src/pkg/exp/ssh/client.go - go - Git at Google

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package ssh

 import (
 	"big"
 	"crypto"
 	"crypto/rand"
 	"fmt"
 	"io"
 	"os"
 	"net"
 	"sync"
 )

 // clientVersion is the fixed identification string that the client will use.
 var clientVersion = []byte("SSH-2.0-Go\r\n")

 // ClientConn represents the client side of an SSH connection.
 type ClientConn struct {
 	*transport
 	config *ClientConfig
 	chanlist
 }

 // Client returns a new SSH client connection using c as the underlying transport.
 func Client(c net.Conn, config *ClientConfig) (*ClientConn, os.Error) {
 	conn := &ClientConn{
 		transport: newTransport(c, config.rand()),
 		config:    config,
 	}
 	if err := conn.handshake(); err != nil {
 		conn.Close()
 		return nil, err
 	}
 	if err := conn.authenticate(); err != nil {
 		conn.Close()
 		return nil, err
 	}
 	go conn.mainLoop()
 	return conn, nil
 }

 // handshake performs the client side key exchange. See RFC 4253 Section 7.
 func (c *ClientConn) handshake() os.Error {
 	var magics handshakeMagics

 	if _, err := c.Write(clientVersion); err != nil {
 		return err
 	}
 	if err := c.Flush(); err != nil {
 		return err
 	}
 	magics.clientVersion = clientVersion[:len(clientVersion)-2]

 	// read remote server version
 	version, err := readVersion(c)
 	if err != nil {
 		return err
 	}
 	magics.serverVersion = version
 	clientKexInit := kexInitMsg{
 		KexAlgos:                supportedKexAlgos,
 		ServerHostKeyAlgos:      supportedHostKeyAlgos,
 		CiphersClientServer:     supportedCiphers,
 		CiphersServerClient:     supportedCiphers,
 		MACsClientServer:        supportedMACs,
 		MACsServerClient:        supportedMACs,
 		CompressionClientServer: supportedCompressions,
 		CompressionServerClient: supportedCompressions,
 	}
 	kexInitPacket := marshal(msgKexInit, clientKexInit)
 	magics.clientKexInit = kexInitPacket

 	if err := c.writePacket(kexInitPacket); err != nil {
 		return err
 	}
 	packet, err := c.readPacket()
 	if err != nil {
 		return err
 	}

 	magics.serverKexInit = packet

 	var serverKexInit kexInitMsg
 	if err = unmarshal(&serverKexInit, packet, msgKexInit); err != nil {
 		return err
 	}

 	kexAlgo, hostKeyAlgo, ok := findAgreedAlgorithms(c.transport, &clientKexInit, &serverKexInit)
 	if !ok {
 		return os.NewError("ssh: no common algorithms")
 	}

 	if serverKexInit.FirstKexFollows && kexAlgo != serverKexInit.KexAlgos[0] {
 		// The server sent a Kex message for the wrong algorithm,
 		// which we have to ignore.
 		if _, err := c.readPacket(); err != nil {
 			return err
 		}
 	}

 	var H, K []byte
 	var hashFunc crypto.Hash
 	switch kexAlgo {
 	case kexAlgoDH14SHA1:
 		hashFunc = crypto.SHA1
 		dhGroup14Once.Do(initDHGroup14)
 		H, K, err = c.kexDH(dhGroup14, hashFunc, &magics, hostKeyAlgo)
 	default:
 		err = fmt.Errorf("ssh: unexpected key exchange algorithm %v", kexAlgo)
 	}
 	if err != nil {
 		return err
 	}

 	if err = c.writePacket([]byte{msgNewKeys}); err != nil {
 		return err
 	}
 	if err = c.transport.writer.setupKeys(clientKeys, K, H, H, hashFunc); err != nil {
 		return err
 	}
 	if packet, err = c.readPacket(); err != nil {
 		return err
 	}
 	if packet[0] != msgNewKeys {
 		return UnexpectedMessageError{msgNewKeys, packet[0]}
 	}
 	return c.transport.reader.setupKeys(serverKeys, K, H, H, hashFunc)
 }

 // authenticate authenticates with the remote server. See RFC 4252.
 // Only "password" authentication is supported.
 func (c *ClientConn) authenticate() os.Error {
 	if err := c.writePacket(marshal(msgServiceRequest, serviceRequestMsg{serviceUserAuth})); err != nil {
 		return err
 	}
 	packet, err := c.readPacket()
 	if err != nil {
 		return err
 	}

 	var serviceAccept serviceAcceptMsg
 	if err = unmarshal(&serviceAccept, packet, msgServiceAccept); err != nil {
 		return err
 	}

 	// TODO(dfc) support proper authentication method negotation
 	method := "none"
 	if c.config.Password != "" {
 		method = "password"
 	}
 	if err := c.sendUserAuthReq(method); err != nil {
 		return err
 	}

 	if packet, err = c.readPacket(); err != nil {
 		return err
 	}

 	if packet[0] != msgUserAuthSuccess {
 		return UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
 	}
 	return nil
 }

 func (c *ClientConn) sendUserAuthReq(method string) os.Error {
 	length := stringLength([]byte(c.config.Password)) + 1
 	payload := make([]byte, length)
 	// always false for password auth, see RFC 4252 Section 8.
 	payload[0] = 0
 	marshalString(payload[1:], []byte(c.config.Password))

 	return c.writePacket(marshal(msgUserAuthRequest, userAuthRequestMsg{
 		User:    c.config.User,
 		Service: serviceSSH,
 		Method:  method,
 		Payload: payload,
 	}))
 }

 // kexDH performs Diffie-Hellman key agreement on a ClientConn. The
 // returned values are given the same names as in RFC 4253, section 8.
 func (c *ClientConn) kexDH(group *dhGroup, hashFunc crypto.Hash, magics *handshakeMagics, hostKeyAlgo string) ([]byte, []byte, os.Error) {
 	x, err := rand.Int(c.config.rand(), group.p)
 	if err != nil {
 		return nil, nil, err
 	}
 	X := new(big.Int).Exp(group.g, x, group.p)
 	kexDHInit := kexDHInitMsg{
 		X: X,
 	}
 	if err := c.writePacket(marshal(msgKexDHInit, kexDHInit)); err != nil {
 		return nil, nil, err
 	}

 	packet, err := c.readPacket()
 	if err != nil {
 		return nil, nil, err
 	}

 	var kexDHReply = new(kexDHReplyMsg)
 	if err = unmarshal(kexDHReply, packet, msgKexDHReply); err != nil {
 		return nil, nil, err
 	}

 	if kexDHReply.Y.Sign() == 0 || kexDHReply.Y.Cmp(group.p) >= 0 {
 		return nil, nil, os.NewError("server DH parameter out of bounds")
 	}

 	kInt := new(big.Int).Exp(kexDHReply.Y, x, group.p)
 	h := hashFunc.New()
 	writeString(h, magics.clientVersion)
 	writeString(h, magics.serverVersion)
 	writeString(h, magics.clientKexInit)
 	writeString(h, magics.serverKexInit)
 	writeString(h, kexDHReply.HostKey)
 	writeInt(h, X)
 	writeInt(h, kexDHReply.Y)
 	K := make([]byte, intLength(kInt))
 	marshalInt(K, kInt)
 	h.Write(K)

 	H := h.Sum()

 	return H, K, nil
 }

 // openChan opens a new client channel. The most common session type is "session".
 // The full set of valid session types are listed in RFC 4250 4.9.1.
 func (c *ClientConn) openChan(typ string) (*clientChan, os.Error) {
 	ch := c.newChan(c.transport)
 	if err := c.writePacket(marshal(msgChannelOpen, channelOpenMsg{
 		ChanType:      typ,
 		PeersId:       ch.id,
 		PeersWindow:   1 << 14,
 		MaxPacketSize: 1 << 15, // RFC 4253 6.1
 	})); err != nil {
 		c.chanlist.remove(ch.id)
 		return nil, err
 	}
 	// wait for response
 	switch msg := (<-ch.msg).(type) {
 	case *channelOpenConfirmMsg:
 		ch.peersId = msg.MyId
 	case *channelOpenFailureMsg:
 		c.chanlist.remove(ch.id)
 		return nil, os.NewError(msg.Message)
 	default:
 		c.chanlist.remove(ch.id)
 		return nil, os.NewError("Unexpected packet")
 	}
 	return ch, nil
 }

 // mainloop reads incoming messages and routes channel messages
 // to their respective ClientChans.
 func (c *ClientConn) mainLoop() {
 	// TODO(dfc) signal the underlying close to all channels
 	defer c.Close()
 	for {
 		packet, err := c.readPacket()
 		if err != nil {
 			break
 		}
 		// TODO(dfc) A note on blocking channel use.
 		// The msg, win, data and dataExt channels of a clientChan can
 		// cause this loop to block indefinately if the consumer does
 		// not service them.
 		switch packet[0] {
 		case msgChannelData:
 			if len(packet) < 9 {
 				// malformed data packet
 				break
 			}
 			peersId := uint32(packet[1])<<24 | uint32(packet[2])<<16 | uint32(packet[3])<<8 | uint32(packet[4])
 			if length := int(packet[5])<<24 | int(packet[6])<<16 | int(packet[7])<<8 | int(packet[8]); length > 0 {
 				packet = packet[9:]
 				c.getChan(peersId).data <- packet[:length]
 			}
 		case msgChannelExtendedData:
 			if len(packet) < 13 {
 				// malformed data packet
 				break
 			}
 			peersId := uint32(packet[1])<<24 | uint32(packet[2])<<16 | uint32(packet[3])<<8 | uint32(packet[4])
 			datatype := uint32(packet[5])<<24 | uint32(packet[6])<<16 | uint32(packet[7])<<8 | uint32(packet[8])
 			if length := int(packet[9])<<24 | int(packet[10])<<16 | int(packet[11])<<8 | int(packet[12]); length > 0 {
 				packet = packet[13:]
 				// RFC 4254 5.2 defines data_type_code 1 to be data destined
 				// for stderr on interactive sessions. Other data types are
 				// silently discarded.
 				if datatype == 1 {
 					c.getChan(peersId).dataExt <- packet[:length]
 				}
 			}
 		default:
 			switch msg := decode(packet).(type) {
 			case *channelOpenMsg:
 				c.getChan(msg.PeersId).msg <- msg
 			case *channelOpenConfirmMsg:
 				c.getChan(msg.PeersId).msg <- msg
 			case *channelOpenFailureMsg:
 				c.getChan(msg.PeersId).msg <- msg
 			case *channelCloseMsg:
 				ch := c.getChan(msg.PeersId)
 				close(ch.win)
 				close(ch.data)
 				close(ch.dataExt)
 				c.chanlist.remove(msg.PeersId)
 			case *channelEOFMsg:
 				c.getChan(msg.PeersId).msg <- msg
 			case *channelRequestSuccessMsg:
 				c.getChan(msg.PeersId).msg <- msg
 			case *channelRequestFailureMsg:
 				c.getChan(msg.PeersId).msg <- msg
 			case *channelRequestMsg:
 				c.getChan(msg.PeersId).msg <- msg
 			case *windowAdjustMsg:
 				c.getChan(msg.PeersId).win <- int(msg.AdditionalBytes)
 			default:
 				fmt.Printf("mainLoop: unhandled %#v\n", msg)
 			}
 		}
 	}
 }

 // Dial connects to the given network address using net.Dial and
 // then initiates a SSH handshake, returning the resulting client connection.
 func Dial(network, addr string, config *ClientConfig) (*ClientConn, os.Error) {
 	conn, err := net.Dial(network, addr)
 	if err != nil {
 		return nil, err
 	}
 	return Client(conn, config)
 }

 // A ClientConfig structure is used to configure a ClientConn. After one has
 // been passed to an SSH function it must not be modified.
 type ClientConfig struct {
 	// Rand provides the source of entropy for key exchange. If Rand is
 	// nil, the cryptographic random reader in package crypto/rand will
 	// be used.
 	Rand io.Reader

 	// The username to authenticate.
 	User string

 	// Used for "password" method authentication.
 	Password string
 }

 func (c *ClientConfig) rand() io.Reader {
 	if c.Rand == nil {
 		return rand.Reader
 	}
 	return c.Rand
 }

 // A clientChan represents a single RFC 4254 channel that is multiplexed
 // over a single SSH connection.
 type clientChan struct {
 	packetWriter
 	id, peersId uint32
 	data        chan []byte      // receives the payload of channelData messages
 	dataExt     chan []byte      // receives the payload of channelExtendedData messages
 	win         chan int         // receives window adjustments
 	msg         chan interface{} // incoming messages
 }

 func newClientChan(t *transport, id uint32) *clientChan {
 	return &clientChan{
 		packetWriter: t,
 		id:           id,
 		data:         make(chan []byte, 16),
 		dataExt:      make(chan []byte, 16),
 		win:          make(chan int, 16),
 		msg:          make(chan interface{}, 16),
 	}
 }

 // Close closes the channel. This does not close the underlying connection.
 func (c *clientChan) Close() os.Error {
 	return c.writePacket(marshal(msgChannelClose, channelCloseMsg{
 		PeersId: c.id,
 	}))
 }

 func (c *clientChan) sendChanReq(req channelRequestMsg) os.Error {
 	if err := c.writePacket(marshal(msgChannelRequest, req)); err != nil {
 		return err
 	}
 	msg := <-c.msg
 	if _, ok := msg.(*channelRequestSuccessMsg); ok {
 		return nil
 	}
 	return fmt.Errorf("failed to complete request: %s, %#v", req.Request, msg)
 }

 // Thread safe channel list.
 type chanlist struct {
 	// protects concurrent access to chans
 	sync.Mutex
 	// chans are indexed by the local id of the channel, clientChan.id.
 	// The PeersId value of messages received by ClientConn.mainloop is
 	// used to locate the right local clientChan in this slice.
 	chans []*clientChan
 }

 // Allocate a new ClientChan with the next avail local id.
 func (c *chanlist) newChan(t *transport) *clientChan {
 	c.Lock()
 	defer c.Unlock()
 	for i := range c.chans {
 		if c.chans[i] == nil {
 			ch := newClientChan(t, uint32(i))
 			c.chans[i] = ch
 			return ch
 		}
 	}
 	i := len(c.chans)
 	ch := newClientChan(t, uint32(i))
 	c.chans = append(c.chans, ch)
 	return ch
 }

 func (c *chanlist) getChan(id uint32) *clientChan {
 	c.Lock()
 	defer c.Unlock()
 	return c.chans[int(id)]
 }

 func (c *chanlist) remove(id uint32) {
 	c.Lock()
 	defer c.Unlock()
 	c.chans[int(id)] = nil
 }

 // A chanWriter represents the stdin of a remote process.
 type chanWriter struct {
 	win          chan int // receives window adjustments
 	id           uint32   // this channel's id
 	rwin         int      // current rwin size
 	packetWriter          // for sending channelDataMsg
 }

 // Write writes data to the remote process's standard input.
 func (w *chanWriter) Write(data []byte) (n int, err os.Error) {
 	for {
 		if w.rwin == 0 {
 			win, ok := <-w.win
 			if !ok {
 				return 0, os.EOF
 			}
 			w.rwin += win
 			continue
 		}
 		n = len(data)
 		packet := make([]byte, 0, 9+n)
 		packet = append(packet, msgChannelData,
 			byte(w.id)>>24, byte(w.id)>>16, byte(w.id)>>8, byte(w.id),
 			byte(n)>>24, byte(n)>>16, byte(n)>>8, byte(n))
 		err = w.writePacket(append(packet, data...))
 		w.rwin -= n
 		return
 	}
 	panic("unreachable")
 }

 func (w *chanWriter) Close() os.Error {
 	return w.writePacket(marshal(msgChannelEOF, channelEOFMsg{w.id}))
 }

 // A chanReader represents stdout or stderr of a remote process.
 type chanReader struct {
 	// TODO(dfc) a fixed size channel may not be the right data structure.
 	// If writes to this channel block, they will block mainLoop, making
 	// it unable to receive new messages from the remote side.
 	data         chan []byte // receives data from remote
 	id           uint32
 	packetWriter // for sending windowAdjustMsg
 	buf          []byte
 }

 // Read reads data from the remote process's stdout or stderr.
 func (r *chanReader) Read(data []byte) (int, os.Error) {
 	var ok bool
 	for {
 		if len(r.buf) > 0 {
 			n := copy(data, r.buf)
 			r.buf = r.buf[n:]
 			msg := windowAdjustMsg{
 				PeersId:         r.id,
 				AdditionalBytes: uint32(n),
 			}
 			return n, r.writePacket(marshal(msgChannelWindowAdjust, msg))
 		}
 		r.buf, ok = <-r.data
 		if !ok {
 			return 0, os.EOF
 		}
 	}
 	panic("unreachable")
 }

 func (r *chanReader) Close() os.Error {
 	return r.writePacket(marshal(msgChannelEOF, channelEOFMsg{r.id}))
 }
	// Copyright 2011 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package ssh

	import (
	"big"
	"crypto"
	"crypto/rand"
	"fmt"
	"io"
	"os"
	"net"
	"sync"
	)

	// clientVersion is the fixed identification string that the client will use.
	var clientVersion = []byte("SSH-2.0-Go\r\n")

	// ClientConn represents the client side of an SSH connection.
	type ClientConn struct {
	*transport
	config *ClientConfig
	chanlist
	}

	// Client returns a new SSH client connection using c as the underlying transport.
	func Client(c net.Conn, config ClientConfig) (ClientConn, os.Error) {
	conn := &ClientConn{
	transport: newTransport(c, config.rand()),
	config: config,
	}
	if err := conn.handshake(); err != nil {
	conn.Close()
	return nil, err
	}
	if err := conn.authenticate(); err != nil {
	conn.Close()
	return nil, err
	}
	go conn.mainLoop()
	return conn, nil
	}

	// handshake performs the client side key exchange. See RFC 4253 Section 7.
	func (c *ClientConn) handshake() os.Error {
	var magics handshakeMagics

	if _, err := c.Write(clientVersion); err != nil {
	return err
	}
	if err := c.Flush(); err != nil {
	return err
	}
	magics.clientVersion = clientVersion[:len(clientVersion)-2]

	// read remote server version
	version, err := readVersion(c)
	if err != nil {
	return err
	}
	magics.serverVersion = version
	clientKexInit := kexInitMsg{
	KexAlgos: supportedKexAlgos,
	ServerHostKeyAlgos: supportedHostKeyAlgos,
	CiphersClientServer: supportedCiphers,
	CiphersServerClient: supportedCiphers,
	MACsClientServer: supportedMACs,
	MACsServerClient: supportedMACs,
	CompressionClientServer: supportedCompressions,
	CompressionServerClient: supportedCompressions,
	}
	kexInitPacket := marshal(msgKexInit, clientKexInit)
	magics.clientKexInit = kexInitPacket

	if err := c.writePacket(kexInitPacket); err != nil {
	return err
	}
	packet, err := c.readPacket()
	if err != nil {
	return err
	}

	magics.serverKexInit = packet

	var serverKexInit kexInitMsg
	if err = unmarshal(&serverKexInit, packet, msgKexInit); err != nil {
	return err
	}

	kexAlgo, hostKeyAlgo, ok := findAgreedAlgorithms(c.transport, &clientKexInit, &serverKexInit)
	if !ok {
	return os.NewError("ssh: no common algorithms")
	}

	if serverKexInit.FirstKexFollows && kexAlgo != serverKexInit.KexAlgos[0] {
	// The server sent a Kex message for the wrong algorithm,
	// which we have to ignore.
	if _, err := c.readPacket(); err != nil {
	return err
	}
	}

	var H, K []byte
	var hashFunc crypto.Hash
	switch kexAlgo {
	case kexAlgoDH14SHA1:
	hashFunc = crypto.SHA1
	dhGroup14Once.Do(initDHGroup14)
	H, K, err = c.kexDH(dhGroup14, hashFunc, &magics, hostKeyAlgo)
	default:
	err = fmt.Errorf("ssh: unexpected key exchange algorithm %v", kexAlgo)
	}
	if err != nil {
	return err
	}

	if err = c.writePacket([]byte{msgNewKeys}); err != nil {
	return err
	}
	if err = c.transport.writer.setupKeys(clientKeys, K, H, H, hashFunc); err != nil {
	return err
	}
	if packet, err = c.readPacket(); err != nil {
	return err
	}
	if packet[0] != msgNewKeys {
	return UnexpectedMessageError{msgNewKeys, packet[0]}
	}
	return c.transport.reader.setupKeys(serverKeys, K, H, H, hashFunc)
	}

	// authenticate authenticates with the remote server. See RFC 4252.
	// Only "password" authentication is supported.
	func (c *ClientConn) authenticate() os.Error {
	if err := c.writePacket(marshal(msgServiceRequest, serviceRequestMsg{serviceUserAuth})); err != nil {
	return err
	}
	packet, err := c.readPacket()
	if err != nil {
	return err
	}

	var serviceAccept serviceAcceptMsg
	if err = unmarshal(&serviceAccept, packet, msgServiceAccept); err != nil {
	return err
	}

	// TODO(dfc) support proper authentication method negotation
	method := "none"
	if c.config.Password != "" {
	method = "password"
	}
	if err := c.sendUserAuthReq(method); err != nil {
	return err
	}

	if packet, err = c.readPacket(); err != nil {
	return err
	}

	if packet[0] != msgUserAuthSuccess {
	return UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
	}
	return nil
	}

	func (c *ClientConn) sendUserAuthReq(method string) os.Error {
	length := stringLength([]byte(c.config.Password)) + 1
	payload := make([]byte, length)
	// always false for password auth, see RFC 4252 Section 8.
	payload[0] = 0
	marshalString(payload[1:], []byte(c.config.Password))

	return c.writePacket(marshal(msgUserAuthRequest, userAuthRequestMsg{
	User: c.config.User,
	Service: serviceSSH,
	Method: method,
	Payload: payload,
	}))
	}

	// kexDH performs Diffie-Hellman key agreement on a ClientConn. The
	// returned values are given the same names as in RFC 4253, section 8.
	func (c ClientConn) kexDH(group dhGroup, hashFunc crypto.Hash, magics *handshakeMagics, hostKeyAlgo string) ([]byte, []byte, os.Error) {
	x, err := rand.Int(c.config.rand(), group.p)
	if err != nil {
	return nil, nil, err
	}
	X := new(big.Int).Exp(group.g, x, group.p)
	kexDHInit := kexDHInitMsg{
	X: X,
	}
	if err := c.writePacket(marshal(msgKexDHInit, kexDHInit)); err != nil {
	return nil, nil, err
	}

	packet, err := c.readPacket()
	if err != nil {
	return nil, nil, err
	}

	var kexDHReply = new(kexDHReplyMsg)
	if err = unmarshal(kexDHReply, packet, msgKexDHReply); err != nil {
	return nil, nil, err
	}

	if kexDHReply.Y.Sign() == 0 \|\| kexDHReply.Y.Cmp(group.p) >= 0 {
	return nil, nil, os.NewError("server DH parameter out of bounds")
	}

	kInt := new(big.Int).Exp(kexDHReply.Y, x, group.p)
	h := hashFunc.New()
	writeString(h, magics.clientVersion)
	writeString(h, magics.serverVersion)
	writeString(h, magics.clientKexInit)
	writeString(h, magics.serverKexInit)
	writeString(h, kexDHReply.HostKey)
	writeInt(h, X)
	writeInt(h, kexDHReply.Y)
	K := make([]byte, intLength(kInt))
	marshalInt(K, kInt)
	h.Write(K)

	H := h.Sum()

	return H, K, nil
	}

	// openChan opens a new client channel. The most common session type is "session".
	// The full set of valid session types are listed in RFC 4250 4.9.1.
	func (c ClientConn) openChan(typ string) (clientChan, os.Error) {
	ch := c.newChan(c.transport)
	if err := c.writePacket(marshal(msgChannelOpen, channelOpenMsg{
	ChanType: typ,
	PeersId: ch.id,
	PeersWindow: 1 << 14,
	MaxPacketSize: 1 << 15, // RFC 4253 6.1
	})); err != nil {
	c.chanlist.remove(ch.id)
	return nil, err
	}
	// wait for response
	switch msg := (<-ch.msg).(type) {
	case *channelOpenConfirmMsg:
	ch.peersId = msg.MyId
	case *channelOpenFailureMsg:
	c.chanlist.remove(ch.id)
	return nil, os.NewError(msg.Message)
	default:
	c.chanlist.remove(ch.id)
	return nil, os.NewError("Unexpected packet")
	}
	return ch, nil
	}

	// mainloop reads incoming messages and routes channel messages
	// to their respective ClientChans.
	func (c *ClientConn) mainLoop() {
	// TODO(dfc) signal the underlying close to all channels
	defer c.Close()
	for {
	packet, err := c.readPacket()
	if err != nil {
	break
	}
	// TODO(dfc) A note on blocking channel use.
	// The msg, win, data and dataExt channels of a clientChan can
	// cause this loop to block indefinately if the consumer does
	// not service them.
	switch packet[0] {
	case msgChannelData:
	if len(packet) < 9 {
	// malformed data packet
	break
	}
	peersId := uint32(packet[1])<<24 \| uint32(packet[2])<<16 \| uint32(packet[3])<<8 \| uint32(packet[4])
	if length := int(packet[5])<<24 \| int(packet[6])<<16 \| int(packet[7])<<8 \| int(packet[8]); length > 0 {
	packet = packet[9:]
	c.getChan(peersId).data <- packet[:length]
	}
	case msgChannelExtendedData:
	if len(packet) < 13 {
	// malformed data packet
	break
	}
	peersId := uint32(packet[1])<<24 \| uint32(packet[2])<<16 \| uint32(packet[3])<<8 \| uint32(packet[4])
	datatype := uint32(packet[5])<<24 \| uint32(packet[6])<<16 \| uint32(packet[7])<<8 \| uint32(packet[8])
	if length := int(packet[9])<<24 \| int(packet[10])<<16 \| int(packet[11])<<8 \| int(packet[12]); length > 0 {
	packet = packet[13:]
	// RFC 4254 5.2 defines data_type_code 1 to be data destined
	// for stderr on interactive sessions. Other data types are
	// silently discarded.
	if datatype == 1 {
	c.getChan(peersId).dataExt <- packet[:length]
	}
	}
	default:
	switch msg := decode(packet).(type) {
	case *channelOpenMsg:
	c.getChan(msg.PeersId).msg <- msg
	case *channelOpenConfirmMsg:
	c.getChan(msg.PeersId).msg <- msg
	case *channelOpenFailureMsg:
	c.getChan(msg.PeersId).msg <- msg
	case *channelCloseMsg:
	ch := c.getChan(msg.PeersId)
	close(ch.win)
	close(ch.data)
	close(ch.dataExt)
	c.chanlist.remove(msg.PeersId)
	case *channelEOFMsg:
	c.getChan(msg.PeersId).msg <- msg
	case *channelRequestSuccessMsg:
	c.getChan(msg.PeersId).msg <- msg
	case *channelRequestFailureMsg:
	c.getChan(msg.PeersId).msg <- msg
	case *channelRequestMsg:
	c.getChan(msg.PeersId).msg <- msg
	case *windowAdjustMsg:
	c.getChan(msg.PeersId).win <- int(msg.AdditionalBytes)
	default:
	fmt.Printf("mainLoop: unhandled %#v\n", msg)
	}
	}
	}
	}

	// Dial connects to the given network address using net.Dial and
	// then initiates a SSH handshake, returning the resulting client connection.
	func Dial(network, addr string, config ClientConfig) (ClientConn, os.Error) {
	conn, err := net.Dial(network, addr)
	if err != nil {
	return nil, err
	}
	return Client(conn, config)
	}

	// A ClientConfig structure is used to configure a ClientConn. After one has
	// been passed to an SSH function it must not be modified.
	type ClientConfig struct {
	// Rand provides the source of entropy for key exchange. If Rand is
	// nil, the cryptographic random reader in package crypto/rand will
	// be used.
	Rand io.Reader

	// The username to authenticate.
	User string

	// Used for "password" method authentication.
	Password string
	}

	func (c *ClientConfig) rand() io.Reader {
	if c.Rand == nil {
	return rand.Reader
	}
	return c.Rand
	}

	// A clientChan represents a single RFC 4254 channel that is multiplexed
	// over a single SSH connection.
	type clientChan struct {
	packetWriter
	id, peersId uint32
	data chan []byte // receives the payload of channelData messages
	dataExt chan []byte // receives the payload of channelExtendedData messages
	win chan int // receives window adjustments
	msg chan interface{} // incoming messages
	}

	func newClientChan(t transport, id uint32) clientChan {
	return &clientChan{
	packetWriter: t,
	id: id,
	data: make(chan []byte, 16),
	dataExt: make(chan []byte, 16),
	win: make(chan int, 16),
	msg: make(chan interface{}, 16),
	}
	}

	// Close closes the channel. This does not close the underlying connection.
	func (c *clientChan) Close() os.Error {
	return c.writePacket(marshal(msgChannelClose, channelCloseMsg{
	PeersId: c.id,
	}))
	}

	func (c *clientChan) sendChanReq(req channelRequestMsg) os.Error {
	if err := c.writePacket(marshal(msgChannelRequest, req)); err != nil {
	return err
	}
	msg := <-c.msg
	if _, ok := msg.(*channelRequestSuccessMsg); ok {
	return nil
	}
	return fmt.Errorf("failed to complete request: %s, %#v", req.Request, msg)
	}

	// Thread safe channel list.
	type chanlist struct {
	// protects concurrent access to chans
	sync.Mutex
	// chans are indexed by the local id of the channel, clientChan.id.
	// The PeersId value of messages received by ClientConn.mainloop is
	// used to locate the right local clientChan in this slice.
	chans []*clientChan
	}

	// Allocate a new ClientChan with the next avail local id.
	func (c chanlist) newChan(t transport) *clientChan {
	c.Lock()
	defer c.Unlock()
	for i := range c.chans {
	if c.chans[i] == nil {
	ch := newClientChan(t, uint32(i))
	c.chans[i] = ch
	return ch
	}
	}
	i := len(c.chans)
	ch := newClientChan(t, uint32(i))
	c.chans = append(c.chans, ch)
	return ch
	}

	func (c chanlist) getChan(id uint32) clientChan {
	c.Lock()
	defer c.Unlock()
	return c.chans[int(id)]
	}

	func (c *chanlist) remove(id uint32) {
	c.Lock()
	defer c.Unlock()
	c.chans[int(id)] = nil
	}

	// A chanWriter represents the stdin of a remote process.
	type chanWriter struct {
	win chan int // receives window adjustments
	id uint32 // this channel's id
	rwin int // current rwin size
	packetWriter // for sending channelDataMsg
	}

	// Write writes data to the remote process's standard input.
	func (w *chanWriter) Write(data []byte) (n int, err os.Error) {
	for {
	if w.rwin == 0 {
	win, ok := <-w.win
	if !ok {
	return 0, os.EOF
	}
	w.rwin += win
	continue
	}
	n = len(data)
	packet := make([]byte, 0, 9+n)
	packet = append(packet, msgChannelData,
	byte(w.id)>>24, byte(w.id)>>16, byte(w.id)>>8, byte(w.id),
	byte(n)>>24, byte(n)>>16, byte(n)>>8, byte(n))
	err = w.writePacket(append(packet, data...))
	w.rwin -= n
	return
	}
	panic("unreachable")
	}

	func (w *chanWriter) Close() os.Error {
	return w.writePacket(marshal(msgChannelEOF, channelEOFMsg{w.id}))
	}

	// A chanReader represents stdout or stderr of a remote process.
	type chanReader struct {
	// TODO(dfc) a fixed size channel may not be the right data structure.
	// If writes to this channel block, they will block mainLoop, making
	// it unable to receive new messages from the remote side.
	data chan []byte // receives data from remote
	id uint32
	packetWriter // for sending windowAdjustMsg
	buf []byte
	}

	// Read reads data from the remote process's stdout or stderr.
	func (r *chanReader) Read(data []byte) (int, os.Error) {
	var ok bool
	for {
	if len(r.buf) > 0 {
	n := copy(data, r.buf)
	r.buf = r.buf[n:]
	msg := windowAdjustMsg{
	PeersId: r.id,
	AdditionalBytes: uint32(n),
	}
	return n, r.writePacket(marshal(msgChannelWindowAdjust, msg))
	}
	r.buf, ok = <-r.data
	if !ok {
	return 0, os.EOF
	}
	}
	panic("unreachable")
	}

	func (r *chanReader) Close() os.Error {
	return r.writePacket(marshal(msgChannelEOF, channelEOFMsg{r.id}))
	}