Hubitat HomeKit across segmented VLANs — the two traffic paths

When a Hubitat hub lives on an isolated IoT VLAN and the Apple Home controllers live on a trusted Main VLAN, "enable mDNS" alone does not make HomeKit work. HomeKit needs two distinct paths, and a prebaked "allow mDNS" firewall exception satisfies only the first half of the first one. Verified working on a live UniFi deployment: Apps Hubitat at 192.168.30.15 (IoT 192.168.30.0/24) publishing HomeKit bridges to an Apple Home controller at 192.168.10.158 (Main 192.168.10.0/24), across a UniFi gateway. All Hubitat hubs here are production — this is a live-network procedure, not a lab (multi-hub-topology rule; the same production-safety stance as device-lifecycle).

Path 1 — Discovery (mDNS)

HomeKit bridges advertise _hap._tcp over mDNS, which is link-local — it does not cross a VLAN boundary on its own. A reflector/proxy has to carry it:

• Turn on the UniFi mDNS Proxy between Main and IoT, and confirm it includes _hap._tcp.
• Specific mDNS allowances must precede any broad IoT → Gateway deny — a blanket IoT-to-gateway block eats the advertisements before the proxy ever sees them.

The firewall exception that actually worked, placed before Block IoT to Gateway:

Destination 5353 is mandatory, not optional. The high range 32768-60999 covers the stateful replies to legacy-unicast queries the reflector sends — but unsolicited HomeKit/mDNS advertisements arrive on 5353, so a rule allowing only the high range passes the query replies and silently drops the advertisements. That is the trap the out-of-the-box "allow mDNS" preset falls into.

Coexistence, verified: Block IoT to Gateway Management and Block IoT to Default Management both stay enabled with the mDNS exception above in front of them.

Path 2 — HAP sessions (TCP)

The advertisement carries the bridge's address and its HAP TCP port in the _hap._tcp SRV record. Discovery finding the bridge is not the same as a controller being able to talk to it:

• Apple Home hubs/controllers on Main must be allowed to initiate TCP to that advertised port on the IoT bridge, and established/related return traffic from IoT must be permitted.
• Never hard-code the HAP port. Read it from the SRV record — it differs between bridges and changes when the integration is recreated.

Block IoT to Main (NAS isolation) is the rule that severs this path. It is currently paused. Do not re-enable it without first proving all three: Main can open TCP to each bridge's advertised _hap._tcp port, established/related return traffic from IoT is allowed, and HomeKit reconnects after an integration restart.

Switch-port / VLAN placement

For a Hubitat on a UniFi access port:

• Native network: IoT. Tagged VLANs: Block All. No client-level Virtual Network Override. A fixed IP is fine.
• Do not combine an IoT client override with a port that blocks the required tagged VLAN — UniFi will report the client has no connectivity, and it means it. Treat that warning as a real configuration error, not noise.

Validation — from the Main network

After any firewall change, use Hubitat's non-destructive "Restart integration" action only, then verify from a Main-network host:

dns-sd -B _hap._tcp local.        # the Hubitat bridges appear in the browse list
dns-sd -G v4 hubitat.local.       # resolves to 192.168.30.15

Then confirm Hubitat's HomeKit page shows an active connection from a Main-network controller. Known-good result on this deployment: Apps HomeKit connected from 192.168.10.158. A full service check restarts only the bridge's HomeKit service and confirms it disappears, republishes, and reconnects.

Troubleshooting order

Work the path, not the symptom — top to bottom, stop when it breaks:

1. Verify the switch-port VLAN config and the device's effective network first.
2. Verify mDNS advertisements reach Main (dns-sd -B).
3. Verify the advertised HAP TCP port is reachable from Main.
4. If isolating a firewall rule: pause the custom policies (only if authorized), record their original states, then restore incrementally and retest after each group.
5. Fix the narrow exception, re-enable the deny rule, retest.

What not to do

A blocked mDNS or HAP path looks exactly like a broken integration — and none of the destructive "fixes" repair a network problem. Until the paths above are proven, do not: factory-reset a hub, reset HomeKit pairing, regenerate the QR/pairing code, reboot or power-cycle a hub, or move a hub between VLANs. "Devices" Hubitat is production, not a disposable test hub. Every one of these needs explicit authorization, and none of them can undo a firewall rule.