---
type: concept
tags: [parking, architecture, devices, entry-flow]
sources: []
updated: 2026-06-15
---

# Device Input Flow (button → backend → relay)

How a physical button press drives the entry lane. The **backend is the source of truth**: the
device only *reports* the press; the host decides and commands the relay. This is the host-in-the-
loop flow the [[dingtian-relay]] makes possible (and the [[uhppote-controller]] could not).

## The path (no polling)

```
car arrives → driver presses button (input I_N, dry contact to GND)
  → device HTTP-pushes  GET …/api/devices/dingtian/<deviceId>/input/<N>/on
  → backend: emit internal device event (device-events bus)
  → backend entry flow: create + sign an entry event, print the ticket
  → backend: pulseOpen(N) over UDP  → barrier opens
  → (on release) device pushes …/input/<N>/off
```

- **Push, not poll.** The device's `input_link_url` feature is configured (by the driver's
  `configureInputPush()`) to call the backend on each input edge — see [[dingtian-relay]]. The
  driver's poll path remains only as a dev/fallback aid.
- **Per-input path** carries the input number in the URL (`…/input/3/on`), so routing needs no
  body parsing. Both edges (`on`/`off`) are sent.
- **Internal event bus** (`device-events.ts`, a Node `EventEmitter`) decouples the HTTP/transport
  layer from business logic — drivers/pushes emit; the entry flow subscribes. Keeps the app
  [[device-adapter-pattern|device-agnostic]].

## Trust model (important — flat network, no VLAN)

The site is a **flat network with no VLAN** ([[network-isolation]] is not yet enforceable here),
so we do **not** trust the device or the network. Both directions now have defence-in-depth, but
neither is the real boundary:

- **Relay control (host → device)** — UDP, now via the Dingtian **binary protocol on :60000 with a
  `relay_pw`** (the only authenticated relay option; the string protocol has none). Set on the
  device + stored in `devices` by the harden step (below).
- **Input push (device → host)** — guarded by **HTTP Digest auth** + a **source-IP allowlist**.
- **The real guarantee is the signed log:** every barrier open is a host decision, recorded as a
  signed event BEFORE the relay fires ([[append-only-event-chain]]). An out-of-band open (which a
  flat network allows) has **no matching signed event → a detectable anomaly**. Device/network
  auth is just speed bumps; both are plaintext over a sniffable network.
- This sharpens under the [[autonomous-direction|unmanned]] roadmap: with no operator, tamper
  detection via the signed log matters more than perimeter auth.

## Device hardening (on assign)

The assign/Save step configures the device end-to-end (admin never touches the device web UI):
fix preconditions (disable `input_link_relay`) → **harden** → set up input push. The `harden`
capability ([[device-registry|HardenableDevice]]):

- **Sets a random `relay_pw`** (1–9999) so binary relay commands need it; stores it in
  `devices` so the backend can keep commanding the relay.
- **Disables unused protocol channels** (rs485, can, tcp×2, mqtt → `p:255`), keeping only UDP1
  binary (relay control) + UDP2 string (status read) — fewer open doors.

> **⚠️ Lesson (the hard way):** do **NOT** enable the device's HTTP CGI session check
> (`session_en`). On this firmware (DT-R004) it makes the config-**read** API drop connections
> (`ECONNRESET`), locking the backend out of the very API it depends on — it required a **factory
> reset** to recover. The harden step deliberately leaves `session_en` off. The CGI config API
> being open is accepted as part of the flat-network reality (the signed log is the guarantee);
> the proper fix is network isolation, not this fragile device feature.

## Push authentication — Digest (decided by hardware testing)

The secret must not be in the URL (sniffable, logged) and the password must not cross the wire in
the clear. We **empirically tested the device** to pick the strongest achievable option:

| Option | Device result |
| --- | --- |
| HTTPS (self-signed) | ❌ device won't push to a self-signed cert |
| **Digest auth** (`auth=2`) | ✅ **works** — full 401-nonce challenge/response |
| Basic auth | ✅ works (but password base64 on the wire) |
| URL token | rejected by design (visible in URL/logs) |

→ **HTTP Digest** (MD5, qop=auth). The password is never sent (only a nonce-keyed hash); nonces
are **single-use** (replay resistance). Per-device credentials (`pushUser`/`pushPassword`) are
generated by the backend on **device assign**, written to the device's `input_link_url` config,
and stored in `devices` — the admin never types a URL or secret. HTTPS would be stronger but
the device can't do it here; Digest + the signed log is the practical answer on a flat network.
See `apps/server/src/digest-auth.ts`.

## Dingtian config-write gotchas (cost a lot of debugging)

Writing the device's config API (`/api/v2/config_set.cgi`) has two non-obvious traps — both now
handled in the driver:

1. **Content-Length is mandatory.** The device's embedded HTTP server does **not** accept chunked
   request bodies. Node uses chunked encoding when `Content-Length` is absent, so the device
   silently ignores the body and returns `{"status":0}` anyway — the write looks successful but
   nothing changes. Always set `Content-Length`.
2. **The `pass` field caps at 31 chars** (longer is silently truncated → Digest mismatch). The
   generated push password is 24 hex chars (96 bits).
3. (Also: the device reboots on apply, so the driver writes then **polls until the change is
   verified**, retrying — back-to-back writes onto a rebooting device are lost.)

## Status

Input push **verified on hardware** with Digest auth (all 4 inputs, real presses authenticated, no
failures). The entry
flow itself (signed event + ticket print + `pulseOpen`) is the next build — see [[dingtian-relay]].