psiphon-tunnel-core/psiphon/server/tunnelServer.go

/*
 * Copyright (c) 2016, Psiphon Inc.
 * All rights reserved.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

package server

import (
	"crypto/subtle"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"strconv"
	"sync"
	"sync/atomic"
	"time"

	"github.com/Psiphon-Inc/crypto/ssh"
	"github.com/Psiphon-Inc/goarista/monotime"
	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common"
	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/osl"
	"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/protocol"
)

const (
	SSH_HANDSHAKE_TIMEOUT                  = 30 * time.Second
	SSH_CONNECTION_READ_DEADLINE           = 5 * time.Minute
	SSH_TCP_PORT_FORWARD_IP_LOOKUP_TIMEOUT = 30 * time.Second
	SSH_TCP_PORT_FORWARD_DIAL_TIMEOUT      = 30 * time.Second
	SSH_TCP_PORT_FORWARD_COPY_BUFFER_SIZE  = 8192
	SSH_SEND_OSL_INITIAL_RETRY_DELAY       = 30 * time.Second
	SSH_SEND_OSL_RETRY_FACTOR              = 2
)

// TunnelServer is the main server that accepts Psiphon client
// connections, via various obfuscation protocols, and provides
// port forwarding (TCP and UDP) services to the Psiphon client.
// At its core, TunnelServer is an SSH server. SSH is the base
// protocol that provides port forward multiplexing, and transport
// security. Layered on top of SSH, optionally, is Obfuscated SSH
// and meek protocols, which provide further circumvention
// capabilities.
type TunnelServer struct {
	runWaitGroup      *sync.WaitGroup
	listenerError     chan error
	shutdownBroadcast <-chan struct{}
	sshServer         *sshServer
}

// NewTunnelServer initializes a new tunnel server.
func NewTunnelServer(
	support *SupportServices,
	shutdownBroadcast <-chan struct{}) (*TunnelServer, error) {

	sshServer, err := newSSHServer(support, shutdownBroadcast)
	if err != nil {
		return nil, common.ContextError(err)
	}

	return &TunnelServer{
		runWaitGroup:      new(sync.WaitGroup),
		listenerError:     make(chan error),
		shutdownBroadcast: shutdownBroadcast,
		sshServer:         sshServer,
	}, nil
}

// Run runs the tunnel server; this function blocks while running a selection of
// listeners that handle connection using various obfuscation protocols.
//
// Run listens on each designated tunnel port and spawns new goroutines to handle
// each client connection. It halts when shutdownBroadcast is signaled. A list of active
// clients is maintained, and when halting all clients are cleanly shutdown.
//
// Each client goroutine handles its own obfuscation (optional), SSH handshake, SSH
// authentication, and then looping on client new channel requests. "direct-tcpip"
// channels, dynamic port fowards, are supported. When the UDPInterceptUdpgwServerAddress
// config parameter is configured, UDP port forwards over a TCP stream, following
// the udpgw protocol, are handled.
//
// A new goroutine is spawned to handle each port forward for each client. Each port
// forward tracks its bytes transferred. Overall per-client stats for connection duration,
// GeoIP, number of port forwards, and bytes transferred are tracked and logged when the
// client shuts down.
//
// Note: client handler goroutines may still be shutting down after Run() returns. See
// comment in sshClient.stop(). TODO: fully synchronized shutdown.
func (server *TunnelServer) Run() error {

	type sshListener struct {
		net.Listener
		localAddress   string
		tunnelProtocol string
	}

	// TODO: should TunnelServer hold its own support pointer?
	support := server.sshServer.support

	// First bind all listeners; once all are successful,
	// start accepting connections on each.

	var listeners []*sshListener

	for tunnelProtocol, listenPort := range support.Config.TunnelProtocolPorts {

		localAddress := fmt.Sprintf(
			"%s:%d", support.Config.ServerIPAddress, listenPort)

		listener, err := net.Listen("tcp", localAddress)
		if err != nil {
			for _, existingListener := range listeners {
				existingListener.Listener.Close()
			}
			return common.ContextError(err)
		}

		log.WithContextFields(
			LogFields{
				"localAddress":   localAddress,
				"tunnelProtocol": tunnelProtocol,
			}).Info("listening")

		listeners = append(
			listeners,
			&sshListener{
				Listener:       listener,
				localAddress:   localAddress,
				tunnelProtocol: tunnelProtocol,
			})
	}

	for _, listener := range listeners {
		server.runWaitGroup.Add(1)
		go func(listener *sshListener) {
			defer server.runWaitGroup.Done()

			log.WithContextFields(
				LogFields{
					"localAddress":   listener.localAddress,
					"tunnelProtocol": listener.tunnelProtocol,
				}).Info("running")

			server.sshServer.runListener(
				listener.Listener,
				server.listenerError,
				listener.tunnelProtocol)

			log.WithContextFields(
				LogFields{
					"localAddress":   listener.localAddress,
					"tunnelProtocol": listener.tunnelProtocol,
				}).Info("stopped")

		}(listener)
	}

	var err error
	select {
	case <-server.shutdownBroadcast:
	case err = <-server.listenerError:
	}

	for _, listener := range listeners {
		listener.Close()
	}
	server.sshServer.stopClients()
	server.runWaitGroup.Wait()

	log.WithContext().Info("stopped")

	return err
}

// GetLoadStats returns load stats for the tunnel server. The stats are
// broken down by protocol ("SSH", "OSSH", etc.) and type. Types of stats
// include current connected client count, total number of current port
// forwards.
func (server *TunnelServer) GetLoadStats() map[string]map[string]int64 {
	return server.sshServer.getLoadStats()
}

// ResetAllClientTrafficRules resets all established client traffic rules
// to use the latest config and client properties. Any existing traffic
// rule state is lost, including throttling state.
func (server *TunnelServer) ResetAllClientTrafficRules() {
	server.sshServer.resetAllClientTrafficRules()
}

// ResetAllClientOSLConfigs resets all established client OSL state to use
// the latest OSL config. Any existing OSL state is lost, including partial
// progress towards SLOKs.
func (server *TunnelServer) ResetAllClientOSLConfigs() {
	server.sshServer.resetAllClientOSLConfigs()
}

// SetClientHandshakeState sets the handshake state -- that it completed and
// what paramaters were passed -- in sshClient. This state is used for allowing
// port forwards and for future traffic rule selection. SetClientHandshakeState
// also triggers an immediate traffic rule re-selection, as the rules selected
// upon tunnel establishment may no longer apply now that handshake values are
// set.
func (server *TunnelServer) SetClientHandshakeState(
	sessionID string, state handshakeState) error {

	return server.sshServer.setClientHandshakeState(sessionID, state)
}

// SetEstablishTunnels sets whether new tunnels may be established or not.
// When not establishing, incoming connections are immediately closed.
func (server *TunnelServer) SetEstablishTunnels(establish bool) {
	server.sshServer.setEstablishTunnels(establish)
}

// GetEstablishTunnels returns whether new tunnels may be established or not.
func (server *TunnelServer) GetEstablishTunnels() bool {
	return server.sshServer.getEstablishTunnels()
}

type sshServer struct {
	support              *SupportServices
	establishTunnels     int32
	shutdownBroadcast    <-chan struct{}
	sshHostKey           ssh.Signer
	clientsMutex         sync.Mutex
	stoppingClients      bool
	acceptedClientCounts map[string]int64
	clients              map[string]*sshClient
}

func newSSHServer(
	support *SupportServices,
	shutdownBroadcast <-chan struct{}) (*sshServer, error) {

	privateKey, err := ssh.ParseRawPrivateKey([]byte(support.Config.SSHPrivateKey))
	if err != nil {
		return nil, common.ContextError(err)
	}

	// TODO: use cert (ssh.NewCertSigner) for anti-fingerprint?
	signer, err := ssh.NewSignerFromKey(privateKey)
	if err != nil {
		return nil, common.ContextError(err)
	}

	return &sshServer{
		support:              support,
		establishTunnels:     1,
		shutdownBroadcast:    shutdownBroadcast,
		sshHostKey:           signer,
		acceptedClientCounts: make(map[string]int64),
		clients:              make(map[string]*sshClient),
	}, nil
}

func (sshServer *sshServer) setEstablishTunnels(establish bool) {

	// Do nothing when the setting is already correct. This avoids
	// spurious log messages when setEstablishTunnels is called
	// periodically with the same setting.
	if establish == sshServer.getEstablishTunnels() {
		return
	}

	establishFlag := int32(1)
	if !establish {
		establishFlag = 0
	}
	atomic.StoreInt32(&sshServer.establishTunnels, establishFlag)

	log.WithContextFields(
		LogFields{"establish": establish}).Info("establishing tunnels")
}

func (sshServer *sshServer) getEstablishTunnels() bool {
	return atomic.LoadInt32(&sshServer.establishTunnels) == 1
}

// runListener is intended to run an a goroutine; it blocks
// running a particular listener. If an unrecoverable error
// occurs, it will send the error to the listenerError channel.
func (sshServer *sshServer) runListener(
	listener net.Listener,
	listenerError chan<- error,
	tunnelProtocol string) {

	handleClient := func(clientConn net.Conn) {

		// Note: establish tunnel limiter cannot simply stop TCP
		// listeners in all cases (e.g., meek) since SSH tunnel can
		// span multiple TCP connections.

		if !sshServer.getEstablishTunnels() {
			log.WithContext().Debug("not establishing tunnels")
			clientConn.Close()
			return
		}

		// process each client connection concurrently
		go sshServer.handleClient(tunnelProtocol, clientConn)
	}

	// Note: when exiting due to a unrecoverable error, be sure
	// to try to send the error to listenerError so that the outer
	// TunnelServer.Run will properly shut down instead of remaining
	// running.

	if protocol.TunnelProtocolUsesMeekHTTP(tunnelProtocol) ||
		protocol.TunnelProtocolUsesMeekHTTPS(tunnelProtocol) {

		meekServer, err := NewMeekServer(
			sshServer.support,
			listener,
			protocol.TunnelProtocolUsesMeekHTTPS(tunnelProtocol),
			protocol.TunnelProtocolUsesObfuscatedSessionTickets(tunnelProtocol),
			handleClient,
			sshServer.shutdownBroadcast)
		if err != nil {
			select {
			case listenerError <- common.ContextError(err):
			default:
			}
			return
		}

		meekServer.Run()

	} else {

		for {
			conn, err := listener.Accept()

			select {
			case <-sshServer.shutdownBroadcast:
				if err == nil {
					conn.Close()
				}
				return
			default:
			}

			if err != nil {
				if e, ok := err.(net.Error); ok && e.Temporary() {
					log.WithContextFields(LogFields{"error": err}).Error("accept failed")
					// Temporary error, keep running
					continue
				}

				select {
				case listenerError <- common.ContextError(err):
				default:
				}
				return
			}

			handleClient(conn)
		}
	}
}

// An accepted client has completed a direct TCP or meek connection and has a net.Conn. Registration
// is for tracking the number of connections.
func (sshServer *sshServer) registerAcceptedClient(tunnelProtocol string) {

	sshServer.clientsMutex.Lock()
	defer sshServer.clientsMutex.Unlock()

	sshServer.acceptedClientCounts[tunnelProtocol] += 1
}

func (sshServer *sshServer) unregisterAcceptedClient(tunnelProtocol string) {

	sshServer.clientsMutex.Lock()
	defer sshServer.clientsMutex.Unlock()

	sshServer.acceptedClientCounts[tunnelProtocol] -= 1
}

// An established client has completed its SSH handshake and has a ssh.Conn. Registration is
// for tracking the number of fully established clients and for maintaining a list of running
// clients (for stopping at shutdown time).
func (sshServer *sshServer) registerEstablishedClient(client *sshClient) bool {

	sshServer.clientsMutex.Lock()

	if sshServer.stoppingClients {
		sshServer.clientsMutex.Unlock()
		return false
	}

	// In the case of a duplicate client sessionID, the previous client is closed.
	// - Well-behaved clients generate pick a random sessionID that should be
	//   unique (won't accidentally conflict) and hard to guess (can't be targetted
	//   by a malicious client).
	// - Clients reuse the same sessionID when a tunnel is unexpectedly disconnected
	//   and resestablished. In this case, when the same server is selected, this logic
	//   will be hit; closing the old, dangling client is desirable.
	// - Multi-tunnel clients should not normally use one server for multiple tunnels.
	existingClient := sshServer.clients[client.sessionID]

	sshServer.clients[client.sessionID] = client
	sshServer.clientsMutex.Unlock()

	// Call stop() outside the mutex to avoid deadlock.
	if existingClient != nil {
		existingClient.stop()
	}

	return true
}

func (sshServer *sshServer) unregisterEstablishedClient(sessionID string) {

	sshServer.clientsMutex.Lock()
	client := sshServer.clients[sessionID]
	delete(sshServer.clients, sessionID)
	sshServer.clientsMutex.Unlock()

	// Call stop() outside the mutex to avoid deadlock.
	if client != nil {
		client.stop()
	}
}

func (sshServer *sshServer) getLoadStats() map[string]map[string]int64 {

	sshServer.clientsMutex.Lock()
	defer sshServer.clientsMutex.Unlock()

	loadStats := make(map[string]map[string]int64)

	// Explicitly populate with zeros to get 0 counts in log messages derived from getLoadStats()

	for tunnelProtocol, _ := range sshServer.support.Config.TunnelProtocolPorts {
		loadStats[tunnelProtocol] = make(map[string]int64)
		loadStats[tunnelProtocol]["accepted_clients"] = 0
		loadStats[tunnelProtocol]["established_clients"] = 0
		loadStats[tunnelProtocol]["tcp_port_forwards"] = 0
		loadStats[tunnelProtocol]["total_tcp_port_forwards"] = 0
		loadStats[tunnelProtocol]["udp_port_forwards"] = 0
		loadStats[tunnelProtocol]["total_udp_port_forwards"] = 0
	}

	// Note: as currently tracked/counted, each established client is also an accepted client

	for tunnelProtocol, acceptedClientCount := range sshServer.acceptedClientCounts {
		loadStats[tunnelProtocol]["accepted_clients"] = acceptedClientCount
	}

	var aggregatedQualityMetrics qualityMetrics

	for _, client := range sshServer.clients {
		// Note: can't sum trafficState.peakConcurrentPortForwardCount to get a global peak
		loadStats[client.tunnelProtocol]["established_clients"] += 1

		client.Lock()

		loadStats[client.tunnelProtocol]["tcp_port_forwards"] += client.tcpTrafficState.concurrentPortForwardCount
		loadStats[client.tunnelProtocol]["total_tcp_port_forwards"] += client.tcpTrafficState.totalPortForwardCount
		loadStats[client.tunnelProtocol]["udp_port_forwards"] += client.udpTrafficState.concurrentPortForwardCount
		loadStats[client.tunnelProtocol]["total_udp_port_forwards"] += client.udpTrafficState.totalPortForwardCount

		aggregatedQualityMetrics.tcpPortForwardDialedCount += client.qualityMetrics.tcpPortForwardDialedCount
		aggregatedQualityMetrics.tcpPortForwardDialedDuration +=
			client.qualityMetrics.tcpPortForwardDialedDuration / time.Millisecond
		aggregatedQualityMetrics.tcpPortForwardFailedCount += client.qualityMetrics.tcpPortForwardFailedCount
		aggregatedQualityMetrics.tcpPortForwardFailedDuration +=
			client.qualityMetrics.tcpPortForwardFailedDuration / time.Millisecond
		client.qualityMetrics.tcpPortForwardDialedCount = 0
		client.qualityMetrics.tcpPortForwardDialedDuration = 0
		client.qualityMetrics.tcpPortForwardFailedCount = 0
		client.qualityMetrics.tcpPortForwardFailedDuration = 0

		client.Unlock()
	}

	// Calculate and report totals across all protocols. It's easier to do this here
	// than futher down the stats stack. Also useful for glancing at log files.

	allProtocolsStats := make(map[string]int64)
	allProtocolsStats["accepted_clients"] = 0
	allProtocolsStats["established_clients"] = 0
	allProtocolsStats["tcp_port_forwards"] = 0
	allProtocolsStats["total_tcp_port_forwards"] = 0
	allProtocolsStats["udp_port_forwards"] = 0
	allProtocolsStats["total_udp_port_forwards"] = 0
	allProtocolsStats["tcp_port_forward_dialed_count"] = aggregatedQualityMetrics.tcpPortForwardDialedCount
	allProtocolsStats["tcp_port_forward_dialed_duration"] = int64(aggregatedQualityMetrics.tcpPortForwardDialedDuration)
	allProtocolsStats["tcp_port_forward_failed_count"] = aggregatedQualityMetrics.tcpPortForwardFailedCount
	allProtocolsStats["tcp_port_forward_failed_duration"] = int64(aggregatedQualityMetrics.tcpPortForwardFailedDuration)

	for _, stats := range loadStats {
		for name, value := range stats {
			allProtocolsStats[name] += value
		}
	}

	loadStats["ALL"] = allProtocolsStats

	return loadStats
}

func (sshServer *sshServer) resetAllClientTrafficRules() {

	sshServer.clientsMutex.Lock()
	clients := make(map[string]*sshClient)
	for sessionID, client := range sshServer.clients {
		clients[sessionID] = client
	}
	sshServer.clientsMutex.Unlock()

	for _, client := range clients {
		client.setTrafficRules()
	}
}

func (sshServer *sshServer) resetAllClientOSLConfigs() {

	sshServer.clientsMutex.Lock()
	clients := make(map[string]*sshClient)
	for sessionID, client := range sshServer.clients {
		clients[sessionID] = client
	}
	sshServer.clientsMutex.Unlock()

	for _, client := range clients {
		client.setOSLConfig()
	}
}

func (sshServer *sshServer) setClientHandshakeState(
	sessionID string, state handshakeState) error {

	sshServer.clientsMutex.Lock()
	client := sshServer.clients[sessionID]
	sshServer.clientsMutex.Unlock()

	if client == nil {
		return common.ContextError(errors.New("unknown session ID"))
	}

	err := client.setHandshakeState(state)
	if err != nil {
		return common.ContextError(err)
	}

	return nil
}

func (sshServer *sshServer) stopClients() {

	sshServer.clientsMutex.Lock()
	sshServer.stoppingClients = true
	clients := sshServer.clients
	sshServer.clients = make(map[string]*sshClient)
	sshServer.clientsMutex.Unlock()

	for _, client := range clients {
		client.stop()
	}
}

func (sshServer *sshServer) handleClient(tunnelProtocol string, clientConn net.Conn) {

	sshServer.registerAcceptedClient(tunnelProtocol)
	defer sshServer.unregisterAcceptedClient(tunnelProtocol)

	geoIPData := sshServer.support.GeoIPService.Lookup(
		common.IPAddressFromAddr(clientConn.RemoteAddr()))

	sshClient := newSshClient(sshServer, tunnelProtocol, geoIPData)

	sshClient.run(clientConn)
}

type sshClient struct {
	sync.Mutex
	sshServer              *sshServer
	tunnelProtocol         string
	sshConn                ssh.Conn
	activityConn           *common.ActivityMonitoredConn
	throttledConn          *common.ThrottledConn
	geoIPData              GeoIPData
	sessionID              string
	supportsServerRequests bool
	handshakeState         handshakeState
	udpChannel             ssh.Channel
	trafficRules           TrafficRules
	tcpTrafficState        trafficState
	udpTrafficState        trafficState
	qualityMetrics         qualityMetrics
	tcpPortForwardLRU      *common.LRUConns
	oslClientSeedState     *osl.ClientSeedState
	signalIssueSLOKs       chan struct{}
	stopBroadcast          chan struct{}
}

type trafficState struct {
	bytesUp                        int64
	bytesDown                      int64
	concurrentPortForwardCount     int64
	peakConcurrentPortForwardCount int64
	totalPortForwardCount          int64
}

// qualityMetrics records upstream TCP dial attempts and
// elapsed time. Elapsed time includes the full TCP handshake
// and, in aggregate, is a measure of the quality of the
// upstream link. These stats are recorded by each sshClient
// and then reported and reset in sshServer.getLoadStats().
type qualityMetrics struct {
	tcpPortForwardDialedCount    int64
	tcpPortForwardDialedDuration time.Duration
	tcpPortForwardFailedCount    int64
	tcpPortForwardFailedDuration time.Duration
}

type handshakeState struct {
	completed   bool
	apiProtocol string
	apiParams   requestJSONObject
}

func newSshClient(
	sshServer *sshServer, tunnelProtocol string, geoIPData GeoIPData) *sshClient {
	return &sshClient{
		sshServer:         sshServer,
		tunnelProtocol:    tunnelProtocol,
		geoIPData:         geoIPData,
		tcpPortForwardLRU: common.NewLRUConns(),
		signalIssueSLOKs:  make(chan struct{}, 1),
		stopBroadcast:     make(chan struct{}),
	}
}

func (sshClient *sshClient) run(clientConn net.Conn) {

	// Set initial traffic rules, pre-handshake, based on currently known info.
	sshClient.setTrafficRules()

	// Wrap the base client connection with an ActivityMonitoredConn which will
	// terminate the connection if no data is received before the deadline. This
	// timeout is in effect for the entire duration of the SSH connection. Clients
	// must actively use the connection or send SSH keep alive requests to keep
	// the connection active. Writes are not considered reliable activity indicators
	// due to buffering.

	activityConn, err := common.NewActivityMonitoredConn(
		clientConn,
		SSH_CONNECTION_READ_DEADLINE,
		false,
		nil,
		nil)
	if err != nil {
		clientConn.Close()
		log.WithContextFields(LogFields{"error": err}).Error("NewActivityMonitoredConn failed")
		return
	}
	clientConn = activityConn

	// Further wrap the connection in a rate limiting ThrottledConn.

	throttledConn := common.NewThrottledConn(clientConn, sshClient.rateLimits())
	clientConn = throttledConn

	// Run the initial [obfuscated] SSH handshake in a goroutine so we can both
	// respect shutdownBroadcast and implement a specific handshake timeout.
	// The timeout is to reclaim network resources in case the handshake takes
	// too long.

	type sshNewServerConnResult struct {
		conn     net.Conn
		sshConn  *ssh.ServerConn
		channels <-chan ssh.NewChannel
		requests <-chan *ssh.Request
		err      error
	}

	resultChannel := make(chan *sshNewServerConnResult, 2)

	if SSH_HANDSHAKE_TIMEOUT > 0 {
		time.AfterFunc(time.Duration(SSH_HANDSHAKE_TIMEOUT), func() {
			resultChannel <- &sshNewServerConnResult{err: errors.New("ssh handshake timeout")}
		})
	}

	go func(conn net.Conn) {
		sshServerConfig := &ssh.ServerConfig{
			PasswordCallback: sshClient.passwordCallback,
			AuthLogCallback:  sshClient.authLogCallback,
			ServerVersion:    sshClient.sshServer.support.Config.SSHServerVersion,
		}
		sshServerConfig.AddHostKey(sshClient.sshServer.sshHostKey)

		result := &sshNewServerConnResult{}

		// Wrap the connection in an SSH deobfuscator when required.

		if protocol.TunnelProtocolUsesObfuscatedSSH(sshClient.tunnelProtocol) {
			// Note: NewObfuscatedSshConn blocks on network I/O
			// TODO: ensure this won't block shutdown
			conn, result.err = common.NewObfuscatedSshConn(
				common.OBFUSCATION_CONN_MODE_SERVER,
				conn,
				sshClient.sshServer.support.Config.ObfuscatedSSHKey)
			if result.err != nil {
				result.err = common.ContextError(result.err)
			}
		}

		if result.err == nil {
			result.sshConn, result.channels, result.requests, result.err =
				ssh.NewServerConn(conn, sshServerConfig)
		}

		resultChannel <- result

	}(clientConn)

	var result *sshNewServerConnResult
	select {
	case result = <-resultChannel:
	case <-sshClient.sshServer.shutdownBroadcast:
		// Close() will interrupt an ongoing handshake
		// TODO: wait for goroutine to exit before returning?
		clientConn.Close()
		return
	}

	if result.err != nil {
		clientConn.Close()
		// This is a Debug log due to noise. The handshake often fails due to I/O
		// errors as clients frequently interrupt connections in progress when
		// client-side load balancing completes a connection to a different server.
		log.WithContextFields(LogFields{"error": result.err}).Debug("handshake failed")
		return
	}

	sshClient.Lock()
	sshClient.sshConn = result.sshConn
	sshClient.activityConn = activityConn
	sshClient.throttledConn = throttledConn
	sshClient.Unlock()

	if !sshClient.sshServer.registerEstablishedClient(sshClient) {
		clientConn.Close()
		log.WithContext().Warning("register failed")
		return
	}

	sshClient.runTunnel(result.channels, result.requests)

	// Note: sshServer.unregisterEstablishedClient calls sshClient.stop(),
	// which also closes underlying transport Conn.

	sshClient.sshServer.unregisterEstablishedClient(sshClient.sessionID)

	sshClient.logTunnel()

	// Initiate cleanup of the GeoIP session cache. To allow for post-tunnel
	// final status requests, the lifetime of cached GeoIP records exceeds the
	// lifetime of the sshClient.
	sshClient.sshServer.support.GeoIPService.MarkSessionCacheToExpire(sshClient.sessionID)
}

func (sshClient *sshClient) passwordCallback(conn ssh.ConnMetadata, password []byte) (*ssh.Permissions, error) {

	expectedSessionIDLength := 2 * protocol.PSIPHON_API_CLIENT_SESSION_ID_LENGTH
	expectedSSHPasswordLength := 2 * SSH_PASSWORD_BYTE_LENGTH

	var sshPasswordPayload protocol.SSHPasswordPayload
	err := json.Unmarshal(password, &sshPasswordPayload)
	if err != nil {

		// Backwards compatibility case: instead of a JSON payload, older clients
		// send the hex encoded session ID prepended to the SSH password.
		// Note: there's an even older case where clients don't send any session ID,
		// but that's no longer supported.
		if len(password) == expectedSessionIDLength+expectedSSHPasswordLength {
			sshPasswordPayload.SessionId = string(password[0:expectedSessionIDLength])
			sshPasswordPayload.SshPassword = string(password[expectedSSHPasswordLength:len(password)])
		} else {
			return nil, common.ContextError(fmt.Errorf("invalid password payload for %q", conn.User()))
		}
	}

	if !isHexDigits(sshClient.sshServer.support, sshPasswordPayload.SessionId) ||
		len(sshPasswordPayload.SessionId) != expectedSessionIDLength {
		return nil, common.ContextError(fmt.Errorf("invalid session ID for %q", conn.User()))
	}

	userOk := (subtle.ConstantTimeCompare(
		[]byte(conn.User()), []byte(sshClient.sshServer.support.Config.SSHUserName)) == 1)

	passwordOk := (subtle.ConstantTimeCompare(
		[]byte(sshPasswordPayload.SshPassword), []byte(sshClient.sshServer.support.Config.SSHPassword)) == 1)

	if !userOk || !passwordOk {
		return nil, common.ContextError(fmt.Errorf("invalid password for %q", conn.User()))
	}

	sessionID := sshPasswordPayload.SessionId

	supportsServerRequests := common.Contains(
		sshPasswordPayload.ClientCapabilities, protocol.CLIENT_CAPABILITY_SERVER_REQUESTS)

	sshClient.Lock()
	sshClient.sessionID = sessionID
	sshClient.supportsServerRequests = supportsServerRequests
	geoIPData := sshClient.geoIPData
	sshClient.Unlock()

	// Store the GeoIP data associated with the session ID. This makes
	// the GeoIP data available to the web server for web API requests.
	// A cache that's distinct from the sshClient record is used to allow
	// for or post-tunnel final status requests.
	// If the client is reconnecting with the same session ID, this call
	// will undo the expiry set by MarkSessionCacheToExpire.
	sshClient.sshServer.support.GeoIPService.SetSessionCache(sessionID, geoIPData)

	return nil, nil
}

func (sshClient *sshClient) authLogCallback(conn ssh.ConnMetadata, method string, err error) {

	if err != nil {

		if method == "none" && err.Error() == "no auth passed yet" {
			// In this case, the callback invocation is noise from auth negotiation
			return
		}

		// Note: here we previously logged messages for fail2ban to act on. This is no longer
		// done as the complexity outweighs the benefits.
		//
		// - The SSH credential is not secret -- it's in the server entry. Attackers targetting
		//   the server likely already have the credential. On the other hand, random scanning and
		//   brute forcing is mitigated with high entropy random passwords, rate limiting
		//   (implemented on the host via iptables), and limited capabilities (the SSH session can
		//   only port forward).
		//
		// - fail2ban coverage was inconsistent; in the case of an unfronted meek protocol through
		//   an upstream proxy, the remote address is the upstream proxy, which should not be blocked.
		//   The X-Forwarded-For header cant be used instead as it may be forged and used to get IPs
		//   deliberately blocked; and in any case fail2ban adds iptables rules which can only block
		//   by direct remote IP, not by original client IP. Fronted meek has the same iptables issue.
		//
		// TODO: random scanning and brute forcing of port 22 will result in log noise. To eliminate
		// this, and to also cover meek protocols, and bad obfuscation keys, and bad inputs to the web
		// server, consider implementing fail2ban-type logic directly in this server, with the ability
		// to use X-Forwarded-For (when trustworthy; e.g, from a CDN).

		log.WithContextFields(LogFields{"error": err, "method": method}).Warning("authentication failed")

	} else {

		log.WithContextFields(LogFields{"error": err, "method": method}).Debug("authentication success")
	}
}

// stop signals the ssh connection to shutdown. After sshConn() returns,
// the connection has terminated but sshClient.run() may still be
// running and in the process of exiting.
func (sshClient *sshClient) stop() {

	sshClient.sshConn.Close()
	sshClient.sshConn.Wait()
}

// runTunnel handles/dispatches new channel and new requests from the client.
// When the SSH client connection closes, both the channels and requests channels
// will close and runClient will exit.
func (sshClient *sshClient) runTunnel(
	channels <-chan ssh.NewChannel, requests <-chan *ssh.Request) {

	waitGroup := new(sync.WaitGroup)

	waitGroup.Add(1)
	go func() {
		defer waitGroup.Done()

		for request := range requests {

			// Requests are processed serially; API responses must be sent in request order.

			var responsePayload []byte
			var err error

			if request.Type == "keepalive@openssh.com" {
				// Keepalive requests have an empty response.
			} else {
				// All other requests are assumed to be API requests.
				responsePayload, err = sshAPIRequestHandler(
					sshClient.sshServer.support,
					sshClient.geoIPData,
					request.Type,
					request.Payload)
			}

			if err == nil {
				err = request.Reply(true, responsePayload)
			} else {
				log.WithContextFields(LogFields{"error": err}).Warning("request failed")
				err = request.Reply(false, nil)
			}
			if err != nil {
				log.WithContextFields(LogFields{"error": err}).Warning("response failed")
			}

		}
	}()

	if sshClient.supportsServerRequests {
		waitGroup.Add(1)
		go func() {
			defer waitGroup.Done()
			sshClient.runOSLSender()
		}()
	}

	for newChannel := range channels {

		if newChannel.ChannelType() != "direct-tcpip" {
			sshClient.rejectNewChannel(newChannel, ssh.Prohibited, "unknown or unsupported channel type")
			continue
		}

		// process each port forward concurrently
		waitGroup.Add(1)
		go func(channel ssh.NewChannel) {
			defer waitGroup.Done()
			sshClient.handleNewPortForwardChannel(channel)
		}(newChannel)
	}

	// The channel loop is interrupted by a client
	// disconnect or by calling sshClient.stop().

	close(sshClient.stopBroadcast)

	waitGroup.Wait()
}

func (sshClient *sshClient) logTunnel() {

	// Note: reporting duration based on last confirmed data transfer, which
	// is reads for sshClient.activityConn.GetActiveDuration(), and not
	// connection closing is important for protocols such as meek. For
	// meek, the connection remains open until the HTTP session expires,
	// which may be some time after the tunnel has closed. (The meek
	// protocol has no allowance for signalling payload EOF, and even if
	// it did the client may not have the opportunity to send a final
	// request with an EOF flag set.)

	sshClient.Lock()

	logFields := getRequestLogFields(
		sshClient.sshServer.support,
		"server_tunnel",
		sshClient.geoIPData,
		sshClient.handshakeState.apiParams,
		baseRequestParams)

	logFields["handshake_completed"] = sshClient.handshakeState.completed
	logFields["start_time"] = sshClient.activityConn.GetStartTime()
	logFields["duration"] = sshClient.activityConn.GetActiveDuration() / time.Millisecond
	logFields["bytes_up_tcp"] = sshClient.tcpTrafficState.bytesUp
	logFields["bytes_down_tcp"] = sshClient.tcpTrafficState.bytesDown
	logFields["peak_concurrent_port_forward_count_tcp"] = sshClient.tcpTrafficState.peakConcurrentPortForwardCount
	logFields["total_port_forward_count_tcp"] = sshClient.tcpTrafficState.totalPortForwardCount
	logFields["bytes_up_udp"] = sshClient.udpTrafficState.bytesUp
	logFields["bytes_down_udp"] = sshClient.udpTrafficState.bytesDown
	logFields["peak_concurrent_port_forward_count_udp"] = sshClient.udpTrafficState.peakConcurrentPortForwardCount
	logFields["total_port_forward_count_udp"] = sshClient.udpTrafficState.totalPortForwardCount

	sshClient.Unlock()

	log.LogRawFieldsWithTimestamp(logFields)
}

func (sshClient *sshClient) runOSLSender() {

	for {
		// Await a signal that there are SLOKs to send
		// TODO: use reflect.SelectCase, and optionally await timer here?
		select {
		case <-sshClient.signalIssueSLOKs:
		case <-sshClient.stopBroadcast:
			return
		}

		retryDelay := SSH_SEND_OSL_INITIAL_RETRY_DELAY
		for {
			err := sshClient.sendOSLRequest()
			if err == nil {
				break
			}
			log.WithContextFields(LogFields{"error": err}).Warning("sendOSLRequest failed")

			// If the request failed, retry after a delay (with exponential backoff)
			// or when signaled that there are additional SLOKs to send
			retryTimer := time.NewTimer(retryDelay)
			select {
			case <-retryTimer.C:
			case <-sshClient.signalIssueSLOKs:
			case <-sshClient.stopBroadcast:
				retryTimer.Stop()
				return
			}
			retryTimer.Stop()
			retryDelay *= SSH_SEND_OSL_RETRY_FACTOR
		}
	}
}

// sendOSLRequest will invoke osl.GetSeedPayload to issue SLOKs and
// generate a payload, and send an OSL request to the client when
// there are new SLOKs in the payload.
func (sshClient *sshClient) sendOSLRequest() error {

	seedPayload := sshClient.getOSLSeedPayload()

	// Don't send when no SLOKs. This will happen when signalIssueSLOKs
	// is received but no new SLOKs are issued.
	if len(seedPayload.SLOKs) == 0 {
		return nil
	}

	oslRequest := protocol.OSLRequest{
		SeedPayload: seedPayload,
	}
	requestPayload, err := json.Marshal(oslRequest)
	if err != nil {
		return common.ContextError(err)
	}

	ok, _, err := sshClient.sshConn.SendRequest(
		protocol.PSIPHON_API_OSL_REQUEST_NAME,
		true,
		requestPayload)
	if err != nil {
		return common.ContextError(err)
	}
	if !ok {
		return common.ContextError(errors.New("client rejected request"))
	}

	sshClient.clearOSLSeedPayload()

	return nil
}

func (sshClient *sshClient) rejectNewChannel(newChannel ssh.NewChannel, reason ssh.RejectionReason, logMessage string) {

	// Note: Debug level, as logMessage may contain user traffic destination address information
	log.WithContextFields(
		LogFields{
			"channelType":  newChannel.ChannelType(),
			"logMessage":   logMessage,
			"rejectReason": reason.String(),
		}).Debug("reject new channel")

	// Note: logMessage is internal, for logging only; just the RejectionReason is sent to the client
	newChannel.Reject(reason, reason.String())
}

func (sshClient *sshClient) handleNewPortForwardChannel(newChannel ssh.NewChannel) {

	// http://tools.ietf.org/html/rfc4254#section-7.2
	var directTcpipExtraData struct {
		HostToConnect       string
		PortToConnect       uint32
		OriginatorIPAddress string
		OriginatorPort      uint32
	}

	err := ssh.Unmarshal(newChannel.ExtraData(), &directTcpipExtraData)
	if err != nil {
		sshClient.rejectNewChannel(newChannel, ssh.Prohibited, "invalid extra data")
		return
	}

	// Intercept TCP port forwards to a specified udpgw server and handle directly.
	// TODO: also support UDP explicitly, e.g. with a custom "direct-udp" channel type?
	isUDPChannel := sshClient.sshServer.support.Config.UDPInterceptUdpgwServerAddress != "" &&
		sshClient.sshServer.support.Config.UDPInterceptUdpgwServerAddress ==
			net.JoinHostPort(directTcpipExtraData.HostToConnect, strconv.Itoa(int(directTcpipExtraData.PortToConnect)))

	if isUDPChannel {
		sshClient.handleUDPChannel(newChannel)
	} else {
		sshClient.handleTCPChannel(
			directTcpipExtraData.HostToConnect, int(directTcpipExtraData.PortToConnect), newChannel)
	}
}

// setHandshakeState records that a client has completed a handshake API request.
// Some parameters from the handshake request may be used in future traffic rule
// selection. Port forwards are disallowed until a handshake is complete. The
// handshake parameters are included in the session summary log recorded in
// sshClient.stop().
func (sshClient *sshClient) setHandshakeState(state handshakeState) error {

	sshClient.Lock()
	completed := sshClient.handshakeState.completed
	if !completed {
		sshClient.handshakeState = state
	}
	sshClient.Unlock()

	// Client must only perform one handshake
	if completed {
		return common.ContextError(errors.New("handshake already completed"))
	}

	sshClient.setTrafficRules()
	sshClient.setOSLConfig()

	return nil
}

// setTrafficRules resets the client's traffic rules based on the latest server config
// and client properties. As sshClient.trafficRules may be reset by a concurrent
// goroutine, trafficRules must only be accessed within the sshClient mutex.
func (sshClient *sshClient) setTrafficRules() {
	sshClient.Lock()
	defer sshClient.Unlock()

	sshClient.trafficRules = sshClient.sshServer.support.TrafficRulesSet.GetTrafficRules(
		sshClient.tunnelProtocol, sshClient.geoIPData, sshClient.handshakeState)

	if sshClient.throttledConn != nil {
		// Any existing throttling state is reset.
		sshClient.throttledConn.SetLimits(
			sshClient.trafficRules.RateLimits.CommonRateLimits())
	}
}

// setOSLConfig resets the client's OSL seed state based on the latest OSL config
// As sshClient.oslClientSeedState may be reset by a concurrent goroutine,
// oslClientSeedState must only be accessed within the sshClient mutex.
func (sshClient *sshClient) setOSLConfig() {
	sshClient.Lock()
	defer sshClient.Unlock()

	propagationChannelID, err := getStringRequestParam(
		sshClient.handshakeState.apiParams, "propagation_channel_id")
	if err != nil {
		// This should not fail as long as client has sent valid handshake
		return
	}

	// Two limitations when setOSLConfig() is invoked due to an
	// OSL config hot reload:
	//
	// 1. any partial progress towards SLOKs is lost.
	//
	// 2. all existing osl.ClientSeedPortForwards for existing
	//    port forwards will not send progress to the new client
	//    seed state.

	sshClient.oslClientSeedState = sshClient.sshServer.support.OSLConfig.NewClientSeedState(
		sshClient.geoIPData.Country,
		propagationChannelID,
		sshClient.signalIssueSLOKs)
}

// newClientSeedPortForward will return nil when no seeding is
// associated with the specified ipAddress.
func (sshClient *sshClient) newClientSeedPortForward(ipAddress net.IP) *osl.ClientSeedPortForward {
	sshClient.Lock()
	defer sshClient.Unlock()

	// Will not be initialized before handshake.
	if sshClient.oslClientSeedState == nil {
		return nil
	}

	return sshClient.oslClientSeedState.NewClientSeedPortForward(ipAddress)
}

// getOSLSeedPayload returns a payload containing all seeded SLOKs for
// this client's session.
func (sshClient *sshClient) getOSLSeedPayload() *osl.SeedPayload {
	sshClient.Lock()
	defer sshClient.Unlock()

	// Will not be initialized before handshake.
	if sshClient.oslClientSeedState == nil {
		return &osl.SeedPayload{SLOKs: make([]*osl.SLOK, 0)}
	}

	return sshClient.oslClientSeedState.GetSeedPayload()
}

func (sshClient *sshClient) clearOSLSeedPayload() {
	sshClient.Lock()
	defer sshClient.Unlock()

	sshClient.oslClientSeedState.ClearSeedPayload()
}

func (sshClient *sshClient) rateLimits() common.RateLimits {
	sshClient.Lock()
	defer sshClient.Unlock()

	return sshClient.trafficRules.RateLimits.CommonRateLimits()
}

func (sshClient *sshClient) idleTCPPortForwardTimeout() time.Duration {
	sshClient.Lock()
	defer sshClient.Unlock()

	return time.Duration(*sshClient.trafficRules.IdleTCPPortForwardTimeoutMilliseconds) * time.Millisecond
}

func (sshClient *sshClient) idleUDPPortForwardTimeout() time.Duration {

	sshClient.Lock()
	defer sshClient.Unlock()

	return time.Duration(*sshClient.trafficRules.IdleUDPPortForwardTimeoutMilliseconds) * time.Millisecond
}

const (
	portForwardTypeTCP = iota
	portForwardTypeUDP
	portForwardTypeTransparentDNS
)

func (sshClient *sshClient) isPortForwardPermitted(
	portForwardType int,
	isTransparentDNSForwarding bool,
	remoteIP net.IP,
	port int) bool {

	sshClient.Lock()
	defer sshClient.Unlock()

	if !sshClient.handshakeState.completed {
		return false
	}

	// Disallow connection to loopback. This is a failsafe. The server
	// should be run on a host with correctly configured firewall rules.
	// And exception is made in the case of tranparent DNS forwarding,
	// where the remoteIP has been rewritten.
	if !isTransparentDNSForwarding && remoteIP.IsLoopback() {
		return false
	}

	var allowPorts []int
	if portForwardType == portForwardTypeTCP {
		allowPorts = sshClient.trafficRules.AllowTCPPorts
	} else {
		allowPorts = sshClient.trafficRules.AllowUDPPorts
	}

	if len(allowPorts) == 0 {
		return true
	}

	// TODO: faster lookup?
	if len(allowPorts) > 0 {
		for _, allowPort := range allowPorts {
			if port == allowPort {
				return true
			}
		}
	}

	for _, subnet := range sshClient.trafficRules.AllowSubnets {
		// Note: ignoring error as config has been validated
		_, network, _ := net.ParseCIDR(subnet)
		if network.Contains(remoteIP) {
			return true
		}
	}

	return false
}

func (sshClient *sshClient) isPortForwardLimitExceeded(
	portForwardType int) (int, bool) {

	sshClient.Lock()
	defer sshClient.Unlock()

	var maxPortForwardCount int
	var state *trafficState
	if portForwardType == portForwardTypeTCP {
		maxPortForwardCount = *sshClient.trafficRules.MaxTCPPortForwardCount
		state = &sshClient.tcpTrafficState
	} else {
		maxPortForwardCount = *sshClient.trafficRules.MaxUDPPortForwardCount
		state = &sshClient.udpTrafficState
	}

	if maxPortForwardCount > 0 && state.concurrentPortForwardCount >= int64(maxPortForwardCount) {
		return maxPortForwardCount, true
	}
	return maxPortForwardCount, false
}

func (sshClient *sshClient) openedPortForward(
	portForwardType int) {

	sshClient.Lock()
	defer sshClient.Unlock()

	var state *trafficState
	if portForwardType == portForwardTypeTCP {
		state = &sshClient.tcpTrafficState
	} else {
		state = &sshClient.udpTrafficState
	}

	state.concurrentPortForwardCount += 1
	if state.concurrentPortForwardCount > state.peakConcurrentPortForwardCount {
		state.peakConcurrentPortForwardCount = state.concurrentPortForwardCount
	}
	state.totalPortForwardCount += 1
}

func (sshClient *sshClient) updateQualityMetrics(
	tcpPortForwardDialSuccess bool, dialDuration time.Duration) {

	sshClient.Lock()
	defer sshClient.Unlock()

	if tcpPortForwardDialSuccess {
		sshClient.qualityMetrics.tcpPortForwardDialedCount += 1
		sshClient.qualityMetrics.tcpPortForwardDialedDuration += dialDuration

	} else {
		sshClient.qualityMetrics.tcpPortForwardFailedCount += 1
		sshClient.qualityMetrics.tcpPortForwardFailedDuration += dialDuration
	}
}

func (sshClient *sshClient) closedPortForward(
	portForwardType int, bytesUp, bytesDown int64) {

	sshClient.Lock()
	defer sshClient.Unlock()

	var state *trafficState
	if portForwardType == portForwardTypeTCP {
		state = &sshClient.tcpTrafficState
	} else {
		state = &sshClient.udpTrafficState
	}

	state.concurrentPortForwardCount -= 1
	state.bytesUp += bytesUp
	state.bytesDown += bytesDown
}

func (sshClient *sshClient) handleTCPChannel(
	hostToConnect string,
	portToConnect int,
	newChannel ssh.NewChannel) {

	isWebServerPortForward := false
	config := sshClient.sshServer.support.Config
	if config.WebServerPortForwardAddress != "" {
		destination := net.JoinHostPort(hostToConnect, strconv.Itoa(portToConnect))
		if destination == config.WebServerPortForwardAddress {
			isWebServerPortForward = true
			if config.WebServerPortForwardRedirectAddress != "" {
				// Note: redirect format is validated when config is loaded
				host, portStr, _ := net.SplitHostPort(config.WebServerPortForwardRedirectAddress)
				port, _ := strconv.Atoi(portStr)
				hostToConnect = host
				portToConnect = port
			}
		}
	}

	type lookupIPResult struct {
		IP  net.IP
		err error
	}
	lookupResultChannel := make(chan *lookupIPResult, 1)

	go func() {
		// TODO: explicit timeout for DNS resolution?
		IPs, err := net.LookupIP(hostToConnect)
		// TODO: shuffle list to try other IPs
		// TODO: IPv6 support
		var IP net.IP
		for _, ip := range IPs {
			if ip.To4() != nil {
				IP = ip
			}
		}
		if err == nil && IP == nil {
			err = errors.New("no IP address")
		}
		lookupResultChannel <- &lookupIPResult{IP, err}
	}()

	var lookupResult *lookupIPResult
	select {
	case lookupResult = <-lookupResultChannel:
	case <-sshClient.stopBroadcast:
		// Note: may leave LookupIP in progress
		return
	}

	if lookupResult.err != nil {
		sshClient.rejectNewChannel(
			newChannel, ssh.ConnectionFailed, fmt.Sprintf("LookupIP failed: %s", lookupResult.err))
		return
	}

	if !isWebServerPortForward &&
		!sshClient.isPortForwardPermitted(
			portForwardTypeTCP,
			false,
			lookupResult.IP,
			portToConnect) {

		sshClient.rejectNewChannel(
			newChannel, ssh.Prohibited, "port forward not permitted")
		return
	}

	var bytesUp, bytesDown int64
	sshClient.openedPortForward(portForwardTypeTCP)
	defer func() {
		sshClient.closedPortForward(
			portForwardTypeTCP, atomic.LoadInt64(&bytesUp), atomic.LoadInt64(&bytesDown))
	}()

	// TOCTOU note: important to increment the port forward count (via
	// openPortForward) _before_ checking isPortForwardLimitExceeded
	// otherwise, the client could potentially consume excess resources
	// by initiating many port forwards concurrently.
	// TODO: close LRU connection (after successful Dial) instead of
	// rejecting new connection?
	if maxCount, exceeded := sshClient.isPortForwardLimitExceeded(portForwardTypeTCP); exceeded {

		// Close the oldest TCP port forward. CloseOldest() closes
		// the conn and the port forward's goroutine will complete
		// the cleanup asynchronously.
		//
		// Some known limitations:
		//
		// - Since CloseOldest() closes the upstream socket but does not
		//   clean up all resources associated with the port forward. These
		//   include the goroutine(s) relaying traffic as well as the SSH
		//   channel. Closing the socket will interrupt the goroutines which
		//   will then complete the cleanup. But, since the full cleanup is
		//   asynchronous, there exists a possibility that a client can consume
		//   more than max port forward resources -- just not upstream sockets.
		//
		// - An LRU list entry for this port forward is not added until
		//   after the dial completes, but the port forward is counted
		//   towards max limits. This means many dials in progress will
		//   put established connections in jeopardy.
		//
		// - We're closing the oldest open connection _before_ successfully
		//   dialing the new port forward. This means we are potentially
		//   discarding a good connection to make way for a failed connection.
		//   We cannot simply dial first and still maintain a limit on
		//   resources used, so to address this we'd need to add some
		//   accounting for connections still establishing.

		sshClient.tcpPortForwardLRU.CloseOldest()

		log.WithContextFields(
			LogFields{
				"maxCount": maxCount,
			}).Debug("closed LRU TCP port forward")
	}

	// Dial the target remote address. This is done in a goroutine to
	// ensure the shutdown signal is handled immediately.

	remoteAddr := net.JoinHostPort(lookupResult.IP.String(), strconv.Itoa(portToConnect))

	log.WithContextFields(LogFields{"remoteAddr": remoteAddr}).Debug("dialing")

	type dialTCPResult struct {
		conn net.Conn
		err  error
	}
	dialResultChannel := make(chan *dialTCPResult, 1)

	dialStartTime := monotime.Now()

	go func() {
		// TODO: on EADDRNOTAVAIL, temporarily suspend new clients
		conn, err := net.DialTimeout(
			"tcp", remoteAddr, SSH_TCP_PORT_FORWARD_DIAL_TIMEOUT)
		dialResultChannel <- &dialTCPResult{conn, err}
	}()

	var dialResult *dialTCPResult
	select {
	case dialResult = <-dialResultChannel:
	case <-sshClient.stopBroadcast:
		// Note: may leave Dial in progress
		// TODO: use net.Dialer.DialContext to be able to cancel
		return
	}

	sshClient.updateQualityMetrics(
		dialResult.err == nil, monotime.Since(dialStartTime))

	if dialResult.err != nil {
		sshClient.rejectNewChannel(
			newChannel, ssh.ConnectionFailed, fmt.Sprintf("DialTimeout failed: %s", dialResult.err))
		return
	}

	// The upstream TCP port forward connection has been established. Schedule
	// some cleanup and notify the SSH client that the channel is accepted.

	fwdConn := dialResult.conn
	defer fwdConn.Close()

	fwdChannel, requests, err := newChannel.Accept()
	if err != nil {
		log.WithContextFields(LogFields{"error": err}).Warning("accept new channel failed")
		return
	}
	go ssh.DiscardRequests(requests)
	defer fwdChannel.Close()

	// ActivityMonitoredConn monitors the TCP port forward I/O and updates
	// its LRU status. ActivityMonitoredConn also times out I/O on the port
	// forward if both reads and writes have been idle for the specified
	// duration.

	lruEntry := sshClient.tcpPortForwardLRU.Add(fwdConn)
	defer lruEntry.Remove()

	// Ensure nil interface if newClientSeedPortForward returns nil
	var updater common.ActivityUpdater
	seedUpdater := sshClient.newClientSeedPortForward(lookupResult.IP)
	if seedUpdater != nil {
		updater = seedUpdater
	}

	fwdConn, err = common.NewActivityMonitoredConn(
		fwdConn,
		sshClient.idleTCPPortForwardTimeout(),
		true,
		updater,
		lruEntry)
	if err != nil {
		log.WithContextFields(LogFields{"error": err}).Error("NewActivityMonitoredConn failed")
		return
	}

	// Relay channel to forwarded connection.

	log.WithContextFields(LogFields{"remoteAddr": remoteAddr}).Debug("relaying")

	// TODO: relay errors to fwdChannel.Stderr()?
	relayWaitGroup := new(sync.WaitGroup)
	relayWaitGroup.Add(1)
	go func() {
		defer relayWaitGroup.Done()
		// io.Copy allocates a 32K temporary buffer, and each port forward relay uses
		// two of these buffers; using io.CopyBuffer with a smaller buffer reduces the
		// overall memory footprint.
		bytes, err := io.CopyBuffer(
			fwdChannel, fwdConn, make([]byte, SSH_TCP_PORT_FORWARD_COPY_BUFFER_SIZE))
		atomic.AddInt64(&bytesDown, bytes)
		if err != nil && err != io.EOF {
			// Debug since errors such as "connection reset by peer" occur during normal operation
			log.WithContextFields(LogFields{"error": err}).Debug("downstream TCP relay failed")
		}
		// Interrupt upstream io.Copy when downstream is shutting down.
		// TODO: this is done to quickly cleanup the port forward when
		// fwdConn has a read timeout, but is it clean -- upstream may still
		// be flowing?
		fwdChannel.Close()
	}()
	bytes, err := io.CopyBuffer(
		fwdConn, fwdChannel, make([]byte, SSH_TCP_PORT_FORWARD_COPY_BUFFER_SIZE))
	atomic.AddInt64(&bytesUp, bytes)
	if err != nil && err != io.EOF {
		log.WithContextFields(LogFields{"error": err}).Debug("upstream TCP relay failed")
	}
	// Shutdown special case: fwdChannel will be closed and return EOF when
	// the SSH connection is closed, but we need to explicitly close fwdConn
	// to interrupt the downstream io.Copy, which may be blocked on a
	// fwdConn.Read().
	fwdConn.Close()

	relayWaitGroup.Wait()

	log.WithContextFields(
		LogFields{
			"remoteAddr": remoteAddr,
			"bytesUp":    atomic.LoadInt64(&bytesUp),
			"bytesDown":  atomic.LoadInt64(&bytesDown)}).Debug("exiting")
}