snowflake/proxy/snowflake.go

package main

import (
	"bytes"
	"crypto/rand"
	"encoding/base64"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"net/url"
	"os"
	"regexp"
	"strings"
	"sync"
	"time"

	"git.torproject.org/pluggable-transports/snowflake.git/common/messages"
	"git.torproject.org/pluggable-transports/snowflake.git/common/nat"
	"git.torproject.org/pluggable-transports/snowflake.git/common/safelog"
	"git.torproject.org/pluggable-transports/snowflake.git/common/util"
	"git.torproject.org/pluggable-transports/snowflake.git/common/websocketconn"
	"github.com/gorilla/websocket"
	"github.com/pion/webrtc/v2"
)

const defaultBrokerURL = "https://snowflake-broker.bamsoftware.com/"
const defaultRelayURL = "wss://snowflake.bamsoftware.com/"
const defaultSTUNURL = "stun:stun.l.google.com:19302"
const pollInterval = 5 * time.Second
const (
	NATUnknown      = "unknown"
	NATRestricted   = "restricted"
	NATUnrestricted = "unrestricted"
)

//amount of time after sending an SDP answer before the proxy assumes the
//client is not going to connect
const dataChannelTimeout = 20 * time.Second

const readLimit = 100000 //Maximum number of bytes to be read from an HTTP request

var broker *Broker
var relayURL string

var currentNATType = NATUnknown

const (
	sessionIDLength = 16
)

var (
	tokens chan bool
	config webrtc.Configuration
	client http.Client
)

var remoteIPPatterns = []*regexp.Regexp{
	/* IPv4 */
	regexp.MustCompile(`(?m)^c=IN IP4 ([\d.]+)(?:(?:\/\d+)?\/\d+)?(:? |\r?\n)`),
	/* IPv6 */
	regexp.MustCompile(`(?m)^c=IN IP6 ([0-9A-Fa-f:.]+)(?:\/\d+)?(:? |\r?\n)`),
}

// https://tools.ietf.org/html/rfc4566#section-5.7
func remoteIPFromSDP(sdp string) net.IP {
	for _, pattern := range remoteIPPatterns {
		m := pattern.FindStringSubmatch(sdp)
		if m != nil {
			// Ignore parsing errors, ParseIP returns nil.
			return net.ParseIP(m[1])
		}
	}
	return nil
}

type Broker struct {
	url                *url.URL
	transport          http.RoundTripper
	keepLocalAddresses bool
}

type webRTCConn struct {
	dc *webrtc.DataChannel
	pc *webrtc.PeerConnection
	pr *io.PipeReader

	lock sync.Mutex // Synchronization for DataChannel destruction
	once sync.Once  // Synchronization for PeerConnection destruction
}

func (c *webRTCConn) Read(b []byte) (int, error) {
	return c.pr.Read(b)
}

func (c *webRTCConn) Write(b []byte) (int, error) {
	c.lock.Lock()
	defer c.lock.Unlock()
	if c.dc != nil {
		c.dc.Send(b)
	}
	return len(b), nil
}

func (c *webRTCConn) Close() (err error) {
	c.once.Do(func() {
		err = c.pc.Close()
	})
	return
}

func (c *webRTCConn) LocalAddr() net.Addr {
	return nil
}

func (c *webRTCConn) RemoteAddr() net.Addr {
	//Parse Remote SDP offer and extract client IP
	clientIP := remoteIPFromSDP(c.pc.RemoteDescription().SDP)
	if clientIP == nil {
		return nil
	}
	return &net.IPAddr{IP: clientIP, Zone: ""}
}

func (c *webRTCConn) SetDeadline(t time.Time) error {
	// nolint: golint
	return fmt.Errorf("SetDeadline not implemented")
}

func (c *webRTCConn) SetReadDeadline(t time.Time) error {
	// nolint: golint
	return fmt.Errorf("SetReadDeadline not implemented")
}

func (c *webRTCConn) SetWriteDeadline(t time.Time) error {
	// nolint: golint
	return fmt.Errorf("SetWriteDeadline not implemented")
}

func getToken() {
	<-tokens
}

func retToken() {
	tokens <- true
}

func genSessionID() string {
	buf := make([]byte, sessionIDLength)
	_, err := rand.Read(buf)
	if err != nil {
		panic(err.Error())
	}
	return strings.TrimRight(base64.StdEncoding.EncodeToString(buf), "=")
}

func limitedRead(r io.Reader, limit int64) ([]byte, error) {
	p, err := ioutil.ReadAll(&io.LimitedReader{R: r, N: limit + 1})
	if err != nil {
		return p, err
	} else if int64(len(p)) == limit+1 {
		return p[0:limit], io.ErrUnexpectedEOF
	}
	return p, err
}

func (b *Broker) pollOffer(sid string) *webrtc.SessionDescription {
	brokerPath := b.url.ResolveReference(&url.URL{Path: "proxy"})
	timeOfNextPoll := time.Now()
	for {
		// Sleep until we're scheduled to poll again.
		now := time.Now()
		time.Sleep(timeOfNextPoll.Sub(now))
		// Compute the next time to poll -- if it's in the past, that
		// means that the POST took longer than pollInterval, so we're
		// allowed to do another one immediately.
		timeOfNextPoll = timeOfNextPoll.Add(pollInterval)
		if timeOfNextPoll.Before(now) {
			timeOfNextPoll = now
		}

		body, err := messages.EncodePollRequest(sid, "standalone", currentNATType)
		if err != nil {
			log.Printf("Error encoding poll message: %s", err.Error())
			return nil
		}
		req, _ := http.NewRequest("POST", brokerPath.String(), bytes.NewBuffer(body))
		resp, err := b.transport.RoundTrip(req)
		if err != nil {
			log.Printf("error polling broker: %s", err)
		} else {
			defer resp.Body.Close()
			if resp.StatusCode != http.StatusOK {
				log.Printf("broker returns: %d", resp.StatusCode)
			} else {
				body, err := limitedRead(resp.Body, readLimit)
				if err != nil {
					log.Printf("error reading broker response: %s", err)
				} else {

					offer, _, err := messages.DecodePollResponse(body)
					if err != nil {
						log.Printf("error reading broker response: %s", err.Error())
						log.Printf("body: %s", body)
						return nil
					}
					if offer != "" {
						offer, err := util.DeserializeSessionDescription(offer)
						if err != nil {
							log.Printf("Error processing session description: %s", err.Error())
							return nil
						}
						return offer

					}
				}
			}
		}
	}
}

func (b *Broker) sendAnswer(sid string, pc *webrtc.PeerConnection) error {
	brokerPath := b.url.ResolveReference(&url.URL{Path: "answer"})
	ld := pc.LocalDescription()
	if !b.keepLocalAddresses {
		ld = &webrtc.SessionDescription{
			Type: ld.Type,
			SDP:  util.StripLocalAddresses(ld.SDP),
		}
	}
	answer, err := util.SerializeSessionDescription(ld)
	if err != nil {
		return err
	}
	body, err := messages.EncodeAnswerRequest(answer, sid)
	if err != nil {
		return err
	}
	req, _ := http.NewRequest("POST", brokerPath.String(), bytes.NewBuffer(body))
	resp, err := b.transport.RoundTrip(req)
	if err != nil {
		return err
	}
	if resp.StatusCode != http.StatusOK {
		return fmt.Errorf("broker returned %d", resp.StatusCode)
	}

	body, err = limitedRead(resp.Body, readLimit)
	if err != nil {
		return fmt.Errorf("error reading broker response: %s", err)
	}
	success, err := messages.DecodeAnswerResponse(body)
	if err != nil {
		return err
	}
	if !success {
		return fmt.Errorf("broker returned client timeout")
	}

	return nil
}

func CopyLoop(c1 io.ReadWriteCloser, c2 io.ReadWriteCloser) {
	var wg sync.WaitGroup
	copyer := func(dst io.ReadWriteCloser, src io.ReadWriteCloser) {
		defer wg.Done()
		if _, err := io.Copy(dst, src); err != nil {
			log.Printf("io.Copy inside CopyLoop generated an error: %v", err)
		}
		dst.Close()
		src.Close()
	}
	wg.Add(2)
	go copyer(c1, c2)
	go copyer(c2, c1)
	wg.Wait()
}

// We pass conn.RemoteAddr() as an additional parameter, rather than calling
// conn.RemoteAddr() inside this function, as a workaround for a hang that
// otherwise occurs inside of conn.pc.RemoteDescription() (called by
// RemoteAddr). https://bugs.torproject.org/18628#comment:8
func datachannelHandler(conn *webRTCConn, remoteAddr net.Addr) {
	defer conn.Close()
	defer retToken()

	u, err := url.Parse(relayURL)
	if err != nil {
		log.Fatalf("invalid relay url: %s", err)
	}

	// Retrieve client IP address
	if remoteAddr != nil {
		// Encode client IP address in relay URL
		q := u.Query()
		clientIP := remoteAddr.String()
		q.Set("client_ip", clientIP)
		u.RawQuery = q.Encode()
	} else {
		log.Printf("no remote address given in websocket")
	}

	ws, _, err := websocket.DefaultDialer.Dial(u.String(), nil)
	if err != nil {
		log.Printf("error dialing relay: %s", err)
		return
	}
	wsConn := websocketconn.New(ws)
	log.Printf("connected to relay")
	defer wsConn.Close()
	CopyLoop(conn, wsConn)
	log.Printf("datachannelHandler ends")
}

// Create a PeerConnection from an SDP offer. Blocks until the gathering of ICE
// candidates is complete and the answer is available in LocalDescription.
// Installs an OnDataChannel callback that creates a webRTCConn and passes it to
// datachannelHandler.
func makePeerConnectionFromOffer(sdp *webrtc.SessionDescription, config webrtc.Configuration, dataChan chan struct{}) (*webrtc.PeerConnection, error) {
	pc, err := webrtc.NewPeerConnection(config)
	if err != nil {
		return nil, fmt.Errorf("accept: NewPeerConnection: %s", err)
	}
	pc.OnDataChannel(func(dc *webrtc.DataChannel) {
		log.Println("OnDataChannel")
		close(dataChan)

		pr, pw := io.Pipe()
		conn := &webRTCConn{pc: pc, dc: dc, pr: pr}

		dc.OnOpen(func() {
			log.Println("OnOpen channel")
		})
		dc.OnClose(func() {
			conn.lock.Lock()
			defer conn.lock.Unlock()
			log.Println("OnClose channel")
			conn.dc = nil
			dc.Close()
			pw.Close()
		})
		dc.OnMessage(func(msg webrtc.DataChannelMessage) {
			var n int
			n, err = pw.Write(msg.Data)
			if err != nil {
				if inerr := pw.CloseWithError(err); inerr != nil {
					log.Printf("close with error generated an error: %v", inerr)
				}
			}
			if n != len(msg.Data) {
				panic("short write")
			}
		})

		go datachannelHandler(conn, conn.RemoteAddr())
	})

	err = pc.SetRemoteDescription(*sdp)
	if err != nil {
		if inerr := pc.Close(); inerr != nil {
			log.Printf("unable to call pc.Close after pc.SetRemoteDescription with error: %v", inerr)
		}
		return nil, fmt.Errorf("accept: SetRemoteDescription: %s", err)
	}
	log.Println("sdp offer successfully received.")

	log.Println("Generating answer...")
	answer, err := pc.CreateAnswer(nil)
	// blocks on ICE gathering. we need to add a timeout if needed
	// not putting this in a separate go routine, because we need
	// SetLocalDescription(answer) to be called before sendAnswer
	if err != nil {
		if inerr := pc.Close(); inerr != nil {
			log.Printf("ICE gathering has generated an error when calling pc.Close: %v", inerr)
		}
		return nil, err
	}

	err = pc.SetLocalDescription(answer)
	if err != nil {
		if err = pc.Close(); err != nil {
			log.Printf("pc.Close after setting local description returned : %v", err)
		}
		return nil, err
	}

	return pc, nil
}

func runSession(sid string) {
	offer := broker.pollOffer(sid)
	if offer == nil {
		log.Printf("bad offer from broker")
		retToken()
		return
	}
	dataChan := make(chan struct{})
	pc, err := makePeerConnectionFromOffer(offer, config, dataChan)
	if err != nil {
		log.Printf("error making WebRTC connection: %s", err)
		retToken()
		return
	}
	err = broker.sendAnswer(sid, pc)
	if err != nil {
		log.Printf("error sending answer to client through broker: %s", err)
		if inerr := pc.Close(); inerr != nil {
			log.Printf("error calling pc.Close: %v", inerr)
		}
		retToken()
		return
	}
	// Set a timeout on peerconnection. If the connection state has not
	// advanced to PeerConnectionStateConnected in this time,
	// destroy the peer connection and return the token.
	select {
	case <-dataChan:
		log.Println("Connection successful.")
	case <-time.After(dataChannelTimeout):
		log.Println("Timed out waiting for client to open data channel.")
		if err := pc.Close(); err != nil {
			log.Printf("error calling pc.Close: %v", err)
		}
		retToken()
	}
}

func main() {
	var capacity uint
	var stunURL string
	var logFilename string
	var rawBrokerURL string
	var unsafeLogging bool
	var keepLocalAddresses bool

	flag.UintVar(&capacity, "capacity", 10, "maximum concurrent clients")
	flag.StringVar(&rawBrokerURL, "broker", defaultBrokerURL, "broker URL")
	flag.StringVar(&relayURL, "relay", defaultRelayURL, "websocket relay URL")
	flag.StringVar(&stunURL, "stun", defaultSTUNURL, "stun URL")
	flag.StringVar(&logFilename, "log", "", "log filename")
	flag.BoolVar(&unsafeLogging, "unsafe-logging", false, "prevent logs from being scrubbed")
	flag.BoolVar(&keepLocalAddresses, "keep-local-addresses", false, "keep local LAN address ICE candidates")
	flag.Parse()

	var logOutput io.Writer = os.Stderr
	log.SetFlags(log.LstdFlags | log.LUTC)
	if logFilename != "" {
		f, err := os.OpenFile(logFilename, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0600)
		if err != nil {
			log.Fatal(err)
		}
		defer f.Close()
		logOutput = io.MultiWriter(os.Stderr, f)
	}
	if unsafeLogging {
		log.SetOutput(logOutput)
	} else {
		// We want to send the log output through our scrubber first
		log.SetOutput(&safelog.LogScrubber{Output: logOutput})
	}

	log.Println("starting")

	var err error
	broker = new(Broker)
	broker.keepLocalAddresses = keepLocalAddresses
	broker.url, err = url.Parse(rawBrokerURL)
	if err != nil {
		log.Fatalf("invalid broker url: %s", err)
	}
	_, err = url.Parse(stunURL)
	if err != nil {
		log.Fatalf("invalid stun url: %s", err)
	}
	_, err = url.Parse(relayURL)
	if err != nil {
		log.Fatalf("invalid relay url: %s", err)
	}

	broker.transport = http.DefaultTransport.(*http.Transport)
	config = webrtc.Configuration{
		ICEServers: []webrtc.ICEServer{
			{
				URLs: []string{stunURL},
			},
		},
	}
	tokens = make(chan bool, capacity)
	for i := uint(0); i < capacity; i++ {
		tokens <- true
	}

	// determine NAT type before polling
	updateNATType(config.ICEServers)
	log.Printf("NAT type: %s", currentNATType)

	for {
		getToken()
		sessionID := genSessionID()
		runSession(sessionID)
	}
}

// use provided STUN server(s) to determine NAT type
func updateNATType(servers []webrtc.ICEServer) {

	var restrictedNAT bool
	var err error
	for _, server := range servers {
		addr := strings.TrimPrefix(server.URLs[0], "stun:")
		restrictedNAT, err = nat.CheckIfRestrictedNAT(addr)
		if err == nil {
			if restrictedNAT {
				currentNATType = NATRestricted
			} else {
				currentNATType = NATUnrestricted
			}
			break
		}
	}
	if err != nil {
		currentNATType = NATUnknown
	}
}