go.mod added

If -uid is set, we fork a child process with the uid, the gid and the
supplimentary gids for that user.

go.mod

@@ -0,0 +1,3 @@
+module tlsserver
+
+go 1.21.6

tlsserver.go

@@ -2,34 +2,109 @@ package main
 
 import (
 	"crypto/tls"
-	"log"
-	"net"
 	"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.Int("port", 1234, "Port to bind to")
-	args []string
-	nargs int
+	port  = flag.String("port", ":1234", "Port to bind to")
+	uid   = flag.Int("uid", -1, "UID to run under")
+	args  []string
 )
 
-func main() {
+const TLSSERVER = "[tlsserver]"
 
+func main() {
 	flag.Parse()
-	 args = flag.Args()
-	nargs = flag.NArg()
-	if nargs < 1 {
+
+	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)
@@ -42,8 +117,7 @@ func main() {
 	tconf.BuildNameToCertificate()
 
 	// start listening
-	sport := fmt.Sprintf(":%d", *port)
-	sock, err := tls.Listen("tcp", sport , tconf)
+	sock, err := tls.Listen("tcp", *port, tconf)
 	if err != nil {
 		fmt.Println("error with tcp-socket:", err)
 		os.Exit(3)
@@ -58,11 +132,11 @@ func main() {
 			continue
 		}
 		log.Println("Got connection:", conn.RemoteAddr())
-		go handleConnection(conn)
+		go handleConnection(conn, args...)
 	}
 }
 
-func handleConnection(conn net.Conn) {
+func handleConnection(conn net.Conn, args ...string) {
 	defer conn.Close()
 
 	// setup cmd
@@ -71,9 +145,51 @@ func handleConnection(conn net.Conn) {
 	cmd.Stdin = conn
 	cmd.Stdout = conn
 	cmd.Stderr = os.Stderr
-	err := cmd.Run()
+
+	// prepare environment according to tcp-environ(5)
+	lh, lp, err := net.SplitHostPort(conn.LocalAddr().String())
 	if err != nil {
-		log.Println(err)
+		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
+}