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udp.go
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udp.go
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/*
* 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 (
"bytes"
"encoding/binary"
"fmt"
"io"
"net"
"strconv"
"sync"
"sync/atomic"
"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common"
"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/crypto/ssh"
"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/errors"
"github.com/Psiphon-Labs/psiphon-tunnel-core/psiphon/common/monotime"
)
// handleUdpgwChannel implements UDP port forwarding. A single UDP
// SSH channel follows the udpgw protocol, which multiplexes many
// UDP port forwards.
//
// The udpgw protocol and original server implementation:
// Copyright (c) 2009, Ambroz Bizjak <ambrop7@gmail.com>
// https://github.com/ambrop72/badvpn
//
func (sshClient *sshClient) handleUdpgwChannel(newChannel ssh.NewChannel) {
// Accept this channel immediately. This channel will replace any
// previously existing udpgw channel for this client.
sshChannel, requests, err := newChannel.Accept()
if err != nil {
if !isExpectedTunnelIOError(err) {
log.WithTraceFields(LogFields{"error": err}).Warning("accept new channel failed")
}
return
}
go ssh.DiscardRequests(requests)
defer sshChannel.Close()
multiplexer := &udpgwPortForwardMultiplexer{
sshClient: sshClient,
sshChannel: sshChannel,
portForwards: make(map[uint16]*udpgwPortForward),
portForwardLRU: common.NewLRUConns(),
relayWaitGroup: new(sync.WaitGroup),
runWaitGroup: new(sync.WaitGroup),
}
multiplexer.runWaitGroup.Add(1)
// setUdpgwChannelHandler will close any existing
// udpgwPortForwardMultiplexer, waiting for all run/relayDownstream
// goroutines to first terminate and all UDP socket resources to be
// cleaned up.
//
// This synchronous shutdown also ensures that the
// concurrentPortForwardCount is reduced to 0 before installing the new
// udpgwPortForwardMultiplexer and its LRU object. If the older handler
// were to dangle with open port forwards, and concurrentPortForwardCount
// were to hit the max, the wrong LRU, the new one, would be used to
// close the LRU port forward.
//
// Call setUdpgwHandler only after runWaitGroup is initialized, to ensure
// runWaitGroup.Wait() cannot be invoked (by some subsequent new udpgw
// channel) before initialized.
if !sshClient.setUdpgwChannelHandler(multiplexer) {
// setUdpgwChannelHandler returns false if some other SSH channel
// calls setUdpgwChannelHandler in the middle of this call. In that
// case, discard this channel: the client's latest udpgw channel is
// retained.
return
}
multiplexer.run()
multiplexer.runWaitGroup.Done()
}
type udpgwPortForwardMultiplexer struct {
sshClient *sshClient
sshChannelWriteMutex sync.Mutex
sshChannel ssh.Channel
portForwardsMutex sync.Mutex
portForwards map[uint16]*udpgwPortForward
portForwardLRU *common.LRUConns
relayWaitGroup *sync.WaitGroup
runWaitGroup *sync.WaitGroup
}
func (mux *udpgwPortForwardMultiplexer) stop() {
// udpgwPortForwardMultiplexer must be initialized by handleUdpgwChannel.
//
// stop closes the udpgw SSH channel, which will cause the run goroutine
// to exit its message read loop and await closure of all relayDownstream
// goroutines. Closing all port forward UDP conns will cause all
// relayDownstream to exit.
_ = mux.sshChannel.Close()
mux.portForwardsMutex.Lock()
for _, portForward := range mux.portForwards {
_ = portForward.conn.Close()
}
mux.portForwardsMutex.Unlock()
mux.runWaitGroup.Wait()
}
func (mux *udpgwPortForwardMultiplexer) run() {
// In a loop, read udpgw messages from the client to this channel. Each
// message contains a UDP packet to send upstream either via a new port
// forward, or on an existing port forward.
//
// A goroutine is run to read downstream packets for each UDP port forward. All read
// packets are encapsulated in udpgw protocol and sent down the channel to the client.
//
// When the client disconnects or the server shuts down, the channel will close and
// readUdpgwMessage will exit with EOF.
buffer := make([]byte, udpgwProtocolMaxMessageSize)
for {
// Note: message.packet points to the reusable memory in "buffer".
// Each readUdpgwMessage call will overwrite the last message.packet.
message, err := readUdpgwMessage(mux.sshChannel, buffer)
if err != nil {
if err != io.EOF {
// Debug since I/O errors occur during normal operation
log.WithTraceFields(LogFields{"error": err}).Debug("readUdpgwMessage failed")
}
break
}
mux.portForwardsMutex.Lock()
portForward := mux.portForwards[message.connID]
mux.portForwardsMutex.Unlock()
// In the udpgw protocol, an existing port forward is closed when
// either the discard flag is set or the remote address has changed.
if portForward != nil &&
(message.discardExistingConn ||
!bytes.Equal(portForward.remoteIP, message.remoteIP) ||
portForward.remotePort != message.remotePort) {
// The port forward's goroutine will complete cleanup, including
// tallying stats and calling sshClient.closedPortForward.
// portForward.conn.Close() will signal this shutdown.
portForward.conn.Close()
// Synchronously await the termination of the relayDownstream
// goroutine. This ensures that the previous goroutine won't
// invoke removePortForward, with the connID that will be reused
// for the new port forward, after this point.
//
// Limitation: this synchronous shutdown cannot prevent a "wrong
// remote address" error on the badvpn udpgw client, which occurs
// when the client recycles a port forward (setting discard) but
// receives, from the server, a udpgw message containing the old
// remote address for the previous port forward with the same
// conn ID. That downstream message from the server may be in
// flight in the SSH channel when the client discard message arrives.
portForward.relayWaitGroup.Wait()
portForward = nil
}
if portForward == nil {
// Create a new port forward
dialIP := net.IP(message.remoteIP)
dialPort := int(message.remotePort)
// Validate DNS packets and check the domain blocklist both when the client
// indicates DNS or when DNS is _not_ indicated and the destination port is
// 53.
if message.forwardDNS || message.remotePort == 53 {
domain, err := common.ParseDNSQuestion(message.packet)
if err != nil {
log.WithTraceFields(LogFields{"error": err}).Debug("ParseDNSQuestion failed")
// Drop packet
continue
}
if domain != "" {
ok, _ := mux.sshClient.isDomainPermitted(domain)
if !ok {
// Drop packet
continue
}
}
}
if message.forwardDNS {
// Transparent DNS forwarding. In this case, isPortForwardPermitted
// traffic rules checks are bypassed, since DNS is essential.
dialIP = mux.sshClient.sshServer.support.DNSResolver.Get()
dialPort = DNS_RESOLVER_PORT
} else if !mux.sshClient.isPortForwardPermitted(
portForwardTypeUDP, dialIP, int(message.remotePort)) {
// The udpgw protocol has no error response, so
// we just discard the message and read another.
continue
}
// Note: UDP port forward counting has no dialing phase
// establishedPortForward increments the concurrent UDP port
// forward counter and closes the LRU existing UDP port forward
// when already at the limit.
mux.sshClient.establishedPortForward(portForwardTypeUDP, mux.portForwardLRU)
// Can't defer sshClient.closedPortForward() here;
// relayDownstream will call sshClient.closedPortForward()
// Pre-check log level to avoid overhead of rendering log for
// every DNS query and other UDP port forward.
if IsLogLevelDebug() {
log.WithTraceFields(
LogFields{
"remoteAddr": net.JoinHostPort(dialIP.String(), strconv.Itoa(dialPort)),
"connID": message.connID}).Debug("dialing")
}
udpConn, err := net.DialUDP(
"udp", nil, &net.UDPAddr{IP: dialIP, Port: dialPort})
if err != nil {
mux.sshClient.closedPortForward(portForwardTypeUDP, 0, 0)
// Monitor for low resource error conditions
mux.sshClient.sshServer.monitorPortForwardDialError(err)
// Note: Debug level, as logMessage may contain user traffic destination address information
log.WithTraceFields(LogFields{"error": err}).Debug("DialUDP failed")
continue
}
lruEntry := mux.portForwardLRU.Add(udpConn)
// Can't defer lruEntry.Remove() here;
// relayDownstream will call lruEntry.Remove()
// 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.
var activityUpdaters []common.ActivityUpdater
// Don't incur activity monitor overhead for DNS requests
if !message.forwardDNS {
activityUpdaters = mux.sshClient.getActivityUpdaters(portForwardTypeUDP, dialIP)
}
conn, err := common.NewActivityMonitoredConn(
udpConn,
mux.sshClient.idleUDPPortForwardTimeout(),
true,
lruEntry,
activityUpdaters...)
if err != nil {
lruEntry.Remove()
mux.sshClient.closedPortForward(portForwardTypeUDP, 0, 0)
log.WithTraceFields(LogFields{"error": err}).Error("NewActivityMonitoredConn failed")
continue
}
portForward = &udpgwPortForward{
connID: message.connID,
preambleSize: message.preambleSize,
remoteIP: message.remoteIP,
remotePort: message.remotePort,
dialIP: dialIP,
conn: conn,
lruEntry: lruEntry,
bytesUp: 0,
bytesDown: 0,
relayWaitGroup: new(sync.WaitGroup),
mux: mux,
}
if message.forwardDNS {
portForward.dnsFirstWriteTime = int64(monotime.Now())
}
mux.portForwardsMutex.Lock()
mux.portForwards[portForward.connID] = portForward
mux.portForwardsMutex.Unlock()
portForward.relayWaitGroup.Add(1)
mux.relayWaitGroup.Add(1)
go portForward.relayDownstream()
}
// Note: assumes UDP writes won't block (https://golang.org/pkg/net/#UDPConn.WriteToUDP)
_, err = portForward.conn.Write(message.packet)
if err != nil {
// Debug since errors such as "write: operation not permitted" occur during normal operation
log.WithTraceFields(LogFields{"error": err}).Debug("upstream UDP relay failed")
// The port forward's goroutine will complete cleanup
portForward.conn.Close()
}
portForward.lruEntry.Touch()
atomic.AddInt64(&portForward.bytesUp, int64(len(message.packet)))
}
// Cleanup all udpgw port forward workers when exiting
mux.portForwardsMutex.Lock()
for _, portForward := range mux.portForwards {
// The port forward's goroutine will complete cleanup
portForward.conn.Close()
}
mux.portForwardsMutex.Unlock()
mux.relayWaitGroup.Wait()
}
func (mux *udpgwPortForwardMultiplexer) removePortForward(connID uint16) {
mux.portForwardsMutex.Lock()
delete(mux.portForwards, connID)
mux.portForwardsMutex.Unlock()
}
type udpgwPortForward struct {
// Note: 64-bit ints used with atomic operations are placed
// at the start of struct to ensure 64-bit alignment.
// (https://golang.org/pkg/sync/atomic/#pkg-note-BUG)
dnsFirstWriteTime int64
dnsFirstReadTime int64
bytesUp int64
bytesDown int64
connID uint16
preambleSize int
remoteIP []byte
remotePort uint16
dialIP net.IP
conn net.Conn
lruEntry *common.LRUConnsEntry
relayWaitGroup *sync.WaitGroup
mux *udpgwPortForwardMultiplexer
}
func (portForward *udpgwPortForward) relayDownstream() {
defer portForward.relayWaitGroup.Done()
defer portForward.mux.relayWaitGroup.Done()
// Downstream UDP packets are read into the reusable memory
// in "buffer" starting at the offset past the udpgw message
// header and address, leaving enough space to write the udpgw
// values into the same buffer and use for writing to the ssh
// channel.
//
// Note: there is one downstream buffer per UDP port forward,
// while for upstream there is one buffer per client.
// TODO: is the buffer size larger than necessary?
buffer := make([]byte, udpgwProtocolMaxMessageSize)
packetBuffer := buffer[portForward.preambleSize:udpgwProtocolMaxMessageSize]
for {
// TODO: if read buffer is too small, excess bytes are discarded?
packetSize, err := portForward.conn.Read(packetBuffer)
if packetSize > udpgwProtocolMaxPayloadSize {
err = fmt.Errorf("unexpected packet size: %d", packetSize)
}
if err != nil {
if err != io.EOF {
// Debug since errors such as "use of closed network connection" occur during normal operation
log.WithTraceFields(LogFields{"error": err}).Debug("downstream UDP relay failed")
}
break
}
if atomic.LoadInt64(&portForward.dnsFirstWriteTime) > 0 &&
atomic.LoadInt64(&portForward.dnsFirstReadTime) == 0 { // Check if already set before invoking Now.
atomic.CompareAndSwapInt64(&portForward.dnsFirstReadTime, 0, int64(monotime.Now()))
}
err = writeUdpgwPreamble(
portForward.preambleSize,
0,
portForward.connID,
portForward.remoteIP,
portForward.remotePort,
uint16(packetSize),
buffer)
if err == nil {
// ssh.Channel.Write cannot be called concurrently.
// See: https://github.com/Psiphon-Inc/crypto/blob/82d98b4c7c05e81f92545f6fddb45d4541e6da00/ssh/channel.go#L272,
// https://codereview.appspot.com/136420043/diff/80002/ssh/channel.go
portForward.mux.sshChannelWriteMutex.Lock()
_, err = portForward.mux.sshChannel.Write(buffer[0 : portForward.preambleSize+packetSize])
portForward.mux.sshChannelWriteMutex.Unlock()
}
if err != nil {
// Close the channel, which will interrupt the main loop.
portForward.mux.sshChannel.Close()
log.WithTraceFields(LogFields{"error": err}).Debug("downstream UDP relay failed")
break
}
portForward.lruEntry.Touch()
atomic.AddInt64(&portForward.bytesDown, int64(packetSize))
}
portForward.mux.removePortForward(portForward.connID)
portForward.lruEntry.Remove()
portForward.conn.Close()
bytesUp := atomic.LoadInt64(&portForward.bytesUp)
bytesDown := atomic.LoadInt64(&portForward.bytesDown)
portForward.mux.sshClient.closedPortForward(portForwardTypeUDP, bytesUp, bytesDown)
dnsStartTime := monotime.Time(atomic.LoadInt64(&portForward.dnsFirstWriteTime))
if dnsStartTime > 0 {
// Record DNS metrics using a heuristic: if a UDP packet was written and
// then a packet was read, assume the DNS request successfully received a
// valid response; failure occurs when the resolver fails to provide a
// response; a "no such host" response is still a success. Limitations: we
// assume a resolver will not respond when, e.g., rate limiting; we ignore
// subsequent requests made via the same UDP port forward.
dnsEndTime := monotime.Time(atomic.LoadInt64(&portForward.dnsFirstReadTime))
dnsSuccess := true
if dnsEndTime == 0 {
dnsSuccess = false
dnsEndTime = monotime.Now()
}
resolveElapsedTime := dnsEndTime.Sub(dnsStartTime)
portForward.mux.sshClient.updateQualityMetricsWithDNSResult(
dnsSuccess,
resolveElapsedTime,
net.IP(portForward.dialIP))
}
log.WithTraceFields(
LogFields{
"remoteAddr": net.JoinHostPort(
net.IP(portForward.remoteIP).String(), strconv.Itoa(int(portForward.remotePort))),
"bytesUp": bytesUp,
"bytesDown": bytesDown,
"connID": portForward.connID}).Debug("exiting")
}
// TODO: express and/or calculate udpgwProtocolMaxPayloadSize as function of MTU?
const (
udpgwProtocolFlagKeepalive = 1 << 0
udpgwProtocolFlagRebind = 1 << 1
udpgwProtocolFlagDNS = 1 << 2
udpgwProtocolFlagIPv6 = 1 << 3
udpgwProtocolMaxPreambleSize = 23
udpgwProtocolMaxPayloadSize = 32768
udpgwProtocolMaxMessageSize = udpgwProtocolMaxPreambleSize + udpgwProtocolMaxPayloadSize
)
type udpgwProtocolMessage struct {
connID uint16
preambleSize int
remoteIP []byte
remotePort uint16
discardExistingConn bool
forwardDNS bool
packet []byte
}
func readUdpgwMessage(
reader io.Reader, buffer []byte) (*udpgwProtocolMessage, error) {
// udpgw message layout:
//
// | 2 byte size | 3 byte header | 6 or 18 byte address | variable length packet |
for {
// Read message
_, err := io.ReadFull(reader, buffer[0:2])
if err != nil {
if err != io.EOF {
err = errors.Trace(err)
}
return nil, err
}
size := binary.LittleEndian.Uint16(buffer[0:2])
if size < 3 || int(size) > len(buffer)-2 {
return nil, errors.TraceNew("invalid udpgw message size")
}
_, err = io.ReadFull(reader, buffer[2:2+size])
if err != nil {
if err != io.EOF {
err = errors.Trace(err)
}
return nil, err
}
flags := buffer[2]
connID := binary.LittleEndian.Uint16(buffer[3:5])
// Ignore udpgw keep-alive messages -- read another message
if flags&udpgwProtocolFlagKeepalive == udpgwProtocolFlagKeepalive {
continue
}
// Read address
var remoteIP []byte
var remotePort uint16
var packetStart, packetEnd int
if flags&udpgwProtocolFlagIPv6 == udpgwProtocolFlagIPv6 {
if size < 21 {
return nil, errors.TraceNew("invalid udpgw message size")
}
remoteIP = make([]byte, 16)
copy(remoteIP, buffer[5:21])
remotePort = binary.BigEndian.Uint16(buffer[21:23])
packetStart = 23
packetEnd = 23 + int(size) - 21
} else {
if size < 9 {
return nil, errors.TraceNew("invalid udpgw message size")
}
remoteIP = make([]byte, 4)
copy(remoteIP, buffer[5:9])
remotePort = binary.BigEndian.Uint16(buffer[9:11])
packetStart = 11
packetEnd = 11 + int(size) - 9
}
// Assemble message
// Note: udpgwProtocolMessage.packet references memory in the input buffer
message := &udpgwProtocolMessage{
connID: connID,
preambleSize: packetStart,
remoteIP: remoteIP,
remotePort: remotePort,
discardExistingConn: flags&udpgwProtocolFlagRebind == udpgwProtocolFlagRebind,
forwardDNS: flags&udpgwProtocolFlagDNS == udpgwProtocolFlagDNS,
packet: buffer[packetStart:packetEnd],
}
return message, nil
}
}
func writeUdpgwPreamble(
preambleSize int,
flags uint8,
connID uint16,
remoteIP []byte,
remotePort uint16,
packetSize uint16,
buffer []byte) error {
if preambleSize != 7+len(remoteIP) {
return errors.TraceNew("invalid udpgw preamble size")
}
size := uint16(preambleSize-2) + packetSize
// size
binary.LittleEndian.PutUint16(buffer[0:2], size)
// flags
buffer[2] = flags
// connID
binary.LittleEndian.PutUint16(buffer[3:5], connID)
// addr
copy(buffer[5:5+len(remoteIP)], remoteIP)
binary.BigEndian.PutUint16(buffer[5+len(remoteIP):7+len(remoteIP)], remotePort)
return nil
}