tlsserver/tlsserver.go

package main

import (
	"crypto/tls"
	"flag"
	"fmt"
	"log"
	"net"
	"os"
	"os/exec"
	"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.Int("port", 1234, "Port to bind to")
	/*
		Rather than using setuid/setgid we rely on setcap CAP_NET_BIND_SERVICE
		uid   = flag.Int("uid", -1, "UID to run under")
		gid   = flag.Int("gid", -1, "GID to run under")
	*/
	args  []string
	nargs int
)

func main() {

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

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

	/*
		The right way to handle/drop privileges is to start with a
		low-privileged user and use setcap CAP_NET_BIND_SERVICE on the
		binary to allow for the listen-operation.

		// set uid/gid
		if *gid >= 0 {
			err := setgid(*gid) // syscall.Setgid(*gid)
			if err != nil {
				fmt.Println("Couldn't setgid to", *gid, ":", err)
				os.Exit(4)
			}
		}

		if *uid >= 0 {
			err := setuid(*uid) // syscall.Setuid(*uid)
			if err != nil {
				fmt.Println("Couldn't setuid to", *uid, ":", err)
				os.Exit(4)
			}
		}
	*/

	// 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)
	}
}

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

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

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