Add mDNS support for MQTT server #4769

blazoncek · 2025-07-12T17:05:08Z

fixes Resolve mDNS address for MQTT broker #4671
reduce some topic string parsing
moves LWT into onConnect

Summary by CodeRabbit

New Features
- Improved mDNS support for MQTT server resolution on ESP32 devices.
Improvements
- Unified and streamlined MQTT topic formatting for consistent and reliable topic generation.
- Enhanced reliability of online status reporting via MQTT by centralizing Last Will and Testament message setup.
Documentation
- Added and clarified comments for better understanding of MQTT payload parsing and logic.

- fixes #4671 - reduce some topic string parsing - moves LWT into onConnect

coderabbitai · 2025-07-12T17:05:13Z

"""

Walkthrough

The update refactors MQTT topic string construction in wled00/mqtt.cpp to use a centralized format string and snprintf_P for consistency. It also adds mDNS resolution support for the MQTT broker on ESP32 platforms and centralizes the setup of the MQTT Last Will and Testament (LWT) message. No changes were made to public interfaces.

Changes

Files / File Groups	Change Summary
wled00/mqtt.cpp	Refactored topic string construction to use a constant format string and `snprintf_P`; improved mDNS support for MQTT broker resolution on ESP32; centralized LWT message setup; added/clarified comments; minor formatting changes.

Assessment against linked issues

Objective (Issue #)	Addressed	Explanation
Resolve mDNS address for MQTT broker (#4671)	✅

Assessment against linked issues: Out-of-scope changes

No out-of-scope changes found.

Possibly related PRs

Change to set LWT only once #4697: Both PRs modify the handling of the MQTT "online" status message and its publishing location within onMqttConnect and publishMqtt, focusing on centralizing and improving the LWT and status topic setup.
"""

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schildbach · 2025-07-14T19:49:22Z

So far, I had no luck with this PR. I installed WLED_0.16.0-alpha_ESP32.bin from this PR to my Athom LS8P, replaced Sync Settings → MQTT → Broker with "assistant.local" and rebooted. It does not seem to connect to my MQTT server.

There is no such problem if I use a regular DNS name or IP address. And a variety of ESPHome devices can connect using the mDNS name without problem as well.

blazoncek · 2025-07-15T04:33:00Z

You don't need to add .local to host name.

schildbach · 2025-07-15T07:28:26Z

You don't need to add .local to host name.

How do you mean? That's what my mDNS domain is called, and broadcasted via avahi-publish.

blazoncek · 2025-07-15T07:50:30Z

.local is needed for your host computer to know it needs to do mDNS. querying mDNS directly does not need it.

schildbach · 2025-07-15T08:11:41Z

.local is needed for your host computer to know it needs to do mDNS. querying mDNS directly does not need it.

Yes exactly. How does WLED know I want to resolve an mDNS address if I don't add the correct .local domain? Domains without .local are plain DNS so those would try to get resolved via my DNS server – and fail, right?

blazoncek · 2025-07-15T08:16:33Z

Just enter the hostname. What else? Similar as, if you are old enough to remember it, WINS.
Will add stripping of .local

schildbach · 2025-07-15T08:19:20Z

Just enter the hostname. What else? Similar as, if you are old enough to remember it, WINS. Will add stripping of .local

Again, how will it know it's an mDNS domain if I strip the .local?

blazoncek · 2025-07-15T08:23:20Z

See the code.

schildbach · 2025-07-15T08:30:46Z

I see changes to MQTT topic parsing which seem irrelevant to this PR. However I'm a Java dev with limited understanding of CPP code so I can't say for sure.

Can you describe in words?

Typically, in these kind of hostname fields there are these cases:

formatted as IPv4 → use IP
formatted as IPv6 → use IP
ending with .local → resolve IP via mDNS
just host, no domain → add DNS search domain, resove IP via DNS
host.domain → resolve IP via DNS

schildbach · 2025-07-15T08:51:24Z

wled00/mqtt.cpp

+    mqttMDNS.replace(F(".local"), ""); // remove .local if present
+    if (strlen(cmDNS) > 0 && mqttMDNS.length() > 0 && mqttMDNS.indexOf('.') < 0) { // if mDNS is enabled and server does not have domain


Is .replace() limited to the end of strings? Otherwise it would strip in the middle of DNS names too which is probably not wanted.

In general, I'd recommend explicitly checking for .local at the end, and only if present resolve via mDNS (stripping the domain if it isn't done by the mDNS component anyway). It seems the above code just checks for the presence of dots, which is imho not correct. Plain hostnames (without dots) are typically expected to be resolved via DNS (using the search domain).

Also, the impl from this PR would fail for those who use dots in their mDNS name. Yes, some people do this and I understand this is bending the mDNS spec a bit, but it would be nice to support these cases anyway.

One more comment about .replace(): is it case-insensitive? Both DNS and mDNS names/domains are case-insensitive.

You are welcome to add on to this at a later time. For now just test if it fulfills your needs with .local domain.
You have to understand that I'm doing that in my spare time and I have no need for this feature and limited resources in ESP may need a few compromises.

FYI resolving mDNS host only requires hostname, no domain. Domain is only there to determine if regular DNS is needed or not.

Fair enough, and thanks for your time investment. I was just reviewing this PR as you asked me to do.

schildbach · 2025-07-15T11:29:26Z

I retested the 24f2306 binary and assistant.local now works for me.

schildbach · 2025-07-15T11:32:45Z

However, assistant.LOCAL doesn't work although I think it should.

ASSISTANT.local works.

schildbach

Are you sure "removing anything behind .local" is the correct fix?

A user could have a perfectly conformant DNS name "server1.localservers.example.com" and this code would strip it to just "server1" and interpret it as an mDNS name.

I think a more correct fix would make sure the .local part needs to appear at the end of the string to be considered as a mDNS name.

blazoncek · 2025-07-15T13:00:30Z

.local part needs to appear at the end of the string

Agreed, but that is a bit more complex. However, that stripping is only done for mDNS resolution. If host is not found, regular DNS query is made. See else.

Add mDNS support for MQTT server

79b3bc2

- fixes #4671 - reduce some topic string parsing - moves LWT into onConnect

blazoncek mentioned this pull request Jul 14, 2025

Resolve mDNS address for MQTT broker #4671

Closed

Strip .local from mDNS resolution

24f2306

schildbach reviewed Jul 15, 2025

View reviewed changes

Remove anything behind .local and ignore case

07e303b

schildbach reviewed Jul 15, 2025

View reviewed changes

Revert status message change

ecc3eae

blazoncek added enhancement connectivity Issue regarding protocols, WiFi connection or availability of interfaces labels Jul 26, 2025

blazoncek merged commit 93e011d into main Aug 2, 2025
42 checks passed

blazoncek deleted the fix-4671 branch August 2, 2025 22:17

This was referenced Aug 31, 2025

Hostname unification #4751

Closed

Hostname unification 2: The Force Push Strikes Back #4893

Open

		mqttMDNS.replace(F(".local"), ""); // remove .local if present
		if (strlen(cmDNS) > 0 && mqttMDNS.length() > 0 && mqttMDNS.indexOf('.') < 0) { // if mDNS is enabled and server does not have domain

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Add mDNS support for MQTT server #4769

Add mDNS support for MQTT server #4769

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