port knocker by chatgpt

client.go

// Client
package main

import (
	"fmt"
	"net"
	"time"
)

func main() {
	ip := net.ParseIP("127.0.0.1")
	knock := []string{"1111", "2222", "3333"}

	for _, port := range knock {
		conn, err := net.DialTimeout("tcp", ip.String()+":"+port, 1*time.Second)
		if err != nil {
			fmt.Println("Port Knocking failed:", err)
		}
		if conn != nil {
			conn.Close()
		}
	}
	fmt.Println("Port Knocking successful!")
}

server.go

// Server
package main

import (
	"fmt"
	"net"
)

var knockCounter int

func main() {
	conn, err := net.Listen("tcp", ":1111")
	if err != nil {
		fmt.Println("Error listening:", err.Error())
		return
	}
	defer conn.Close()

	fmt.Println("Listening on port 1111...")

	for {
		c, err := conn.Accept()
		if err != nil {
			fmt.Println("Error accepting:", err.Error())
			return
		}
		go handleRequest(c)
	}
}

func handleRequest(c net.Conn) {
	defer c.Close()

	knockCounter++
	fmt.Println("Connection opened. Knock counter:", knockCounter)

	if knockCounter == 3 {
		fmt.Println("All knocks received. Opening port 2222...")
		openPort("2222")
		knockCounter = 0
	}
}

func openPort(port string) {
	listener, err := net.Listen("tcp", ":"+port)
	if err != nil {
		fmt.Println("Error listening on port", port, err.Error())
		return
	}
	defer listener.Close()

	fmt.Println("Listening on port", port)
	for {
		conn, err := listener.Accept()
		if err != nil {
			fmt.Println("Error accepting: ", err.Error())
			return
		}
		go handleConnection(conn)
	}
}

func handleConnection(conn net.Conn) {
	defer conn.Close()

	fmt.Println("Connection from: ", conn.RemoteAddr())
}