Everyone deserves an entrance music to announce their arrival into a room. I think this was the biggest perk given to WWE wrestlers. In an attempt to replicate this behaviour, I wrote a fun little program in Golang that you can view here.
Basic Design
A user's phone will invariably connect to your home/office Wifi network before they reach the door. I've tried to leverage this fact in building the application. The workflow goes as follows:
- Continuously poll the Wifi network to check for any new devices that have connected recently.
- If a new device is found, compare it's mac address to a pre-configured list of devices.
- If a match is found, play a pre-configured sound file on the speakers.
This program can be run on a Raspberry Pi or a discarded laptop in your possession. It just needs to be online and connected to the Wifi and speakers at all times.
Challenges
One of the biggest hurdles I faced was to reliably get a list of devices connected on a local network. While there are many tools such as nmap and arp-scan, they weren't proving to be reliable. For example, nmap (by default) relies on PING scans to locate devices. Most devices on the other hand don't respond to ICMP requests reliably.
Infact, most devices deem such network scanning behaviour as malicious and the defaults are set to protect the user from such behaviour. iOS was notorioiusly hard to detect reliably. There are work-arounds, but I was hoping to not complicate the code too much. At one frustrating juncture, I even considered scraping the router admin interface to get a list of connected clients.
I have a DLink DIR-800 router which doesn't support DD-WRT firmware. I hear that DD-WRT has the capability to publish this information over some queue, but unfortunately, this wasn't an option for me.
Solution
If the arp-scan from my machine wasn't reliable, would it be any different if I performed it on the router itself? Luckily, my router does support Telnet connections. I logged into my router via telnet and surprisingly it opened a full fledged shell! That's incredible.
$ telnet 192.168.0.1 23
Trying 192.168.0.1...
Connected to ralink.dlink.com.
Escape character is '^]'.
ralink login: admin
Password:
BusyBox v1.12.1 (2014-12-11 15:09:57 CST) built-in shell (ash)
Enter 'help' for a list of built-in commands.
# echo $SHELL
/bin/sh
Great, so now all I needed to do was perform an arp-scan from the router and parse the results.
On Linux systems, arp-scan reads the values from the file /proc/net/arp. So, instead of using a command, I simply read that file and parsed it's contents.
# cat /proc/net/arp
IP address HW type Flags HW address Mask Device
192.168.0.103 0x1 0x2 11:11:11:11:11:11 * br0
192.168.0.102 0x1 0x2 22:22:22:22:22:22 * br0
192.168.0.104 0x1 0x0 44:44:44:44:44:44 * br0
A 0x2 flag indicates that the device is connected, while 0x0 indicates that it's not.
N.B: It does take a little bit of time for the ARP file to reflect any disconnections. The ARP cache on some devices may even be 4 hours. Your mileage may vary. But new connections were fairly quick to be reflected here.
End Result
I tied it all together in a tiny, fun utility (source code available here). I had an old Sony Vaio laptop lying around which I dusted and brought out of retirement to run this code. Now whenever I enter the room/office, I'm preceded by my favourite song!
Handy Side-Effect: After configuring a few friends phones, I know who's visiting me before they even ring the door bell.
Other Applications
Your could also use this utility to add some personalization for colleagues in your office. Can be a nerdy differentiator for smaller companies and startups.
Future Work
Since, I only have 1 router at home, I can't test this program with other models and manufacturers. If you do find this project interesting, I'd love to hear how you can connect to your Wifi router via CLI. I'll make enhancements to the project accordingly.
Top comments (3)
In your code, why do you skip disconnected devices (continue if flag!=0x2)?
All you need to do (as I understand) is to get list of devices that are in proximity to your router regardless of their connection status. By dropping this condition you can react to device appearance before it connects to your network (couple seconds earlier).
I added that check to mark devices that have disconnected. For example, if I leave the room, the entry still stays in the arp proc file but it's status changes to 0x0. It will be removed according to whatever caching rules exist on the router. Hence, I decided to handle the flag and only honour devices that are connected.
Also, new connections were fairly fast to detect. There was less than a seconds delay everytime for my router. OTOH, disconnections are much slower to detect. On average, it was taking 30 mins to detect a disconnection.
Add a wicked light-show, crowd and the Shawn-Michaels-Entrance-Song from the 90ies and I'm sold!