dissertation-2-code/main.go

package main

import (
	"errors"
	"fmt"
	"log"
	"mpbl3p/config"
	"mpbl3p/flags"
	"mpbl3p/tun"
	"os"
	"os/signal"
	"strconv"
	"syscall"
)

const (
	ENV_NC_TUN_FD      = "NC_TUN_FD"
	ENV_NC_CONFIG_PATH = "NC_CONFIG_PATH"
)

func main() {
	log.SetFlags(log.Ldate | log.Ltime | log.Llongfile)

	if _, exists := os.LookupEnv(ENV_NC_TUN_FD); !exists {
		// we are the parent process
		// 1) process arguments
		// 2) validate config
		// 2) create a tun adapter
		// 3) spawn a child
		// 4) exit

		o, err := flags.ParseFlags()
		if err != nil {
			if errors.Is(err, flags.PrintedHelpErr) {
				return
			}
			panic(err)
		}

		log.Println("loading config...")

		c, err := config.LoadConfig(o.ConfigFile)
		if err != nil {
			panic(err)
		}

		log.Println("creating tun adapter...")
		t, err := tun.NewTun(o.Positional.InterfaceName, int(c.Host.MTU))
		if err != nil {
			panic(err)
		}

		if o.Foreground {
			if err := os.Setenv(ENV_NC_TUN_FD, fmt.Sprintf("%d", t.File().Fd())); err != nil {
				panic(err)
			}
			if err := os.Setenv(ENV_NC_CONFIG_PATH, o.ConfigFile); err != nil {
				panic(err)
			}
			log.Println("switching to foreground")
			goto FOREGROUND
		}

		files := make([]*os.File, 4)
		files[0], _ = os.Open(os.DevNull) // stdin
		files[1], _ = os.Open(os.DevNull) // stderr
		files[2], _ = os.Open(os.DevNull) // stdout
		files[3] = t.File()

		env := os.Environ()
		env = append(env, fmt.Sprintf("%s=3", ENV_NC_TUN_FD))
		env = append(env, fmt.Sprintf("%s=%s", ENV_NC_CONFIG_PATH, o.ConfigFile))

		attr := os.ProcAttr{
			Env:   env,
			Files: files,
		}

		path, err := os.Executable()
		if err != nil {
			panic(err)
		}

		process, err := os.StartProcess(
			path,
			os.Args,
			&attr,
		)
		if err != nil {
			panic(err)
		}

		pidFile, err := os.Create(o.PidFile)
		if err != nil {
			panic(err)
		}
		if _, err := fmt.Fprintf(pidFile, "%d", process.Pid); err != nil {
			panic(err)
		}

		_ = process.Release()
		return
	}

	// we are the child process
	// 1) recreate tun adapter from file descriptor
	// 2) launch proxy

FOREGROUND:

	log.Println("loading config...")

	c, err := config.LoadConfig(os.Getenv(ENV_NC_CONFIG_PATH))
	if err != nil {
		panic(err)
	}

	log.Println("connecting tun adapter...")
	tunFd, err := strconv.ParseUint(os.Getenv(ENV_NC_TUN_FD), 10, 64)
	if err != nil {
		panic(err)
	}

	t, err := tun.NewFromFile(uintptr(tunFd), int(c.Host.MTU))
	if err != nil {
		panic(err)
	}
	defer func() {
		if err := t.Close(); err != nil {
			panic(err)
		}
	}()

	log.Println("building config...")
	p, err := c.Build(t, t)
	if err != nil {
		panic(err)
	}

	log.Println("starting proxy...")
	p.Start()

	log.Println("proxy started")

	signals := make(chan os.Signal)
	signal.Notify(signals, syscall.SIGTERM, syscall.SIGINT)

	<-signals
	log.Println("exiting...")
}