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// 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 (
// Client implements a traditional SSH client that supports shells,
// subprocesses, port forwarding and tunneled dialing.
type Client struct {
forwards forwardList // forwarded tcpip connections from the remote side
mu sync.Mutex
channelHandlers map[string]chan NewChannel
// HandleChannelOpen returns a channel on which NewChannel requests
// for the given type are sent. If the type already is being handled,
// nil is returned. The channel is closed when the connection is closed.
func (c *Client) HandleChannelOpen(channelType string) <-chan NewChannel {
if c.channelHandlers == nil {
// The SSH channel has been closed.
c := make(chan NewChannel)
return c
ch := c.channelHandlers[channelType]
if ch != nil {
return nil
ch = make(chan NewChannel, 16)
c.channelHandlers[channelType] = ch
return ch
// NewClient creates a Client on top of the given connection.
func NewClient(c Conn, chans <-chan NewChannel, reqs <-chan *Request) *Client {
conn := &Client{
Conn: c,
channelHandlers: make(map[string]chan NewChannel, 1),
go conn.handleGlobalRequests(reqs)
go conn.handleChannelOpens(chans)
go func() {
go conn.forwards.handleChannels(conn.HandleChannelOpen("forwarded-tcpip"))
return conn
// NewClientConn establishes an authenticated SSH connection using c
// as the underlying transport. The Request and NewChannel channels
// must be serviced or the connection will hang.
func NewClientConn(c net.Conn, addr string, config *ClientConfig) (Conn, <-chan NewChannel, <-chan *Request, error) {
fullConf := *config
conn := &connection{
sshConn: sshConn{conn: c},
if err := conn.clientHandshake(addr, &fullConf); err != nil {
return nil, nil, nil, fmt.Errorf("ssh: handshake failed: %v", err)
conn.mux = newMux(conn.transport)
return conn, conn.mux.incomingChannels, conn.mux.incomingRequests, nil
// clientHandshake performs the client side key exchange. See RFC 4253 Section
// 7.
func (c *connection) clientHandshake(dialAddress string, config *ClientConfig) error {
if config.ClientVersion != "" {
c.clientVersion = []byte(config.ClientVersion)
} else {
c.clientVersion = []byte(packageVersion)
var err error
c.serverVersion, err = exchangeVersions(c.sshConn.conn, c.clientVersion)
if err != nil {
return err
c.transport = newClientTransport(
newTransport(c.sshConn.conn, config.Rand, true /* is client */),
c.clientVersion, c.serverVersion, config, dialAddress, c.sshConn.RemoteAddr())
if err := c.transport.requestKeyChange(); err != nil {
return err
if packet, err := c.transport.readPacket(); err != nil {
return err
} else if packet[0] != msgNewKeys {
return unexpectedMessageError(msgNewKeys, packet[0])
// We just did the key change, so the session ID is established.
c.sessionID = c.transport.getSessionID()
return c.clientAuthenticate(config)
// verifyHostKeySignature verifies the host key obtained in the key
// exchange.
func verifyHostKeySignature(hostKey PublicKey, result *kexResult) error {
sig, rest, ok := parseSignatureBody(result.Signature)
if len(rest) > 0 || !ok {
return errors.New("ssh: signature parse error")
return hostKey.Verify(result.H, sig)
// NewSession opens a new Session for this client. (A session is a remote
// execution of a program.)
func (c *Client) NewSession() (*Session, error) {
ch, in, err := c.OpenChannel("session", nil)
if err != nil {
return nil, err
return newSession(ch, in)
func (c *Client) handleGlobalRequests(incoming <-chan *Request) {
for r := range incoming {
// This handles keepalive messages and matches
// the behaviour of OpenSSH.
r.Reply(false, nil)
// handleChannelOpens channel open messages from the remote side.
func (c *Client) handleChannelOpens(in <-chan NewChannel) {
for ch := range in {
handler := c.channelHandlers[ch.ChannelType()]
if handler != nil {
handler <- ch
} else {
ch.Reject(UnknownChannelType, fmt.Sprintf("unknown channel type: %v", ch.ChannelType()))
for _, ch := range c.channelHandlers {
c.channelHandlers = nil
// Dial starts a client connection to the given SSH server. It is a
// convenience function that connects to the given network address,
// initiates the SSH handshake, and then sets up a Client. For access
// to incoming channels and requests, use net.Dial with NewClientConn
// instead.
func Dial(network, addr string, config *ClientConfig) (*Client, error) {
conn, err := net.Dial(network, addr)
if err != nil {
return nil, err
c, chans, reqs, err := NewClientConn(conn, addr, config)
if err != nil {
return nil, err
return NewClient(c, chans, reqs), nil
// A ClientConfig structure is used to configure a Client. It must not be
// modified after having been passed to an SSH function.
type ClientConfig struct {
// Config contains configuration that is shared between clients and
// servers.
// User contains the username to authenticate as.
User string
// Auth contains possible authentication methods to use with the
// server. Only the first instance of a particular RFC 4252 method will
// be used during authentication.
Auth []AuthMethod
// HostKeyCallback, if not nil, is called during the cryptographic
// handshake to validate the server's host key. A nil HostKeyCallback
// implies that all host keys are accepted.
HostKeyCallback func(hostname string, remote net.Addr, key PublicKey) error
// ClientVersion contains the version identification string that will
// be used for the connection. If empty, a reasonable default is used.
ClientVersion string
// HostKeyAlgorithms lists the key types that the client will
// accept from the server as host key, in order of
// preference. If empty, a reasonable default is used. Any
// string returned from PublicKey.Type method may be used, or
// any of the CertAlgoXxxx and KeyAlgoXxxx constants.
HostKeyAlgorithms []string