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arp-watch

The arp-watch tool detects changes to the local ARP (Address Resolution Protocol) cache. It currently supports Linux, UNIX, and MacOS.

Installation

Make sure you have Go installed and your GOPATH set.

go get github.com/st3v/arp-watch

Usage

The arp-watch tool will report changes to MAC addresses inside the local ARP cache. A change is reported using an event key in the form of net.arp.<key>.<action>. Thereby, <key> represents a sanitized IP or an IP alias (see config options). <action> represents the type of change, i.e.:

  • set: a previously unknown entry has been detected in the ARP cache
  • unset: a previously observed entry has disappered from the ARP cache
  • changed: a previously observed entry has changed in the ARP cache (i.e. the MAC address for a given IP changed)

The tool reports change events on the command line and optionally sends them as a metric to a Cloud Foundry Metron agent.

Example:

$ arp-watch -stateFilePath /tmp/arp-watch.state -configPath /tmp/arp-watch.config
net.arp.192_168_0_1.set: '' -> '00:11::22:33:44:55'
net.arp.192_168_0_2.changed: '66:77::88:99:aa:bb' -> '66:77::88:99:aa:bb'
net.arp.192_168_0_3.unset: 'aa:bb::cc:dd:ee:ff' -> ''

Command Line Parameters

$ arp-watch --help
Usage of ./arp-watch:
  -configPath string
    	Path to config file. Optional.
  -stateFilePath string
    	Path to state file. Optional.

If -configPath is not being passed, the tool will check the ARP cache exactly once and return immediately afterwards. The tool will not apply filters and aliases (see below).

If -stateFilePath is not being passed, the tool will initially report the currently cached addresses as being set. (i.e. there is not initial state). If the -stateFilePath flag is set and it points to an existing state file, the tool will set its initial state accordingly. Upon termination the tool will write its state to the specified -statefilePath.

Config File Options

The config file uses JSON as its format. The following options are available. All of them are optional.

frequency

If frequency is set the tool will keep running indefinitely and continuously check for changes in the ARP cache. The frequency defines how often the tool checks the ARP cache. It's a string that is a sequnce of decimal numbers each with and optional fraction and a unit suffix, e.g. "300ms", "1.5h" or "2h45m". Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h". If frequency is not set, the tool will check the ARP cache once and terminate immediately.

filters

This is an array of strings, each representing an IP that should be considered when watching the ARP cache. If filters is not set or set to an empty array, the tool will consider all cached IPs.

aliases

This option can be used to map IP addresses to arbitrary keys, e.g. '192.168.0.1': 'node-1'. The tool uses this mapping when reporting changes in the ARP cache, e.g. net.arp.node-1.set. If a given IP does not have an alias, the tool will report the IP with its dots (.) replaced by dashes (-), e.g. net.arp.192-168-0-1.set.

metron.endpoint

This option specifies the endpoint for an existing Metron agent. Everytime the tool detects a change in the ARP cache, it will send a corresponding state change metric to Metron. net.arp.<key>.<action> will be used as the format for the metric key (see above). The value for the metric is always 1 and its unit is count.

metron.origin

This option specifies the origin string used when sending metrics to the Metron agent.

Example Config File:

{
  "metron": {
    "endpoint": "localhost:3457",
    "origin": "node-1"
  },
  "frequency": "1s",
  "filters": [
    "192.168.0.1", 
    "192.168.0.2" 
  ],
  "aliases": {
    "192.168.0.1": "host-1", 
    "192.168.0.2": "host-2"
  }
}

Licensing

Translator is licensed under the Apache License, Version 2.0. See LICENSE for the full license text.