// Example usage:
// Terminal 1: python -m http.server 8000
// Terminal 2: go run tcpproxy.go 9000 localhost:8000
// Terminal 3: curl localhost:9000
package main

import (
	"fmt"
	"io"
	"net"
	"os"
)

func main() {
	if len(os.Args) != 3 {
		fmt.Println("Usage: go run tcpproxy.go <listen_port> <target_host:port>")
		os.Exit(1)
	}
	
	listenPort := os.Args[1]   // port to listen on
	targetAddr := os.Args[2]   // target server address
	
	// create TCP listener on specified port
	listener, err := net.Listen("tcp", ":"+listenPort)
	if err != nil {
		fmt.Printf("Failed to listen on port %s: %v\n", listenPort, err)
		os.Exit(1)
	}
	defer listener.Close()
	
	fmt.Printf("TCP proxy listening on port %s, forwarding to %s\n", listenPort, targetAddr)
	
	// accept incoming connections
	for {
		clientConn, err := listener.Accept()
		if err != nil {
			fmt.Printf("Failed to accept connection: %v\n", err)
			continue
		}
		
		// handle each client connection concurrently
		go handleConnection(clientConn, targetAddr)
	}
}

func handleConnection(clientConn net.Conn, targetAddr string) {
	defer clientConn.Close() // ensure client connection is closed
	
	// connect to target server
	serverConn, err := net.Dial("tcp", targetAddr)
	if err != nil {
		fmt.Printf("Failed to connect to target %s: %v\n", targetAddr, err)
		return
	}
	defer serverConn.Close() // ensure server connection is closed
	
	// start bidirectional data copying
	go io.Copy(serverConn, clientConn) // client -> server
	io.Copy(clientConn, serverConn)    // server -> client (blocks until done)
}