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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, TCP port/streamlocal forwarding and tunneled dialing.
type Client struct {
handleForwardsOnce sync.Once // guards calling (*Client).handleForwards
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, chanSize)
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() {
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
if fullConf.HostKeyCallback == nil {
return nil, nil, nil, errors.New("ssh: must specify HostKeyCallback")
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.waitSession(); err != nil {
return err
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.DialTimeout(network, addr, config.Timeout)
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
// HostKeyCallback is the function type used for verifying server
// keys. A HostKeyCallback must return nil if the host key is OK, or
// an error to reject it. It receives the hostname as passed to Dial
// or NewClientConn. The remote address is the RemoteAddr of the
// net.Conn underlying the SSH connection.
type HostKeyCallback func(hostname string, remote net.Addr, key PublicKey) error
// BannerCallback is the function type used for treat the banner sent by
// the server. A BannerCallback receives the message sent by the remote server.
type BannerCallback func(message string) error
// 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 is called during the cryptographic
// handshake to validate the server's host key. The client
// configuration must supply this callback for the connection
// to succeed. The functions InsecureIgnoreHostKey or
// FixedHostKey can be used for simplistic host key checks.
HostKeyCallback HostKeyCallback
// BannerCallback is called during the SSH dance to display a custom
// server's message. The client configuration can supply this callback to
// handle it as wished. The function BannerDisplayStderr can be used for
// simplistic display on Stderr.
BannerCallback BannerCallback
// 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
// Timeout is the maximum amount of time for the TCP connection to establish.
// A Timeout of zero means no timeout.
Timeout time.Duration
// InsecureIgnoreHostKey returns a function that can be used for
// ClientConfig.HostKeyCallback to accept any host key. It should
// not be used for production code.
func InsecureIgnoreHostKey() HostKeyCallback {
return func(hostname string, remote net.Addr, key PublicKey) error {
return nil
type fixedHostKey struct {
key PublicKey
func (f *fixedHostKey) check(hostname string, remote net.Addr, key PublicKey) error {
if f.key == nil {
return fmt.Errorf("ssh: required host key was nil")
if !bytes.Equal(key.Marshal(), f.key.Marshal()) {
return fmt.Errorf("ssh: host key mismatch")
return nil
// FixedHostKey returns a function for use in
// ClientConfig.HostKeyCallback to accept only a specific host key.
func FixedHostKey(key PublicKey) HostKeyCallback {
hk := &fixedHostKey{key}
return hk.check
// BannerDisplayStderr returns a function that can be used for
// ClientConfig.BannerCallback to display banners on os.Stderr.
func BannerDisplayStderr() BannerCallback {
return func(banner string) error {
_, err := os.Stderr.WriteString(banner)
return err