netwatch/README.md

NetWatch is an MIT-licensed JavaScript single-page application by
[@sneak](https://sneak.berlin) that provides real-time network latency
monitoring to common internet hosts, displayed with color-coded figures and
sparkline graphs, served from a static bucket or from its Docker image, where a
small Go backend stores the measurements the page reports.

## Getting Started

```bash
# Install the dependencies and the git pre-commit hook
make setup

# Run the page on the Vite dev server
make dev

# Build netwatch-server and run it on port 8080
make run

# Run the tests, both linters and the format check
make check

# Build the page into dist/ and netwatch-server into the repo root
make build

# Build the image and run it
make docker
docker run -p 8080:8080 netwatch
```

`make check` and `make docker` need Docker. `make dev` passes `/api` to
`http://127.0.0.1:8080`, where `make run` starts `netwatch-server` with its
defaults, so the reports the page posts are stored in `data/reports`.

## Entrypoints

This repository adheres to the
[Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
standard: normalized scripts in `script/` are the entrypoints for the
development workflow, and the Makefile targets are thin shims that call them.
The scripts cover both the frontend and the Go backend, so `make check` fails if
either one is broken. We provide:

- `script/bootstrap` — install all dependencies (the pinned node via nvm unless
  one new enough for the frontend's dependencies is installed, yarn via
  corepack, `yarn install --frozen-lockfile`, Go `GO_VERSION` unless the `go` on
  `PATH` is exactly that version, and the Go modules), linking what it installs
  itself into `~/.local/bin`, which has to be on `PATH`. It installs no Go
  linter and not Docker: `make test` and `make lint` run in Docker
- `script/setup` — make a fresh clone ready for development: bootstrap plus the
  git pre-commit hook
- `script/dev` — run the Vite dev server, which proxies `/api` to a locally
  running `netwatch-server`
- `script/build` — build the frontend for production into `dist/`, then the
  static `netwatch-server` binary into the repo root with its version stamped
  in: `VERSION` from the environment, or when that is unset or empty,
  `git describe` inside a git checkout, then `dev`
- `script/run` — build with `script/build`, then run `netwatch-server` with its
  defaults (see [Settings](#settings))
- `script/clean` — remove what `script/build` writes
- `script/projectname` — print the project name (used for the Docker image tag)
- `script/test` — build the `test` phase of `Dockerfile` without the cache: the
  frontend's unit tests and production build in its `frontend` stage, and the
  backend's Go tests with the race detector and coverage
- `script/lint` — build the `lint` phase of `Dockerfile` without the cache:
  eslint in its `frontend-lint` stage, and golangci-lint over the Go packages
- `script/fmt` — format all files (writes): prettier over the JavaScript, CSS,
  HTML and Markdown, then gofmt over `cmd/` and `internal/`. It runs on the
  host, as `script/fmt-check` does, with `~/.local/bin` put on `PATH`
- `script/fmt-check` — check formatting (read-only): prettier, then gofmt
- `script/check` — run test, lint, and fmt-check
- `script/add-dependency` — add a frontend package, or move one to another
  version: `make add-dependency PACKAGE=<name>@<version>` runs `yarn add --dev`,
  which changes `package.json` and `yarn.lock` together, then
  `yarn install --frozen-lockfile`
- `script/tidy` — run `go mod tidy`: to add a Go module, import it and run
  `make tidy`; to move one to another version, edit its `require` line in
  `go.mod`, then run `make tidy`
- `script/frontend-test` — run the unit tests in `test/unit/` on the host with
  Node's built-in test runner, through the `test` script in `package.json`, and
  if any fails, run them again listing every test, and fail; then the production
  build. Each test run has a 90-second timeout. `make test` runs the same in
  Docker
- `script/frontend-fmt` — format the JavaScript, CSS, HTML and Markdown with
  prettier (writes)
- `script/frontend-fmt-check` — check prettier formatting (read-only)
- `script/frontend-viewport-test` — responsive-layout verification of the built
  frontend in a containerised headless Chrome (see
  [test/viewport/README.md](test/viewport/README.md)). Not part of
  `script/check`: it takes minutes.
- `script/docker` — build the image from `Dockerfile` without the build cache,
  tagged `netwatch` via `script/projectname`
- `script/cibuild` — CI entrypoint: runs `script/bootstrap` and `script/check`,
  then builds the image as `script/docker` does, without the build cache
- `script/precommit` — run by the git pre-commit hook; runs `script/check`
- `script/install-precommit` — install the git pre-commit hook

## Responsive layout

The narrow-viewport layout lives in the `max-width: 768px` media block in
`web/src/styles.css`. It is verified automatically by
`make frontend-viewport-test`, which drives a digest-pinned headless Chrome
against the built `dist/` and asserts on computed layout at widths derived from
that CSS — on every breakpoint it declares and one pixel either side of it, plus
a 320px floor, a desktop baseline and two landscape sizes. See
[test/viewport/README.md](test/viewport/README.md) for what it covers and what
it genuinely cannot.

## Rationale

When debugging network issues, it's useful to have a persistent at-a-glance view
of latency and reachability to multiple well-known internet endpoints. NetWatch
provides this as a single page that does all its measuring in the browser, so it
can be served from anywhere static files are served. The backend in its Docker
image only stores the measurements the page reports; without it, the page works
the same and nothing is stored.

## Design

The page's source is in `web/`, the Go backend's in `cmd/` and `internal/`, and
nginx's configuration in `configs/`.

The page is built with Vite and Tailwind CSS v4. Its code is all in
`web/src/main.js`, with a class-based architecture:

- **`CONFIG`**: Configuration object (update interval, timeouts, axis ticks,
  etc.). The interval menu sets `updateInterval`, the one value the page writes
  into `CONFIG`; the timeouts, the time the history spans and the x-axis ticks
  are computed from it
- **`HostState`**: Per-host state management — history buffer, latency tracking,
  status transitions
- **`AppState`**: Top-level state container — WAN hosts, local hosts, pause
  state, aggregate stats
- **`SparklineRenderer`**: Canvas 2D sparkline drawing with fixed axes,
  color-coded line segments, error regions, and DPR-aware scaling
- **UI functions**: `buildUI()` constructs the DOM, `updateHostRow()` /
  `updateSummary()` / `updateHealthBox()` handle incremental updates
- **`tick()`**: Main loop — measures all hosts in parallel, pushing each host's
  sample and redrawing its row as soon as its check ends, then redraws every
  row, the summary and the health box once the last check ends. The first round,
  after loading or an interval change, is discarded. The rows are sorted when
  the last check ends in round 2, the first one kept, and in rounds 11, 21, 31
  and so on. When paused, pushes blank markers (no probes, no false outage)
- **`Reporter`**: Posts collected samples to the backend

### Reporting

Every `reportInterval` (default 60s) the page POSTs a JSON report to the
same-origin path `/api/v1/reports`: a random per-browser `clientId` kept in
`localStorage`, `geo` sent as null, and each host's unreported, non-paused
samples (timestamp, latency, error). A per-host high-water mark makes every
report a delta, so only new samples are sent; the mark advances only on a
delivered report, and while paused nothing is sent. Delivery failure is quiet —
one debug-log line per outage, retried at the next interval, never blocking
probing. The report-building step is a pure function of host state.

### Backend

`netwatch-server` is a small Go HTTP server that stores the reports the page
posts. It keeps them in memory and writes them to `DATA_DIR` as zstd-compressed
files of JSON lines: every minute, whenever 10 MiB are waiting, and when it
stops. Its routes:

- `POST /api/v1/reports` — takes a report, without credentials; each client
  address may send a limited number a minute, and the report files are capped in
  size, the oldest deleted first
- `GET /.well-known/healthcheck` — answers 200 with `"status":"ok"`, the
  server's version and its uptime
- `GET /metrics` — Prometheus metrics behind basic auth, only when
  `METRICS_USERNAME` and `METRICS_PASSWORD` are set; each client address may
  make a limited number of requests to it a minute

It is an `fx`-wired Go application: `cmd/netwatch-server/` starts it, and its
packages are in `internal/`:

- **`config`**: Loads configuration from environment variables and config files
  via Viper.
- **`handlers`**: HTTP request handlers for the API (health check, report
  ingestion).
- **`reportbuf`**: The in-memory buffer of reports and the files it writes them
  to.
- **`server`**: Chi-based HTTP server with middleware wiring and route
  registration.
- **`healthcheck`**, **`middleware`**, **`logger`**, **`globals`**: Supporting
  infrastructure.

In the image, the `test` phase of `Dockerfile` tests it, the `builder` stage
builds it, and `bin/entrypoint.sh` runs it as user `netwatch` on
`127.0.0.1:8081`, behind nginx (see [Deployment](#deployment)). Outside the
image, `make run` builds it and runs it on port 8080.

#### Settings

| Variable               | Default              | Description                                                                                              |
| ---------------------- | -------------------- | -------------------------------------------------------------------------------------------------------- |
| `BIND_ADDRESS`         | empty                | IP address to listen on; empty listens on every interface                                                |
| `PORT`                 | `8080`               | HTTP listen port                                                                                         |
| `DATA_DIR`             | `./data/reports`     | Directory for compressed reports                                                                         |
| `DATA_DIR_MAX_BYTES`   | `1073741824` (1 GiB) | Most bytes of report files kept in `DATA_DIR`, oldest deleted first; see [Report limits](#report-limits) |
| `DEBUG`                | `false`              | Enable debug logging                                                                                     |
| `TRUSTED_PROXIES`      | loopback + RFC1918   | Comma-separated CIDRs whose `X-Forwarded-For` / `X-Real-IP` headers are trusted for client IP resolution |
| `REPORTS_PER_MINUTE`   | `60`                 | Reports each client address may send a minute; see [Report limits](#report-limits)                       |
| `CORS_ALLOWED_ORIGINS` | empty                | Comma-separated origins whose pages may call the API; see [CORS](#cors)                                  |
| `METRICS_USERNAME`     | empty                | Basic auth user name for `/metrics`; see [Metrics](#metrics)                                             |
| `METRICS_PASSWORD`     | empty                | Basic auth password for `/metrics`; see [Metrics](#metrics)                                              |
| `SENTRY_DSN`           | empty                | DSN of the Sentry project to send errors to; see [Sentry](#sentry)                                       |

`TRUSTED_PROXIES` defaults to
`127.0.0.1/32,::1/128,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`. The loopback
entries cover a reverse proxy on the same host. A request whose direct peer is
outside this set has its forwarded headers ignored, and the direct peer is
logged and rate-limited instead. The container image does not use this default;
see [Deployment](#deployment).

A variable set to a value the server cannot use, such as `PORT=abc`,
`DEBUG=maybe` or a `BIND_ADDRESS` that is not an IP address, stops it from
starting, with an error naming the variable. An empty variable counts as unset.

#### Report storage

Reports are written as `reports-<timestamp>-<number>.jsonl.zst` files in
`DATA_DIR`. The timestamp is in UTC to the millisecond, so the names sort by
time. The number starts at 1 when the server starts and goes up by one for each
file the server starts to write, so two files written in the same millisecond
still get different names. A failed write uses up its number and leaves a gap in
the numbers: its file, if it was created, is removed. The file stays, counted
toward `DATA_DIR_MAX_BYTES` from the next start, only if removing it fails too.
Files are created with `O_EXCL` to prevent overwrites.

#### Report limits

`POST /api/v1/reports` takes reports from anyone who can reach it, without
credentials, so it is bounded instead. Both refusals below answer with the same
`{"status":"error"}` body as any other error.

- **Rate limit.** Each client address, resolved through `TRUSTED_PROXIES`, may
  send `REPORTS_PER_MINUTE` reports a minute; past that it gets 429 with
  `Retry-After: 60`. The minute slides: reports from the minute before still
  count, fading out over the current one, so an address is sure never to be
  refused only while it sends at most half of `REPORTS_PER_MINUTE` in any 60
  seconds. The page sends one report a minute from each open tab, so the default
  of 60 refuses nothing from up to 30 tabs behind one address, such as a
  household or an office sharing it, however their reports bunch up. Report
  responses also carry `X-RateLimit-Limit`, `X-RateLimit-Remaining` and
  `X-RateLimit-Reset` headers.
- **Size cap.** The report files in `DATA_DIR` may total at most
  `DATA_DIR_MAX_BYTES`, counting the files already there at start. Reports
  waiting in memory count at their uncompressed size until they are written;
  those lost to a failed write stop counting, and the part of its file written
  is removed. When a report would take the total past the cap, the oldest report
  files are deleted to make room, and each deletion is logged with the file's
  name and size; a file still being written is never deleted. A report is
  refused with 507, and nothing of it is stored, only when the reports waiting
  to be written fill the cap on their own, and then no file is deleted. At
  start, report files past the cap, as after lowering it, are deleted the same
  way. So the cap is how much of the newest reports is kept: the default of 1
  GiB is small enough for any host; set it to the space you can give `DATA_DIR`.

#### CORS

The page calls the API from the origin it is served from, so by default the
server sends no CORS headers, and browsers let no other origin's pages call it.
To serve the page from elsewhere, list that origin in `CORS_ALLOWED_ORIGINS`
(for example `https://netwatch.example.com`); pages from a listed origin may
`GET` and `POST` with a `Content-Type` header. Each entry must be a plain
origin, `scheme://host` with an optional `:port`, as browsers send it: no path,
not even a trailing `/`, and no `*`. Any other entry stops the server from
starting, with an error naming `CORS_ALLOWED_ORIGINS`.

#### Metrics

With both `METRICS_USERNAME` and `METRICS_PASSWORD` set, the server serves
Prometheus metrics at `GET /metrics` to requests with those as their basic auth
credentials, and answers any other with 401. For each request that reaches the
health check or `POST /api/v1/reports`, those the rate limit refuses included,
the metrics record its duration and response size, labelled with its path,
method and status; they also count those requests in progress, and include Go's
runtime and process metrics. No other request is recorded: not those to
`/metrics` itself, and not those answered before they reach either route, such
as a CORS preflight, or a request refused with 404 for a path no route has, 405
for a method its route does not take, or 413 for declaring a body length over
the 1 MiB limit. A report whose body goes over the limit without declaring its
length reaches the route, is answered 413 there, and is recorded with that
status. Clients can make up any number of paths and methods, and each would add
labels to the metrics for as long as the server runs.

`/metrics` is rate limited, so that its password cannot be guessed quickly: each
client address, resolved through `TRUSTED_PROXIES`, may make 60 requests to it a
minute, whatever their credentials. Past that it gets 429 with
`Retry-After: 60`, and its credentials are not checked. The minute slides as it
does for reports (see [Report limits](#report-limits)), so a scraper polling
every 2 seconds or less often is never refused. This allowance is apart from the
one for reports.

#### Sentry

With `SENTRY_DSN` set, each panic in a handler is reported to that Sentry
project under the release `netwatch-server-` followed by the server's version.
On shutdown the server waits up to 2 seconds for Sentry to finish sending.

### Monitoring targets

- **26 WAN hosts**: datavi.be (pinned at start), Anthropic API, OpenAI API, AWS
  Console, Google Cloud Console, Microsoft Azure, Cloudflare, Fastly CDN,
  Akamai, Google, GitHub, B2, 8 S3 regional endpoints (Cape Town, London,
  Bahrain, Tokyo, Singapore, Sydney, Oregon, São Paulo) and 6 Hetzner speed test
  servers (Nuremberg, Falkenstein, Helsinki, Ashburn, Hillsboro, Singapore)
- **Local CPE**: Cable modem at 192.168.100.1 (always monitored)
- **Local Gateway**: Auto-detected on startup by probing common default gateway
  addresses (192.168.1.1, 192.168.0.1, 192.168.8.1, 10.0.0.1); first responder
  wins. Note: modern browsers enforce Private Network Access restrictions that
  block public-origin pages from reaching RFC1918 addresses, so local targets
  only work when NetWatch is served from localhost or a private address.

Local hosts are tracked separately from WAN stats.

### Latency measurement

GET requests to each target's URL as written, with `mode: 'no-cors'` and
`cache: 'no-store'`, timed with `performance.now()`. No query string is added:
the Hetzner speed-test servers close the connection without an answer when the
URL has one, and `no-store` keeps the browser's cache out of the measurement.
Each check times out after 80% of the refresh interval (24 seconds at 30
seconds) and is then recorded as a timeout, so a round's checks have all
finished before the next round is due. When no WAN host answers, a recovery
probe checks 4 WAN hosts, picked at random when it starts, every half second,
giving up the checks it started half a second before. As soon as one answers, a
new round starts at once, as it does after an interval change. A round started
early gives up the last round's checks if they are still waiting, and that round
records nothing more, so rounds never overlap. The browser chooses between IPv4
and IPv6 for each target, as for any request; the local targets are IPv4
addresses.

Each recorded check that fails writes one line to the browser console with
`console.error`, and the same line to the debug log: the target's name and URL,
the time, whether it timed out, answered over the time limit or hit a network
error (with the error the browser gives the page, such as
`TypeError: Failed to fetch`, which does not say why), and how long the request
took. A target that answers after failed checks writes one `console.info` line
with its latency and how many checks in a row had failed.

### Color coding

| Latency     | Color  |
| ----------- | ------ |
| < 50ms      | Green  |
| < 100ms     | Lime   |
| < 200ms     | Yellow |
| < 500ms     | Orange |
| >= 500ms    | Red    |
| Unreachable | Gray   |

### Output structure

```
dist/
├── index.html
└── assets/
    ├── index-*.css
    └── index-*.js
```

## Features

- A round of checks every 3 seconds by default; the interval menu sets 1, 2, 3,
  5, 10, 15, 30 or 60 seconds and clears the history
- Sparklines of each target's last 100 rounds: 300 seconds at 3 seconds
- The first round after loading or an interval change is discarded, as DNS and
  TLS setup inflate its latencies
- Health indicator from the WAN hosts' latest results: OFFLINE (red) when more
  than 10 fail and at most 4 answer, otherwise DEGRADED (orange) when more than
  4 fail, otherwise SLOW (yellow) when more than 3 take over 1000ms, otherwise
  HEALTHY (green)
- Summary stats across WAN hosts only: how many answered, the min, median,
  average and max of their latest latencies, the min and max over the whole
  history, and the number of rounds run (`Checks`)
- Fixed chart axes: Y-axis 0–1000ms, higher latencies drawn at the top; X-axis
  the time the history spans
- Color-coded latency figures and sparkline line segments
- WAN host rows sorted by latest latency, unreachable last; pinned rows stay on
  top, in name order
- Play/pause: pause stops probes but history keeps scrolling (blank gaps, no
  false outage)
- Debug log panel, behind a checkbox in the footer, with five levels (error,
  warning, notice, info, debug) and the last 1000 lines
- Local and UTC clocks
- Clickable service URLs
- A footer link to the commit the page was built from
- Canvas-based sparkline rendering with devicePixelRatio scaling
- Zero runtime dependencies: all resources bundled into build artifacts

## Deployment

`make build` writes the page to `dist/`, which any static file host (S3, GCS,
Cloudflare Pages, Vercel, Netlify, GitHub Pages) can serve; with no backend
there, its reports fail quietly and nothing is stored. Or run the Docker image
behind a reverse proxy.

The Docker image, built from `Dockerfile` by `make docker`, is the whole service
in one container: nginx serves the built frontend and passes `/api/`,
`/.well-known/healthcheck` and `/metrics` to the Go backend, `netwatch-server`.
`bin/entrypoint.sh` starts it listening only inside the container, on
`127.0.0.1:8081`, with `TRUSTED_PROXIES=127.0.0.1/32`, so it takes the client
address nginx passes on and no other. The image:

- Listens on port 8080 by default (override with `PORT` env var)
- Takes the client address from `X-Forwarded-For` only on requests from the
  reverse proxies named in `TRUSTED_PROXIES`, and by default from none.
  `bin/entrypoint.sh` checks each entry with `netwatch-server check-cidr`, which
  parses it as the backend parses its own `TRUSTED_PROXIES`
- Sends access logs to stdout
- Caches static assets with immutable headers
- Sends the security headers `REPO_POLICIES.md` requires on every response, as
  `configs/security-headers.conf` sets them, in place of the backend's own
- Stores reports in `DATA_DIR`, `/data/reports` by default, on the `/data`
  volume. Before the backend starts, the image creates `DATA_DIR` and gives it
  and `/data` to user `netwatch` (uid 1000), which the backend runs as, with
  `netwatch-server prepare-data-dir`, which acts on nothing outside `/data`, so
  a host directory bind-mounted at `/data` ends up owned by uid 1000
- Writes buffered reports to disk on `docker stop`, and exits non-zero if nginx
  or the backend exits on its own, so the platform restarts it

## Running under upaas

What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:

- **Port:** container port `8080`.
- **Volume:** container path `/data`; the reports are kept in `/data/reports`.
- **Environment variables:** none is required. An empty one counts as unset, and
  one set to a value netwatch cannot use stops the container at start, with the
  reason in its log.
    - `PORT`, default `8080`: the container port, from 1 to 65535. `8081` cannot
      be used: the backend listens on it inside the container
    - `REPORTS_PER_MINUTE`, default `60`: reports each client address may send a
      minute
    - `DATA_DIR_MAX_BYTES`, default `1073741824` (1 GiB): the most room the
      report files may take; the oldest are deleted to stay under it
    - `CORS_ALLOWED_ORIGINS`, default empty: other origins whose pages may call
      the API
    - `DEBUG`, default `false`: debug logging
    - `DATA_DIR`, default `/data/reports`: the directory the reports are kept
      in: `/data` or a path below it, with no `.` or `..` part and no extra `/`.
      The container also stops if the path goes through a symbolic link that
      leads out of `/data` or is written as a full path
    - `TRUSTED_PROXIES`, default empty: set it to the address the reverse proxy
      in front of the container connects from, as an IP address or CIDR; several
      are separated by commas. nginx takes the client address from
      `X-Forwarded-For` only on a request from one of them, and the rate limit
      counts that address. Unset, `X-Forwarded-For` is ignored and every client
      behind the proxy shares the proxy's one allowance of `REPORTS_PER_MINUTE`.
      Name only addresses nothing but the proxy connects from: any client that
      connects from one can write its own `X-Forwarded-For`, and through a port
      Docker publishes, every client may connect from the Docker network's
      gateway, such as `172.17.0.1`.
    - `METRICS_USERNAME` and `METRICS_PASSWORD`, default empty: with both set,
      the backend records Prometheus metrics of its requests and serves them at
      `/metrics` on the container port, to requests with this user name and
      password as their basic auth credentials. With neither set, there are no
      metrics and `/metrics` is not found. One set without the other, or a user
      name containing `:`, stops the container
    - `SENTRY_DSN`, default empty: set to a Sentry project's DSN, the backend
      sends its errors to that Sentry project: each request whose handling
      crashes, which still gets a 500 response. A value Sentry does not accept
      stops the container. Empty, the backend sends nothing to Sentry
- **Health check:** the image's `HEALTHCHECK` requests
  `/.well-known/healthcheck` through nginx every 30 seconds, so it fails unless
  both nginx and the backend answer. upaas reads the container's health 60
  seconds after a deploy and fails the deploy unless it is `healthy`. The
  container also stops when either process exits.

## Browser Compatibility

Requires a modern browser with ES modules, Fetch API, Canvas API, and CSS custom
properties.

## Limitations

- **CORS**: The checks are cross-origin requests in `no-cors` mode, so the page
  cannot read the answer, only time it: any answer counts as reachable, an error
  page included.
- **Local targets**: The cable modem at 192.168.100.1 and the detected gateway
  answer only on a network that has them, and only when NetWatch is served from
  localhost or a private address (see Monitoring targets).
- **Network conditions**: Measurements reflect browser-to-endpoint latency,
  which includes your local network, ISP, and internet routing.

## TODO

The to-do list is [TODO.md](TODO.md): where the work stands, the next step, the
open work, and what has been done.

## License

MIT. See [LICENSE](LICENSE).

## Author

[@sneak](https://sneak.berlin)