xs/hkexshd/hkexshd.go

// hkexshd server
//
// Copyright (c) 2017-2018 Russell Magee
// Licensed under the terms of the MIT license (see LICENSE.mit in this
// distribution)
//
// golang implementation by Russ Magee (rmagee_at_gmail.com)
package main

import (
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
	"os/user"
	"runtime"
	"syscall"

	"blitter.com/go/goutmp"
	hkexsh "blitter.com/go/hkexsh"
	"blitter.com/go/hkexsh/spinsult"
	"github.com/kr/pty"
)

type cmdSpec struct {
	op         []byte
	who        []byte
	cmd        []byte
	authCookie []byte
	status     int
}

/* -------------------------------------------------------------- */

/*
 // Run a command (via os.exec) as a specific user
//
// Uses ptys to support commands which expect a terminal.
func runCmdAs(who string, cmd string, conn hkex.Conn) (err error) {
	u, _ := user.Lookup(who)
	var uid, gid uint32
	fmt.Sscanf(u.Uid, "%d", &uid)
	fmt.Sscanf(u.Gid, "%d", &gid)
	fmt.Println("uid:", uid, "gid:", gid)

	args := strings.Split(cmd, " ")
	arg0 := args[0]
	args = args[1:]
	c := exec.Command(arg0, args...)
	c.SysProcAttr = &syscall.SysProcAttr{}
	c.SysProcAttr.Credential = &syscall.Credential{Uid: uid, Gid: gid}
	c.Stdin = conn
	c.Stdout = conn
	c.Stderr = conn

	// Start the command with a pty.
	ptmx, err := pty.Start(c) // returns immediately with ptmx file
	if err != nil {
		return err
	}
	// Make sure to close the pty at the end.
	defer func() { _ = ptmx.Close() }() // Best effort.
	// Copy stdin to the pty and the pty to stdout.
	go func() { _, _ = io.Copy(ptmx, conn) }()
	_, _ = io.Copy(conn, ptmx)

	//err = c.Run()  // returns when c finishes.

	if err != nil {
		log.Printf("Command finished with error: %v", err)
		log.Printf("[%s]\n", cmd)
	}
	return
}
*/

// Run a command (via default shell) as a specific user
//
// Uses ptys to support commands which expect a terminal.
func runShellAs(who string, cmd string, interactive bool, conn hkexsh.Conn, chaffing bool) (err error) {
	u, _ := user.Lookup(who)
	var uid, gid uint32
	fmt.Sscanf(u.Uid, "%d", &uid)
	fmt.Sscanf(u.Gid, "%d", &gid)
	log.Println("uid:", uid, "gid:", gid)

	// Need to clear server's env and set key vars of the
	// target user. This isn't perfect (TERM doesn't seem to
	// work 100%; ANSI/xterm colour isn't working even
	// if we set "xterm" or "ansi" here; and line count
	// reported by 'stty -a' defaults to 24 regardless
	// of client shell window used to run client.
	// Investigate -- rlm 2018-01-26)
	os.Clearenv()
	os.Setenv("HOME", u.HomeDir)
	os.Setenv("TERM", "vt102") // TODO: server or client option?

	var c *exec.Cmd
	if interactive {
		c = exec.Command("/bin/bash", "-i", "-l")
	} else {
		c = exec.Command("/bin/bash", "-c", cmd)
	}
	//If os.Clearenv() isn't called by server above these will be seen in the
	//client's session env.
	//c.Env = []string{"HOME=" + u.HomeDir, "SUDO_GID=", "SUDO_UID=", "SUDO_USER=", "SUDO_COMMAND=", "MAIL=", "LOGNAME="+who}
	c.Dir = u.HomeDir
	c.SysProcAttr = &syscall.SysProcAttr{}
	c.SysProcAttr.Credential = &syscall.Credential{Uid: uid, Gid: gid}
	c.Stdin = conn
	c.Stdout = conn
	c.Stderr = conn

	// Start the command with a pty.
	ptmx, err := pty.Start(c) // returns immediately with ptmx file
	if err != nil {
		return err
	}
	// Make sure to close the pty at the end.
	defer func() { _ = ptmx.Close() }() // Best effort.

	// Watch for term resizes
	go func() {
		for sz := range conn.WinCh {
			log.Printf("[Setting term size to: %v %v]\n", sz.Rows, sz.Cols)
			pty.Setsize(ptmx, &pty.Winsize{Rows: sz.Rows, Cols: sz.Cols})
		}
	}()

	// Copy stdin to the pty.. (bgnd goroutine)
	go func() {
		_, e := io.Copy(ptmx, conn)
		if e != nil {
			log.Printf("** std->pty ended **\n")
			return
		}
	}()

	if chaffing {
		conn.EnableChaff()
	}
	defer conn.DisableChaff()
	defer conn.ShutdownChaff()

	// ..and the pty to stdout.
	_, e := io.Copy(conn, ptmx)
	if e != nil {
		log.Printf("** pty->stdout ended **\n")
		return
	}

	//err = c.Run()  // returns when c finishes.

	log.Printf("[%s]\n", cmd)
	if err != nil {
		log.Printf("Command finished with error: %v", err)
	}
	return
}

func rejectUserMsg() string {
	return "Begone, " + spinsult.GetSentence() + "\r\n"
}

// Demo of a simple server that listens and spawns goroutines for each
// connecting client. Note this code is identical to standard tcp
// server code, save for declaring 'hkex' rather than 'net'
// Listener and Conns. The KEx and encrypt/decrypt is done within the type.
// Compare to 'serverp.go' in this directory to see the equivalence.
func main() {
	version := "0.1pre (NO WARRANTY)"
	var vopt bool
	var chaffEnabled bool
	var chaffFreqMin uint
	var chaffFreqMax uint
	var chaffBytesMax uint
	var dbg bool
	var laddr string

	flag.BoolVar(&vopt, "v", false, "show version")
	flag.StringVar(&laddr, "l", ":2000", "interface[:port] to listen")
	flag.BoolVar(&chaffEnabled, "cE", true, "enabled chaff pkts (default true)")
	flag.UintVar(&chaffFreqMin, "cfm", 100, "chaff pkt freq min (msecs)")
	flag.UintVar(&chaffFreqMax, "cfM", 5000, "chaff pkt freq max (msecs)")
	flag.UintVar(&chaffBytesMax, "cbM", 64, "chaff pkt size max (bytes)")
	flag.BoolVar(&dbg, "d", false, "debug logging")
	flag.Parse()

	if vopt {
		fmt.Printf("version v%s\n", version)
		os.Exit(0)
	}

	if dbg {
		log.SetOutput(os.Stdout)
	} else {
		log.SetOutput(ioutil.Discard)
	}

	// Listen on TCP port 2000 on all available unicast and
	// anycast IP addresses of the local system.
	l, err := hkexsh.Listen("tcp", laddr)
	if err != nil {
		log.Fatal(err)
	}
	defer l.Close()

	log.Println("Serving on", laddr)
	for {
		// Wait for a connection.
		conn, err := l.Accept()
		if err != nil {
			log.Printf("Accept() got error(%v), hanging up.\n", err)
			conn.Close()
			//log.Fatal(err)
		} else {
			log.Println("Accepted client")

			// Set up chaffing to client
			// Will only start when runShellAs() is called
			// after stdin/stdout are hooked up
			conn.SetupChaff(chaffFreqMin, chaffFreqMax, chaffBytesMax) // configure server->client chaffing

			// Handle the connection in a new goroutine.
			// The loop then returns to accepting, so that
			// multiple connections may be served concurrently.
			go func(c hkexsh.Conn) (e error) {
				defer c.Close()

				//We use io.ReadFull() here to guarantee we consume
				//just the data we want for the cmdSpec, and no more.
				//Otherwise data will be sitting in the channel that isn't
				//passed down to the command handlers.
				var rec cmdSpec
				var len1, len2, len3, len4 uint32

				n, err := fmt.Fscanf(c, "%d %d %d %d\n", &len1, &len2, &len3, &len4)
				log.Printf("cmdSpec read:%d %d %d %d\n", len1, len2, len3, len4)

				if err != nil || n < 4 {
					log.Println("[Bad cmdSpec fmt]")
					return err
				}
				//fmt.Printf("  lens:%d %d %d %d\n", len1, len2, len3, len4)

				rec.op = make([]byte, len1, len1)
				_, err = io.ReadFull(c, rec.op)
				if err != nil {
					log.Println("[Bad cmdSpec.op]")
					return err
				}
				rec.who = make([]byte, len2, len2)
				_, err = io.ReadFull(c, rec.who)
				if err != nil {
					log.Println("[Bad cmdSpec.who]")
					return err
				}

				rec.cmd = make([]byte, len3, len3)
				_, err = io.ReadFull(c, rec.cmd)
				if err != nil {
					log.Println("[Bad cmdSpec.cmd]")
					return err
				}

				rec.authCookie = make([]byte, len4, len4)
				_, err = io.ReadFull(c, rec.authCookie)
				if err != nil {
					log.Println("[Bad cmdSpec.authCookie]")
					return err
				}

				log.Printf("[cmdSpec: op:%c who:%s cmd:%s auth:****]\n",
					rec.op[0], string(rec.who), string(rec.cmd))

				valid, allowedCmds := hkexsh.AuthUser(string(rec.who), string(rec.authCookie), "/etc/hkexsh.passwd")
				// Security scrub
				for i := range rec.authCookie {
					rec.authCookie[i] = 0
				}
				runtime.GC()

				if !valid {
					log.Println("Invalid user", string(rec.who))
					c.Write([]byte(rejectUserMsg()))
					return
				}
				log.Printf("[allowedCmds:%s]\n", allowedCmds)

				if rec.op[0] == 'c' {
					// Non-interactive command
					addr := c.RemoteAddr()
					hname := goutmp.GetHost(addr.String())

					log.Println("[Running command for [%s@%s]]\n", rec.who, hname)
					runShellAs(string(rec.who), string(rec.cmd), false, conn, chaffEnabled)
					// Returned hopefully via an EOF or exit/logout;
					// Clear current op so user can enter next, or EOF
					rec.op[0] = 0
					log.Printf("[Command completed for [%s@%s]\n", rec.who, hname)
				} else if rec.op[0] == 's' {
					// Interactive session
					addr := c.RemoteAddr()
					hname := goutmp.GetHost(addr.String())
					log.Println("[Running shell for [%s@%s]]\n", rec.who, hname)

					utmpx := goutmp.Put_utmp(string(rec.who), hname)
					defer func() { goutmp.Unput_utmp(utmpx) }()
					goutmp.Put_lastlog_entry("hkexsh", string(rec.who), hname)
					runShellAs(string(rec.who), string(rec.cmd), true, conn, chaffEnabled)
					// Returned hopefully via an EOF or exit/logout;
					// Clear current op so user can enter next, or EOF
					rec.op[0] = 0
					log.Printf("[Exiting shell for [%s@%s]]\n", rec.who, hname)
				} else {
					log.Println("[Bad cmdSpec]")
				}
				return
			}(conn)
		} // Accept() success
	} //endfor
	log.Println("[Exiting]")
}