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

I thought, 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.