) {
//
}
throw e;
}
}
catch (Exception e) {
try {
client.close();
}
catch (Throwable t) {
//
}
throw e;
}
}
 
public void run() {
byte buf[] = new byte[4096];
int av, r;
while (true) {
try {
av = clientIn.available();
if (av > 0) {
r = clientIn.read(buf);
targetOut.write(buf, 0, r);
}
av = targetIn.available();
if (av > 0) {
r = targetIn.read(buf);
clientOut.write(buf, 0, r);
}
Thread.sleep(1);
if (target.isClosed() || !target.isConnected()
|| client.isClosed() || !client.isConnected()) {
System.out.println("closed connection from "
+ client.getInetAddress() + ":"
+ client.getPort());
try {
target.close();
}
catch (Throwable t) {
//
}
try {
client.close();
}
catch (Throwable t) {
//
}
}
}
catch (Exception e) {
e.printStackTrace();
}
}
}
}
 
/**
 * 
 * @param args
 * @throws IOException
 */
public static void main(String args[]) throws IOException {
int localPort = Integer.parseInt(args[0]);
String remoteHost = args[1];
int remotePort = Integer.parseInt(args[2]);
System.out.println("TCP proxy");
System.out.println("local port: " + localPort);
System.out.println("remote host: " + remoteHost);
System.out.println("remote port: " + remotePort);
 
ServerSocket server = new ServerSocket(localPort, 50);
new Listener(server, remoteHost, remotePort).start();
}
}