ESP32 mDNS Not Working

ESP32 mDNS Not Working #

You're trying to access your ESP32 via http://esp32.local, but the browser says “server not found” or ping fails. Yet when you type in the IP address directly (e.g., http://192.168.1.104), everything works.

This is a sign that multicast DNS (mDNS) — the system that makes .local names work — isn't functioning correctly on your network or device. This guide will help you understand how ESP32 mDNS works and what to do when it fails.

What Is mDNS? #

Multicast DNS lets devices on a local network resolve names like esp32.local without a central DNS server. It uses multicast traffic on port 5353, and is often used in LANs for discovery of IoT devices, printers, etc.

ESP32 supports mDNS via the ESPmDNS library. In code:

#include <ESPmDNS.h>

if (!MDNS.begin("esp32")) {
  Serial.println("Error starting mDNS");
}

This should make esp32.local reachable — but only if the network and your host system allow it.

Common Symptoms of Broken mDNS #

esp32.local Fails to Resolve #

You can ping the ESP32’s IP directly but:

ping esp32.local

results in Host not found.

Only Some Devices Work #

mDNS may work on one laptop (e.g., a MacBook) but not on another (e.g., Windows PC), indicating OS-level or service differences.

Works with IP Address, Not Hostname #

All functions via IP (HTTP, OTA, etc.) work perfectly, but .local never resolves. This is common on Windows if Bonjour is missing.

Root Causes and Fixes #

🧯 MDNS Not Initialized Correctly #

MDNS.begin() must be called after the ESP32 connects to Wi-Fi:

WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED) {
  delay(500);
}
MDNS.begin("esp32");

If it runs too early (before network stack is up), it silently fails.

🔒 Bonjour or Avahi Not Installed #

On Windows, mDNS requires Apple Bonjour (bundled with iTunes or available standalone).

On Linux, install avahi-daemon and nss-mdns:

sudo apt install avahi-daemon libnss-mdns

macOS has built-in mDNS support via Bonjour.

🌐 Network Doesn’t Allow Multicast #

Many guest Wi-Fi networks or mobile hotspots block client-to-client traffic or multicast packets.

Fixes:

• Use a home router, not a phone hotspot

• Disable “AP Isolation” in router settings

• Ensure all devices are on the same subnet

🛡 Firewall Blocking Port 5353 #

On some systems, firewalls block inbound or outbound UDP packets on port 5353. Check firewall rules or try temporarily disabling them to test.

Workarounds and Alternatives #

• Print the IP in Serial Monitor and use it:

Serial.println(WiFi.localIP());

• Use a captive portal or MQTT broker with a fixed IP instead of relying on mDNS.

• For production, consider using a static IP or fallback to DHCP-assigned IP discovery via a known endpoint.

Conclusion #

mDNS on ESP32 is a powerful feature, but it depends on:

• Correct timing in your firmware

• Host OS support (Bonjour, Avahi)

• Network policies allowing multicast

If esp32.local isn’t working, check the basics:

• Is MDNS.begin() called after Wi-Fi connects?

• Does your OS support mDNS?

• Are you on a network that allows local discovery?

With the right setup, mDNS can make your ESP32 discoverable instantly on any local network — no IP memorization required.