mc-router/server/connector.go

package server

import (
	"bytes"
	"context"
	"io"
	"net"
	"strconv"
	"time"

	"github.com/go-kit/kit/metrics"
	"github.com/itzg/mc-router/mcproto"
	"github.com/juju/ratelimit"
	"github.com/pires/go-proxyproto"
	"github.com/sirupsen/logrus"
)

const (
	handshakeTimeout = 5 * time.Second
)

var noDeadline time.Time

type Connector interface {
	StartAcceptingConnections(ctx context.Context, listenAddress string, connRateLimit int) error
}

type ConnectorMetrics struct {
	Errors            metrics.Counter
	BytesTransmitted  metrics.Counter
	Connections       metrics.Counter
	ActiveConnections metrics.Gauge
}

func NewConnector(metrics *ConnectorMetrics, sendProxyProto bool) Connector {

	return &connectorImpl{
		metrics:        metrics,
		sendProxyProto: sendProxyProto,
	}
}

type connectorImpl struct {
	state          mcproto.State
	metrics        *ConnectorMetrics
	sendProxyProto bool
}

func (c *connectorImpl) StartAcceptingConnections(ctx context.Context, listenAddress string, connRateLimit int) error {

	ln, err := net.Listen("tcp", listenAddress)
	if err != nil {
		logrus.WithError(err).Fatal("Unable to start listening")
		return err
	}
	logrus.WithField("listenAddress", listenAddress).Info("Listening for Minecraft client connections")

	go c.acceptConnections(ctx, ln, connRateLimit)

	return nil
}

func (c *connectorImpl) acceptConnections(ctx context.Context, ln net.Listener, connRateLimit int) {
	//noinspection GoUnhandledErrorResult
	defer ln.Close()

	bucket := ratelimit.NewBucketWithRate(float64(connRateLimit), int64(connRateLimit*2))

	for {
		select {
		case <-ctx.Done():
			return

		case <-time.After(bucket.Take(1)):
			conn, err := ln.Accept()
			if err != nil {
				logrus.WithError(err).Error("Failed to accept connection")
			} else {
				go c.HandleConnection(ctx, conn)
			}
		}
	}
}

func (c *connectorImpl) HandleConnection(ctx context.Context, frontendConn net.Conn) {
	c.metrics.Connections.With("side", "frontend").Add(1)
	//noinspection GoUnhandledErrorResult
	defer frontendConn.Close()

	clientAddr := frontendConn.RemoteAddr()
	logrus.
		WithField("client", clientAddr).
		Info("Got connection")
	defer logrus.WithField("client", clientAddr).Debug("Closing frontend connection")

	inspectionBuffer := new(bytes.Buffer)

	inspectionReader := io.TeeReader(frontendConn, inspectionBuffer)

	if err := frontendConn.SetReadDeadline(time.Now().Add(handshakeTimeout)); err != nil {
		logrus.
			WithError(err).
			WithField("client", clientAddr).
			Error("Failed to set read deadline")
		c.metrics.Errors.With("type", "read_deadline").Add(1)
		return
	}
	packet, err := mcproto.ReadPacket(inspectionReader, clientAddr, c.state)
	if err != nil {
		logrus.WithError(err).WithField("clientAddr", clientAddr).Error("Failed to read packet")
		c.metrics.Errors.With("type", "read").Add(1)
		return
	}

	logrus.
		WithField("client", clientAddr).
		WithField("length", packet.Length).
		WithField("packetID", packet.PacketID).
		Debug("Got packet")

	if packet.PacketID == mcproto.PacketIdHandshake {
		handshake, err := mcproto.ReadHandshake(packet.Data)
		if err != nil {
			logrus.WithError(err).WithField("clientAddr", clientAddr).
				Error("Failed to read handshake")
			c.metrics.Errors.With("type", "read").Add(1)
			return
		}

		logrus.
			WithField("client", clientAddr).
			WithField("handshake", handshake).
			Debug("Got handshake")

		serverAddress := handshake.ServerAddress

		c.findAndConnectBackend(ctx, frontendConn, clientAddr, inspectionBuffer, serverAddress)
	} else if packet.PacketID == mcproto.PacketIdLegacyServerListPing {
		handshake, ok := packet.Data.(*mcproto.LegacyServerListPing)
		if !ok {
			logrus.
				WithField("client", clientAddr).
				WithField("packet", packet).
				Warn("Unexpected data type for PacketIdLegacyServerListPing")
			c.metrics.Errors.With("type", "unexpected_content").Add(1)
			return
		}

		logrus.
			WithField("client", clientAddr).
			WithField("handshake", handshake).
			Debug("Got legacy server list ping")

		serverAddress := handshake.ServerAddress

		c.findAndConnectBackend(ctx, frontendConn, clientAddr, inspectionBuffer, serverAddress)
	} else {
		logrus.
			WithField("client", clientAddr).
			WithField("packetID", packet.PacketID).
			Error("Unexpected packetID, expected handshake")
		c.metrics.Errors.With("type", "unexpected_content").Add(1)
		return
	}
}

func (c *connectorImpl) findAndConnectBackend(ctx context.Context, frontendConn net.Conn,
	clientAddr net.Addr, preReadContent io.Reader, serverAddress string) {

	backendHostPort, resolvedHost := Routes.FindBackendForServerAddress(serverAddress)
	if backendHostPort == "" {
		logrus.WithField("serverAddress", serverAddress).Warn("Unable to find registered backend")
		c.metrics.Errors.With("type", "missing_backend").Add(1)
		return
	}
	logrus.
		WithField("client", clientAddr).
		WithField("server", serverAddress).
		WithField("backendHostPort", backendHostPort).
		Info("Connecting to backend")
	backendConn, err := net.Dial("tcp", backendHostPort)
	if err != nil {
		logrus.
			WithError(err).
			WithField("client", clientAddr).
			WithField("serverAddress", serverAddress).
			WithField("backend", backendHostPort).
			Warn("Unable to connect to backend")
		c.metrics.Errors.With("type", "backend_failed").Add(1)
		return
	}

	c.metrics.Connections.With("side", "backend", "host", resolvedHost).Add(1)
	c.metrics.ActiveConnections.Add(1)
	defer c.metrics.ActiveConnections.Add(-1)

	// PROXY protocol implementation
	if c.sendProxyProto {
		remoteHostStr, _, _ := net.SplitHostPort(backendHostPort)
		sourceAddrStr, sourcePortStr, _ := net.SplitHostPort(clientAddr.String())
		sourcePort, _ := strconv.Atoi(sourcePortStr)

		header := &proxyproto.Header{
			Version:           2,
			Command:           proxyproto.PROXY,
			TransportProtocol: proxyproto.TCPv4,
			SourceAddr: &net.TCPAddr{
				IP:   net.ParseIP(sourceAddrStr),
				Port: sourcePort,
			},
			DestinationAddr: &net.TCPAddr{
				IP: net.ParseIP(remoteHostStr),
			},
		}

		_, err = header.WriteTo(backendConn)
		if err != nil {
			logrus.
				WithError(err).
				WithField("clientAddr", header.SourceAddr).
				WithField("destAddr", header.DestinationAddr).
				Error("Failed to write PROXY header")
			c.metrics.Errors.With("type", "proxy_write").Add(1)
			_ = backendConn.Close()
			return
		}
	}

	amount, err := io.Copy(backendConn, preReadContent)
	if err != nil {
		logrus.WithError(err).Error("Failed to write handshake to backend connection")
		c.metrics.Errors.With("type", "backend_failed").Add(1)
		return
	}

	logrus.WithField("amount", amount).Debug("Relayed handshake to backend")
	if err = frontendConn.SetReadDeadline(noDeadline); err != nil {
		logrus.
			WithError(err).
			WithField("client", clientAddr).
			Error("Failed to clear read deadline")
		c.metrics.Errors.With("type", "read_deadline").Add(1)
		return
	}

	c.pumpConnections(ctx, frontendConn, backendConn)
}

func (c *connectorImpl) pumpConnections(ctx context.Context, frontendConn, backendConn net.Conn) {
	//noinspection GoUnhandledErrorResult
	defer backendConn.Close()

	clientAddr := frontendConn.RemoteAddr()
	defer logrus.WithField("client", clientAddr).Debug("Closing backend connection")

	errors := make(chan error, 2)

	go c.pumpFrames(backendConn, frontendConn, errors, "backend", "frontend", clientAddr)
	go c.pumpFrames(frontendConn, backendConn, errors, "frontend", "backend", clientAddr)

	select {
	case err := <-errors:
		if err != io.EOF {
			logrus.WithError(err).
				WithField("client", clientAddr).
				Error("Error observed on connection relay")
			c.metrics.Errors.With("type", "relay").Add(1)
		}

	case <-ctx.Done():
		logrus.Debug("Observed context cancellation")
	}
}

func (c *connectorImpl) pumpFrames(incoming io.Reader, outgoing io.Writer, errors chan<- error, from, to string, clientAddr net.Addr) {
	amount, err := io.Copy(outgoing, incoming)
	logrus.
		WithField("client", clientAddr).
		WithField("amount", amount).
		Infof("Finished relay %s->%s", from, to)

	c.metrics.BytesTransmitted.Add(float64(amount))

	if err != nil {
		errors <- err
	} else {
		// successful io.Copy return nil error, not EOF...to simulate that to trigger outer handling
		errors <- io.EOF
	}
}