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mDNS Workshop Note

Summary

Workshop notes on mDNS (multicast DNS, RFC 6762) — how devices on a local network resolve hostnames like printer.local without a central DNS server, and how the paired DNS-SD (RFC 6763) mechanism does zero-config service discovery on top of it. Covers why this is the auto-discovery mechanism behind Bonjour/Avahi and a lot of smart-home gear, including ESPHome devices, and the most common way it silently breaks.

Key Takeaways

mDNS lets each device answer for its own hostname via multicast (224.0.0.251 / UDP 5353) instead of a central DNS server
DNS-SD (RFC 6763), layered on mDNS, is how a device advertises and discovers services (e.g. “who offers _http._tcp here?”)
Bonjour, Avahi, and ESPHome’s device auto-discovery all build on this pair of protocols
mDNS traffic is link-local and does not cross routers by default — a device moving to a different VLAN or a guest network silently loses discoverability
Symptom pattern: “why can’t I see my printer/ESPHome device” tickets very often trace back to exactly this segmentation issue
Some routers offer an mDNS reflector/repeater to bridge segments deliberately

Content

Quick note from a workshop talk on mDNS (multicast DNS, RFC 6762). It lets devices on a local network resolve hostnames like printer.local without a central DNS server — each device answers for its own name by listening on the multicast address 224.0.0.251 (or the IPv6 equivalent) on UDP port 5353. Paired with DNS-SD (RFC 6763) it’s how mDNS does service discovery too: a device can ask “who offers _http._tcp on this network?” and get a list back, which is the mechanism Bonjour/Avahi and a lot of smart-home gear (like ESPHome devices) build on for auto-discovery. The talk’s main caution: mDNS traffic doesn’t cross routers by default (multicast is link-local), so it silently breaks the moment a device moves to a different VLAN or a guest network — a lot of “why can’t I see my printer/ESPHome device” tickets trace back to exactly this. Some routers offer an mDNS reflector/repeater to bridge segments on purpose.

Sources

No public URL — captured from workshop notes. See raw/validation-test-note.md for the original dictation.

Created as part of a 2026-08-29 end-to-end workflow validation exercise — safe to remove.