arper.py

   1 #!/usr/bin/env python3
   2
   3 """A caching layer around the kernel's network mapping between IPs and MACs"""
   4
   5 import datetime
   6 import logging
   7 import os
   8 from typing import Any, Optional
   9
  10 from overrides import overrides
  11
  12 import argparse_utils
  13 from collect.bidict import BiDict
  14 import config
  15 import exec_utils
  16 import file_utils
  17 import persistent
  18 import string_utils
  19 import site_config
  20
  21 logger = logging.getLogger(__name__)
  22
  23 cfg = config.add_commandline_args(
  24     f'MAC <--> IP Address mapping table cache ({__file__})',
  25     'Commandline args related to MAC <--> IP Address mapping',
  26 )
  27 cfg.add_argument(
  28     '--arper_cache_location',
  29     default=f'{os.environ["HOME"]}/cache/.arp_table_cache',
  30     metavar='FILENAME',
  31     help='Where to cache the kernel ARP table',
  32 )
  33 cfg.add_argument(
  34     '--arper_cache_max_staleness',
  35     type=argparse_utils.valid_duration,
  36     default=datetime.timedelta(seconds=60 * 60),
  37     metavar='DURATION',
  38     help='Max acceptable age of the kernel arp table cache'
  39 )
  40 cfg.add_argument(
  41     '--arper_min_entries_to_be_valid',
  42     type=int,
  43     default=site_config.get_config().arper_minimum_device_count,
  44     help='Min number of arp entries to bother persisting.'
  45 )
  46
  47
  48 @persistent.persistent_autoloaded_singleton()
  49 class Arper(persistent.Persistent):
  50     def __init__(
  51             self, cached_state: Optional[BiDict[str, str]] = None
  52     ) -> None:
  53         self.state = BiDict()
  54         if cached_state is not None:
  55             logger.debug('Loading Arper map from cached state.')
  56             self.state = cached_state
  57         else:
  58             logger.debug('No usable cached state; calling /usr/sbin/arp')
  59             self.update()
  60
  61     def update(self):
  62         output = exec_utils.cmd(
  63             '/usr/sbin/arp -a',
  64             timeout_seconds=5.0
  65         )
  66         for line in output.split('\n'):
  67             ip = string_utils.extract_ip_v4(line)
  68             mac = string_utils.extract_mac_address(line)
  69             if ip is not None and mac is not None:
  70                 mac = mac.lower()
  71                 logger.debug(f'    {mac} => {ip}')
  72                 self.state[mac] = ip
  73
  74     def get_ip_by_mac(self, mac: str) -> Optional[str]:
  75         mac = mac.lower()
  76         return self.state.get(mac, None)
  77
  78     def get_mac_by_ip(self, ip: str) -> Optional[str]:
  79         return self.state.inverse.get(ip, None)
  80
  81     @overrides
  82     def save(self) -> bool:
  83         if len(self.state) > config.config['arper_min_entries_to_be_valid']:
  84             logger.debug(
  85                 f'Persisting state to {config.config["arper_cache_location"]}'
  86             )
  87             with file_utils.FileWriter(config.config['arper_cache_location']) as wf:
  88                 for (mac, ip) in self.state.items():
  89                     mac = mac.lower()
  90                     print(f'{mac}, {ip}', file=wf)
  91             return True
  92         else:
  93             logger.warning(
  94                 f'Only saw {len(self.state)} entries; needed at least {config.config["arper_min_entries_to_be_valid"]} to bother persisting.'
  95             )
  96             return False
  97
  98     @classmethod
  99     @overrides
 100     def load(cls) -> Any:
 101         cache_file = config.config['arper_cache_location']
 102         if persistent.was_file_written_within_n_seconds(
 103                 cache_file,
 104                 config.config['arper_cache_max_staleness'].total_seconds(),
 105         ):
 106             logger.debug(f'Loading state from {cache_file}')
 107             cached_state = BiDict()
 108             with open(cache_file, 'r') as rf:
 109                 contents = rf.readlines()
 110                 for line in contents:
 111                     line = line[:-1]
 112                     logger.debug(f'ARPER:{cache_file}> {line}')
 113                     (mac, ip) = line.split(',')
 114                     mac = mac.strip()
 115                     mac = mac.lower()
 116                     ip = ip.strip()
 117                     cached_state[mac] = ip
 118             if len(cached_state) > config.config['arper_min_entries_to_be_valid']:
 119                 return cls(cached_state)
 120             else:
 121                 logger.warning(
 122                     f'{cache_file} sucks, only {len(cached_state)} entries.  Deleting it.'
 123                 )
 124                 os.remove(cache_file)
 125
 126         logger.debug('No usable saved state found')
 127         return None