ssh: add openssh controlmaster socket support

ssh/client.go

@@ -88,6 +88,21 @@ func NewClientConn(c net.Conn, addr string, config *ClientConfig) (Conn, <-chan
 	return conn, conn.mux.incomingChannels, conn.mux.incomingRequests, nil
 }
 
+// NewControlClientConn establishes an SSH connection over an OpenSSH
+// ControlMaster socket c in proxy mode. The Request and NewChannel channels
+// must be serviced or the connection will hang.
+func NewControlClientConn(c net.Conn) (Conn, <-chan NewChannel, <-chan *Request, error) {
+	conn := &connection{
+		sshConn: sshConn{conn: c},
+	}
+	var err error
+	if conn.transport, err = handshakeControlProxy(c); err != nil {
+		return nil, nil, nil, fmt.Errorf("ssh: control proxy 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 {

ssh/connection.go

@@ -84,9 +84,15 @@ func DiscardRequests(in <-chan *Request) {
 	}
 }
 
+type connTransport interface {
+	packetConn
+	getSessionID() []byte
+	waitSession() error
+}
+
 // A connection represents an incoming connection.
 type connection struct {
-	transport *handshakeTransport
+	transport connTransport
 	sshConn
 
 	// The connection protocol.

ssh/control.go

@@ -0,0 +1,167 @@
+// Copyright 2022 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 (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+const (
+	MUX_MSG_HELLO = 0x00000001
+	MUX_C_PROXY   = 0x1000000f
+	MUX_S_PROXY   = 0x8000000f
+	MUX_S_FAILURE = 0x80000003
+)
+
+// handshakeControlProxy attempts to establish a transport connection with an
+// OpenSSH ControlMaster socket in proxy mode. For details see:
+// https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.mux
+func handshakeControlProxy(rw io.ReadWriteCloser) (connTransport, error) {
+	b := &controlBuffer{}
+	b.WriteUint32(MUX_MSG_HELLO)
+	b.WriteUint32(4) // Protocol Version
+	if _, err := rw.Write(b.LengthPrefixedBytes()); err != nil {
+		return nil, fmt.Errorf("mux hello write failed: %v", err)
+	}
+
+	b.Reset()
+	b.WriteUint32(MUX_C_PROXY)
+	b.WriteUint32(0) // Request ID
+	if _, err := rw.Write(b.LengthPrefixedBytes()); err != nil {
+		return nil, fmt.Errorf("mux client proxy write failed: %v", err)
+	}
+
+	r := controlReader{rw}
+	m, err := r.Next()
+	if err != nil {
+		return nil, fmt.Errorf("mux hello read failed: %v", err)
+	}
+	if m.messageType != MUX_MSG_HELLO {
+		return nil, fmt.Errorf("mux reply not hello")
+	}
+	if v, err := m.ReadUint32(); err != nil || v != 4 {
+		return nil, fmt.Errorf("mux reply hello has bad protocol version")
+	}
+	m, err = r.Next()
+	if err != nil {
+		return nil, fmt.Errorf("error reading mux server proxy: %v", err)
+	}
+	if m.messageType != MUX_S_PROXY {
+		return nil, fmt.Errorf("expected server proxy response got %d", m.messageType)
+	}
+	return &controlProxyTransport{rw}, nil
+}
+
+// controlProxyTransport implements the connTransport interface for
+// ControlMaster connections. Each controlMessage has zero length padding and
+// no MAC.
+type controlProxyTransport struct {
+	rw io.ReadWriteCloser
+}
+
+func (p *controlProxyTransport) Close() error {
+	return p.Close()
+}
+
+func (p *controlProxyTransport) getSessionID() []byte {
+	return nil
+}
+
+func (p *controlProxyTransport) readPacket() ([]byte, error) {
+	var l uint32
+	err := binary.Read(p.rw, binary.BigEndian, &l)
+	if err == nil {
+		buf := &bytes.Buffer{}
+		_, err = io.CopyN(buf, p.rw, int64(l))
+		if err == nil {
+			// Discard the padding byte.
+			buf.ReadByte()
+			return buf.Bytes(), nil
+		}
+	}
+	return nil, err
+}
+
+func (p *controlProxyTransport) writePacket(controlMessage []byte) error {
+	l := uint32(len(controlMessage)) + 1
+	b := &bytes.Buffer{}
+	binary.Write(b, binary.BigEndian, &l) // controlMessage Length.
+	b.WriteByte(0)                        // Padding Length.
+	b.Write(controlMessage)
+	_, err := p.rw.Write(b.Bytes())
+	return err
+}
+
+func (p *controlProxyTransport) waitSession() error {
+	return nil
+}
+
+type controlBuffer struct {
+	bytes.Buffer
+}
+
+func (b *controlBuffer) WriteUint32(i uint32) {
+	binary.Write(b, binary.BigEndian, i)
+}
+
+func (b *controlBuffer) LengthPrefixedBytes() []byte {
+	b2 := &bytes.Buffer{}
+	binary.Write(b2, binary.BigEndian, uint32(b.Len()))
+	b2.Write(b.Bytes())
+	return b2.Bytes()
+}
+
+type controlMessage struct {
+	body        bytes.Buffer
+	messageType uint32
+}
+
+func (p controlMessage) ReadUint32() (uint32, error) {
+	var u uint32
+	err := binary.Read(&p.body, binary.BigEndian, &u)
+	return u, err
+}
+
+func (p controlMessage) ReadString() (string, error) {
+	var l uint32
+	err := binary.Read(&p.body, binary.BigEndian, &l)
+	if err != nil {
+		return "", fmt.Errorf("error reading string length: %v", err)
+	}
+	b := p.body.Next(int(l))
+	if len(b) != int(l) {
+		return string(b), fmt.Errorf("EOF on string read")
+	}
+	return string(b), nil
+}
+
+type controlReader struct {
+	r io.Reader
+}
+
+func (r controlReader) Next() (*controlMessage, error) {
+	p := &controlMessage{}
+	var len uint32
+	err := binary.Read(r.r, binary.BigEndian, &len)
+	if err != nil {
+		return nil, fmt.Errorf("error reading message length: %v", err)
+	}
+	_, err = io.CopyN(&p.body, r.r, int64(len))
+	if err != nil {
+		return nil, fmt.Errorf("error reading message payload: %v", err)
+	}
+	err = binary.Read(&p.body, binary.BigEndian, &p.messageType)
+	if err != nil {
+		return nil, fmt.Errorf("error reading message type: %v", err)
+	}
+	if p.messageType == MUX_S_FAILURE {
+		reason, _ := p.ReadString()
+		return nil, fmt.Errorf("server failure: '%s'", reason)
+	}
+	return p, nil
+}