# -*- coding: utf-8 -*-
#
# Copyright (C) 2015 GNS3 Technologies Inc.
#
# 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 3 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, see .
"""
IOU VM management (creates command line, processes, files etc.) in
order to run an IOU instance.
"""
import os
import sys
import signal
import re
import asyncio
import subprocess
import shutil
import argparse
import threading
import configparser
import glob
from pkg_resources import parse_version
from .iou_error import IOUError
from ..adapters.ethernet_adapter import EthernetAdapter
from ..adapters.serial_adapter import SerialAdapter
from ..nios.nio_udp import NIO_UDP
from ..nios.nio_tap import NIO_TAP
from ..base_vm import BaseVM
from .ioucon import start_ioucon
import gns3server.utils.asyncio
import logging
log = logging.getLogger(__name__)
class IOUVM(BaseVM):
module_name = 'iou'
"""
IOU vm implementation.
:param name: name of this IOU vm
:param vm_id: IOU instance identifier
:param project: Project instance
:param manager: parent VM Manager
:param console: TCP console port
:params console_host: TCP console host IP
:params ethernet_adapters: Number of ethernet adapters
:params serial_adapters: Number of serial adapters
:params ram: Ram MB
:params nvram: Nvram KB
:params l1_keepalives: Always up ethernet interface:
:params initial_config: Content of the initial configuration file
"""
def __init__(self, name, vm_id, project, manager,
console=None,
console_host="0.0.0.0",
ram=None,
nvram=None,
ethernet_adapters=None,
serial_adapters=None,
l1_keepalives=None,
initial_config=None):
super().__init__(name, vm_id, project, manager)
self._console = console
self._command = []
self._iouyap_process = None
self._iou_process = None
self._iou_stdout_file = ""
self._started = False
self._path = None
self._iourc_path = None
self._ioucon_thread = None
self._console_host = console_host
# IOU settings
self._iourc = None
self._ethernet_adapters = []
self._serial_adapters = []
self.ethernet_adapters = 2 if ethernet_adapters is None else ethernet_adapters # one adapter = 4 interfaces
self.serial_adapters = 2 if serial_adapters is None else serial_adapters # one adapter = 4 interfaces
self._use_default_iou_values = True # for RAM & NVRAM values
self._nvram = 128 if nvram is None else nvram # Kilobytes
self._initial_config = ""
self._ram = 256 if ram is None else ram # Megabytes
self._l1_keepalives = False if l1_keepalives is None else l1_keepalives # used to overcome the always-up Ethernet interfaces (not supported by all IOSes).
if initial_config is not None:
self.initial_config = initial_config
if self._console is not None:
self._console = self._manager.port_manager.reserve_console_port(self._console)
else:
self._console = self._manager.port_manager.get_free_console_port()
@asyncio.coroutine
def close(self):
yield from self.stop()
if self._console:
self._manager.port_manager.release_console_port(self._console)
self._console = None
@property
def path(self):
"""Path of the iou binary"""
return self._path
@path.setter
def path(self, path):
"""
Path of the iou binary
:params path: Path to the binary
"""
self._path = path
if not os.path.isfile(self._path) or not os.path.exists(self._path):
if os.path.islink(self._path):
raise IOUError("IOU image '{}' linked to '{}' is not accessible".format(self._path, os.path.realpath(self._path)))
else:
raise IOUError("IOU image '{}' is not accessible".format(self._path))
try:
with open(self._path, "rb") as f:
# read the first 7 bytes of the file.
elf_header_start = f.read(7)
except OSError as e:
raise IOUError("Cannot read ELF header for IOU image '{}': {}".format(self._path, e))
# IOU images must start with the ELF magic number, be 32-bit, little endian
# and have an ELF version of 1 normal IOS image are big endian!
if elf_header_start != b'\x7fELF\x01\x01\x01':
raise IOUError("'{}' is not a valid IOU image".format(self._path))
if not os.access(self._path, os.X_OK):
raise IOUError("IOU image '{}' is not executable".format(self._path))
@property
def iourc_path(self):
"""
Returns the path to the iourc file.
:returns: path to the iourc file
"""
return self._iourc_path
@iourc_path.setter
def iourc_path(self, path):
"""
Set path to IOURC file
"""
self._iourc_path = path
log.info("IOU {name} [id={id}]: iourc file path set to {path}".format(name=self._name,
id=self._id,
path=self._iourc_path))
@property
def use_default_iou_values(self):
"""
Returns if this device uses the default IOU image values.
:returns: boolean
"""
return self._use_default_iou_values
@use_default_iou_values.setter
def use_default_iou_values(self, state):
"""
Sets if this device uses the default IOU image values.
:param state: boolean
"""
self._use_default_iou_values = state
if state:
log.info("IOU {name} [id={id}]: uses the default IOU image values".format(name=self._name, id=self._id))
else:
log.info("IOU {name} [id={id}]: does not use the default IOU image values".format(name=self._name, id=self._id))
def _check_requirements(self):
"""
Check if IOUYAP is available
"""
path = self.iouyap_path
if not path:
raise IOUError("No path to a IOU executable has been set")
if not os.path.isfile(path):
raise IOUError("IOU program '{}' is not accessible".format(path))
if not os.access(path, os.X_OK):
raise IOUError("IOU program '{}' is not executable".format(path))
def __json__(self):
return {"name": self.name,
"vm_id": self.id,
"console": self._console,
"project_id": self.project.id,
"path": self.path,
"ethernet_adapters": len(self._ethernet_adapters),
"serial_adapters": len(self._serial_adapters),
"ram": self._ram,
"nvram": self._nvram,
"l1_keepalives": self._l1_keepalives,
}
@property
def iouyap_path(self):
"""
Returns the IOUYAP executable path.
:returns: path to IOUYAP
"""
path = self._manager.config.get_section_config("IOU").get("iouyap_path", "iouyap")
if path == "iouyap":
path = shutil.which("iouyap")
return path
@property
def console(self):
"""
Returns the console port of this IOU vm.
:returns: console port
"""
return self._console
@console.setter
def console(self, console):
"""
Change console port
:params console: Console port (integer)
"""
if console == self._console:
return
if self._console:
self._manager.port_manager.release_console_port(self._console)
self._console = self._manager.port_manager.reserve_console_port(console)
@property
def ram(self):
"""
Returns the amount of RAM allocated to this IOU instance.
:returns: amount of RAM in Mbytes (integer)
"""
return self._ram
@ram.setter
def ram(self, ram):
"""
Sets amount of RAM allocated to this IOU instance.
:param ram: amount of RAM in Mbytes (integer)
"""
if self._ram == ram:
return
log.info("IOU {name} [id={id}]: RAM updated from {old_ram}MB to {new_ram}MB".format(name=self._name,
id=self._id,
old_ram=self._ram,
new_ram=ram))
self._ram = ram
@property
def nvram(self):
"""
Returns the mount of NVRAM allocated to this IOU instance.
:returns: amount of NVRAM in Kbytes (integer)
"""
return self._nvram
@nvram.setter
def nvram(self, nvram):
"""
Sets amount of NVRAM allocated to this IOU instance.
:param nvram: amount of NVRAM in Kbytes (integer)
"""
if self._nvram == nvram:
return
log.info("IOU {name} [id={id}]: NVRAM updated from {old_nvram}KB to {new_nvram}KB".format(name=self._name,
id=self._id,
old_nvram=self._nvram,
new_nvram=nvram))
self._nvram = nvram
@BaseVM.name.setter
def name(self, new_name):
"""
Sets the name of this IOU vm.
:param new_name: name
"""
if self.initial_config_file:
content = self.initial_config
content = content.replace(self._name, new_name)
self.initial_config = content
super(IOUVM, IOUVM).name.__set__(self, new_name)
@property
def application_id(self):
return self._manager.get_application_id(self.id)
@asyncio.coroutine
def _library_check(self):
"""
Checks for missing shared library dependencies in the IOU image.
"""
try:
output = yield from gns3server.utils.asyncio.subprocess_check_output("ldd", self._path)
except (FileNotFoundError, subprocess.SubprocessError) as e:
log.warn("Could not determine the shared library dependencies for {}: {}".format(self._path, e))
return
print(output)
p = re.compile("([\.\w]+)\s=>\s+not found")
missing_libs = p.findall(output)
if missing_libs:
raise IOUError("The following shared library dependencies cannot be found for IOU image {}: {}".format(self._path,
", ".join(missing_libs)))
@asyncio.coroutine
def start(self):
"""
Starts the IOU process.
"""
self._check_requirements()
if not self.is_running():
yield from self._library_check()
self._rename_nvram_file()
if self._iourc_path and not os.path.isfile(self._iourc_path):
raise IOUError("A valid iourc file is necessary to start IOU")
iouyap_path = self.iouyap_path
if not iouyap_path or not os.path.isfile(iouyap_path):
raise IOUError("iouyap is necessary to start IOU")
self._create_netmap_config()
# created a environment variable pointing to the iourc file.
env = os.environ.copy()
if self._iourc_path:
env["IOURC"] = self._iourc_path
self._command = yield from self._build_command()
try:
log.info("Starting IOU: {}".format(self._command))
self._iou_stdout_file = os.path.join(self.working_dir, "iou.log")
log.info("Logging to {}".format(self._iou_stdout_file))
with open(self._iou_stdout_file, "w") as fd:
self._iou_process = yield from asyncio.create_subprocess_exec(*self._command,
stdout=fd,
stderr=subprocess.STDOUT,
cwd=self.working_dir,
env=env)
log.info("IOU instance {} started PID={}".format(self._id, self._iou_process.pid))
self._started = True
except FileNotFoundError as e:
raise IOUError("could not start IOU: {}: 32-bit binary support is probably not installed".format(e))
except (OSError, subprocess.SubprocessError) as e:
iou_stdout = self.read_iou_stdout()
log.error("could not start IOU {}: {}\n{}".format(self._path, e, iou_stdout))
raise IOUError("could not start IOU {}: {}\n{}".format(self._path, e, iou_stdout))
# start console support
self._start_ioucon()
# connections support
yield from self._start_iouyap()
def _rename_nvram_file(self):
"""
Before start the VM rename the nvram file to the correct application id
"""
destination = os.path.join(self.working_dir, "nvram_{:05d}".format(self.application_id))
for file_path in glob.glob(os.path.join(self.working_dir, "nvram_*")):
shutil.move(file_path, destination)
@asyncio.coroutine
def _start_iouyap(self):
"""
Starts iouyap (handles connections to and from this IOU device).
"""
try:
self._update_iouyap_config()
command = [self.iouyap_path, "-q", str(self.application_id + 512)] # iouyap has always IOU ID + 512
log.info("starting iouyap: {}".format(command))
self._iouyap_stdout_file = os.path.join(self.working_dir, "iouyap.log")
log.info("logging to {}".format(self._iouyap_stdout_file))
with open(self._iouyap_stdout_file, "w") as fd:
self._iouyap_process = yield from asyncio.create_subprocess_exec(*command,
stdout=fd,
stderr=subprocess.STDOUT,
cwd=self.working_dir)
log.info("iouyap started PID={}".format(self._iouyap_process.pid))
except (OSError, subprocess.SubprocessError) as e:
iouyap_stdout = self.read_iouyap_stdout()
log.error("could not start iouyap: {}\n{}".format(e, iouyap_stdout))
raise IOUError("Could not start iouyap: {}\n{}".format(e, iouyap_stdout))
def _update_iouyap_config(self):
"""
Updates the iouyap.ini file.
"""
iouyap_ini = os.path.join(self.working_dir, "iouyap.ini")
config = configparser.ConfigParser()
config["default"] = {"netmap": "NETMAP",
"base_port": "49000"}
bay_id = 0
for adapter in self._slots:
unit_id = 0
for unit in adapter.ports.keys():
nio = adapter.get_nio(unit)
if nio:
connection = None
if isinstance(nio, NIO_UDP):
# UDP tunnel
connection = {"tunnel_udp": "{lport}:{rhost}:{rport}".format(lport=nio.lport,
rhost=nio.rhost,
rport=nio.rport)}
elif isinstance(nio, NIO_TAP):
# TAP interface
connection = {"tap_dev": "{tap_device}".format(tap_device=nio.tap_device)}
elif isinstance(nio, NIO_GenericEthernet):
# Ethernet interface
connection = {"eth_dev": "{ethernet_device}".format(ethernet_device=nio.ethernet_device)}
if connection:
interface = "{iouyap_id}:{bay}/{unit}".format(iouyap_id=str(self.application_id + 512), bay=bay_id, unit=unit_id)
config[interface] = connection
if nio.capturing:
pcap_data_link_type = nio.pcap_data_link_type.upper()
if pcap_data_link_type == "DLT_PPP_SERIAL":
pcap_protocol = "ppp"
elif pcap_data_link_type == "DLT_C_HDLC":
pcap_protocol = "hdlc"
elif pcap_data_link_type == "DLT_FRELAY":
pcap_protocol = "fr"
else:
pcap_protocol = "ethernet"
capture_info = {"pcap_file": "{pcap_file}".format(pcap_file=nio.pcap_output_file),
"pcap_protocol": pcap_protocol,
"pcap_overwrite": "y"}
config[interface].update(capture_info)
unit_id += 1
bay_id += 1
try:
with open(iouyap_ini, "w") as config_file:
config.write(config_file)
log.info("IOU {name} [id={id}]: iouyap.ini updated".format(name=self._name,
id=self._id))
except OSError as e:
raise IOUError("Could not create {}: {}".format(iouyap_ini, e))
@asyncio.coroutine
def stop(self):
"""
Stops the IOU process.
"""
if self.is_running():
# stop console support
if self._ioucon_thread:
self._ioucon_thread_stop_event.set()
if self._ioucon_thread.is_alive():
self._ioucon_thread.join(timeout=3.0) # wait for the thread to free the console port
self._ioucon_thread = None
self._terminate_process_iou()
try:
yield from asyncio.wait_for(self._iou_process.wait(), timeout=3)
except asyncio.TimeoutError:
self._iou_process.kill()
if self._iou_process.returncode is None:
log.warn("IOU process {} is still running".format(self._iou_process.pid))
self._iou_process = None
if self._iouyap_process is not None:
self._terminate_process_iouyap()
try:
yield from asyncio.wait_for(self._iouyap_process.wait(), timeout=3)
except asyncio.TimeoutError:
self._iou_process.kill()
if self._iouyap_process.returncode is None:
log.warn("IOUYAP process {} is still running".format(self._iou_process.pid))
self._started = False
def _terminate_process_iouyap(self):
"""Terminate the process if running"""
if self._iou_process:
log.info("Stopping IOUYAP instance {} PID={}".format(self.name, self._iouyap_process.pid))
try:
self._iouyap_process.terminate()
# Sometime the process can already be dead when we garbage collect
except ProcessLookupError:
pass
def _terminate_process_iou(self):
"""Terminate the process if running"""
if self._iou_process:
log.info("Stopping IOU instance {} PID={}".format(self.name, self._iou_process.pid))
try:
self._iou_process.terminate()
# Sometime the process can already be dead when we garbage collect
except ProcessLookupError:
pass
@asyncio.coroutine
def reload(self):
"""
Reload the IOU process. (Stop / Start)
"""
yield from self.stop()
yield from self.start()
def is_running(self):
"""
Checks if the IOU process is running
:returns: True or False
"""
if self._iou_process:
return True
return False
def is_iouyap_running(self):
"""
Checks if the IOUYAP process is running
:returns: True or False
"""
if self._iouyap_process:
return True
return False
def _create_netmap_config(self):
"""
Creates the NETMAP file.
"""
netmap_path = os.path.join(self.working_dir, "NETMAP")
try:
with open(netmap_path, "w") as f:
for bay in range(0, 16):
for unit in range(0, 4):
f.write("{iouyap_id}:{bay}/{unit}{iou_id:>5d}:{bay}/{unit}\n".format(iouyap_id=str(self.application_id + 512),
bay=bay,
unit=unit,
iou_id=self.application_id))
log.info("IOU {name} [id={id}]: NETMAP file created".format(name=self._name,
id=self._id))
except OSError as e:
raise IOUError("Could not create {}: {}".format(netmap_path, e))
@asyncio.coroutine
def _build_command(self):
"""
Command to start the IOU process.
(to be passed to subprocess.Popen())
IOU command line:
Usage: [options]
: unix-js-m | unix-is-m | unix-i-m | ...
: instance identifier (0 < id <= 1024)
Options:
-e Number of Ethernet interfaces (default 2)
-s Number of Serial interfaces (default 2)
-n Size of nvram in Kb (default 64KB)
-b IOS debug string
-c Configuration file name
-d Generate debug information
-t Netio message trace
-q Suppress informational messages
-h Display this help
-C Turn off use of host clock
-m Megabytes of router memory (default 256MB)
-L Disable local console, use remote console
-l Enable Layer 1 keepalive messages
-u UDP port base for distributed networks
-R Ignore options from the IOURC file
-U Disable unix: file system location
-W Disable watchdog timer
-N Ignore the NETMAP file
"""
command = [self._path]
if len(self._ethernet_adapters) != 2:
command.extend(["-e", str(len(self._ethernet_adapters))])
if len(self._serial_adapters) != 2:
command.extend(["-s", str(len(self._serial_adapters))])
if not self.use_default_iou_values:
command.extend(["-n", str(self._nvram)])
command.extend(["-m", str(self._ram)])
command.extend(["-L"]) # disable local console, use remote console
initial_config_file = self.initial_config_file
if initial_config_file:
command.extend(["-c", initial_config_file])
if self._l1_keepalives:
yield from self._enable_l1_keepalives(command)
command.extend([str(self.application_id)])
return command
def read_iou_stdout(self):
"""
Reads the standard output of the IOU process.
Only use when the process has been stopped or has crashed.
"""
output = ""
if self._iou_stdout_file:
try:
with open(self._iou_stdout_file, errors="replace") as file:
output = file.read()
except OSError as e:
log.warn("could not read {}: {}".format(self._iou_stdout_file, e))
return output
def read_iouyap_stdout(self):
"""
Reads the standard output of the iouyap process.
Only use when the process has been stopped or has crashed.
"""
output = ""
if self._iouyap_stdout_file:
try:
with open(self._iouyap_stdout_file, errors="replace") as file:
output = file.read()
except OSError as e:
log.warn("could not read {}: {}".format(self._iouyap_stdout_file, e))
return output
def _start_ioucon(self):
"""
Starts ioucon thread (for console connections).
"""
if not self._ioucon_thread:
telnet_server = "{}:{}".format(self._console_host, self.console)
log.info("Starting ioucon for IOU instance {} to accept Telnet connections on {}".format(self._name, telnet_server))
args = argparse.Namespace(appl_id=str(self.application_id), debug=False, escape='^^', telnet_limit=0, telnet_server=telnet_server)
self._ioucon_thread_stop_event = threading.Event()
self._ioucon_thread = threading.Thread(target=start_ioucon, args=(args, self._ioucon_thread_stop_event))
self._ioucon_thread.start()
@property
def ethernet_adapters(self):
"""
Returns the number of Ethernet adapters for this IOU instance.
:returns: number of adapters
"""
return len(self._ethernet_adapters)
@ethernet_adapters.setter
def ethernet_adapters(self, ethernet_adapters):
"""
Sets the number of Ethernet adapters for this IOU instance.
:param ethernet_adapters: number of adapters
"""
self._ethernet_adapters.clear()
for _ in range(0, ethernet_adapters):
self._ethernet_adapters.append(EthernetAdapter(interfaces=4))
log.info("IOU {name} [id={id}]: number of Ethernet adapters changed to {adapters}".format(name=self._name,
id=self._id,
adapters=len(self._ethernet_adapters)))
self._slots = self._ethernet_adapters + self._serial_adapters
@property
def serial_adapters(self):
"""
Returns the number of Serial adapters for this IOU instance.
:returns: number of adapters
"""
return len(self._serial_adapters)
@serial_adapters.setter
def serial_adapters(self, serial_adapters):
"""
Sets the number of Serial adapters for this IOU instance.
:param serial_adapters: number of adapters
"""
self._serial_adapters.clear()
for _ in range(0, serial_adapters):
self._serial_adapters.append(SerialAdapter(interfaces=4))
log.info("IOU {name} [id={id}]: number of Serial adapters changed to {adapters}".format(name=self._name,
id=self._id,
adapters=len(self._serial_adapters)))
self._slots = self._ethernet_adapters + self._serial_adapters
def slot_add_nio_binding(self, adapter_number, port_number, nio):
"""
Adds a slot NIO binding.
:param adapter_number: slot ID
:param port_number: port ID
:param nio: NIO instance to add to the slot/port
"""
try:
adapter = self._slots[adapter_number]
except IndexError:
raise IOUError("Slot {adapter_number} doesn't exist on IOU {name}".format(name=self._name,
adapter_number=adapter_number))
if not adapter.port_exists(port_number):
raise IOUError("Port {port_number} doesn't exist in adapter {adapter}".format(adapter=adapter,
port_number=port_number))
adapter.add_nio(port_number, nio)
log.info("IOU {name} [id={id}]: {nio} added to {adapter_number}/{port_number}".format(name=self._name,
id=self._id,
nio=nio,
adapter_number=adapter_number,
port_number=port_number))
if self.is_iouyap_running():
self._update_iouyap_config()
os.kill(self._iouyap_process.pid, signal.SIGHUP)
def slot_remove_nio_binding(self, adapter_number, port_number):
"""
Removes a slot NIO binding.
:param adapter_number: slot ID
:param port_number: port ID
:returns: NIO instance
"""
try:
adapter = self._slots[adapter_number]
except IndexError:
raise IOUError("Slot {adapter_number} doesn't exist on IOU {name}".format(name=self._name,
adapter_number=adapter_number))
if not adapter.port_exists(port_number):
raise IOUError("Port {port_number} doesn't exist in adapter {adapter}".format(adapter=adapter,
port_number=port_number))
nio = adapter.get_nio(port_number)
adapter.remove_nio(port_number)
log.info("IOU {name} [id={id}]: {nio} removed from {adapter_number}/{port_number}".format(name=self._name,
id=self._id,
nio=nio,
adapter_number=adapter_number,
port_number=port_number))
if self.is_iouyap_running():
self._update_iouyap_config()
os.kill(self._iouyap_process.pid, signal.SIGHUP)
return nio
@property
def l1_keepalives(self):
"""
Returns either layer 1 keepalive messages option is enabled or disabled.
:returns: boolean
"""
return self._l1_keepalives
@l1_keepalives.setter
def l1_keepalives(self, state):
"""
Enables or disables layer 1 keepalive messages.
:param state: boolean
"""
self._l1_keepalives = state
if state:
log.info("IOU {name} [id={id}]: has activated layer 1 keepalive messages".format(name=self._name, id=self._id))
else:
log.info("IOU {name} [id={id}]: has deactivated layer 1 keepalive messages".format(name=self._name, id=self._id))
@asyncio.coroutine
def _enable_l1_keepalives(self, command):
"""
Enables L1 keepalive messages if supported.
:param command: command line
"""
env = os.environ.copy()
env["IOURC"] = self._iourc
try:
output = yield from gns3server.utils.asyncio.subprocess_check_output(self._path, "-h", cwd=self.working_dir, env=env)
if re.search("-l\s+Enable Layer 1 keepalive messages", output):
command.extend(["-l"])
else:
raise IOUError("layer 1 keepalive messages are not supported by {}".format(os.path.basename(self._path)))
except (OSError, subprocess.SubprocessError) as e:
log.warn("could not determine if layer 1 keepalive messages are supported by {}: {}".format(os.path.basename(self._path), e))
@property
def initial_config(self):
"""Return the content of the current initial-config file"""
config_file = self.initial_config_file
if config_file is None:
return None
try:
with open(config_file) as f:
return f.read()
except OSError as e:
raise IOUError("Can't read configuration file '{}'".format(config_file))
@initial_config.setter
def initial_config(self, initial_config):
"""
Update the initial config
:param initial_config: The content of the initial configuration file
"""
try:
script_file = os.path.join(self.working_dir, "initial-config.cfg")
with open(script_file, 'w+') as f:
if initial_config is None:
f.write('')
else:
initial_config = initial_config.replace("%h", self._name)
f.write(initial_config)
except OSError as e:
raise IOUError("Can't write initial configuration file '{}'".format(self.script_file))
@property
def initial_config_file(self):
"""
Returns the initial config file for this IOU instance.
:returns: path to config file. None if the file doesn't exist
"""
path = os.path.join(self.working_dir, 'initial-config.cfg')
if os.path.exists(path):
return path
else:
return None