// handleUDPChannel 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 <*****@*****.**>
// https://github.com/ambrop72/badvpn
//
func (sshClient *sshClient) handleUDPChannel(newChannel ssh.NewChannel) {
// Accept this channel immediately. This channel will replace any
// previously existing UDP channel for this client.
sshChannel, requests, err := newChannel.Accept()
if err != nil {
log.WithContextFields(LogFields{"error": err}).Warning("accept new channel failed")
return
}
go ssh.DiscardRequests(requests)
defer sshChannel.Close()
sshClient.setUDPChannel(sshChannel)
multiplexer := &udpPortForwardMultiplexer{
sshClient: sshClient,
sshChannel: sshChannel,
portForwards: make(map[uint16]*udpPortForward),
portForwardLRU: common.NewLRUConns(),
relayWaitGroup: new(sync.WaitGroup),
}
multiplexer.run()
}
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
}
}
}
if !isWebServerPortForward && !sshClient.isPortForwardPermitted(
portForwardTypeTCP, hostToConnect, 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 := fmt.Sprintf("%s:%d", hostToConnect, portToConnect)
log.WithContextFields(LogFields{"remoteAddr": remoteAddr}).Debug("dialing")
type dialTcpResult struct {
conn net.Conn
err error
}
resultChannel := make(chan *dialTcpResult, 1)
go func() {
// TODO: on EADDRNOTAVAIL, temporarily suspend new clients
// TODO: IPv6 support
conn, err := net.DialTimeout(
"tcp4", remoteAddr, SSH_TCP_PORT_FORWARD_DIAL_TIMEOUT)
resultChannel <- &dialTcpResult{conn, err}
}()
var result *dialTcpResult
select {
case result = <-resultChannel:
case <-sshClient.stopBroadcast:
// Note: may leave dial in progress (TODO: use DialContext to cancel)
return
}
if result.err != nil {
sshClient.rejectNewChannel(newChannel, ssh.ConnectionFailed, result.err.Error())
return
}
// The upstream TCP port forward connection has been established. Schedule
// some cleanup and notify the SSH client that the channel is accepted.
fwdConn := result.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()
fwdConn, err = common.NewActivityMonitoredConn(
fwdConn,
sshClient.idleTCPPortForwardTimeout(),
true,
lruEntry)
if result.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")
}