pixa

pixa is a GPL-3.0-licensed Go web server by @sneak 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

# 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 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, owned by root or by uid 65532 and gid 65532. The server runs as the container's pixad user, which has that uid and gid, and the container gives the directory to pixad 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. Toward a target of 1-5k r/s, pixa keeps in memory the content types of the 10,000 transformed images most recently cached or served, so a cache hit on one of them reads only the image file from disk and not the metadata file stored beside it.

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)

An image is served with Cache-Control: public, max-age=<seconds>, immutable. When the URL has an expiry (an exp, or the TTL of an encrypted URL), max-age is the whole seconds left until then, at most one year, so no browser or proxy cache keeps the image after pixa would refuse the URL. A URL with no expiry gets one year. immutable only stops a client revalidating while its copy is fresh.

The login form (POST /) is limited to 5 attempts per minute per client address, counting an IPv6 client by its /64; an attempt over the limit is refused with 429 and a Retry-After header. Behind a reverse proxy the client address comes from X-Forwarded-For only when the address pixa sees for requests that come through the proxy is in trusted_proxies; otherwise all users behind the proxy are counted as one client. That address is not always the proxy's own: a proxy on the Docker host that connects to pixa over 127.0.0.1 is seen as the gateway of the container's Docker network, such as 172.17.0.1 on the default bridge, and one that connects through another of the host's addresses is seen with that address. To be sure, read it as remoteIP in pixa's request log while it is not in trusted_proxies (see trusted_proxies under Configuration). With the default trusted_proxies (the RFC 1918 ranges), a client with a private address can choose the address it is counted by through its own X-Forwarded-For, whether it connects directly or through the proxy, because its own address is trusted too. Setting trusted_proxies to only the address pixa sees for requests that come through the proxy closes this.

Image Metadata

pixa decodes and re-encodes every image it serves, and removes all metadata from the output: EXIF (GPS position, camera make, model and serial number, capture time, embedded thumbnail), XMP, IPTC and the ICC colour profile. This cannot be turned off.

• The orig format means the source's own format, not the source's bytes: an orig image is re-encoded and stripped like any other.
• An image with an EXIF orientation is turned upright first, so it displays the same without the tag; a requested size applies to the upright image.
• An image with an ICC profile is converted to sRGB first, since clients show an image with no profile as sRGB. Colours outside sRGB, such as the most saturated ones in a Display P3 photo, are clipped.

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

The URL's sig is the HMAC-SHA256 result in base64url (the URL-safe alphabet of RFC 4648) with the trailing = padding kept, 44 characters in all. pixa compares it exactly, so a signature encoded without padding, as Node's base64url and Go's base64.RawURLEncoding do, is refused with 401.

Example: with the signing key example-signing-key-for-documentation, 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 of it with the signing key
3. Base64URL-encode the result, keeping the = padding: -ay7KHpfqmtIGbibDGbUuBDkymi-Ymdn0NkC6j5EJag=
4. URL: /v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=-ay7KHpfqmtIGbibDGbUuBDkymi-Ymdn0NkC6j5EJag=&exp=1704067200

For the same image at quality 40 with fit contain, the input ends in :40:contain, the signature is 5IwXUx6vf7yefhaUvFzgXZvG2o0Df4RJxPTK3pKq5VU=, and the URL is /v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=5IwXUx6vf7yefhaUvFzgXZvG2o0Df4RJxPTK3pKq5VU=&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_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 — CORS origin
• 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 from an address outside these ranges 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 the address pixa sees for requests that come through your proxy, such as 172.17.0.1/32, when the defaults do not cover it, or to trust nothing else (see the login limit under Routes). For a proxy on the Docker host that connects to pixa over 127.0.0.1, that address is the gateway of the container's Docker network (172.17.0.1 on the default bridge), not the proxy's own address; a proxy that connects through another of the host's addresses is seen with that address. To be sure which address it is, set this to [] (or PIXA_TRUSTED_PROXIES to empty), send a request through the proxy, and read remoteIP in pixa's request log line for it
• upstream_fetch_timeout — timeout for origin requests
• upstream_max_response_size — max origin response size
• downstream_timeout — client response 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 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 for the full prioritized task list.

License

GPL-3.0. See LICENSE.

Author

@sneak