Synnefo » snf-vncauthproxy

#!/usr/bin/env python

"""

vncauthproxy - a VNC authentication proxy

"""

#

# Copyright (c) 2010-2013 Greek Research and Technology Network S.A.

#

# This program is free software; you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation; either version 2 of the License, or

# (at your option) any later version.

#

# This program is distributed in the hope that it will be useful, but

# WITHOUT ANY WARRANTY; without even the implied warranty of

# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

# General Public License for more details.

#

# You should have received a copy of the GNU General Public License

# along with this program; if not, write to the Free Software

# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

# 02110-1301, USA.

# Daemon files

DEFAULT_LOG_FILE = "/var/log/vncauthproxy/vncauthproxy.log"

DEFAULT_PID_FILE = "/var/run/vncauthproxy/vncauthproxy.pid"

# By default, bind / listen for control connections to TCP(v4) 127.0.0.1:24999

DEFAULT_LISTEN_ADDRESS = "127.0.0.1"

DEFAULT_LISTEN_PORT = 24999

# Backlog for the control socket

DEFAULT_BACKLOG = 256

# Timeout for the VNC server connection establishment / RFB handshake

DEFAULT_SERVER_TIMEOUT = 60.0

# Connect retries and delay between retries for the VNC server socket

DEFAULT_CONNECT_RETRIES = 3

DEFAULT_RETRY_WAIT = 0.1

# Connect timeout for the listening sockets

DEFAULT_CONNECT_TIMEOUT = 30

# Port range for the listening sockets

#

# We must take care not to fall into the ephemeral port range,

# this can lead to transient failures to bind a chosen port.

#

# By default, Linux uses 32768 to 61000, see:

# http://www.ncftp.com/ncftpd/doc/misc/ephemeral_ports.html#Linux

# so 25000-30000 seems to be a sensible default.

#

# We also take into account the ports that Ganeti daemons bind to, the port

# range used by DRBD etc.

DEFAULT_MIN_PORT = 25000

DEFAULT_MAX_PORT = 30000

# SSL certificate / key files

DEFAULT_CERT_FILE = "/var/lib/vncauthproxy/cert.pem"

DEFAULT_KEY_FILE = "/var/lib/vncauthproxy/key.pem"

# Auth file

DEFAULT_AUTH_FILE = "/var/lib/vncauthproxy/users"

import os

import sys

import logging

import gevent

import gevent.event

import daemon

import random

import daemon.runner

import hashlib

import re

import rfb

try:

    import simplejson as json

except ImportError:

    import json

from gevent import socket, ssl

from signal import SIGINT, SIGTERM

from gevent.select import select

from lockfile import LockTimeout, AlreadyLocked

# Take care of differences between python-daemon versions.

try:

    from daemon import pidfile as pidlockfile

except ImportError:

    from daemon import pidlockfile

logger = None

class InternalError(Exception):

    """Exception for internal vncauthproxy errors"""

    pass

# Currently, gevent uses libevent-dns for asynchronous DNS resolution,

# which opens a socket upon initialization time. Since we can't get the fd

# reliably, We have to maintain all file descriptors open (which won't harm

# anyway)

class AllFilesDaemonContext(daemon.DaemonContext):

    """DaemonContext class keeping all file descriptors open"""

    def _get_exclude_file_descriptors(self):

        class All:

            def __contains__(self, value):

                return True

        return All()

class VncAuthProxy(gevent.Greenlet):

    """

    Simple class implementing a VNC Forwarder with MITM authentication as a

    Greenlet

    VncAuthProxy forwards VNC traffic from a specified port of the local host

    to a specified remote host:port. Furthermore, it implements VNC

    Authentication, intercepting the client/server handshake and asking the

    client for authentication even if the backend requires none.

    It is primarily intended for use in virtualization environments, as a VNC

    ``switch''.

    """

    id = 1

    def __init__(self, logger, client):

        """

        @type logger: logging.Logger

        @param logger: the logger to use

        @type client: socket.socket

        @param listeners: the client control connection socket

        """

        gevent.Greenlet.__init__(self)

        self.id = VncAuthProxy.id

        VncAuthProxy.id += 1

        self.log = logger

        self.client = client

        # A list of worker/forwarder greenlets, one for each direction

        self.workers = []

        self.sport = None

        self.pool = None

        self.daddr = None

        self.dport = None

        self.server = None

        self.password = None

    def _cleanup(self):

        """Cleanup everything: workers, sockets, ports

        Kill all remaining forwarder greenlets, close all active sockets,

        return the source port to the pool if applicable, then exit

        gracefully.

        """

        # Make sure all greenlets are dead, then clean them up

        self.debug("Cleaning up %d workers", len(self.workers))

        for g in self.workers:

            g.kill()

        gevent.joinall(self.workers)

        del self.workers

        self.debug("Cleaning up sockets")

        while self.listeners:

            self.listeners.pop().close()

        if self.server:

            self.server.close()

        if self.client:

            self.client.close()

        # Reintroduce the port number of the client socket in

        # the port pool, if applicable.

        if not self.pool is None:

            self.pool.append(self.sport)

            self.debug("Returned port %d to port pool, contains %d ports",

                       self.sport, len(self.pool))

        self.info("Cleaned up connection, all done")

        raise gevent.GreenletExit

    def __str__(self):

        return "VncAuthProxy: %d -> %s:%d" % (self.sport, self.daddr,

                                              self.dport)

    def _forward(self, source, dest):

        """

        Forward traffic from source to dest

        @type source: socket

        @param source: source socket

        @type dest: socket

        @param dest: destination socket

        """

        while True:

            d = source.recv(16384)

            if d == '':

                if source == self.client:

                    self.info("Client connection closed")

                else:

                    self.info("Server connection closed")

                break

            dest.sendall(d)

        # No need to close the source and dest sockets here.

        # They are owned by and will be closed by the original greenlet.

    def _perform_server_handshake(self):

        """

        Initiate a connection with the backend server and perform basic

        RFB 3.8 handshake with it.

        Return a socket connected to the backend server.

        """

        server = None

        tries = VncAuthProxy.connect_retries

        while tries:

            tries -= 1

            # Initiate server connection

            for res in socket.getaddrinfo(self.daddr, self.dport,

                                          socket.AF_UNSPEC,

                                          socket.SOCK_STREAM, 0,

                                          socket.AI_PASSIVE):

                af, socktype, proto, canonname, sa = res

                try:

                    server = socket.socket(af, socktype, proto)

                except socket.error:

                    server = None

                    continue

                # Set socket timeout for the initial handshake

                server.settimeout(VncAuthProxy.server_timeout)

                try:

                    self.debug("Connecting to %s:%s", *sa[:2])

                    server.connect(sa)

                    self.debug("Connection to %s:%s successful", *sa[:2])

                except socket.error:

                    self.debug("Failed to perform sever hanshake, retrying...")

                    server.close()

                    server = None

                    continue

                # We succesfully connected to the server

                tries = 0

                break

            # Wait and retry

            gevent.sleep(VncAuthProxy.retry_wait)

        if server is None:

            raise InternalError("Failed to connect to server")

        version = server.recv(1024)

        if not rfb.check_version(version):

            raise InternalError("Unsupported RFB version: %s"

                                % version.strip())

        server.send(rfb.RFB_VERSION_3_8 + "\n")

        res = server.recv(1024)

        types = rfb.parse_auth_request(res)

        if not types:

            raise InternalError("Error handshaking with the server")

        else:

            self.debug("Supported authentication types: %s",

                       " ".join([str(x) for x in types]))

        if rfb.RFB_AUTHTYPE_NONE not in types:

            raise InternalError("Error, server demands authentication")

        server.send(rfb.to_u8(rfb.RFB_AUTHTYPE_NONE))

        # Check authentication response

        res = server.recv(4)

        res = rfb.from_u32(res)

        if res != 0:

            raise InternalError("Authentication error")

        # Reset the timeout for the rest of the session

        server.settimeout(None)

        self.server = server

    def _establish_connection(self):

        client = self.client

        ports = VncAuthProxy.ports

        # Receive and parse a client request.

        response = {

            "source_port": 0,

            "status": "FAILED",

        }

        try:

            # Control request, in JSON:

            #

            # {

            #     "source_port":

            #         <source port or 0 for automatic allocation>,

            #     "destination_address":

            #         <destination address of backend server>,

            #     "destination_port":

            #         <destination port>

            #     "password":

            #         <the password to use to authenticate clients>

            #     "auth_user":

            #         <user for control connection authentication>,

            #      "auth_password":

            #         <password for control connection authentication>,

            # }

            #

            # The <password> is used for MITM authentication of clients

            # connecting to <source_port>, who will subsequently be

            # forwarded to a VNC server listening at

            # <destination_address>:<destination_port>

            #

            # Control reply, in JSON:

            # {

            #     "source_port": <the allocated source port>

            #     "status": <one of "OK" or "FAILED">

            # }

            #

            buf = client.recv(1024)

            req = json.loads(buf)

            auth_user = req['auth_user']

            auth_password = req['auth_password']

            sport_orig = int(req['source_port'])

            self.daddr = req['destination_address']

            self.dport = int(req['destination_port'])

            self.password = req['password']

            if auth_user not in VncAuthProxy.authdb:

                msg = "Authentication failure: user not found"

                raise InternalError(msg)

            (cipher, authdb_password) = VncAuthProxy.authdb[auth_user]

            if cipher == 'HA1':

                message = auth_user + ':vncauthproxy:' + auth_password

                auth_password = hashlib.md5(message).hexdigest()

            if auth_password != authdb_password:

                msg = "Authentication failure: wrong password"

                raise InternalError(msg)

        except KeyError:

            msg = "Malformed request: %s" % buf

            raise InternalError(msg)

        except InternalError as err:

            self.warn(err)

            response['reason'] = str(err)

            client.send(json.dumps(response))

            client.close()

            raise gevent.GreenletExit

        except Exception as err:

            self.exception(err)

            self.error("Unexpected error")

            client.send(json.dumps(response))

            client.close()

            raise gevent.GreenletExit

        server = None

        pool = None

        sport = sport_orig

        try:

            # If the client has so indicated, pick an ephemeral source port

            # randomly, and remove it from the port pool.

            if sport_orig == 0:

                while True:

                    try:

                        sport = random.choice(ports)

                        ports.remove(sport)

                        break

                    except ValueError:

                        self.debug("Port %d already taken", sport)

                self.debug("Got port %d from pool, %d remaining",

                           sport, len(ports))

                pool = ports

            self.sport = sport

            self.pool = pool

            self.listeners = get_listening_sockets(sport)

            self._perform_server_handshake()

            self.info("New forwarding: %d (client req'd: %d) -> %s:%d",

                      sport, sport_orig, self.daddr, self.dport)

            response = {"source_port": sport,

                        "status": "OK"}

        except IndexError:

            self.error("FAILED forwarding, out of ports for [req'd by "

                       "client: %d -> %s:%d]",

                       sport_orig, self.daddr, self.dport)

            raise gevent.GreenletExit

        except InternalError as err:

            self.error(err)

            self.error(("FAILED forwarding: %d (client req'd: %d) -> "

                        "%s:%d"), sport, sport_orig, self.daddr, self.dport)

            if pool:

                pool.append(sport)

                self.debug("Returned port %d to pool, %d remanining",

                           sport, len(pool))

            if server:

                server.close()

            raise gevent.GreenletExit

        except Exception as err:

            self.exception(err)

            self.error("Unexpected error")

            self.error(("FAILED forwarding: %d (client req'd: %d) -> "

                        "%s:%d"), sport, sport_orig, self.daddr, self.dport)

            if pool:

                pool.append(sport)

                self.debug("Returned port %d to pool, %d remanining",

                           sport, len(pool))

            if server:

                server.close()

            raise gevent.GreenletExit

        finally:

            client.send(json.dumps(response))

            client.close()

    def _client_handshake(self):

        """

        Perform handshake/authentication with a connecting client

        Outline:

        1. Client connects

        2. We fake RFB 3.8 protocol and require VNC authentication

           [processing also supports RFB 3.3]

        3. Client accepts authentication method

        4. We send an authentication challenge

        5. Client sends the authentication response

        6. We check the authentication

        Upon return, self.client socket is connected to the client.

        """

        self.client.send(rfb.RFB_VERSION_3_8 + "\n")

        client_version_str = self.client.recv(1024)

        client_version = rfb.check_version(client_version_str)

        if not client_version:

            self.error("Invalid version: %s", client_version_str)

            raise gevent.GreenletExit

        # Both for RFB 3.3 and 3.8

        self.debug("Requesting authentication")

        auth_request = rfb.make_auth_request(rfb.RFB_AUTHTYPE_VNC,

                                             version=client_version)

        self.client.send(auth_request)

        # The client gets to propose an authtype only for RFB 3.8

        if client_version == rfb.RFB_VERSION_3_8:

            res = self.client.recv(1024)

            type = rfb.parse_client_authtype(res)

            if type == rfb.RFB_AUTHTYPE_ERROR:

                self.warn("Client refused authentication: %s", res[1:])

            else:

                self.debug("Client requested authtype %x", type)

            if type != rfb.RFB_AUTHTYPE_VNC:

                self.error("Wrong auth type: %d", type)

                self.client.send(rfb.to_u32(rfb.RFB_AUTH_ERROR))

                raise gevent.GreenletExit

        # Generate the challenge

        challenge = os.urandom(16)

        self.client.send(challenge)

        response = self.client.recv(1024)

        if len(response) != 16:

            self.error("Wrong response length %d, should be 16", len(response))

            raise gevent.GreenletExit

        if rfb.check_password(challenge, response, self.password):

            self.debug("Authentication successful")

        else:

            self.warn("Authentication failed")

            self.client.send(rfb.to_u32(rfb.RFB_AUTH_ERROR))

            raise gevent.GreenletExit

        # Accept the authentication

        self.client.send(rfb.to_u32(rfb.RFB_AUTH_SUCCESS))

    def _proxy(self):

        try:

            self.info("Waiting for a client to connect at %s",

                      ", ".join(["%s:%d" % s.getsockname()[:2]

                                 for s in self.listeners]))

            rlist, _, _ = select(self.listeners, [], [],

                                 timeout=VncAuthProxy.connect_timeout)

            if not rlist:

                self.info("Timed out, no connection after %d sec",

                          VncAuthProxy.connect_timeout)

                raise gevent.GreenletExit

            for sock in rlist:

                self.client, addrinfo = sock.accept()

                self.info("Connection from %s:%d", *addrinfo[:2])

                # Close all listening sockets, we only want a one-shot

                # connection from a single client.

                while self.listeners:

                    sock = self.listeners.pop().close()

                break

            # Perform RFB handshake with the client.

            self._client_handshake()

            # Bridge both connections through two "forwarder" greenlets.

            # This greenlet will wait until any of the workers dies.

            # Final cleanup will take place in _cleanup().

            dead = gevent.event.Event()

            dead.clear()

            # This callback will get called if any of the two workers dies.

            def callback(g):

                self.debug("Worker %d/%d died", self.workers.index(g),

                           len(self.workers))

                dead.set()

            self.workers.append(gevent.spawn(self._forward,

                                             self.client, self.server))

            self.workers.append(gevent.spawn(self._forward,

                                             self.server, self.client))

            for g in self.workers:

                g.link(callback)

            # Wait until any of the workers dies

            self.debug("Waiting for any of %d workers to die",

                       len(self.workers))

            dead.wait()

            # We can go now, _cleanup() will take care of

            # all worker, socket and port cleanup

            self.debug("A forwarder died, our work here is done")

            raise gevent.GreenletExit

        except Exception as err:

            # Any unhandled exception in the previous block

            # is an error and must be logged accordingly

            if not isinstance(err, gevent.GreenletExit):

                self.exception(err)

                self.error("Unexpected error")

            raise err

        finally:

            self._cleanup()

    def _run(self):

        self._establish_connection()

        self._proxy()

# Logging support inside VncAuthproxy

# Wrap all common logging functions in logging-specific methods

for funcname in ["info", "debug", "warn", "error", "critical",

                 "exception"]:

    def gen(funcname):

        def wrapped_log_func(self, *args, **kwargs):

            func = getattr(self.log, funcname)

            func("[C%d] %s" % (self.id, args[0]), *args[1:], **kwargs)

        return wrapped_log_func

    setattr(VncAuthProxy, funcname, gen(funcname))

def fatal_signal_handler(signame):

    logger.info("Caught %s, will raise SystemExit", signame)

    raise SystemExit

def get_listening_sockets(sport, saddr=None, reuse_addr=False):

    sockets = []

    # Use two sockets, one for IPv4, one for IPv6. IPv4-to-IPv6 mapped

    # addresses do not work reliably everywhere (under linux it may have

    # been disabled in /proc/sys/net/ipv6/bind_ipv6_only).

    for res in socket.getaddrinfo(saddr, sport, socket.AF_UNSPEC,

                                  socket.SOCK_STREAM, 0, socket.AI_PASSIVE):

        af, socktype, proto, canonname, sa = res

        try:

            s = None

            s = socket.socket(af, socktype, proto)

            if af == socket.AF_INET6:

                # Bind v6 only when AF_INET6, otherwise either v4 or v6 bind

                # will fail.

                s.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, 1)

            if reuse_addr:

                s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

            s.bind(sa)

            s.listen(1)

            sockets.append(s)

            logger.debug("Listening on %s:%d", *sa[:2])

        except socket.error as err:

            logger.error("Error binding to %s:%d: %s", sa[0], sa[1], err[1])

            if s:

                s.close()

            while sockets:

                sockets.pop().close()

            # Make sure we fail immediately if we cannot get a socket

            raise InternalError(err)

    return sockets

def parse_auth_file(auth_file):

    supported_ciphers = ('cleartext', 'HA1', None)

    regexp = re.compile(r'^\s*(?P<user>\S+)\s+({(?P<cipher>\S+)})?'

                        '(?P<pass>\S+)\s*$')

    users = {}

    try:

        with open(auth_file) as f:

            lines = [l.strip() for l in f.readlines()]

            for line in lines:

                if not line or line.startswith('#'):

                    continue

                m = regexp.match(line)

                if not m:

                    raise InternalError("Invaild entry in auth file: %s"

                                        % line)

                user = m.group('user')

                cipher = m.group('cipher')

                if cipher not in supported_ciphers:

                    raise InternalError("Unsupported cipher in auth file: "

                                        "%s" % line)

                password = (cipher, m.group('pass'))

                if user in users:

                    raise InternalError("Duplicate user entry in auth file")

                users[user] = password

    except IOError as err:

        logger.critical("Couldn't read auth file")

        raise InternalError(err)

    if not users:

        raise InternalError("No users defined")

    return users

def parse_arguments(args):

    from optparse import OptionParser

    parser = OptionParser()

    parser.add_option("-d", "--debug", action="store_true", dest="debug",

                      help="Enable debugging information")

    parser.add_option("--log", dest="log_file",

                      default=DEFAULT_LOG_FILE,

                      metavar="FILE",

                      help=("Write log to FILE (default: %s)" %

                            DEFAULT_LOG_FILE))

    parser.add_option('--pid-file', dest="pid_file",

                      default=DEFAULT_PID_FILE,

                      metavar='PIDFILE',

                      help=("Save PID to file (default: %s)" %

                            DEFAULT_PID_FILE))

    parser.add_option("--listen-address", dest="listen_address",

                      default=DEFAULT_LISTEN_ADDRESS,

                      metavar="LISTEN_ADDRESS",

                      help=("Address to listen for control connections"

                            "(default: 127.0.0.1)"))

    parser.add_option("--listen-port", dest="listen_port",

                      default=DEFAULT_LISTEN_PORT,

                      metavar="LISTEN_PORT",

                      help=("Port to listen for control connections"

                            "(default: %d)" % DEFAULT_LISTEN_PORT))

    parser.add_option("--server-timeout", dest="server_timeout",

                      default=DEFAULT_SERVER_TIMEOUT, type="float",

                      metavar="N",

                      help=("Wait for N seconds for the VNC server RFB "

                            "handshake (default %s)" % DEFAULT_SERVER_TIMEOUT))

    parser.add_option("--connect-retries", dest="connect_retries",

                      default=DEFAULT_CONNECT_RETRIES, type="int",

                      metavar="N",

                      help=("Retry N times to connect to the "

                            "server (default: %d)" %

                            DEFAULT_CONNECT_RETRIES))

    parser.add_option("--retry-wait", dest="retry_wait",

                      default=DEFAULT_RETRY_WAIT, type="float",

                      metavar="N",

                      help=("Wait N seconds before retrying "

                            "to connect to the server (default: %s)" %

                            DEFAULT_RETRY_WAIT))

    parser.add_option("--connect-timeout", dest="connect_timeout",

                      default=DEFAULT_CONNECT_TIMEOUT, type="int",

                      metavar="N",

                      help=("Wait N seconds for a client "

                            "to connect (default: %d)"

                            % DEFAULT_CONNECT_TIMEOUT))

    parser.add_option("-p", "--min-port", dest="min_port",

                      default=DEFAULT_MIN_PORT, type="int", metavar="MIN_PORT",

                      help=("The minimum port number to use for automatically-"

                            "allocated ephemeral ports (default: %s)" %

                            DEFAULT_MIN_PORT))

    parser.add_option("-P", "--max-port", dest="max_port",

                      default=DEFAULT_MAX_PORT, type="int", metavar="MAX_PORT",

                      help=("The maximum port number to use for automatically-"

                            "allocated ephemeral ports (default: %s)" %

                            DEFAULT_MAX_PORT))

    parser.add_option('--enable-ssl', dest="enable_ssl",

                      default=False, action='store_true',

                      help=("Enable SSL/TLS for control connections "

                            "(default: False"))

    parser.add_option('--cert-file', dest="cert_file",

                      default=DEFAULT_CERT_FILE,

                      metavar='CERTFILE',

                      help=("SSL certificate (default: %s)" %

                            DEFAULT_CERT_FILE))

    parser.add_option('--key-file', dest="key_file",

                      default=DEFAULT_KEY_FILE,

                      metavar='KEYFILE',

                      help=("SSL key (default: %s)" %

                            DEFAULT_KEY_FILE))

    parser.add_option('--auth-file', dest="auth_file",

                      default=DEFAULT_AUTH_FILE,

                      metavar='AUTHFILE',

                      help=("Authentication file (default: %s)" %

                            DEFAULT_AUTH_FILE))

    (opts, args) = parser.parse_args(args)

    if args:

        parser.print_help()

        sys.exit(1)

    return opts

def main():

    """Run the daemon from the command line"""

    opts = parse_arguments(sys.argv[1:])

    # Initialize logger

    lvl = logging.DEBUG if opts.debug else logging.INFO

    global logger

    logger = logging.getLogger("vncauthproxy")

    logger.setLevel(lvl)

    formatter = logging.Formatter(("%(asctime)s %(module)s[%(process)d] "

                                   " %(levelname)s: %(message)s"),

                                  "%Y-%m-%d %H:%M:%S")

    handler = logging.FileHandler(opts.log_file)

    handler.setFormatter(formatter)

    logger.addHandler(handler)

    try:

        # Create pidfile

        pidf = pidlockfile.TimeoutPIDLockFile(opts.pid_file, 10)

        # Init ephemeral port pool

        ports = range(opts.min_port, opts.max_port + 1)

        # Init VncAuthProxy class attributes

        VncAuthProxy.server_timeout = opts.server_timeout

        VncAuthProxy.connect_retries = opts.connect_retries

        VncAuthProxy.retry_wait = opts.retry_wait

        VncAuthProxy.connect_timeout = opts.connect_timeout

        VncAuthProxy.ports = ports

        VncAuthProxy.authdb = parse_auth_file(opts.auth_file)

        sockets = get_listening_sockets(opts.listen_port, opts.listen_address,

                                        reuse_addr=True)

        wrap_ssl = lambda sock: sock

        if opts.enable_ssl:

            ssl_prot = ssl.PROTOCOL_TLSv1

            wrap_ssl = lambda sock: ssl.wrap_socket(sock, server_side=True,

                                                    keyfile=opts.key_file,

                                                    certfile=opts.cert_file,

                                                    ssl_version=ssl_prot)

        # Become a daemon:

        # Redirect stdout and stderr to handler.stream to catch

        # early errors in the daemonization process [e.g., pidfile creation]

        # which will otherwise go to /dev/null.

        daemon_context = AllFilesDaemonContext(

            pidfile=pidf,

            umask=0022,

            stdout=handler.stream,

            stderr=handler.stream,

            files_preserve=[handler.stream])

        # Remove any stale PID files, left behind by previous invocations

        if daemon.runner.is_pidfile_stale(pidf):

            logger.warning("Removing stale PID lock file %s", pidf.path)

            pidf.break_lock()

        try:

            daemon_context.open()

        except (AlreadyLocked, LockTimeout):

            raise InternalError(("Failed to lock PID file %s, another "

                                 "instance running?"), pidf.path)

        logger.info("Became a daemon")

        # A fork() has occured while daemonizing,

        # we *must* reinit gevent

        gevent.reinit()

        # Catch signals to ensure graceful shutdown,

        #

        # Uses gevent.signal so the handler fires even during

        # gevent.socket.accept()

        gevent.signal(SIGINT, fatal_signal_handler, "SIGINT")

        gevent.signal(SIGTERM, fatal_signal_handler, "SIGTERM")

    except InternalError as err:

        logger.critical(err)

        sys.exit(1)

    except Exception as err:

        logger.critical("Unexpected error:")

        logger.exception(err)

        sys.exit(1)

    while True:

        try:

            client = None

            rlist, _, _ = select(sockets, [], [])

            for ctrl in rlist:

                client, _ = ctrl.accept()

                client = wrap_ssl(client)

                logger.info("New control connection")

                VncAuthProxy.spawn(logger, client)

            continue

        except Exception as err:

            logger.error("Unexpected error:")

            logger.exception(err)

            if client:

                client.close()

            continue

        except SystemExit:

            break

    try:

        logger.info("Closing control sockets")

        while sockets:

            sock = sockets.pop()

            sock.close()

        daemon_context.close()

        sys.exit(0)

    except Exception as err:

        logger.critical("Unexpected error:")

        logger.exception(err)

        sys.exit(1)