tlsserver/tlsserver.go

package main

import (
	"crypto/tls"
	"flag"
	"fmt"
	"log"
	"net"
	"os"
	"os/exec"
	"os/user"
	"strconv"
	"syscall"
)

var (
	cfile = flag.String("cert", "cert.pem", "Certificate file in PEM format")
	kfile = flag.String("key", "key.pem", "Key file in PEM format")
	port  = flag.String("port", ":1234", "Port to bind to")
	uid   = flag.Int("uid", -1, "UID to run under")
	args  []string
)

const TLSSERVER = "[tlsserver]"

func main() {
	flag.Parse()

	if flag.NArg() < 1 {
		fmt.Println("Usage: tlsserver [options] cmd [flags for cmd]")
		fmt.Println("options:")
		flag.PrintDefaults()
		os.Exit(1)
	}

	if os.Args[0] == TLSSERVER {
		log.Fatal(serve())
	} else {
		log.Fatal(exc())
	}
}

func exc() error {
	if *uid < 0 {
		return serve()
	}

	// Collect information about the user
	user, err := user.LookupId(strconv.Itoa(*uid))
	if err != nil {
		return err
	}

	// Get the primary and secondary groups
	gids := []int{}

	gid, err := strconv.Atoi(user.Gid)
	if err != nil {
		return fmt.Errorf("couldn't parse user.Gid, strconv.Atoi(%s): %v", user.Gid, err)
	}

	if sgids, err := user.GroupIds(); err != nil {
		log.Println("weird, couldn't get supplimentary groups: %v", err)
	} else {
		for _, s := range sgids {
			v, e := strconv.Atoi(s)
			if e != nil {
				log.Println("weird, strconv.Atoi(%s): %v", s, e)
				continue
			} else if v != gid {
				gids = append(gids, v)
			}
		}
	}

	if err := setgid(gid); err != nil {
		return err
	}

	if len(gids) > 0 {
		if err = syscall.Setgroups(gids); err != nil {
			return fmt.Errorf("syscall.Setgroups: %w", err)
		}
	}

	if err = setuid(*uid); err != nil {
		return err
	}

	// Setup the arguments that are needed
	args := []string{TLSSERVER} // name of the command

	for _, arg := range []string{"cert", "key", "port"} {
		fl := flag.Lookup(arg)
		// Only set the flag if it's value are different from the default
		if fl != nil && fl.Value.String() != fl.DefValue {
			args = append(args, "-"+arg, fl.Value.String())
		}
	}
	args = append(args, flag.Args()...)

	return syscall.Exec(os.Args[0], args, os.Environ())
}

func serve() error {
	args = flag.Args()

	// setup certs etc. for TLS-socket
	tconf := new(tls.Config)
	cert, err := tls.LoadX509KeyPair(*cfile, *kfile)
	if err != nil {
		fmt.Println("error with certs:", err)
		os.Exit(2)
	}

	tconf.Certificates = append(tconf.Certificates, cert)
	tconf.BuildNameToCertificate()

	// start listening
	sock, err := tls.Listen("tcp", *port, tconf)
	if err != nil {
		fmt.Println("error with tcp-socket:", err)
		os.Exit(3)
	}
	defer sock.Close()

	// accept-loop
	for {
		conn, err := sock.Accept()
		if err != nil {
			log.Println("error during Accept()", err)
			continue
		}
		log.Println("Got connection:", conn.RemoteAddr())
		go handleConnection(conn, args...)
	}
}

func handleConnection(conn net.Conn, args ...string) {
	defer conn.Close()

	// setup cmd
	cmd := exec.Command(args[0])
	cmd.Args = args
	cmd.Stdin = conn
	cmd.Stdout = conn
	cmd.Stderr = os.Stderr

	// prepare environment according to tcp-environ(5)
	lh, lp, err := net.SplitHostPort(conn.LocalAddr().String())
	if err != nil {
		log.Println("net.SplitHostPort(conn.LocalAddr()):", err)
		return
	}

	rh, rp, err := net.SplitHostPort(conn.RemoteAddr().String())
	if err != nil {
		log.Println("net.SplitHostPort(conn.RemoteAddr()):", err)
		return
	}

	cmd.Env = make([]string, 0)
	cmd.Env = append(cmd.Env, "PATH="+os.Getenv("PATH"))
	cmd.Env = append(cmd.Env, "PROTO=TCP")
	cmd.Env = append(cmd.Env, "TCPLOCALIP="+lh)
	cmd.Env = append(cmd.Env, "TCPLOCALPORT="+lp)
	cmd.Env = append(cmd.Env, "TCPREMOTEIP="+rh)
	cmd.Env = append(cmd.Env, "TCPREMOTEPORT="+rp)

	err = cmd.Run()
	if err != nil {
		log.Println("after Run: ", err)
	}
	log.Println("Done with connection", conn.RemoteAddr())
}

// Since go1.4 the setgid syscall is deliberatelly not supported anymore, as it
// only applies to the calling thread.  So we try this here:
func setgid(gid int) error {
	// RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
	_, _, e := syscall.RawSyscall(syscall.SYS_SETGID, uintptr(gid), 0, 0)
	if e != 0 {
		return fmt.Errorf("setgid: %w", e)
	}
	return nil
}

func setuid(uid int) error {
	// RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
	_, _, e := syscall.RawSyscall(syscall.SYS_SETUID, uintptr(uid), 0, 0)
	if e != 0 {
		return fmt.Errorf("setuid: %w", e)
	}
	return nil
}