UPnP: The art of connecting devices

So you built a shiny new mobile app and you want it to chat with your smart camera. Nice! You slap in the camera’s IP address and boom — it works.

Until… it doesn’t. One day you restart your router, and suddenly it's like:

“Who even are you? I don’t know you anymore.”

And if you’ve got multiple cameras? It’s chaos. Like trying to find Waldo in a sea of IP addresses.

So how do the pros do it? How does your app find smart devices automagically, no matter what?

🎉 Welcome to the wonderful world of UPnP! — aka “plug me in and let me party on the network.”

It’s a protocol used in almost all smart home gear (Philips Hue, Google Nest, the fancy fridge that tweets when the milk’s gone bad… you name it). Let’s dive in — no scary technical stuff, promise!

🧠 UPnP in a nutshell: "I'm here!!"

When a smart device joins a network, it yells out:

“Hey everyone! I’m a smart lightbulb!”
“Here’s my IP: 192.168.1.12 💅”
“My unique ID is 56a5c464-24eb-485a-8825-920a5a6a0e0f — don’t wear it out.”
“I’m version 1.12 ✨”
“My API lives on port 3200 — come visit!”

And just like that, every other device on the network knows how to talk to it. No drama, no manual config.

Let’s build a (not-so-bright) smart bulb

We’ll play with Node.js and a little library called node-ssdp.

💡 The Smart Lightbulb (aka “the fake server”)

Let's write the code that will be living in your Smart Lightbulb. It is a simple HTTP server with a /toggle endpoint that can be called to turn the light on or off.

import express from "express";

const port = 3200;
const app = express();

// Toggle the bulb (or pretend to)
app.post("/toggle", (req, res) => {
  toggleTheLight();
  res.send("Toggled!");
});

app.listen(port);

Then we want to start the UPnP server that can broadcast our lightbulb precense to the network.

import { Server } from "node-ssdp";

const server = new Server({
  udn: "uuid:56a5c464-24eb-485a-8825-920a5a6a0e0f",
});

server.addUSN("urn:schemas-upnp-org:service:bulb:1.12");

The UDN is like the social security number of your Lightbulb. It’s unique to the smart object and will allow other devices to identify it after restarts.

And the USN? That’s how it tells the world, “Hey, I’m a bulb. A fabulous one. Version 1.12, thank you very much.”

📱 The Mobile App

Let's now write the code for our mobile app that can turn on our lightbulb using it's /toggle HTTP endpoint.

const devices = [];

function toggleTheLight() {
  for (const device of devices) {
    axios.post(`http://${device.address}:3200/toggle`);
  }
}

We now need to discover our Lightbulbs on the network to fill our devices. We will use the same node-ssdp library by instanciating a client.

import { Client } from "node-ssdp";

const client = new Client();

client.on("response", function (headers, statusCode, rinfo) {
  devices.push({
    address: rinfo.address,
    uuid: headers.USN,
  });
});

client.search("urn:schemas-upnp-org:service:bulb:*");

When using client.search the app will sends out a search like:

“Yo, any bulbs around? I’m not picky.”

And all the bulbs will shout back. We can then collect their information to interact with them and turn on the light. (Or make a disco.)

Real-Life Use Case

If you are interested to discover my work I wrote a case study about my mission for Lacoste where I built a cinema shopping experience.

Conlusion

Asking your user to manually enter the IP address of their smart devices is like asking them to read the TOS. Ain’t nobody got time for that!

UPnP is the magic sauce that makes smart devices discoverable and easy to connect. It’s like the friendly neighbor who always knows where the party is at.