teleport/lib/sshutils/server.go

/*
Copyright 2015 Gravitational, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

This file contains the implementatino of sshutils.Server class.
It is the underlying "base SSH server" for everything in Teleport.

*/

package sshutils

import (
	"bytes"
	"crypto/subtle"
	"encoding/json"
	"fmt"
	"io"
	"net"
	"strings"
	"sync"
	"time"

	"github.com/gravitational/teleport"
	"github.com/gravitational/teleport/lib/defaults"
	"github.com/gravitational/teleport/lib/limiter"
	"github.com/gravitational/teleport/lib/utils"

	log "github.com/Sirupsen/logrus"
	"github.com/gravitational/trace"
	"golang.org/x/crypto/ssh"
)

// Server is a generic implementation of an SSH server. All Teleport
// services (auth, proxy, ssh) use this as a base to accept SSH connections.
type Server struct {
	// component is a name of the facility which uses this server,
	// used for logging/debugging. typically it's "proxy" or "auth api", etc
	component string

	// addr is the address this server binds to and listens on
	addr utils.NetAddr

	// listener is usually the listening TCP/IP socket
	listener net.Listener

	// closeC channel is used to stop the server by closing it
	closeC chan struct{}

	newChanHandler NewChanHandler
	reqHandler     RequestHandler

	cfg          ssh.ServerConfig
	limiter      *limiter.Limiter
	askedToClose bool
}

const (
	// SSHVersionPrefix is the prefix of "server version" string which begins
	// every SSH handshake. It MUST start with "SSH-2.0" according to
	// https://tools.ietf.org/html/rfc4253#page-4
	SSHVersionPrefix = "SSH-2.0-Teleport"

	// ProxyHelloSignature is a string which Teleport proxy will send
	// right after the initial SSH "handshake/version" message if it detects
	// talking to a Teleport server.
	ProxyHelloSignature = "Teleport-Proxy"

	// MaxVersionStringBytes is the maximum number of bytes allowed for a
	// SSH version string
	// https://tools.ietf.org/html/rfc4253
	MaxVersionStringBytes = 255
)

// ServerOption is a functional argument for server
type ServerOption func(cfg *Server) error

func SetLimiter(limiter *limiter.Limiter) ServerOption {
	return func(s *Server) error {
		s.limiter = limiter
		return nil
	}
}

func NewServer(
	component string,
	a utils.NetAddr,
	h NewChanHandler,
	hostSigners []ssh.Signer,
	ah AuthMethods,
	opts ...ServerOption) (*Server, error) {

	err := checkArguments(a, h, hostSigners, ah)
	if err != nil {
		return nil, err
	}
	s := &Server{
		component:      component,
		addr:           a,
		newChanHandler: h,
		closeC:         make(chan struct{}),
	}
	s.limiter, err = limiter.NewLimiter(limiter.LimiterConfig{})
	if err != nil {
		return nil, trace.Wrap(err)
	}

	for _, o := range opts {
		if err := o(s); err != nil {
			return nil, err
		}
	}
	for _, signer := range hostSigners {
		(&s.cfg).AddHostKey(signer)
	}
	s.cfg.PublicKeyCallback = ah.PublicKey
	s.cfg.PasswordCallback = ah.Password
	s.cfg.NoClientAuth = ah.NoClient

	// Teleport SSH server will be sending the following "version string" during
	// SSH handshake (example): "SSH-2.0-T eleport 1.5.1-beta" (space is important!)
	s.cfg.ServerVersion = fmt.Sprintf("%s %s", SSHVersionPrefix, teleport.Version)
	return s, nil
}

func SetSSHConfig(cfg ssh.ServerConfig) ServerOption {
	return func(s *Server) error {
		s.cfg = cfg
		return nil
	}
}

func SetRequestHandler(req RequestHandler) ServerOption {
	return func(s *Server) error {
		s.reqHandler = req
		return nil
	}
}

func (s *Server) Addr() string {
	return s.listener.Addr().String()
}

func (s *Server) Start() error {
	s.askedToClose = false
	socket, err := net.Listen(s.addr.AddrNetwork, s.addr.Addr)
	if err != nil {
		return err
	}
	s.listener = socket
	log.Infof("[SSH:%s] listening socket: %v", s.component, socket.Addr())
	go s.acceptConnections()
	return nil
}

func (s *Server) notifyClosed() {
	close(s.closeC)
}

func (s *Server) Wait() {
	<-s.closeC
}

// Close closes listening socket and stops accepting connections
func (s *Server) Close() error {
	s.askedToClose = true
	if s.listener != nil {
		return s.listener.Close()
	}
	return nil
}

func (s *Server) acceptConnections() {
	defer s.notifyClosed()
	addr := s.Addr()
	log.Infof("[SSH:%v] is listening on %v", s.component, addr)
	for {
		conn, err := s.listener.Accept()
		if err != nil {
			if s.askedToClose {
				log.Infof("[SSH:%v] server %v exited", s.component, addr)
				s.askedToClose = false
				return
			}
			// our best shot to avoid excessive logging
			if op, ok := err.(*net.OpError); ok && !op.Timeout() {
				log.Debugf("[SSH:%v] closed socket %v", s.component, op)
				return
			}
			log.Errorf("SSH:%v accept error: %T %v", s.component, err, err)
			return
		}
		go s.handleConnection(conn)
	}
}

// handleConnection is called every time an SSH server accepts a new
// connection from a client.
//
// this is the foundation of all SSH connections in Teleport (between clients
// and proxies, proxies and servers, servers and auth, etc).
//
func (s *Server) handleConnection(conn net.Conn) {
	// initiate an SSH connection, note that we don't need to close the conn here
	// in case of error as ssh server takes care of this
	remoteAddr, _, err := net.SplitHostPort(conn.RemoteAddr().String())
	if err != nil {
		log.Errorf(err.Error())
	}
	if err := s.limiter.AcquireConnection(remoteAddr); err != nil {
		log.Errorf(err.Error())
		conn.Close()
		return
	}
	defer s.limiter.ReleaseConnection(remoteAddr)

	// apply idle read/write timeout to this connection.
	conn = utils.ObeyIdleTimeout(conn,
		defaults.DefaultIdleConnectionDuration,
		s.component)

	// create a new SSH server which handles the handshake (and pass the custom
	// payload structure which will be populated only when/if this connection
	// comes from another Teleport proxy):
	sconn, chans, reqs, err := ssh.NewServerConn(wrapConnection(conn), &s.cfg)
	if err != nil {
		conn.SetDeadline(time.Time{})
		return
	}

	user := sconn.User()
	if err := s.limiter.RegisterRequest(user); err != nil {
		log.Errorf(err.Error())
		sconn.Close()
		conn.Close()
		return
	}
	// Connection successfully initiated
	log.Infof("[SSH:%v] new connection %v -> %v vesion: %v",
		s.component, sconn.RemoteAddr(), sconn.LocalAddr(), string(sconn.ClientVersion()))

	wg := sync.WaitGroup{}
	wg.Add(2)

	go func() {
		// Handle incoming out-of-band Requests
		s.handleRequests(reqs)
		wg.Done()
	}()
	go func() {
		// Handle channel requests on this connections
		s.handleChannels(conn, sconn, chans)
		wg.Done()
	}()

	wg.Wait()
}

func (s *Server) handleRequests(reqs <-chan *ssh.Request) {
	for req := range reqs {
		log.Infof("[SSH:%v] recieved out-of-band request: %+v", s.component, req)
		if s.reqHandler != nil {
			s.reqHandler.HandleRequest(req)
		}
	}
}

func (s *Server) handleChannels(conn net.Conn, sconn *ssh.ServerConn, chans <-chan ssh.NewChannel) {
	for nch := range chans {
		if nch == nil {
			log.Warningf("nil channel: %v", nch)
			continue
		}
		s.newChanHandler.HandleNewChan(conn, sconn, nch)
	}
}

type RequestHandler interface {
	HandleRequest(r *ssh.Request)
}

type RequestHandlerFunc func(*ssh.Request)

func (f RequestHandlerFunc) HandleRequest(r *ssh.Request) {
	f(r)
}

type NewChanHandler interface {
	HandleNewChan(net.Conn, *ssh.ServerConn, ssh.NewChannel)
}

type NewChanHandlerFunc func(net.Conn, *ssh.ServerConn, ssh.NewChannel)

func (f NewChanHandlerFunc) HandleNewChan(conn net.Conn, sshConn *ssh.ServerConn, ch ssh.NewChannel) {
	f(conn, sshConn, ch)
}

type AuthMethods struct {
	PublicKey PublicKeyFunc
	Password  PasswordFunc
	NoClient  bool
}

func checkArguments(a utils.NetAddr, h NewChanHandler, hostSigners []ssh.Signer, ah AuthMethods) error {
	if a.Addr == "" || a.AddrNetwork == "" {
		return trace.BadParameter("addr: specify network and the address for listening socket")
	}

	if h == nil {
		return trace.BadParameter("missing NewChanHandler")
	}
	if len(hostSigners) == 0 {
		return trace.BadParameter("need at least one signer")
	}
	for _, s := range hostSigners {
		if s == nil {
			return trace.BadParameter("host signer can not be nil")
		}
	}
	if ah.PublicKey == nil && ah.Password == nil && ah.NoClient == false {
		return trace.BadParameter("need at least one auth method")
	}
	return nil
}

type PublicKeyFunc func(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error)
type PasswordFunc func(conn ssh.ConnMetadata, password []byte) (*ssh.Permissions, error)

// KeysEqual is constant time compare of the keys to avoid timing attacks
func KeysEqual(ak, bk ssh.PublicKey) bool {
	a := ssh.Marshal(ak)
	b := ssh.Marshal(bk)
	return (len(a) == len(b) && subtle.ConstantTimeCompare(a, b) == 1)
}

// HandshakePayload structure is sent as a JSON blob by the teleport
// proxy to every SSH server who identifies itself as Teleport server
//
// It allows teleport proxies to communicate additional data to server
type HandshakePayload struct {
	// ClientAddr is the IP address of the remote client
	ClientAddr string `json:"clientAddr,omitempty"`
}

// connectionWrapper allows the SSH server to perform custom handshake which
// allows teleport proxy servers to relay the remote IP address of the client
// to the SSH server (otherwise connection.RemoteAddr would always point to
// a proxy, which is useless for audit purposes)
type connectionWrapper struct {
	net.Conn

	// upstreamReader reads from the underlying (wrapped) connection
	upstreamReader io.Reader

	// clientAddr points to the true client address (client is behind
	// a proxy). Keeping this address is the entire point of the
	// connection wrapper.
	clientAddr net.Addr
}

// RemoteAddr returns the behind-the-proxy client address
func (c *connectionWrapper) RemoteAddr() net.Addr {
	return c.clientAddr
}

// Read implements io.Read() part of net.Connection which allows the
// connection wrapper to peek at the beginning of SSH handshake
func (c *connectionWrapper) Read(b []byte) (int, error) {
	if c.upstreamReader != nil {
		return c.upstreamReader.Read(b)
	}
	// inspect the client's hello message and see if it's a teleport
	// proxy connecting?
	buff := make([]byte, MaxVersionStringBytes)
	n, err := c.Conn.Read(buff)
	if err != nil {
		log.Error(err)
		return n, err
	}
	// teleport proxy handshake:
	if strings.HasPrefix(string(buff), "Teleport-Proxy") {
		c.upstreamReader = c.Conn
		endOfLine := bytes.IndexByte(buff, '\n')
		if endOfLine > 0 {
			// custom payload starts after the newline:
			var hp HandshakePayload
			if err = json.Unmarshal(buff[endOfLine+1:n], &hp); err != nil {
				log.Error(err)
			} else {
				ca, err := utils.ParseAddr(hp.ClientAddr)
				if err != nil {
					log.Error(err)
				} else {
					// replace proxy's client addr with a real client address
					// we just got from the custom payload:
					c.clientAddr = ca
				}
			}
		}
		// not a teleport proxy:
	} else {
		c.upstreamReader = io.MultiReader(bytes.NewBuffer(buff[:n]), c.Conn)
	}
	return c.upstreamReader.Read(b)
}

// wrapConnection takes a network connection, wraps it into connectionWrapper
// object (which overrides Read method) and returns the wrapper.
func wrapConnection(conn net.Conn) net.Conn {
	return &connectionWrapper{
		Conn:       conn,
		clientAddr: conn.RemoteAddr(),
	}
}