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server.go
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/*
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.
*/
// Package sshutils contains contains the implementations of the base SSH
// server used throughout Teleport.
package sshutils
import (
"bytes"
"context"
"crypto/subtle"
"encoding/json"
"io"
"net"
"sync"
"sync/atomic"
"time"
"golang.org/x/crypto/ssh"
"github.com/gravitational/teleport"
"github.com/gravitational/teleport/lib/defaults"
"github.com/gravitational/teleport/lib/limiter"
"github.com/gravitational/teleport/lib/utils"
"github.com/gravitational/trace"
log "github.com/sirupsen/logrus"
)
// 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 {
*log.Entry
sync.RWMutex
// 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
newChanHandler NewChanHandler
reqHandler RequestHandler
cfg ssh.ServerConfig
limiter *limiter.Limiter
listenerClosed bool
closeContext context.Context
closeFunc context.CancelFunc
// conns tracks amount of current active connections
conns int32
// shutdownPollPeriod sets polling period for shutdown
shutdownPollPeriod time.Duration
// insecureSkipHostValidation does not validate the host signers to make sure
// they are a valid certificate. Used in tests.
insecureSkipHostValidation bool
// fips means Teleport started in a FedRAMP/FIPS 140-2 compliant
// configuration.
fips 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
// TrueClientAddrVar environment variable is used by the web UI to pass
// the remote IP (user's IP) from the browser/HTTP session into an SSH session
TrueClientAddrVar = "TELEPORT_CLIENT_ADDR"
)
// 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
}
}
// SetShutdownPollPeriod sets a polling period for graceful shutdowns of SSH servers
func SetShutdownPollPeriod(period time.Duration) ServerOption {
return func(s *Server) error {
s.shutdownPollPeriod = period
return nil
}
}
// SetInsecureSkipHostValidation does not validate the host signers to make sure
// they are a valid certificate. Used in tests.
func SetInsecureSkipHostValidation() ServerOption {
return func(s *Server) error {
s.insecureSkipHostValidation = true
return nil
}
}
func NewServer(
component string,
a utils.NetAddr,
h NewChanHandler,
hostSigners []ssh.Signer,
ah AuthMethods,
opts ...ServerOption) (*Server, error) {
var err error
closeContext, cancel := context.WithCancel(context.TODO())
s := &Server{
Entry: log.WithFields(log.Fields{
trace.Component: "ssh:" + component,
}),
addr: a,
newChanHandler: h,
component: component,
closeContext: closeContext,
closeFunc: cancel,
}
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
}
}
if s.shutdownPollPeriod == 0 {
s.shutdownPollPeriod = defaults.ShutdownPollPeriod
}
err = s.checkArguments(a, h, hostSigners, ah)
if 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 servers need to identify as such to allow passing of the client
// IP from the client to the proxy to the destination node.
s.cfg.ServerVersion = SSHVersionPrefix
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 SetCiphers(ciphers []string) ServerOption {
return func(s *Server) error {
s.Debugf("Supported ciphers: %q.", ciphers)
if ciphers != nil {
s.cfg.Ciphers = ciphers
}
return nil
}
}
func SetKEXAlgorithms(kexAlgorithms []string) ServerOption {
return func(s *Server) error {
s.Debugf("Supported KEX algorithms: %q.", kexAlgorithms)
if kexAlgorithms != nil {
s.cfg.KeyExchanges = kexAlgorithms
}
return nil
}
}
func SetMACAlgorithms(macAlgorithms []string) ServerOption {
return func(s *Server) error {
s.Debugf("Supported MAC algorithms: %q.", macAlgorithms)
if macAlgorithms != nil {
s.cfg.MACs = macAlgorithms
}
return nil
}
}
func SetFIPS(fips bool) ServerOption {
return func(s *Server) error {
s.fips = fips
return nil
}
}
func (s *Server) Addr() string {
return s.listener.Addr().String()
}
func (s *Server) isClosed() bool {
s.RLock()
defer s.RUnlock()
return s.listenerClosed
}
func (s *Server) Serve(listener net.Listener) error {
if err := s.setListener(listener); err != nil {
return trace.Wrap(err)
}
s.acceptConnections()
return nil
}
func (s *Server) Start() error {
listener, err := net.Listen(s.addr.AddrNetwork, s.addr.Addr)
if err != nil {
return trace.ConvertSystemError(err)
}
if err := s.setListener(listener); err != nil {
return trace.Wrap(err)
}
go s.acceptConnections()
return nil
}
func (s *Server) setListener(l net.Listener) error {
s.Lock()
defer s.Unlock()
if s.listener != nil {
return trace.BadParameter("listener is already set to %v", s.listener.Addr())
}
s.listenerClosed = false
s.listener = l
return nil
}
// Wait waits until server stops serving new connections
// on the listener socket
func (s *Server) Wait(ctx context.Context) {
select {
case <-s.closeContext.Done():
case <-ctx.Done():
}
}
// Shutdown initiates graceful shutdown - waiting until all active
// connections will get closed
func (s *Server) Shutdown(ctx context.Context) error {
// close listener to stop receiving new connections
err := s.Close()
s.Wait(ctx)
activeConnections := s.trackConnections(0)
if activeConnections == 0 {
return err
}
s.Infof("Shutdown: waiting for %v connections to finish.", activeConnections)
lastReport := time.Time{}
ticker := time.NewTicker(s.shutdownPollPeriod)
defer ticker.Stop()
for {
select {
case <-ticker.C:
activeConnections = s.trackConnections(0)
if activeConnections == 0 {
return err
}
if time.Now().Sub(lastReport) > 10*s.shutdownPollPeriod {
s.Infof("Shutdown: waiting for %v connections to finish.", activeConnections)
lastReport = time.Now()
}
case <-ctx.Done():
s.Infof("Context cancelled wait, returning.")
return trace.ConnectionProblem(err, "context cancelled")
}
}
}
// Close closes listening socket and stops accepting connections
func (s *Server) Close() error {
s.Lock()
defer s.Unlock()
// If no listener is set, the server is in tunnel mode which means
// closeFunc has to be manually called.
if s.listener == nil {
s.closeFunc()
return nil
}
// listener already closed, nothing to do
if s.listenerClosed {
return nil
}
s.listenerClosed = true
if s.listener != nil {
err := s.listener.Close()
return err
}
return nil
}
func (s *Server) acceptConnections() {
defer s.closeFunc()
backoffTimer := time.NewTicker(5 * time.Second)
defer backoffTimer.Stop()
addr := s.Addr()
s.Debugf("Listening on %v.", addr)
for {
conn, err := s.listener.Accept()
if err != nil {
if s.isClosed() {
s.Debugf("Server %v has closed.", addr)
return
}
select {
case <-s.closeContext.Done():
s.Debugf("Server %v has closed.", addr)
return
case <-backoffTimer.C:
s.Debugf("Backoff on network error: %v.", err)
}
} else {
go s.HandleConnection(conn)
}
}
}
func (s *Server) trackConnections(delta int32) int32 {
return atomic.AddInt32(&s.conns, delta)
}
// 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) {
s.trackConnections(1)
defer s.trackConnections(-1)
// 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)
// Wrap connection with a tracker used to monitor how much data was
// transmitted and received over the connection.
wconn := utils.NewTrackingConn(conn)
// 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(wconn), &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
s.Debugf("Incoming connection %v -> %v vesion: %v.",
sconn.RemoteAddr(), sconn.LocalAddr(), string(sconn.ClientVersion()))
// will be called when the connection is closed
connClosed := func() {
s.Debugf("Closed connection %v.", sconn.RemoteAddr())
}
// The keepalive ticket will ensure that SSH keepalive requests are being sent
// to the client at an interval much shorter than idle connection kill switch
keepAliveTick := time.NewTicker(defaults.DefaultIdleConnectionDuration / 3)
defer keepAliveTick.Stop()
keepAlivePayload := [8]byte{0}
for {
select {
// handle out of band ssh requests
case req := <-reqs:
if req == nil {
connClosed()
return
}
s.Debugf("Received out-of-band request: %+v.", req)
if s.reqHandler != nil {
go s.reqHandler.HandleRequest(req)
}
// handle channels:
case nch := <-chans:
if nch == nil {
connClosed()
return
}
go s.newChanHandler.HandleNewChan(wconn, sconn, nch)
// send keepalive pings to the clients
case <-keepAliveTick.C:
const wantReply = true
sconn.SendRequest(teleport.KeepAliveReqType, wantReply, keepAlivePayload[:])
}
}
}
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 (s *Server) checkArguments(a utils.NetAddr, h NewChanHandler, hostSigners []ssh.Signer, ah AuthMethods) error {
// If the server is not in tunnel mode, an address must be specified.
if s.listener != nil {
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 _, signer := range hostSigners {
if signer == nil {
return trace.BadParameter("host signer can not be nil")
}
if !s.insecureSkipHostValidation {
err := validateHostSigner(s.fips, signer)
if err != nil {
return trace.Wrap(err)
}
}
}
if ah.PublicKey == nil && ah.Password == nil && ah.NoClient == false {
return trace.BadParameter("need at least one auth method")
}
return nil
}
// validateHostSigner make sure the signer is a valid certificate.
func validateHostSigner(fips bool, signer ssh.Signer) error {
cert, ok := signer.PublicKey().(*ssh.Certificate)
if !ok {
return trace.BadParameter("only host certificates supported")
}
if len(cert.ValidPrincipals) == 0 {
return trace.BadParameter("at least one valid principal is required in host certificate")
}
certChecker := utils.CertChecker{
FIPS: fips,
}
err := certChecker.CheckCert(cert.ValidPrincipals[0], cert)
if err != nil {
return trace.Wrap(err)
}
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
// lets teleport proxy servers to relay a true remote client IP address
// to the SSH server.
//
// (otherwise connection.RemoteAddr (client IP) will always point to a proxy IP
// instead of oa true client IP)
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 us
// peek at the beginning of SSH handshake (that's why we're wrapping the connection)
func (c *connectionWrapper) Read(b []byte) (int, error) {
// handshake already took place, forward upstream:
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 {
// EOF happens quite often, don't pollute the logs with it
if !trace.IsEOF(err) {
log.Error(err)
}
return n, err
}
// chop off extra unused bytes at the end of the buffer:
buff = buff[:n]
var skip = 0
// are we reading from a Teleport proxy?
if bytes.HasPrefix(buff, []byte(ProxyHelloSignature)) {
// the JSON paylaod ends with a binary zero:
payloadBoundary := bytes.IndexByte(buff, 0x00)
if payloadBoundary > 0 {
var hp HandshakePayload
payload := buff[len(ProxyHelloSignature):payloadBoundary]
if err = json.Unmarshal(payload, &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
}
}
skip = payloadBoundary + 1
}
}
c.upstreamReader = io.MultiReader(bytes.NewBuffer(buff[skip:]), 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(),
}
}