snowflake/client/lib/rendezvous.go

// WebRTC rendezvous requires the exchange of SessionDescriptions between
// peers in order to establish a PeerConnection.
//
// This file contains the one method currently available to Snowflake:
//
// - Domain-fronted HTTP signaling. The Broker automatically exchange offers
//   and answers between this client and some remote WebRTC proxy.

package lib

import (
	"bytes"
	"errors"
	"io"
	"io/ioutil"
	"log"
	"net/http"
	"net/url"
	"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/util"
	"github.com/pion/webrtc/v3"
)

const (
	BrokerErrorUnexpected string = "Unexpected error, no answer."
	readLimit                    = 100000 //Maximum number of bytes to be read from an HTTP response
)

// Signalling Channel to the Broker.
type BrokerChannel struct {
	// The Host header to put in the HTTP request (optional and may be
	// different from the host name in URL).
	Host               string
	url                *url.URL
	transport          http.RoundTripper // Used to make all requests.
	keepLocalAddresses bool
	NATType            string
	lock               sync.Mutex
}

// We make a copy of DefaultTransport because we want the default Dial
// and TLSHandshakeTimeout settings. But we want to disable the default
// ProxyFromEnvironment setting.
func CreateBrokerTransport() http.RoundTripper {
	transport := http.DefaultTransport.(*http.Transport)
	transport.Proxy = nil
	transport.ResponseHeaderTimeout = 15 * time.Second
	return transport
}

// Construct a new BrokerChannel, where:
// |broker| is the full URL of the facilitating program which assigns proxies
// to clients, and |front| is the option fronting domain.
func NewBrokerChannel(broker string, front string, transport http.RoundTripper, keepLocalAddresses bool) (*BrokerChannel, error) {
	targetURL, err := url.Parse(broker)
	if err != nil {
		return nil, err
	}
	log.Println("Rendezvous using Broker at:", broker)
	bc := new(BrokerChannel)
	bc.url = targetURL
	if front != "" { // Optional front domain.
		log.Println("Domain fronting using:", front)
		bc.Host = bc.url.Host
		bc.url.Host = front
	}

	bc.transport = transport
	bc.keepLocalAddresses = keepLocalAddresses
	bc.NATType = nat.NATUnknown
	return bc, nil
}

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
}

// Roundtrip HTTP POST using WebRTC SessionDescriptions.
//
// Send an SDP offer to the broker, which assigns a proxy and responds
// with an SDP answer from a designated remote WebRTC peer.
func (bc *BrokerChannel) Negotiate(offer *webrtc.SessionDescription) (
	*webrtc.SessionDescription, error) {
	log.Println("Negotiating via BrokerChannel...\nTarget URL: ",
		bc.Host, "\nFront URL:  ", bc.url.Host)
	// Ideally, we could specify an `RTCIceTransportPolicy` that would handle
	// this for us.  However, "public" was removed from the draft spec.
	// See https://developer.mozilla.org/en-US/docs/Web/API/RTCConfiguration#RTCIceTransportPolicy_enum
	if !bc.keepLocalAddresses {
		offer = &webrtc.SessionDescription{
			Type: offer.Type,
			SDP:  util.StripLocalAddresses(offer.SDP),
		}
	}
	offerSDP, err := util.SerializeSessionDescription(offer)
	if err != nil {
		return nil, err
	}

	// Encode client poll request
	bc.lock.Lock()
	req := &messages.ClientPollRequest{
		Offer: offerSDP,
		NAT:   bc.NATType,
	}
	body, err := req.EncodePollRequest()
	bc.lock.Unlock()
	if err != nil {
		return nil, err
	}

	data := bytes.NewReader([]byte(body))
	// Suffix with broker's client registration handler.
	clientURL := bc.url.ResolveReference(&url.URL{Path: "client"})
	request, err := http.NewRequest("POST", clientURL.String(), data)
	if nil != err {
		return nil, err
	}
	if "" != bc.Host { // Set true host if necessary.
		request.Host = bc.Host
	}
	resp, err := bc.transport.RoundTrip(request)
	if nil != err {
		return nil, err
	}
	defer resp.Body.Close()
	log.Printf("BrokerChannel Response:\n%s\n\n", resp.Status)

	switch resp.StatusCode {
	case http.StatusOK:
		body, err := limitedRead(resp.Body, readLimit)
		if nil != err {
			return nil, err
		}
		log.Printf("Received answer: %s", string(body))

		resp, err := messages.DecodeClientPollResponse(body)
		if err != nil {
			return nil, err
		}
		if resp.Error != "" {
			return nil, errors.New(resp.Error)
		}
		return util.DeserializeSessionDescription(resp.Answer)
	default:
		return nil, errors.New(BrokerErrorUnexpected)
	}
}

func (bc *BrokerChannel) SetNATType(NATType string) {
	bc.lock.Lock()
	bc.NATType = NATType
	bc.lock.Unlock()
	log.Printf("NAT Type: %s", NATType)
}

// Implements the |Tongue| interface to catch snowflakes, using BrokerChannel.
type WebRTCDialer struct {
	*BrokerChannel
	webrtcConfig *webrtc.Configuration
	max          int
}

func NewWebRTCDialer(broker *BrokerChannel, iceServers []webrtc.ICEServer, max int) *WebRTCDialer {
	config := webrtc.Configuration{
		ICEServers: iceServers,
	}

	return &WebRTCDialer{
		BrokerChannel: broker,
		webrtcConfig:  &config,
		max:           max,
	}
}

// Initialize a WebRTC Connection by signaling through the broker.
func (w WebRTCDialer) Catch() (*WebRTCPeer, error) {
	// TODO: [#25591] Fetch ICE server information from Broker.
	// TODO: [#25596] Consider TURN servers here too.
	return NewWebRTCPeer(w.webrtcConfig, w.BrokerChannel)
}

// Returns the maximum number of snowflakes to collect
func (w WebRTCDialer) GetMax() int {
	return w.max
}