package main

import (
	"encoding/binary"
	"log"
	"net"
	"os"

	"baalvpn/shared"
)

const SERVER_IP = "172.20.12.47"

var (
	internalIP net.IP = nil
)

func main() {
	if os.Geteuid() != 0 {
		panic("root permissions needed")
	}

	listenAddr, err := net.ResolveUDPAddr("udp", ":38001")
	if err != nil {
		panic(err)
	}
	listener, err := net.ListenUDP("udp", listenAddr)
	if err != nil {
		panic(err)
	}
	defer listener.Close()

	go receivePackets(listener)
	register(listener)

	<-make(chan int) // block forever
}

func register(c *net.UDPConn) {
	serverAddr, err := net.ResolveUDPAddr("udp", SERVER_IP+":38000")
	if err != nil {
		panic(err)
	}

	req := shared.BuildPkt(shared.REQ_REGISTER, []byte{})
	if _, err := c.WriteToUDP(req, serverAddr); err != nil {
		panic(err)
	}
}

func receivePackets(listener *net.UDPConn) {
	for {
		buffer := make([]byte, 50000)
		n, addr, err := listener.ReadFromUDP(buffer)
		if err != nil {
			log.Println(err)
			continue
		}
		if n == 0 {
			continue
		}

		go receivePacket(listener, buffer[:n], addr)
	}
}

func receivePacket(c *net.UDPConn, pkt []byte, addr *net.UDPAddr) {
	if len(pkt) < 4 {
		log.Println("packet too short")
		return
	}
	version := binary.LittleEndian.Uint16(pkt[0:])
	if version != shared.PROTO_VERSION {
		log.Println("mismatched packet version")
		return
	}

	pktType := binary.LittleEndian.Uint16(pkt[2:])

	switch pktType {
	case shared.RESP_REGISTER:
		internalIP = net.IP(pkt[4:])
		log.Println("Received IP from the server:", internalIP.String())
	default:
		log.Println("unknown packet type")
	}
}