smallwebwaf/README.md

smallwebwaf

smallwebwaf is an MIT-licensed web application firewall, written in Go by @sneak, for one self-hosted web app. It runs inside the app's own container, in front of the app. It refuses the requests of scrapers, scanners and other abusers before they reach the app.

A web application firewall reads each request on its way to a web app and refuses the ones that look like abuse. smallwebwaf is also a reverse proxy: it accepts requests on the app's behalf, passes them to the app, and passes the app's answers back.

It needs no setting. Every setting has a default chosen for a service on the open internet. SPEC.md says how smallwebwaf behaves and why. The reference documents under docs/ are listed under "Documents" below.

What it protects against

• Scrapers that fetch every page of a site, all day. Rate limiting gives each client 1000 requests a minute, 10,000 an hour and 50,000 a day. Byte limits give it 10 GiB a minute, 20 GiB an hour and 50 GiB a day. A client over a limit is refused and banned. The first ban lasts an hour. Each repeat within a day makes the next ban three times as long. A ban that would last more than seven days is permanent.
• Scanners that probe for secrets, such as /.env or /.git/config. The default rule file bans such a client for seven days at its first probe. Its next request during the ban makes the ban permanent. The default rule file does not cover probes for other apps, such as a WordPress login. A ban rule of your own, or an entry in SWWAF_TRAP_PATHS, bans those the same way.
• Attacks carried in a request's URL or headers, such as SQL injection and path traversal. The OWASP Core Rule Set is a published set of rules that recognise common web attacks. Coraza, a Go engine, runs those rules (see "Attack detection" in SPEC.md). A request the rules take for an attack is refused.
• Abuse from networks and countries you do not want. You can:
  • refuse whole netblocks and countries;
  • give the clients of chosen networks and countries a fraction of every limit, so that they are banned sooner;
  • refuse or limit clients on a blocklist or a DNS blocklist, or with a high AbuseIPDB score;
  • ban clients named by your own CrowdSec engine.
• Slow and oversized requests. Every request and every answer has a time limit and a size limit.

For the operator, it:

• writes one JSON log line for every request, saying what it did and why (see docs/request-log.md);
• sends alerts to a webhook, Slack or ntfy (see docs/alerts.md);
• serves Prometheus metrics (see docs/metrics.md);
• keeps its bans, counters and history in JSON files you can read and edit, so that a restart loses nothing and a ban can be lifted with an editor (see docs/state-files.md).

Not built, and not planned:

• TLS and routing for several apps. Both stay with traefik, the reverse proxy in front of the container.
• Browser challenges such as captchas.
• A web console.
• Defence against floods large enough to fill the host's network link.

Getting started

Build the image from a clone. make docker runs the tests and the linter, then builds the image, tagged smallwebwaf. It builds for amd64, on an amd64 host only: the Dockerfile checks Ubuntu's package lists against the hashes of the amd64 archive.

git clone https://git.eeqj.de/sneak/smallwebwaf.git
cd smallwebwaf
make docker

Push the image to a registry your hosts pull from. Then build the app's own image on it. The app's Dockerfile starts FROM the smallwebwaf image, named by digest, and adds:

• the app's binary;
• the packages the app needs, from the nixpkgs in the image;
• a user for the app;
• a run script that starts the app listening on 127.0.0.1:8081.

# The smallwebwaf image, pinned by digest.
FROM <registry>/smallwebwaf@sha256:<digest>

# Packages the app needs, if any, from the nixpkgs in the image.
RUN nix-env -iA nixpkgs.git

# The app's binary, and a user of its own to run it.
COPY app /usr/local/bin/app
RUN useradd --system --no-create-home --shell /usr/sbin/nologin app

# The app's runit service.
COPY --chmod=755 app.run /etc/service/app/run

app.run sits beside the Dockerfile. --listen and --trusted-proxies stand for the app's own options:

#!/usr/bin/env bash
set -euo pipefail

main() {
    sleep 1
    exec chpst -u app:app /usr/local/bin/app \
        --listen 127.0.0.1:8081 \
        --trusted-proxies 10.0.0.0/8,172.16.0.0/12,192.168.0.0/16,127.0.0.1/32,::1/128
}

main "$@"

Leave ENTRYPOINT and USER out of the app's Dockerfile. The image's own entrypoint starts smallwebwaf and the app side by side, each as its own user.

Deploy the app's image as you deploy any app. Point traefik's labels at port 8080 (traefik.http.services.<name>.loadbalancer.server.port=8080). Mount a volume at /var/lib/smallwebwaf, so that bans and history survive a deploy. For example, with no setting:

docker run --detach --name app \
    --volume app-smallwebwaf:/var/lib/smallwebwaf \
    <registry>/app@sha256:<digest>

With no setting, the address of every new visitor is sent to GeoJS, a web service, to find its country and network. SWWAF_LOOKUP_SOURCE=off sends nothing. SWWAF_LOOKUP_SOURCE=file, with SWWAF_LOOKUP_DB_PATH naming a database file on your own host, looks addresses up in that file (see "Lookup of AS number and country" in SPEC.md).

deploy/example-app is such an app. make example-app builds it and checks it (see "Checking that it works" below). To run smallwebwaf outside the image while working on it, see "Entrypoints" below.

What happens to a request

smallwebwaf handles each request in fifteen steps, in this order. Once a step refuses a request, the steps after it are skipped, apart from step 15, and nothing of the request reaches the app. "What happens to a request" in SPEC.md gives each step in full, under the same numbers.

1. The health check, GET /_smallwebwaf/healthz, is answered at once.
2. The client is identified.
3. The static lists, SWWAF_ALLOW_NETS and SWWAF_DENY_NETS.
4. The bans.
5. The lookup of the client's AS number and country.
6. The country lists.
7. The reputation sources.
8. The client's limit percentage.
9. The rate limits.
10. The trap paths.
11. The rule files.
12. The Core Rule Set.
13. smallwebwaf's own endpoints, under /_smallwebwaf/.
14. The request is passed to the app.
15. After the answer: the byte limits, the error burst and the client's history.

A refused visitor is answered with SWWAF_BAN_RESPONSE, 403 by default (see "Ban lengths, response and scope" in SPEC.md). A block rule or a Core Rule Set match answers 403. A missing or wrong token gets 401. A request over a size or time limit gets the status for that limit, such as 413 for a body too large (see "Size and time limits" in SPEC.md).

Settings you are likely to touch

Each setting is an environment variable of the app's container, such as docker run -e SWWAF_MODE=observe ..., read at start. docs/settings.md lists every setting, its default and the values it takes. The ones most deployments touch:

• SWWAF_MODE=observe: smallwebwaf makes no ban of its own, and refuses a request only for a size or time limit or a missing or wrong token. A request it would have refused is passed to the app, and its log line says what enforce mode would have done. For trying a configuration before enforcing it.
• SWWAF_ALLOW_NETS: netblocks you trust, such as your monitoring. Their clients are refused only for a size or time limit or a missing or wrong token.
• SWWAF_RATE_LIMIT_EXEMPT_NETS: netblocks exempt from rate limiting and the byte limits, such as a machine that talks to the app all day.
• SWWAF_TRAP_PATHS: paths only scanners ask for, such as /wp-login.php,/xmlrpc.php in front of a code forge. A request for one bans the client.
• SWWAF_WAF_DISABLED_RULES: Core Rule Set rule ids to switch off when a rule refuses your app's ordinary requests. The log line names the rules a request matched. Keep the six default ids in a list you give (see docs/settings.md).
• SWWAF_REQUEST_MAX_BYTES, SWWAF_CLIENT_REQUEST_TIMEOUT and SWWAF_UPSTREAM_REQUEST_TIMEOUT: raise them for an app that takes uploads over 100 MiB or slower than 60 seconds.
• SWWAF_LOOKUP_SOURCE: geojs (the default), file or off, as above.
• SWWAF_ADMIN_TOKEN and SWWAF_METRICS_TOKEN: switch on the ban endpoints and the metrics. Give them as files to keep them from the app.
• SWWAF_ALERT_WEBHOOK_URL, SWWAF_ALERT_SLACK_WEBHOOK_URL or SWWAF_ALERT_NTFY_URL: where alerts go.
• SWWAF_INSTANCE_NAME: the name in every log line, alert and metric, such as fsn1app1/gitea.

Checking that it works

• docker inspect --format '{{.State.Health.Status}}' <container> says healthy once smallwebwaf answers and the app accepts connections. A log line with "msg":"starting" gives the version, the address and every setting in effect, with secrets masked.
• A request to the site through traefik gets the app's answer, and its log line has "action":"forward". https://<site>/_smallwebwaf/healthz answers ok.
• To see a refusal without banning yourself, run the container with SWWAF_MODE=observe and ask for /.env. The request reaches the app. Its log line has "would_action":"banned" and "rule_ids":["env-file"]. In enforce mode the same request bans your address for seven days, and your next request makes the ban permanent. Lift the ban by editing bans.json (see "Bans" in SPEC.md).
• With SWWAF_METRICS_TOKEN set, curl -H "Authorization: Bearer $TOKEN" https://<site>/_smallwebwaf/metrics answers with the metrics.
• make example-app builds the image and the example app on it, runs the app, and checks it. It needs network access. It checks:
  • the health check;
  • a request forwarded to the app;
  • a ban for a broken limit, and a ban for a probe made permanent;
  • orderly stops with sv stop and docker stop;
  • bans surviving a new container on the same volume;
  • a setting given as a file.

Entrypoints

This repository adheres to the Scripts to Rule Them All standard. The scripts in script/ are the entrypoints for working on it, and the Makefile targets are thin shims that call them. The scripts are POSIX sh. The tests and the linter run in Docker, as phases of the Dockerfile, and never on the host.

• script/bootstrap: installs on the host what the other scripts need, with make bootstrap:
  • make, git and curl;
  • Go, for gofmt;
  • node, yarn and prettier.
• script/setup: readies a fresh clone: script/bootstrap, then script/install-precommit. make setup.
• script/projectname: prints smallwebwaf, which the other scripts tag their images with.
• script/test: checks that go.mod and go.sum are tidy, then runs the tests, as the test phase of the Dockerfile. make test.
• script/tidy: writes go.mod and go.sum as go mod tidy writes them, in the tidy stage of the Dockerfile. make tidy.
• script/lint: runs golangci-lint, as the lint phase of the Dockerfile. make lint.
• script/fmt: formats the Go code with gofmt and the Markdown with prettier. make fmt.
• script/fmt-check: checks the formatting and changes nothing. make fmt-check.
• script/check: runs script/test, script/lint and script/fmt-check. make check.
• script/docker: builds the image, whose build runs the tests and the linter first. make docker.
• script/cibuild: what CI runs: script/bootstrap, script/check, then the image build.
• script/precommit: run by the git pre-commit hook; runs script/check.
• script/install-precommit: installs that hook. make hooks.
• script/build: builds bin/smallwebwaf with the Go on your host. make build.
• script/run: builds bin/smallwebwaf and runs it with the settings in your environment, listening on port 8080 in front of an app at SWWAF_UPSTREAM_URL. Its state files go in bin/state and its rule files are those of share/rules.d, unless SWWAF_STATE_DIR and SWWAF_RULES_DIR say otherwise. make run.
• script/example-app: builds the image and the example app on it, runs it with a volume, and checks it as "Checking that it works" above describes. Then it removes the containers, volumes and images. Needs network access. make example-app.
• script/gitea-requests: builds the image and runs gitea 1.27.3 behind it at the defaults. It sends through it a sample of gitea's ordinary use and the requests the Core Rule Set is known to refuse, and prints what smallwebwaf did with each. Needs network access, and a docker daemon that pushes to a registry on localhost over plain HTTP. docs/gitea.md records the results. make gitea-requests.

Rationale

Small self-hosted sites receive a great deal of traffic nobody asked for:

• scrapers that ignore robots.txt and crawl every commit of every repository on a public git server;
• scanners walking through lists of WordPress and .env paths;
• credential-guessing bots.

Most of it comes from a small number of hosting networks and countries. A single-person operation has no abuse desk and no CDN contract. It needs something small that can be put in front of one service and left alone.

The existing tools each solve part of this. Rule-based firewalls catch attack payloads but do not limit request rates. Rate limiters count requests but cannot tell a residential visitor from a rented server farm. The products that do most of it want several containers, a database and a web console. None of them can say "clients from these networks are banned after half as many requests as anyone else". That is the useful thing to say when nearly all abuse comes from a known list of AS numbers. EVALUATION.md goes through the candidates one by one.

smallwebwaf fills that gap with:

• per-client limits that ban;
• lower limits for the networks and countries abuse comes from;
• bans that grow with repetition;
• a log that is easy to search and ship;
• one binary in the app's own container, with no database and no required setting.

Design

The smallwebwaf image is built on Ubuntu 26.04 LTS, pinned by digest, with nixpkgs fixed at one release. An app's Dockerfile installs a package with nix-env -iA nixpkgs.<name> and gets the same package each time it is built. docs/image.md lists what the image holds and how each part is pinned.

• runit, a service supervisor, runs smallwebwaf and the app side by side, each as its own user, and starts either again when it exits. The app cannot touch the state files or the process of smallwebwaf.
• smallwebwaf listens on port 8080 and passes requests to the app on 127.0.0.1:8081. The app listens on 127.0.0.1 only, so nothing reaches it except through smallwebwaf. smallwebwaf's own endpoints are under /_smallwebwaf/, on the same port.
• The app sees the headers traefik sent, with traefik's address added to X-Forwarded-For, from a TCP peer on 127.0.0.1. So the app must trust 127.0.0.1 and ::1 for forwarded headers, besides the private address ranges.
• Both processes start with the container's environment variables, so every setting of smallwebwaf starts with SWWAF_, and the app can read a token given as one. A token given as a file that only the smallwebwaf user can read is out of its reach (see "Settings given as files" in docs/settings.md).
• The state files live in /var/lib/smallwebwaf. A volume mounted there keeps them across a deploy.
• docker stop stops both processes at once. smallwebwaf gives the requests in progress five seconds to finish, so the app must keep answering for five seconds after SIGTERM.

"Deployment" in SPEC.md has the rest: the health check, what happens when a process stops, moving smallwebwaf to another port, and the forwarded headers. "Architecture" in SPEC.md lists the packages and what each does.

TODO

• Planned, as "Planned, not built" in SPEC.md describes:
  • sharing bans between the instances of several apps;
  • sending the log over RELP.

Documents

• SPEC.md: how smallwebwaf behaves, and why.
• EVALUATION.md: what already exists, what each tool covers and misses, and why none was adopted.
• REPO_POLICIES.md: the policies this repository follows.
• docs/settings.md: every setting, its default and the values it takes.
• docs/rules.md: the rule file format and the default file.
• docs/admin-endpoints.md: the endpoints under /_smallwebwaf/, with examples.
• docs/request-log.md: the request log's fields.
• docs/alerts.md: the alert events and what each destination gets.
• docs/metrics.md: the metrics.
• docs/state-files.md: the state files and what each holds.
• docs/image.md: what the image holds and how it is pinned.
• docs/gitea.md: smallwebwaf in front of gitea.

License

MIT. See LICENSE.

Author

@sneak