ARP Presence Sensor

If anyone is using an EdgeRouter on EdgeOS, I've put together some scripts which can help determine if someone has arrived or left your network. This doesn't require you to ping devices. Of course there are some concerns around if people disable wireless on their device, but the same concerns work for the ping detection method.

I'd also recommend this be used with a tool for combining multiple presence sensors into a "master" presence to be more accurate:

Either Presence Governor or [RELEASE] Combined Presence.

There are also other ways of doing this if you are not using EdgeOS: [UPDATED] iPhone WiFi Presence Sensor is one option.

Another option which is related to ARP but doesn't require the wireless device to be on your network would be to sniff WiFi traffic and look for wireless probes. You need a Linux Device/RPi with a spare wireless adapter that supports monitor mode. The other gotcha here is some mobile devices mask their true MAC address for anonymous probes (they return an Admin OUI). Either way, this would be a router-agnostic way to solve this problem, however it needs someone to change the code (GitHub - bdwilson/wuds: Wireless User Detection System) and add the ability to expire devices that haven't been seen in X amount of time. This has been on my list but I haven't had time.



When can we expect you to write something similar using arpwatch for folks not on EdgeOS?

I can add that to my "to do" list when I rebuild this Linux server that handles some of my boring utility functions.

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I can supply the same kind of thing for Orbi routers. Please contact me directly. I'll put it up on Github soon.

The generic Linux version -- not specific to any wireless router -- is trivial, as in:

#! /bin/bash

# This script takes a list of [hostname|MAC|IP] and checks whether the specified device[s] are on the LAN.
# Will fail miserably with non-CIDR routing, multi-homed hosts, etc.

if [ "X$1" = "X" ] ; then
	echo "$0 [hostname|MAC|IP]"
	echo ""
	echo "Checks whether the specified device[s] are on the LAN."
	exit 1

defaultroute=`ip route show|grep default |sed -e "s/default via //" -e "s/\.[0-9][0-9]* .*//" `
subnet=`ip route show |sed -n -e "s,^\(${defaultroute}\.[0-9][0-9]*\)/[0-9].*,\1,p"`

for device in $*
	# inefficient, but simple
	ping -c 2 -W 2  -b $subnet 1> /dev/null 2>&1	# populate the ARP cache
	arp -a | grep -qiw $1 
	if [ $? = 0 ] ; then
		echo "Device $1 is present on the LAN ($subnet)"
		echo "Device $1 is absent from the LAN ($subnet)"
		exitval=$((exitval + 1))	# unnecessary
exit $exitval

When can we expect you to write something similar using arpwatch for folks not on EdgeOS?

arpwatch is not router-brand specific.

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I use arping to do my presence testing. I’m not sure how different it is to the method you use in your script with ping and arp.

I did a bunch of testing with arping and 2 iPhones I have.

The longest an iPhone ever stopped responding to arping was 6s (I logged it for weeks arping every 5s) so I set the timeout for a minute when it left the network. When the phone connects back it picks it up almost instantly.

This was way better than any ping where an iPhone would stop responding for up to 15 minutes once it went to sleep.

I know; was simply pointing out that if folks are going to offer other options, please share them.

Conceptually, no difference. Really, I just wrote the script (in under 10min) as an example, not that it does anything elegant or unique.

You've got a method that need to change it.

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Hi @brianwilson

I realize this is an old thread but I hope I can ask a few questions.

I have both files on my edgerouter 4, in the correct location, and I have added my pushover user & api keys. I haven't changed anything else and have not created a .arpnotify or .arpignore file yet, meaning no makerAPI connections yet. I just thought the first step would be to get it on there and see if I would start getting pushover notifications.


  1. Is there any problem with this initial configuration?
  2. Do I need to restart the router for this script to execute? Or do I need to point to it in some start up file? How does it get initialized?
  3. As long as my devices are using DHCP, this should work even though 90% of my devices are set up with static DHCP reservations?

As long as you’ve got the cron entries it should run. Even if your hosts are static reservations, they will still show up in the logs.

Thank you for the reply. But you lost me with “cron entries”.

I followed the instructions and have the files on my er4. I manually ran it from the admin login and I received a pushover notification but that is it.

I did reboot the router but the script does not seem to be running.

Last step in installation:

#crontab -e

23 11 * * * /config/arp_notify/
*/5 * * * * /config/arp_notify/

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Hahaha. I definitely do not do that.

Thank you. I will give it a whirl tomorrow.

omg I am NOT a linux guy. I went through the CLI console again and I could not fine the arp_notify directory that I created the last time? I must have been in a temporary sandbox??

this time I used putty and ssh'ed and repeated all the steps. I evidently did not have a crontab previously (I had to do some googling that one) and got it entered. I see that the backup runs daily but the arp_notify runs every 5 mins?? I am still watching my logs.

This just popped up, so I think I got it.

Sep 29 13:05:02 teravani [arp_notify]: No new ARP entries detected.

Again, thank you for your help.

Thanks for the script.

I can not get it to work. Logs always show “No new ARP entries detected”.

Can you help?

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