cloudflared-mirror/sshserver/sshserver_unix.go

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//+build !windows
package sshserver
import (
"crypto/ecdsa"
"crypto/elliptic"
"crypto/rand"
"encoding/binary"
"encoding/json"
"fmt"
"io"
"net"
"runtime"
"strings"
"time"
"github.com/cloudflare/cloudflared/sshgen"
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"github.com/cloudflare/cloudflared/sshlog"
"github.com/gliderlabs/ssh"
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"github.com/google/uuid"
"github.com/pkg/errors"
"github.com/sirupsen/logrus"
gossh "golang.org/x/crypto/ssh"
)
const (
auditEventStart = "session_start"
auditEventStop = "session_stop"
auditEventExec = "exec"
auditEventScp = "scp"
auditEventResize = "resize"
auditEventShell = "shell"
sshContextSessionID = "sessionID"
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sshContextEventLogger = "eventLogger"
sshContextPreamble = "sshPreamble"
sshContextSSHClient = "sshClient"
SSHPreambleLength = 2
defaultSSHPort = "22"
)
type auditEvent struct {
Event string `json:"event,omitempty"`
EventType string `json:"event_type,omitempty"`
SessionID string `json:"session_id,omitempty"`
User string `json:"user,omitempty"`
Login string `json:"login,omitempty"`
Datetime string `json:"datetime,omitempty"`
Hostname string `json:"hostname,omitempty"`
Destination string `json:"destination,omitempty"`
}
// sshConn wraps the incoming net.Conn and a cleanup function
// This is done to allow the outgoing SSH client to be retrieved and closed when the conn itself is closed.
type sshConn struct {
net.Conn
cleanupFunc func()
}
// close calls the cleanupFunc before closing the conn
func (c sshConn) Close() error {
c.cleanupFunc()
return c.Conn.Close()
}
type SSHProxy struct {
ssh.Server
hostname string
logger *logrus.Logger
shutdownC chan struct{}
caCert ssh.PublicKey
logManager sshlog.Manager
}
type SSHPreamble struct {
Destination string
JWT string
}
// New creates a new SSHProxy and configures its host keys and authentication by the data provided
func New(logManager sshlog.Manager, logger *logrus.Logger, version, localAddress, hostname string, shutdownC chan struct{}, idleTimeout, maxTimeout time.Duration) (*SSHProxy, error) {
sshProxy := SSHProxy{
hostname: hostname,
logger: logger,
shutdownC: shutdownC,
logManager: logManager,
}
sshProxy.Server = ssh.Server{
Addr: localAddress,
MaxTimeout: maxTimeout,
IdleTimeout: idleTimeout,
Version: fmt.Sprintf("SSH-2.0-Cloudflare-Access_%s_%s", version, runtime.GOOS),
PublicKeyHandler: sshProxy.proxyAuthCallback,
ConnCallback: sshProxy.connCallback,
ChannelHandlers: map[string]ssh.ChannelHandler{
"default": sshProxy.channelHandler,
},
}
if err := sshProxy.configureHostKeys(); err != nil {
return nil, err
}
return &sshProxy, nil
}
// Start the SSH proxy listener to start handling SSH connections from clients
func (s *SSHProxy) Start() error {
s.logger.Infof("Starting SSH server at %s", s.Addr)
go func() {
<-s.shutdownC
if err := s.Close(); err != nil {
s.logger.WithError(err).Error("Cannot close SSH server")
}
}()
return s.ListenAndServe()
}
// proxyAuthCallback attempts to connect to ultimate SSH destination. If successful, it allows the incoming connection
// to connect to the proxy and saves the outgoing SSH client to the context. Otherwise, no connection to the
// the proxy is allowed.
func (s *SSHProxy) proxyAuthCallback(ctx ssh.Context, key ssh.PublicKey) bool {
client, err := s.dialDestination(ctx)
if err != nil {
return false
}
ctx.SetValue(sshContextSSHClient, client)
return true
}
// connCallback reads the preamble sent from the proxy server and saves an audit event logger to the context.
// If any errors occur, the connection is terminated by returning nil from the callback.
func (s *SSHProxy) connCallback(ctx ssh.Context, conn net.Conn) net.Conn {
// AUTH-2050: This is a temporary workaround of a timing issue in the tunnel muxer to allow further testing.
// TODO: Remove this
time.Sleep(10 * time.Millisecond)
preamble, err := s.readPreamble(conn)
if err != nil {
if netErr, ok := err.(net.Error); ok && netErr.Timeout() {
s.logger.Warn("Could not establish session. Client likely does not have --destination set and is using old-style ssh config")
} else if err != io.EOF {
s.logger.WithError(err).Error("failed to read SSH preamble")
}
return nil
}
ctx.SetValue(sshContextPreamble, preamble)
logger, sessionID, err := s.auditLogger()
if err != nil {
s.logger.WithError(err).Error("failed to configure logger")
return nil
}
ctx.SetValue(sshContextEventLogger, logger)
ctx.SetValue(sshContextSessionID, sessionID)
// attempts to retrieve and close the outgoing ssh client when the incoming conn is closed.
// If no client exists, the conn is being closed before the PublicKeyCallback was called (where the client is created).
cleanupFunc := func() {
client, ok := ctx.Value(sshContextSSHClient).(*gossh.Client)
if ok && client != nil {
client.Close()
}
}
return sshConn{conn, cleanupFunc}
}
// channelHandler proxies incoming and outgoing SSH traffic back and forth over an SSH Channel
func (s *SSHProxy) channelHandler(srv *ssh.Server, conn *gossh.ServerConn, newChan gossh.NewChannel, ctx ssh.Context) {
if newChan.ChannelType() != "session" && newChan.ChannelType() != "direct-tcpip" {
msg := fmt.Sprintf("channel type %s is not supported", newChan.ChannelType())
s.logger.Info(msg)
if err := newChan.Reject(gossh.UnknownChannelType, msg); err != nil {
s.logger.WithError(err).Error("Error rejecting SSH channel")
}
return
}
localChan, localChanReqs, err := newChan.Accept()
if err != nil {
s.logger.WithError(err).Error("Failed to accept session channel")
return
}
defer localChan.Close()
// client will be closed when the sshConn is closed
client, ok := ctx.Value(sshContextSSHClient).(*gossh.Client)
if !ok {
s.logger.Error("Could not retrieve client from context")
return
}
remoteChan, remoteChanReqs, err := client.OpenChannel(newChan.ChannelType(), newChan.ExtraData())
if err != nil {
s.logger.WithError(err).Error("Failed to open remote channel")
return
}
defer remoteChan.Close()
// Proxy ssh traffic back and forth between client and destination
s.proxyChannel(localChan, remoteChan, localChanReqs, remoteChanReqs, conn, ctx)
}
// proxyChannel couples two SSH channels and proxies SSH traffic and channel requests back and forth.
func (s *SSHProxy) proxyChannel(localChan, remoteChan gossh.Channel, localChanReqs, remoteChanReqs <-chan *gossh.Request, conn *gossh.ServerConn, ctx ssh.Context) {
done := make(chan struct{}, 2)
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go func() {
if _, err := io.Copy(localChan, remoteChan); err != nil {
s.logger.WithError(err).Error("remote to local copy error")
}
done <- struct{}{}
}()
go func() {
if _, err := io.Copy(remoteChan, localChan); err != nil {
s.logger.WithError(err).Error("local to remote copy error")
}
done <- struct{}{}
}()
s.logAuditEvent(conn, "", auditEventStart, ctx)
defer s.logAuditEvent(conn, "", auditEventStop, ctx)
// Proxy channel requests
for {
select {
case req := <-localChanReqs:
if req == nil {
return
}
if err := s.forwardChannelRequest(remoteChan, req); err != nil {
s.logger.WithError(err).Error("Failed to forward request")
return
}
s.logChannelRequest(req, conn, ctx)
case req := <-remoteChanReqs:
if req == nil {
return
}
if err := s.forwardChannelRequest(localChan, req); err != nil {
s.logger.WithError(err).Error("Failed to forward request")
return
}
case <-done:
return
}
}
}
// readPreamble reads a preamble from the SSH connection before any SSH traffic is sent.
// This preamble is a JSON encoded struct containing the users JWT and ultimate destination.
// The first 4 bytes contain the length of the preamble which follows immediately.
func (s *SSHProxy) readPreamble(conn net.Conn) (*SSHPreamble, error) {
// Set conn read deadline while reading preamble to prevent hangs if preamble wasnt sent.
if err := conn.SetReadDeadline(time.Now().Add(500 * time.Millisecond)); err != nil {
return nil, errors.Wrap(err, "failed to set conn deadline")
}
defer func() {
if err := conn.SetReadDeadline(time.Time{}); err != nil {
s.logger.WithError(err).Error("Failed to unset conn read deadline")
}
}()
size := make([]byte, SSHPreambleLength)
if _, err := io.ReadFull(conn, size); err != nil {
return nil, err
}
payloadLength := binary.BigEndian.Uint16(size)
payload := make([]byte, payloadLength)
if _, err := io.ReadFull(conn, payload); err != nil {
return nil, err
}
var preamble SSHPreamble
err := json.Unmarshal(payload, &preamble)
if err != nil {
return nil, err
}
preamble.Destination, err = canonicalizeDest(preamble.Destination)
if err != nil {
return nil, err
}
return &preamble, nil
}
// canonicalizeDest adds a default port if one doesnt exist
func canonicalizeDest(dest string) (string, error) {
_, _, err := net.SplitHostPort(dest)
// if host and port are split without error, a port exists.
if err != nil {
addrErr, ok := err.(*net.AddrError)
if !ok {
return "", err
}
// If the port is missing, append it.
if addrErr.Err == "missing port in address" {
return fmt.Sprintf("%s:%s", dest, defaultSSHPort), nil
}
// If there are too many colons and address is IPv6, wrap in brackets and append port. Otherwise invalid address
ip := net.ParseIP(dest)
if addrErr.Err == "too many colons in address" && ip != nil && ip.To4() == nil {
return fmt.Sprintf("[%s]:%s", dest, defaultSSHPort), nil
}
return "", addrErr
}
return dest, nil
}
// dialDestination creates a new SSH client and dials the destination server
func (s *SSHProxy) dialDestination(ctx ssh.Context) (*gossh.Client, error) {
preamble, ok := ctx.Value(sshContextPreamble).(*SSHPreamble)
if !ok {
msg := "failed to retrieve SSH preamble from context"
s.logger.Error(msg)
return nil, errors.New(msg)
}
signer, err := s.genSSHSigner(preamble.JWT)
if err != nil {
s.logger.WithError(err).Error("Failed to generate signed short lived cert")
return nil, err
}
clientConfig := &gossh.ClientConfig{
User: ctx.User(),
// AUTH-2103 TODO: proper host key check
HostKeyCallback: gossh.InsecureIgnoreHostKey(),
Auth: []gossh.AuthMethod{gossh.PublicKeys(signer)},
ClientVersion: ctx.ServerVersion(),
}
client, err := gossh.Dial("tcp", preamble.Destination, clientConfig)
if err != nil {
s.logger.WithError(err).Info("Failed to connect to destination SSH server")
return nil, err
}
return client, nil
}
// Generates a key pair and sends public key to get signed by CA
func (s *SSHProxy) genSSHSigner(jwt string) (gossh.Signer, error) {
key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
if err != nil {
return nil, errors.Wrap(err, "failed to generate ecdsa key pair")
}
pub, err := gossh.NewPublicKey(&key.PublicKey)
if err != nil {
return nil, errors.Wrap(err, "failed to convert ecdsa public key to SSH public key")
}
pubBytes := gossh.MarshalAuthorizedKey(pub)
signedCertBytes, err := sshgen.SignCert(jwt, string(pubBytes))
if err != nil {
return nil, errors.Wrap(err, "failed to retrieve cert from SSHCAAPI")
}
signedPub, _, _, _, err := gossh.ParseAuthorizedKey([]byte(signedCertBytes))
if err != nil {
return nil, errors.Wrap(err, "failed to parse SSH public key")
}
cert, ok := signedPub.(*gossh.Certificate)
if !ok {
return nil, errors.Wrap(err, "failed to assert public key as certificate")
}
signer, err := gossh.NewSignerFromKey(key)
if err != nil {
return nil, errors.Wrap(err, "failed to create signer")
}
certSigner, err := gossh.NewCertSigner(cert, signer)
if err != nil {
return nil, errors.Wrap(err, "failed to create cert signer")
}
return certSigner, nil
}
// forwardChannelRequest sends request req to SSH channel sshChan, waits for reply, and sends the reply back.
func (s *SSHProxy) forwardChannelRequest(sshChan gossh.Channel, req *gossh.Request) error {
reply, err := sshChan.SendRequest(req.Type, req.WantReply, req.Payload)
if err != nil {
return errors.Wrap(err, "Failed to send request")
}
if err := req.Reply(reply, nil); err != nil {
return errors.Wrap(err, "Failed to reply to request")
}
return nil
}
// logChannelRequest creates an audit log for different types of channel requests
func (s *SSHProxy) logChannelRequest(req *gossh.Request, conn *gossh.ServerConn, ctx ssh.Context) {
var eventType string
var event string
switch req.Type {
case "exec":
var payload struct{ Value string }
if err := gossh.Unmarshal(req.Payload, &payload); err != nil {
s.logger.WithError(err).Errorf("Failed to unmarshal channel request payload: %s:%s", req.Type, req.Payload)
}
event = payload.Value
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eventType = auditEventExec
if strings.HasPrefix(string(req.Payload), "scp") {
eventType = auditEventScp
}
case "shell":
eventType = auditEventShell
case "window-change":
eventType = auditEventResize
default:
return
}
s.logAuditEvent(conn, event, eventType, ctx)
}
func (s *SSHProxy) auditLogger() (io.WriteCloser, string, error) {
sessionUUID, err := uuid.NewRandom()
if err != nil {
return nil, "", errors.Wrap(err, "failed to create sessionID")
}
sessionID := sessionUUID.String()
writer, err := s.logManager.NewLogger(fmt.Sprintf("%s-event.log", sessionID), s.logger)
if err != nil {
return nil, "", errors.Wrap(err, "failed to create logger")
}
return writer, sessionID, nil
}
func (s *SSHProxy) logAuditEvent(conn *gossh.ServerConn, event, eventType string, ctx ssh.Context) {
sessionID, sessionIDOk := ctx.Value(sshContextSessionID).(string)
writer, writerOk := ctx.Value(sshContextEventLogger).(io.WriteCloser)
if !writerOk || !sessionIDOk {
s.logger.Error("Failed to retrieve audit logger from context")
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return
}
var destination string
preamble, ok := ctx.Value(sshContextPreamble).(*SSHPreamble)
if ok {
destination = preamble.Destination
} else {
s.logger.Error("Failed to retrieve SSH preamble from context")
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}
ae := auditEvent{
Event: event,
EventType: eventType,
SessionID: sessionID,
User: conn.User(),
Login: conn.User(),
Datetime: time.Now().UTC().Format(time.RFC3339),
Hostname: s.hostname,
Destination: destination,
}
data, err := json.Marshal(&ae)
if err != nil {
s.logger.WithError(err).Error("Failed to marshal audit event. malformed audit object")
return
}
line := string(data) + "\n"
if _, err := writer.Write([]byte(line)); err != nil {
s.logger.WithError(err).Error("Failed to write audit event.")
}
}