snowflake/client/lib/snowflake.go

package lib

import (
	"context"
	"errors"
	"io"
	"log"
	"net"
	"time"

	"git.torproject.org/pluggable-transports/snowflake.git/common/turbotunnel"
	"github.com/xtaci/kcp-go/v5"
	"github.com/xtaci/smux"
)

const (
	ReconnectTimeout = 10 * time.Second
	SnowflakeTimeout = 20 * time.Second
	// How long to wait for the OnOpen callback on a DataChannel.
	DataChannelTimeout = 10 * time.Second
)

type dummyAddr struct{}

func (addr dummyAddr) Network() string { return "dummy" }
func (addr dummyAddr) String() string  { return "dummy" }

// newSession returns a new smux.Session and the net.PacketConn it is running
// over. The net.PacketConn successively connects through Snowflake proxies
// pulled from snowflakes.
func newSession(snowflakes SnowflakeCollector) (net.PacketConn, *smux.Session, error) {
	clientID := turbotunnel.NewClientID()

	// We build a persistent KCP session on a sequence of ephemeral WebRTC
	// connections. This dialContext tells RedialPacketConn how to get a new
	// WebRTC connection when the previous one dies. Inside each WebRTC
	// connection, we use EncapsulationPacketConn to encode packets into a
	// stream.
	dialContext := func(ctx context.Context) (net.PacketConn, error) {
		log.Printf("redialing on same connection")
		// Obtain an available WebRTC remote. May block.
		conn := snowflakes.Pop()
		if conn == nil {
			return nil, errors.New("handler: Received invalid Snowflake")
		}
		log.Println("---- Handler: snowflake assigned ----")
		// Send the magic Turbo Tunnel token.
		_, err := conn.Write(turbotunnel.Token[:])
		if err != nil {
			return nil, err
		}
		// Send ClientID prefix.
		_, err = conn.Write(clientID[:])
		if err != nil {
			return nil, err
		}
		return NewEncapsulationPacketConn(dummyAddr{}, dummyAddr{}, conn), nil
	}
	pconn := turbotunnel.NewRedialPacketConn(dummyAddr{}, dummyAddr{}, dialContext)

	// conn is built on the underlying RedialPacketConn—when one WebRTC
	// connection dies, another one will be found to take its place. The
	// sequence of packets across multiple WebRTC connections drives the KCP
	// engine.
	conn, err := kcp.NewConn2(dummyAddr{}, nil, 0, 0, pconn)
	if err != nil {
		pconn.Close()
		return nil, nil, err
	}
	// Permit coalescing the payloads of consecutive sends.
	conn.SetStreamMode(true)
	// Set the maximum send and receive window sizes to a high number
	// Removes KCP bottlenecks: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40026
	conn.SetWindowSize(65535, 65535)
	// Disable the dynamic congestion window (limit only by the
	// maximum of local and remote static windows).
	conn.SetNoDelay(
		0, // default nodelay
		0, // default interval
		0, // default resend
		1, // nc=1 => congestion window off
	)
	// On the KCP connection we overlay an smux session and stream.
	smuxConfig := smux.DefaultConfig()
	smuxConfig.Version = 2
	smuxConfig.KeepAliveTimeout = 10 * time.Minute
	sess, err := smux.Client(conn, smuxConfig)
	if err != nil {
		conn.Close()
		pconn.Close()
		return nil, nil, err
	}

	return pconn, sess, err
}

// Given an accepted SOCKS connection, establish a WebRTC connection to the
// remote peer and exchange traffic.
func Handler(socks net.Conn, tongue Tongue) error {
	// Prepare to collect remote WebRTC peers.
	snowflakes, err := NewPeers(tongue)
	if err != nil {
		return err
	}

	// Use a real logger to periodically output how much traffic is happening.
	snowflakes.BytesLogger = NewBytesSyncLogger()

	log.Printf("---- Handler: begin collecting snowflakes ---")
	go connectLoop(snowflakes)

	// Create a new smux session
	log.Printf("---- Handler: starting a new session ---")
	pconn, sess, err := newSession(snowflakes)
	if err != nil {
		return err
	}

	// On the smux session we overlay a stream.
	stream, err := sess.OpenStream()
	if err != nil {
		return err
	}
	defer stream.Close()

	// Begin exchanging data.
	log.Printf("---- Handler: begin stream %v ---", stream.ID())
	copyLoop(socks, stream)
	log.Printf("---- Handler: closed stream %v ---", stream.ID())
	snowflakes.End()
	log.Printf("---- Handler: end collecting snowflakes ---")
	pconn.Close()
	sess.Close()
	log.Printf("---- Handler: discarding finished session ---")
	return nil
}

// Maintain |SnowflakeCapacity| number of available WebRTC connections, to
// transfer to the Tor SOCKS handler when needed.
func connectLoop(snowflakes SnowflakeCollector) {
	for {
		timer := time.After(ReconnectTimeout)
		_, err := snowflakes.Collect()
		if err != nil {
			log.Printf("WebRTC: %v  Retrying...", err)
		}
		select {
		case <-timer:
			continue
		case <-snowflakes.Melted():
			log.Println("ConnectLoop: stopped.")
			return
		}
	}
}

// Exchanges bytes between two ReadWriters.
// (In this case, between a SOCKS connection and smux stream.)
func copyLoop(socks, stream io.ReadWriter) {
	done := make(chan struct{}, 2)
	go func() {
		if _, err := io.Copy(socks, stream); err != nil {
			log.Printf("copying WebRTC to SOCKS resulted in error: %v", err)
		}
		done <- struct{}{}
	}()
	go func() {
		if _, err := io.Copy(stream, socks); err != nil {
			log.Printf("copying SOCKS to stream resulted in error: %v", err)
		}
		done <- struct{}{}
	}()
	<-done
	log.Println("copy loop ended")
}