pixa/README.md

# pixa

pixa is a GPL-3.0-licensed Go web server by
[@sneak](https://sneak.berlin) that proxies images from upstream
sources, optionally resizing or transforming them, and serves the
results. Both source and transformed images are cached to disk so that
subsequent requests are served without origin fetches or additional
processing.

## Getting Started

```bash
# clone and build
git clone https://git.eeqj.de/sneak/pixa.git
cd pixa
make build

# run with a config file: copy the example and set a real signing key
# (the example placeholder is refused at startup), e.g. with
# openssl rand -base64 32
cp config.example.yml config.yml
$EDITOR config.yml   # replace the signing_key placeholder
./bin/pixad --config config.yml

# or build and run via Docker
make docker
docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixa:latest
```

A container takes its settings from environment variables (see
Configuration below for the list). Only `PIXA_SIGNING_KEY` is required; if
it is unset the container exits at startup naming the variable. Everything
else has a built-in default. A config file mounted at `/etc/pixa/config.yml`
is optional: it is read when present, and an environment variable wins over
the same setting in it.

## Running under upaas

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

- **Port:** pixa listens on container port `8080`.
- **Volume:** container path `/var/lib/pixa`, where pixa keeps its
  database and cache. upaas bind-mounts the host path it is given and
  does not create it, so the host directory must exist before the first
  deploy.
- **Environment variables:**
    - `PIXA_SIGNING_KEY` (required): secret for signed and encrypted URLs
      and login, 32+ characters, for example from
      `openssl rand -base64 32`
    - `PIXA_ALLOWLIST_HOSTS`: upstream hosts served without a signature,
      comma-separated
    - `PIXA_CACHE_MAX_BYTES`: disk cache limit in bytes; `0` disables it;
      default 75% of free space
    - the rest are in the table under Configuration below
- **Health check:** the image's `HEALTHCHECK` requests
  `/.well-known/healthcheck.json`. upaas reads the container's health 60
  seconds after a deploy and marks the deploy failed unless it is
  `healthy`. The probe uses the port from `PORT` (default `8080`), so a
  port changed only in a mounted config file is not seen by it: change
  the port with `PORT`.
- **First run:** create the host directory. It may be owned by root: the
  container gives it to its `pixad` user when it starts.

## Rationale

Image-heavy web applications need a fast, caching reverse proxy that
can resize and transcode images on the fly. pixa fills that role as a
single, self-contained binary with no external runtime dependencies
beyond libvips. It supports HMAC-SHA256 signed URLs with expiration to
prevent abuse, and allowlisted source hosts for open access.

## Design

### Storage

- **Source content**:
  `<statedir>/cache/src-content/<ab>/<cd>/<sha256 of source content>`
- **Source metadata**:
  `<statedir>/cache/src-metadata/<hostname>/<sha256 of path>.json`
  (fetch time, original headers, request, content hash)
- **Database**: `<statedir>/state.sqlite3` (SQLite)
- **Output documents**:
  `<statedir>/cache/dst-content/<ab>/<cd>/<sha256 of output content>`

Multiple source paths may reference the same content blob; the
database tracks references rather than using filesystem refcounting.
In-process caching of request-to-output mappings targets 1-5k r/s.

### Routes

```
/v1/image/<host>/<path>/<size>.<format>?sig=<signature>&exp=<expiration>
```

Images are only fetched from origins using TLS with valid certificates.

A request whose query string cannot be decoded, or gives any parameter more
than once, is refused with 400.

- `<format>`: one of `orig`, `png`, `jpeg`, `webp`
- `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)

### Source Hosts

Source hosts may be allowlisted in the configuration. Non-allowlisted
hosts require an HMAC-SHA256 signature.

#### Signature Specification

Signatures use HMAC-SHA256 and include an expiration timestamp to
prevent replay attacks. Signatures are **exact match only**: every
component (host, path, query, dimensions, format, expiration, quality,
fit) must match exactly what was signed. No suffix matching, wildcard
matching, or partial matching is supported.

**Signed data format** (colon-separated):

```
HMAC-SHA256(secret, "host:path:query:width:height:format:expiration:quality:fit")
```

Where:

- `host` — source origin hostname (e.g. `cdn.example.com`)
- `path` — source path (e.g. `/photos/cat.jpg`)
- `query` — source query string, empty string if none
- `width` — requested width in pixels, `0` for original
- `height` — requested height in pixels, `0` for original
- `format` — output format (jpeg, png, webp, avif, gif, orig)
- `expiration` — the URL's `exp` query parameter, the Unix timestamp when
  the signature expires; a request whose `exp` is not a whole number, an
  empty `exp=` included, is refused with 400
- `quality` — the URL's `q` query parameter, a whole number from 1 to 100,
  or `85` when the URL has no `q`; a request whose `q` is anything else is
  refused with 400
- `fit` — the URL's `fit` query parameter (cover, contain, fill, inside,
  outside), or `cover` when the URL has no `fit`; a request whose `fit` is
  anything else, an empty `fit=` included, is refused with 400

**Example:** resize `https://cdn.example.com/photos/cat.jpg` to 800x600
WebP with expiration 1704067200, default quality and fit:

1. Build input:
   `cdn.example.com:/photos/cat.jpg::800:600:webp:1704067200:85:cover`
2. Compute HMAC-SHA256 with your secret key
3. Base64URL-encode the result
4. URL:
   `/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=<base64url>&exp=1704067200`

For the same image at quality 40 with fit `contain`, the input ends in
`:40:contain` and the URL is
`/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=<base64url>&exp=1704067200&q=40&fit=contain`.

**Allowlist patterns:**

- **Exact match**: `cdn.example.com` — matches only that host
- **Suffix match**: `.example.com` — matches `cdn.example.com`,
  `images.example.com`, and `example.com`

### Configuration

Every setting can be given as an environment variable, in a YAML config
file (`--config`), or both. A variable present in the environment wins over
the file, even when it is empty, and the file wins over the built-in
default. The one exception is a variable named in the file's `env:` section:
it is set while the file loads, so it overrides both the environment the
process was started with and the file's own key. A variable's value is
parsed as the same text in the file would be. The three lists take
comma-separated entries, with the spaces around each trimmed; an empty
variable is an empty list. A value that does not parse or is invalid aborts
startup, naming the variable. A variable whose name starts with `PIXA_` but
is not in the table below, such as a misspelled one or `PIXA_PORT`, aborts
startup naming it, as an unknown config key does. The one other accepted
name is `PIXA_CONFIG_PATH`, the config file's path (like `--config`). The
variables set by the file's `env:` section are checked the same way.

| Variable                             | Config key                      | Meaning                                                                      |
| ------------------------------------ | ------------------------------- | ---------------------------------------------------------------------------- |
| `PIXA_SIGNING_KEY`                   | `signing_key`                   | Required: secret for signed and encrypted URLs and login, 32+ characters     |
| `PORT`                               | `port`                          | Port to listen on; default `8080`                                            |
| `PIXA_STATE_DIR`                     | `state_dir`                     | Directory for the database and the disk cache; default `/var/lib/pixa`       |
| `PIXA_DB_URL`                        | `db_url`                        | SQLite database URL; default `state.sqlite3` in the state directory          |
| `PIXA_CACHE_MAX_BYTES`               | `cache_max_bytes`               | Disk cache limit in bytes; `0` disables it; default 75% of free space        |
| `PIXA_ALLOWLIST_HOSTS`               | `allowlist_hosts`               | Upstream hosts served without a signature                                    |
| `PIXA_BLOCKED_NETWORKS`              | `blocked_networks`              | CIDR ranges never fetched from, on top of the built-in ones                  |
| `PIXA_TRUSTED_PROXIES`               | `trusted_proxies`               | CIDR ranges of proxies whose `X-Forwarded-For` is believed; default RFC 1918 |
| `PIXA_ALLOW_HTTP`                    | `allow_http`                    | Allow plain-HTTP upstreams, for testing only; default `false`                |
| `PIXA_UPSTREAM_CONNECTIONS_PER_HOST` | `upstream_connections_per_host` | Concurrent connections per upstream host; default `20`                       |
| `PIXA_UPSTREAM_FETCH_TIMEOUT`        | `upstream_fetch_timeout`        | Time allowed for one fetch from an upstream host; default `30s`              |
| `PIXA_UPSTREAM_MAX_RESPONSE_SIZE`    | `upstream_max_response_size`    | Largest upstream response accepted, in bytes; default 50 MiB                 |
| `PIXA_DOWNSTREAM_TIMEOUT`            | `downstream_timeout`            | Time allowed for answering one client request; default `60s`                 |
| `PIXA_ACCESS_CONTROL_ALLOW_ORIGIN`   | `access_control_allow_origin`   | CORS origin allowed to read responses: `*` or one origin; default `*`        |
| `PIXA_METRICS_USERNAME`              | `metrics.username`              | Username for `/metrics`, which is served only when both are set              |
| `PIXA_METRICS_PASSWORD`              | `metrics.password`              | Password for `/metrics`; set together with the username                      |
| `PIXA_SENTRY_DSN`                    | `sentry_dsn`                    | Sentry DSN for error reporting; empty disables it                            |
| `PIXA_DEBUG`                         | `debug`                         | Debug logging and plain-HTTP local development; default `false`              |
| `PIXA_MAINTENANCE_MODE`              | `maintenance_mode`              | Maintenance flag reported by the health check; default `false`               |

Key settings in more detail:

- `access_control_allow_origin` — the origin a browser lets read pixa's
  responses, sent as the CORS `Access-Control-Allow-Origin` header: `*`, the
  default, is any site; otherwise one origin, scheme and host only, such as
  `https://example.com`. Anything else aborts startup
- `allowlist_hosts` — list of allowed upstream hosts
- `blocked_networks` — list of CIDR ranges to refuse for SSRF protection,
  added to the always-enforced built-in ranges (loopback, private,
  link-local, CGNAT, benchmark, NAT64, and the like); an invalid CIDR
  aborts startup
- `trusted_proxies` — list of CIDR ranges of the reverse proxies in front
  of pixa. `X-Forwarded-For` is believed only when the direct peer falls
  inside one of these ranges; the logged and login-recorded client
  address is then the rightmost forwarded entry that is not itself a
  trusted proxy. Otherwise the direct peer address is used and the header
  is ignored, so a client connecting directly cannot spoof its address.
  An omitted key defaults to the RFC 1918 private ranges (`10.0.0.0/8`,
  `172.16.0.0/12`, `192.168.0.0/16`), since pixa is deployed behind a
  proxy on a private network; an explicitly empty list (`[]`) trusts no
  one, and an explicit list replaces the default. An invalid CIDR aborts
  startup. Set this to your proxy's address range if it is not already
  covered by the defaults
- `upstream_fetch_timeout` — time allowed for one fetch from an upstream
  host, as a duration such as `30s` (the default) or `2m`
- `upstream_max_response_size` — largest upstream response accepted, in
  bytes; default `52428800` (50 MiB). It also limits the image data pixa
  decodes
- `downstream_timeout` — time allowed for answering one client request, as a
  duration; default `60s`. The upstream fetch counts toward it, so keep it
  longer than `upstream_fetch_timeout`
- `signing_key` — HMAC secret for URL signatures
- `cache_max_bytes` — disk cache size limit in bytes; `0` disables the
  disk cache entirely; omitted defaults to 75% of the free space on
  the filesystem containing `<state_dir>/cache/` (minimum 500 MiB)

See `config.example.yml` for all options with defaults.

### Architecture

- **Dependency injection**: Uber fx
- **HTTP router**: go-chi
- **Image processing**: govips (CGO wrapper for libvips)
- **Database**: SQLite via modernc.org/sqlite
- **Static assets**: embedded via `//go:embed`
- **Metrics**: Prometheus
- **Logging**: stdlib slog

## 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. We provide:

- `script/bootstrap` — install all dependencies (idempotent)
- `script/setup` — make a fresh clone ready for development
  (bootstrap, then install-precommit)
- `script/projectname` — output the project name ("pixa")
- `script/test` — run the test suite
- `script/lint` — run golangci-lint, always in a container (builds
  `Dockerfile.lint` when run outside one)
- `script/fmt` — format all code (writes)
- `script/fmt-check` — check formatting (read-only)
- `script/check` — run test, lint, and fmt-check
- `script/docker` — build the Docker image tagged via `script/projectname`
- `script/docker-smoke` — build the image, start it, wait for it to be healthy
- `script/cibuild` — CI entrypoint: `docker build .` (the Dockerfile
  runs the checks, so a green build implies a green repo)
- `script/precommit` — pre-commit checks (`go mod tidy` guard, then
  `script/check`)
- `script/install-precommit` — install the git pre-commit hook that
  runs `script/precommit`

## TODO

See [TODO.md](TODO.md) for the full prioritized task list.

## License

GPL-3.0. See [LICENSE](LICENSE).

## Author

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