exp/ssh: add direct-tcpip client support

This CL adds experimental support for making proxied
net.Conn connections via the remote server.

nb. Functional tests exist for this feature but CL
5320053 or similar needs to be committed to support them.

R=rsc, agl, n13m3y3r
CC=cw, golang-dev, huin
https://golang.org/cl/5371081

src/pkg/exp/ssh/Makefile

@@ -11,9 +11,10 @@ GOFILES=\
 	client_auth.go\
 	common.go\
 	messages.go\
-	transport.go\
 	server.go\
 	server_shell.go\
 	session.go\
+	tcpip.go\
+	transport.go\
 
 include ../../../Make.pkg

src/pkg/exp/ssh/tcpip.go

+// 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 (
+	"errors"
+	"io"
+	"net"
+)
+// Dial initiates a connection to the addr from the remote host.
+// addr is resolved using net.ResolveTCPAddr before connection. 
+// This could allow an observer to observe the DNS name of the 
+// remote host. Consider using ssh.DialTCP to avoid this.
+func (c *ClientConn) Dial(n, addr string) (net.Conn, error) {
+	raddr, err := net.ResolveTCPAddr(n, addr)
+	if err != nil {
+		return nil, err
+	}
+	return c.DialTCP(n, nil, raddr)
+}
+
+// DialTCP connects to the remote address raddr on the network net,
+// which must be "tcp", "tcp4", or "tcp6".  If laddr is not nil, it is used
+// as the local address for the connection.
+func (c *ClientConn) DialTCP(n string, laddr, raddr *net.TCPAddr) (net.Conn, error) {
+	if laddr == nil {
+		laddr = &net.TCPAddr{
+			IP:   net.IPv4zero,
+			Port: 0,
+		}
+	}
+	ch, err := c.dial(laddr.IP.String(), laddr.Port, raddr.IP.String(), raddr.Port)
+	if err != nil {
+		return nil, err
+	}
+	return &tcpchanconn{
+		tcpchan: ch,
+		laddr:   laddr,
+		raddr:   raddr,
+	}, nil
+}
+
+// dial opens a direct-tcpip connection to the remote server. laddr and raddr are passed as
+// strings and are expected to be resolveable at the remote end.
+func (c *ClientConn) dial(laddr string, lport int, raddr string, rport int) (*tcpchan, error) {
+	// RFC 4254 7.2
+	type channelOpenDirectMsg struct {
+		ChanType      string
+		PeersId       uint32
+		PeersWindow   uint32
+		MaxPacketSize uint32
+		raddr         string
+		rport         uint32
+		laddr         string
+		lport         uint32
+	}
+	ch := c.newChan(c.transport)
+	if err := c.writePacket(marshal(msgChannelOpen, channelOpenDirectMsg{
+		ChanType:      "direct-tcpip",
+		PeersId:       ch.id,
+		PeersWindow:   1 << 14,
+		MaxPacketSize: 1 << 15, // RFC 4253 6.1
+		raddr:         raddr,
+		rport:         uint32(rport),
+		laddr:         laddr,
+		lport:         uint32(lport),
+	})); err != nil {
+		c.chanlist.remove(ch.id)
+		return nil, err
+	}
+	// wait for response
+	switch msg := (<-ch.msg).(type) {
+	case *channelOpenConfirmMsg:
+		ch.peersId = msg.MyId
+		ch.win <- int(msg.MyWindow)
+	case *channelOpenFailureMsg:
+		c.chanlist.remove(ch.id)
+		return nil, errors.New("ssh: error opening remote TCP connection: " + msg.Message)
+	default:
+		c.chanlist.remove(ch.id)
+		return nil, errors.New("ssh: unexpected packet")
+	}
+	return &tcpchan{
+		clientChan: ch,
+		Reader: &chanReader{
+			packetWriter: ch,
+			id:           ch.id,
+			data:         ch.data,
+		},
+		Writer: &chanWriter{
+			packetWriter: ch,
+			id:           ch.id,
+			win:          ch.win,
+		},
+	}, nil
+}
+
+type tcpchan struct {
+	*clientChan // the backing channel
+	io.Reader
+	io.Writer
+}
+
+// tcpchanconn fulfills the net.Conn interface without 
+// the tcpchan having to hold laddr or raddr directly.
+type tcpchanconn struct {
+	*tcpchan
+	laddr, raddr net.Addr
+}
+
+// LocalAddr returns the local network address.
+func (t *tcpchanconn) LocalAddr() net.Addr {
+	return t.laddr
+}
+
+// RemoteAddr returns the remote network address.
+func (t *tcpchanconn) RemoteAddr() net.Addr {
+	return t.raddr
+}
+
+// SetTimeout sets the read and write deadlines associated
+// with the connection.
+func (t *tcpchanconn) SetTimeout(nsec int64) error {
+	if err := t.SetReadTimeout(nsec); err != nil {
+		return err
+	}
+	return t.SetWriteTimeout(nsec)
+}
+
+// SetReadTimeout sets the time (in nanoseconds) that
+// Read will wait for data before returning an error with Timeout() == true.
+// Setting nsec == 0 (the default) disables the deadline.
+func (t *tcpchanconn) SetReadTimeout(nsec int64) error {
+	return errors.New("ssh: tcpchan: timeout not supported")
+}
+
+// SetWriteTimeout sets the time (in nanoseconds) that
+// Write will wait to send its data before returning an error with Timeout() == true.
+// Setting nsec == 0 (the default) disables the deadline.
+// Even if write times out, it may return n > 0, indicating that
+// some of the data was successfully written.
+func (t *tcpchanconn) SetWriteTimeout(nsec int64) error {
+	return errors.New("ssh: tcpchan: timeout not supported")
+}