Walking the hall with a phone.

Companion ships preconfigured with cellular, so a wing install never waits on facility IT, a VLAN ticket, or a Wi-Fi password. But there is still a moment at the bedside where a technician needs to confirm that this device, mounted on this wall, is the one assigned to room 302B. We solved that with mDNS, the same protocol that lets a printer show up in your Finder sidebar.

Bonjour, room 302B

On first boot, the ESP32 firmware brings up a temporary local-only interface and starts advertising an _http._tcp service via mDNS. The hostname is derived from the last four hex digits of the MAC, so a tech sees companion-302b.local resolve from any phone on the same network. Bonjour does the heavy lifting on iOS; Android picks it up through the standard NsdManager. No app to sideload, no QR code to scrape, no captive portal.

For the occasional Windows laptop on a clipboard cart, the firmware also registers a NetBIOS name as a fallback. Same short name, different resolver, so the device is reachable whether the tech's tooling speaks Apple's stack or Microsoft's.

Discovery beats data entry

The old way to do this was a registration form — type the MAC, type the room, hit submit. MAC addresses are twelve hex characters with a colon between every pair. Techs transpose digits, miss a colon, confuse a B for an 8. Then someone walks back through the wing with a flashlight. Service discovery skips the whole class of error: the device announces itself, the tech taps the one that matches the sticker on the bezel, and CareOS binds the pairing server-side.

An install that used to be a half-day of paperwork is now a walk down the hall with a phone. Once the binding is confirmed, the mDNS responder shuts down and Companion goes back to talking only to the CareOS realtime endpoint over cellular — quiet on the local network, exactly where it should be.