ESP32: use mDNS so the chip responds to a friendly name

mDNS (multicast DNS) is the protocol that lets you reach a device on your local network by name instead of IP address. With mDNS, you type kitchen-sensor.local instead of 192.168.1.42. No DNS server, no configuration, no router setup.

It works on the ESP32 (with Wi-Fi) and on the Raspberry Pi (with Avahi). Most modern operating systems (Windows 10+, macOS, iOS, Android) automatically resolve .local names.

What you need

• ESP32 dev board with Wi-Fi
• (For Pi Python section) Raspberry Pi with network

The code: ESP32

#include <WiFi.h>
#include <ESPmDNS.h>

const char* ssid = "your-wifi";
const char* password = "your-password";
const char* hostname = "kitchen-sensor";

void setup() {
  Serial.begin(115200);
  delay(1000);

  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) delay(500);

  // Set hostname
  WiFi.setHostname(hostname);

  // Initialize mDNS
  if (!MDNS.begin(hostname)) {
    Serial.println("mDNS init failed");
    return;
  }
  Serial.print("mDNS responder started: ");
  Serial.print(hostname);
  Serial.println(".local");

  // Add a service (HTTP server on port 80)
  MDNS.addService("http", "tcp", 80);
}

void loop() {
  MDNS.update();
}

Upload. Open Serial Monitor. After Wi-Fi connects, you should see:

mDNS responder started: kitchen-sensor.local

Now from any computer on the same network, you can open http://kitchen-sensor.local/ in a browser and reach the ESP32’s web server.

The code: Raspberry Pi

mDNS on Linux is provided by Avahi. Most Pi OS images have it preinstalled.

sudo apt install avahi-daemon
sudo systemctl enable avahi-daemon

The Pi responds to <hostname>.local automatically once Avahi is running. Set the hostname:

sudo hostnamectl set-hostname kitchen-sensor
sudo reboot

After reboot, kitchen-sensor.local resolves to the Pi’s IP.

Why mDNS is useful

Three scenarios where mDNS shines:

1. DHCP reassigns the IP. Your router can change the ESP32’s IP at any time. With mDNS, the name stays the same.
2. Multiple devices on the same network. IP addresses are hard to remember. Names are not.
3. Zero configuration. No DNS server, no router setup, no static IP assignment.

The downside: mDNS only works on the local network. It does not resolve over the internet. For that, you need a real DNS service.

Custom service names

Beyond HTTP, mDNS can advertise any service:

// Add a custom service
MDNS.addService("_ctrlaltbrian", "_tcp", 8080);

This advertises a _ctrlaltbrian._tcp service on port 8080. Other devices on the network can browse for this service and find your ESP32.

Useful for service discovery in IoT networks:

// On the broker/broadcaster
MDNS.addService("_mqtt", "_tcp", 1883);
MDNS.addService("_ctrlaltbrian-sensor", "_tcp", 8080);

A client looking for sensors can scan the network for the service name:

import zeroconf
zc = zeroconf.Zeroconf()
browser = zeroconf.ServiceBrowser(zc, "_ctrlaltbrian-sensor._tcp.local.", ...)

Browser compatibility

Platform	mDNS support
Windows 10+	Built in. Works automatically.
Windows 7/8	Need Bonjour installed (it comes with iTunes).
macOS	Built in. Works automatically.
iOS	Built in. Works automatically.
Android	Built in since Android 8.0.
Linux	Avahi (install if not present).
Chromebook	Built in.

For older Windows, install Apple’s Bonjour Print Services (free). It ships with iTunes and Adobe software, so most users already have it.

The 250ms timeout

mDNS responses are sent over multicast UDP. Networks with many devices or poor Wi-Fi can drop these packets. If a name does not resolve, try again or check the network.

Tools for testing:

• dns-sd -B _http._tcp local. (macOS): list HTTP services
• avahi-browse -at (Linux): list all services
• nslookup kitchen-sensor.local (Windows 10+): query a specific name

Security

mDNS has no authentication. Any device on the local network can advertise any name. A malicious device could pretend to be kitchen-sensor.local and MITM your connections. For trusted local networks this is fine; for untrusted networks, use HTTPS with proper certificate validation.

What you learned

• mDNS lets you reach devices by name (kitchen-sensor.local).
• It works on the ESP32 (with Wi-Fi) and the Raspberry Pi (with Avahi).
• It is built into most modern operating systems.
• No DNS server or configuration required.

When something breaks

• Name does not resolve. Wi-Fi not connected, mDNS not initialized, or the OS does not support .local resolution.
• Resolves but wrong device. Another device on the network is using the same name. Change the hostname.
• Slow to resolve. Multicast packets dropped. Try again or use a direct IP.

What to build next

• The ESP32 web server tutorial combines with this for named web servers.
• The ESP-NOW tutorial uses direct MAC addressing, which complements mDNS.
• The book ESP32 Smart Home covers service discovery across multiple devices.