text: #!/usr/bin/python
  # Copyright 2016 Google Inc. All Rights Reserved.
  #
  # 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.
  
  """Utilities for configuring IP address forwarding."""
  
  import logging
  import os
  import re
  try:
    import netifaces
  except ImportError:
    netifaces = None
  
  
  MAC_REGEX = re.compile(r'\A([0-9A-Fa-f]{2}[:]){5}([0-9A-Fa-f]{2})\Z')
  
  
  class NetworkUtils(object):
    """System network Ethernet interface utilities."""
  
    def __init__(self, logger=logging):
      """Constructor.
  
      Args:
        logger: logger object, used to write to SysLog and serial port.
      """
      self.logger = logger
      self.interfaces = self._CreateInterfaceMap()
  
    def _CreateInterfaceMap(self):
      """Generate a dictionary mapping MAC address to Ethernet interfaces.
  
      Returns:
        dict, string MAC addresses mapped to the string network interface name.
      """
      if netifaces:
        return self._CreateInterfaceMapNetifaces()
      else:
        return self._CreateInterfaceMapSysfs()
  
    def _CreateInterfaceMapSysfs(self):
      """Generate a dictionary mapping MAC address to Ethernet interfaces.
  
      Returns:
        dict, string MAC addresses mapped to the string network interface name.
      """
      interfaces = {}
      for interface in os.listdir('/sys/class/net'):
        try:
          mac_address = open(
              '/sys/class/net/%s/address' % interface).read().strip()
        except (IOError, OSError) as e:
          message = 'Unable to determine MAC address for %s. %s.'
          self.logger.warning(message, interface, str(e))
        else:
          interfaces[mac_address] = interface
      return interfaces
  
    def _CreateInterfaceMapNetifaces(self):
      """Generate a dictionary mapping MAC address to Ethernet interfaces.
  
      Returns:
        dict, string MAC addresses mapped to the string network interface name.
      """
  
      interfaces = {}
      for interface in netifaces.interfaces():
        af_link = netifaces.ifaddresses(interface).get(netifaces.AF_LINK, [])
        mac_address = next(iter(af_link), {}).get('addr', '')
        # In some systems this field can come with an empty string or with the
        # name of the interface when there is no MAC address associated with it.
        # Check the regex to be sure.
        if MAC_REGEX.match(mac_address):
          interfaces[mac_address] = interface
        else:
          message = 'Unable to determine MAC address for %s.'
          self.logger.warning(message, interface)
      return interfaces
  
    def GetNetworkInterface(self, mac_address):
      """Get the name of the network interface associated with a MAC address.
  
      Args:
        mac_address: string, the hardware address of the network interface.
  
      Returns:
        string, the network interface associated with a MAC address or None.
      """
      return self.interfaces.get(mac_address)
  