NetWatch is an MIT-licensed JavaScript single-page application by @sneak 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

# 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 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)
• 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 in its frontend-test stage, and the backend's Go tests with the race detector and coverage. The frontend's production build is left to the image build, in the frontend-build stage
• 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 tests in Docker, without the production build
• 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). 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 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. Changing the interval clears each host's history on the page; the samples in it not yet reported still go in the next report. 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). 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
DEBUG	false	Enable debug logging
TRUSTED_PROXIES	RFC 1918 ranges	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
CORS_ALLOWED_ORIGINS	empty	Comma-separated origins whose pages may call the API; see CORS
METRICS_USERNAME	empty	Basic auth user name for /metrics; see Metrics
METRICS_PASSWORD	empty	Basic auth password for /metrics; see Metrics
SENTRY_DSN	empty	DSN of the Sentry project to send errors to; see Sentry

TRUSTED_PROXIES defaults to 10.0.0.0/8,172.16.0.0/12,192.168.0.0/16. A list replaces the default; set and empty, the server trusts no proxy. Loopback is not in the default, so a reverse proxy on the same host has to be named, as in TRUSTED_PROXIES=127.0.0.1/32. 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.

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, except TRUSTED_PROXIES.

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.

The reports of a failed write stay in memory, and the next flush, within a minute, tries again to write them. From the failed write until a write succeeds, POST /api/v1/reports refuses every report with 500 and keeps nothing of it, so the page keeps its samples and sends them again at its next report. Reports already answered 200 are lost only if the server stops before a write succeeds; the final flush at stop tries them too, and if it fails, the process exits non-zero.

Report limits

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

• JSON only. A report must be sent with Content-Type: application/json, parameters such as charset allowed, as the page sends it; any other is refused with 415, and nothing of it is stored. A page on another origin can send a text/plain POST, or a form, without a CORS preflight; refusing them keeps any site from making its visitors' browsers store reports (see CORS).
• 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 of a failed write included; 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), 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, by default from any address in 10.0.0.0/8, 172.16.0.0/12 or 192.168.0.0/16. 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 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, except TRUSTED_PROXIES, 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 10.0.0.0/8,172.16.0.0/12,192.168.0.0/16: the addresses the reverse proxy in front of the container connects from, as IP addresses or CIDRs 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. A list replaces the default. Set and empty, X-Forwarded-For is ignored and every client behind the proxy shares the proxy's one allowance of REPORTS_PER_MINUTE. Any client that connects from one of these addresses 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, which the default includes; where that matters, name only the addresses nothing but the proxy connects from.
  • 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: where the work stands, the next step, the open work, and what has been done.

License

MIT. See LICENSE.

Author

@sneak