crypto/ssh/server.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 (
	"bytes"
	"crypto"
	"crypto/rand"
	"crypto/rsa"
	"crypto/x509"
	"encoding/binary"
	"encoding/pem"
	"errors"
	"io"
	"math/big"
	"net"
	"sync"
)

type ServerConfig struct {
	rsa           *rsa.PrivateKey
	rsaSerialized []byte

	// 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

	// NoClientAuth is true if clients are allowed to connect without
	// authenticating.
	NoClientAuth bool

	// PasswordCallback, if non-nil, is called when a user attempts to
	// authenticate using a password. It may be called concurrently from
	// several goroutines.
	PasswordCallback func(conn *ServerConn, user, password string) bool

	// PublicKeyCallback, if non-nil, is called when a client attempts public
	// key authentication. It must return true iff the given public key is
	// valid for the given user.
	PublicKeyCallback func(conn *ServerConn, user, algo string, pubkey []byte) bool

	// Cryptographic-related configuration.
	Crypto CryptoConfig
}

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

// SetRSAPrivateKey sets the private key for a Server. A Server must have a
// private key configured in order to accept connections. The private key must
// be in the form of a PEM encoded, PKCS#1, RSA private key. The file "id_rsa"
// typically contains such a key.
func (s *ServerConfig) SetRSAPrivateKey(pemBytes []byte) error {
	block, _ := pem.Decode(pemBytes)
	if block == nil {
		return errors.New("ssh: no key found")
	}
	var err error
	s.rsa, err = x509.ParsePKCS1PrivateKey(block.Bytes)
	if err != nil {
		return err
	}

	s.rsaSerialized = marshalPrivRSA(s.rsa)
	return nil
}

func parseRSASig(in []byte) (sig []byte, ok bool) {
	algo, in, ok := parseString(in)
	if !ok || string(algo) != hostAlgoRSA {
		return nil, false
	}
	sig, in, ok = parseString(in)
	if len(in) > 0 {
		ok = false
	}
	return
}

// cachedPubKey contains the results of querying whether a public key is
// acceptable for a user. The cache only applies to a single ServerConn.
type cachedPubKey struct {
	user, algo string
	pubKey     []byte
	result     bool
}

const maxCachedPubKeys = 16

// A ServerConn represents an incoming connection.
type ServerConn struct {
	*transport
	config *ServerConfig

	channels   map[uint32]*serverChan
	nextChanId uint32

	// lock protects err and channels.
	lock sync.Mutex
	err  error

	// cachedPubKeys contains the cache results of tests for public keys.
	// Since SSH clients will query whether a public key is acceptable
	// before attempting to authenticate with it, we end up with duplicate
	// queries for public key validity.
	cachedPubKeys []cachedPubKey

	// User holds the successfully authenticated user name.
	// It is empty if no authentication is used.  It is populated before
	// any authentication callback is called and not assigned to after that.
	User string

	// ClientVersion is the client's version, populated after
	// Handshake is called. It should not be modified.
	ClientVersion []byte

	// Initial H used for the session ID. Once assigned this must not change
	// even during subsequent key exchanges.
	sessionId []byte
}

// Server returns a new SSH server connection
// using c as the underlying transport.
func Server(c net.Conn, config *ServerConfig) *ServerConn {
	return &ServerConn{
		transport: newTransport(c, config.rand()),
		channels:  make(map[uint32]*serverChan),
		config:    config,
	}
}

// kexDH performs Diffie-Hellman key agreement on a ServerConnection. The
// returned values are given the same names as in RFC 4253, section 8.
func (s *ServerConn) kexDH(group *dhGroup, hashFunc crypto.Hash, magics *handshakeMagics, hostKeyAlgo string) (H, K []byte, err error) {
	packet, err := s.readPacket()
	if err != nil {
		return
	}
	var kexDHInit kexDHInitMsg
	if err = unmarshal(&kexDHInit, packet, msgKexDHInit); err != nil {
		return
	}

	y, err := rand.Int(s.config.rand(), group.p)
	if err != nil {
		return
	}

	Y := new(big.Int).Exp(group.g, y, group.p)
	kInt, err := group.diffieHellman(kexDHInit.X, y)
	if err != nil {
		return nil, nil, err
	}

	var serializedHostKey []byte
	switch hostKeyAlgo {
	case hostAlgoRSA:
		serializedHostKey = s.config.rsaSerialized
	default:
		return nil, nil, errors.New("ssh: internal error")
	}

	h := hashFunc.New()
	writeString(h, magics.clientVersion)
	writeString(h, magics.serverVersion)
	writeString(h, magics.clientKexInit)
	writeString(h, magics.serverKexInit)
	writeString(h, serializedHostKey)
	writeInt(h, kexDHInit.X)
	writeInt(h, Y)
	K = make([]byte, intLength(kInt))
	marshalInt(K, kInt)
	h.Write(K)

	H = h.Sum(nil)

	h.Reset()
	h.Write(H)
	hh := h.Sum(nil)

	var sig []byte
	switch hostKeyAlgo {
	case hostAlgoRSA:
		sig, err = rsa.SignPKCS1v15(s.config.rand(), s.config.rsa, hashFunc, hh)
		if err != nil {
			return
		}
	default:
		return nil, nil, errors.New("ssh: internal error")
	}

	serializedSig := serializeSignature(hostKeyAlgo, sig)

	kexDHReply := kexDHReplyMsg{
		HostKey:   serializedHostKey,
		Y:         Y,
		Signature: serializedSig,
	}
	packet = marshal(msgKexDHReply, kexDHReply)

	err = s.writePacket(packet)
	return
}

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

// Handshake performs an SSH transport and client authentication on the given ServerConn.
func (s *ServerConn) Handshake() (err error) {
	if _, err = s.Write(serverVersion); err != nil {
		return
	}
	if err = s.Flush(); err != nil {
		return
	}

	s.ClientVersion, err = readVersion(s)
	if err != nil {
		return
	}
	if err = s.clientInitHandshake(nil, nil); err != nil {
		return
	}

	var packet []byte
	if packet, err = s.readPacket(); err != nil {
		return
	}
	var serviceRequest serviceRequestMsg
	if err = unmarshal(&serviceRequest, packet, msgServiceRequest); err != nil {
		return
	}
	if serviceRequest.Service != serviceUserAuth {
		return errors.New("ssh: requested service '" + serviceRequest.Service + "' before authenticating")
	}
	serviceAccept := serviceAcceptMsg{
		Service: serviceUserAuth,
	}
	if err = s.writePacket(marshal(msgServiceAccept, serviceAccept)); err != nil {
		return
	}

	if err = s.authenticate(s.sessionId); err != nil {
		return
	}
	return
}

func (s *ServerConn) clientInitHandshake(clientKexInit *kexInitMsg, clientKexInitPacket []byte) (err error) {
	serverKexInit := kexInitMsg{
		KexAlgos:                supportedKexAlgos,
		ServerHostKeyAlgos:      supportedHostKeyAlgos,
		CiphersClientServer:     s.config.Crypto.ciphers(),
		CiphersServerClient:     s.config.Crypto.ciphers(),
		MACsClientServer:        s.config.Crypto.macs(),
		MACsServerClient:        s.config.Crypto.macs(),
		CompressionClientServer: supportedCompressions,
		CompressionServerClient: supportedCompressions,
	}
	serverKexInitPacket := marshal(msgKexInit, serverKexInit)

	if err = s.writePacket(serverKexInitPacket); err != nil {
		return
	}

	if clientKexInitPacket == nil {
		clientKexInit = new(kexInitMsg)
		if clientKexInitPacket, err = s.readPacket(); err != nil {
			return
		}
		if err = unmarshal(clientKexInit, clientKexInitPacket, msgKexInit); err != nil {
			return
		}
	}

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

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

	var magics handshakeMagics
	magics.serverVersion = serverVersion[:len(serverVersion)-2]
	magics.clientVersion = s.ClientVersion
	magics.serverKexInit = marshal(msgKexInit, serverKexInit)
	magics.clientKexInit = clientKexInitPacket

	var H, K []byte
	var hashFunc crypto.Hash
	switch kexAlgo {
	case kexAlgoDH14SHA1:
		hashFunc = crypto.SHA1
		dhGroup14Once.Do(initDHGroup14)
		H, K, err = s.kexDH(dhGroup14, hashFunc, &magics, hostKeyAlgo)
	case keyAlgoDH1SHA1:
		hashFunc = crypto.SHA1
		dhGroup1Once.Do(initDHGroup1)
		H, K, err = s.kexDH(dhGroup1, hashFunc, &magics, hostKeyAlgo)
	default:
		err = errors.New("ssh: unexpected key exchange algorithm " + kexAlgo)
	}
	if err != nil {
		return
	}
	// sessionId must only be assigned during initial handshake.
	if s.sessionId == nil {
		s.sessionId = H
	}

	var packet []byte

	if err = s.writePacket([]byte{msgNewKeys}); err != nil {
		return
	}
	if err = s.transport.writer.setupKeys(serverKeys, K, H, s.sessionId, hashFunc); err != nil {
		return
	}

	if packet, err = s.readPacket(); err != nil {
		return
	}
	if packet[0] != msgNewKeys {
		return UnexpectedMessageError{msgNewKeys, packet[0]}
	}
	if err = s.transport.reader.setupKeys(clientKeys, K, H, s.sessionId, hashFunc); err != nil {
		return
	}

	return
}

func isAcceptableAlgo(algo string) bool {
	return algo == hostAlgoRSA
}

// testPubKey returns true if the given public key is acceptable for the user.
func (s *ServerConn) testPubKey(user, algo string, pubKey []byte) bool {
	if s.config.PublicKeyCallback == nil || !isAcceptableAlgo(algo) {
		return false
	}

	for _, c := range s.cachedPubKeys {
		if c.user == user && c.algo == algo && bytes.Equal(c.pubKey, pubKey) {
			return c.result
		}
	}

	result := s.config.PublicKeyCallback(s, user, algo, pubKey)
	if len(s.cachedPubKeys) < maxCachedPubKeys {
		c := cachedPubKey{
			user:   user,
			algo:   algo,
			pubKey: make([]byte, len(pubKey)),
			result: result,
		}
		copy(c.pubKey, pubKey)
		s.cachedPubKeys = append(s.cachedPubKeys, c)
	}

	return result
}

func (s *ServerConn) authenticate(H []byte) error {
	var userAuthReq userAuthRequestMsg
	var err error
	var packet []byte

userAuthLoop:
	for {
		if packet, err = s.readPacket(); err != nil {
			return err
		}
		if err = unmarshal(&userAuthReq, packet, msgUserAuthRequest); err != nil {
			return err
		}

		if userAuthReq.Service != serviceSSH {
			return errors.New("ssh: client attempted to negotiate for unknown service: " + userAuthReq.Service)
		}

		switch userAuthReq.Method {
		case "none":
			if s.config.NoClientAuth {
				break userAuthLoop
			}
		case "password":
			if s.config.PasswordCallback == nil {
				break
			}
			payload := userAuthReq.Payload
			if len(payload) < 1 || payload[0] != 0 {
				return ParseError{msgUserAuthRequest}
			}
			payload = payload[1:]
			password, payload, ok := parseString(payload)
			if !ok || len(payload) > 0 {
				return ParseError{msgUserAuthRequest}
			}

			s.User = userAuthReq.User
			if s.config.PasswordCallback(s, userAuthReq.User, string(password)) {
				break userAuthLoop
			}
		case "publickey":
			if s.config.PublicKeyCallback == nil {
				break
			}
			payload := userAuthReq.Payload
			if len(payload) < 1 {
				return ParseError{msgUserAuthRequest}
			}
			isQuery := payload[0] == 0
			payload = payload[1:]
			algoBytes, payload, ok := parseString(payload)
			if !ok {
				return ParseError{msgUserAuthRequest}
			}
			algo := string(algoBytes)

			pubKey, payload, ok := parseString(payload)
			if !ok {
				return ParseError{msgUserAuthRequest}
			}
			if isQuery {
				// The client can query if the given public key
				// would be ok.
				if len(payload) > 0 {
					return ParseError{msgUserAuthRequest}
				}
				if s.testPubKey(userAuthReq.User, algo, pubKey) {
					okMsg := userAuthPubKeyOkMsg{
						Algo:   algo,
						PubKey: string(pubKey),
					}
					if err = s.writePacket(marshal(msgUserAuthPubKeyOk, okMsg)); err != nil {
						return err
					}
					continue userAuthLoop
				}
			} else {
				sig, payload, ok := parseString(payload)
				if !ok || len(payload) > 0 {
					return ParseError{msgUserAuthRequest}
				}
				if !isAcceptableAlgo(algo) {
					break
				}
				rsaSig, ok := parseRSASig(sig)
				if !ok {
					return ParseError{msgUserAuthRequest}
				}
				signedData := buildDataSignedForAuth(H, userAuthReq, algoBytes, pubKey)
				switch algo {
				case hostAlgoRSA:
					hashFunc := crypto.SHA1
					h := hashFunc.New()
					h.Write(signedData)
					digest := h.Sum(nil)
					key, _, ok := parsePubKey(pubKey)
					if !ok {
						return ParseError{msgUserAuthRequest}
					}
					rsaKey, ok := key.(*rsa.PublicKey)
					if !ok {
						return ParseError{msgUserAuthRequest}
					}
					if rsa.VerifyPKCS1v15(rsaKey, hashFunc, digest, rsaSig) != nil {
						return ParseError{msgUserAuthRequest}
					}
				default:
					return errors.New("ssh: isAcceptableAlgo incorrect")
				}
				s.User = userAuthReq.User
				if s.testPubKey(userAuthReq.User, algo, pubKey) {
					break userAuthLoop
				}
			}
		}

		var failureMsg userAuthFailureMsg
		if s.config.PasswordCallback != nil {
			failureMsg.Methods = append(failureMsg.Methods, "password")
		}
		if s.config.PublicKeyCallback != nil {
			failureMsg.Methods = append(failureMsg.Methods, "publickey")
		}

		if len(failureMsg.Methods) == 0 {
			return errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
		}

		if err = s.writePacket(marshal(msgUserAuthFailure, failureMsg)); err != nil {
			return err
		}
	}

	packet = []byte{msgUserAuthSuccess}
	if err = s.writePacket(packet); err != nil {
		return err
	}

	return nil
}

const defaultWindowSize = 32768

// Accept reads and processes messages on a ServerConn. It must be called
// in order to demultiplex messages to any resulting Channels.
func (s *ServerConn) Accept() (Channel, error) {
	// TODO(dfc) s.lock is not held here so visibility of s.err is not guaranteed.
	if s.err != nil {
		return nil, s.err
	}

	for {
		packet, err := s.readPacket()
		if err != nil {

			s.lock.Lock()
			s.err = err
			s.lock.Unlock()

			// TODO(dfc) s.lock protects s.channels but isn't being held here.
			for _, c := range s.channels {
				c.setDead()
				c.handleData(nil)
			}

			return nil, err
		}

		switch packet[0] {
		case msgChannelData:
			if len(packet) < 9 {
				// malformed data packet
				return nil, ParseError{msgChannelData}
			}
			remoteId := binary.BigEndian.Uint32(packet[1:5])
			s.lock.Lock()
			c, ok := s.channels[remoteId]
			if !ok {
				s.lock.Unlock()
				continue
			}
			if length := binary.BigEndian.Uint32(packet[5:9]); length > 0 {
				packet = packet[9:]
				c.handleData(packet[:length])
			}
			s.lock.Unlock()
		default:
			switch msg := decode(packet).(type) {
			case *channelOpenMsg:
				if msg.MaxPacketSize < minPacketLength || msg.MaxPacketSize > 1<<31 {
					return nil, errors.New("ssh: invalid MaxPacketSize from peer")
				}
				c := &serverChan{
					channel: channel{
						conn:      s,
						remoteId:  msg.PeersId,
						remoteWin: window{Cond: newCond()},
						maxPacket: msg.MaxPacketSize,
					},
					chanType:    msg.ChanType,
					extraData:   msg.TypeSpecificData,
					myWindow:    defaultWindowSize,
					serverConn:  s,
					cond:        newCond(),
					pendingData: make([]byte, defaultWindowSize),
				}
				c.remoteWin.add(msg.PeersWindow)
				s.lock.Lock()
				c.localId = s.nextChanId
				s.nextChanId++
				s.channels[c.localId] = c
				s.lock.Unlock()
				return c, nil

			case *channelRequestMsg:
				s.lock.Lock()
				c, ok := s.channels[msg.PeersId]
				if !ok {
					s.lock.Unlock()
					continue
				}
				c.handlePacket(msg)
				s.lock.Unlock()

			case *windowAdjustMsg:
				s.lock.Lock()
				c, ok := s.channels[msg.PeersId]
				if !ok {
					s.lock.Unlock()
					continue
				}
				c.handlePacket(msg)
				s.lock.Unlock()

			case *channelEOFMsg:
				s.lock.Lock()
				c, ok := s.channels[msg.PeersId]
				if !ok {
					s.lock.Unlock()
					continue
				}
				c.handlePacket(msg)
				s.lock.Unlock()

			case *channelCloseMsg:
				s.lock.Lock()
				c, ok := s.channels[msg.PeersId]
				if !ok {
					s.lock.Unlock()
					continue
				}
				c.handlePacket(msg)
				s.lock.Unlock()

			case *globalRequestMsg:
				if msg.WantReply {
					if err := s.writePacket([]byte{msgRequestFailure}); err != nil {
						return nil, err
					}
				}

			case *kexInitMsg:
				s.lock.Lock()
				if err := s.clientInitHandshake(msg, packet); err != nil {
					s.lock.Unlock()
					return nil, err
				}
				s.lock.Unlock()

			case UnexpectedMessageError:
				return nil, msg
			case *disconnectMsg:
				return nil, io.EOF
			default:
				// Unknown message. Ignore.
			}
		}
	}

	panic("unreachable")
}

// A Listener implements a network listener (net.Listener) for SSH connections.
type Listener struct {
	listener net.Listener
	config   *ServerConfig
}

// Addr returns the listener's network address.
func (l *Listener) Addr() net.Addr {
	return l.listener.Addr()
}

// Close closes the listener.
func (l *Listener) Close() error {
	return l.listener.Close()
}

// Accept waits for and returns the next incoming SSH connection.
// The receiver should call Handshake() in another goroutine 
// to avoid blocking the accepter.
func (l *Listener) Accept() (*ServerConn, error) {
	c, err := l.listener.Accept()
	if err != nil {
		return nil, err
	}
	return Server(c, l.config), nil
}

// Listen creates an SSH listener accepting connections on
// the given network address using net.Listen.
func Listen(network, addr string, config *ServerConfig) (*Listener, error) {
	l, err := net.Listen(network, addr)
	if err != nil {
		return nil, err
	}
	return &Listener{
		l,
		config,
	}, nil
}