smallwebwaf/README.md

# smallwebwaf

`smallwebwaf` is an MIT-licensed web application firewall, written in Go by
[@sneak](https://sneak.berlin), 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. A request it does not refuse reaches the app as it came,
apart from a few headers (see "How it sits in front of the app" below).

It needs no setting. Every setting has a default chosen for a service on the
open internet. The design is in [`SPEC.md`](SPEC.md). The survey of existing
tools that led to it is in [`EVALUATION.md`](EVALUATION.md). The reference
documents under [`docs/`](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 that runs those rules, inspects each
  request the earlier checks let through. 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`](docs/request-log.md));
- sends alerts to a webhook, Slack or ntfy (see
  [`docs/alerts.md`](docs/alerts.md));
- serves Prometheus metrics (see [`docs/metrics.md`](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/bans.md`](docs/bans.md) and
  [`docs/state-files.md`](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.

```sh
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`.

```dockerfile
# 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:

```bash
#!/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:

```sh
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` looks addresses up in a database file on
your own host (see [`docs/lookup.md`](docs/lookup.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 "Developing and testing" below.

## How it sits in front of the app

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`](docs/image.md) lists what the image holds and how each part is
pinned.

- The container's first process is `runsvinit`, which starts runit, a service
  supervisor. runit runs the `run` script of each directory under
  `/etc/service`, and runs it again a second after its program exits.
  `smallwebwaf` runs as the `smallwebwaf` user (uid 65532) and the app as a user
  of its own. So the app cannot touch the state files or the process of
  `smallwebwaf`.
- `smallwebwaf` listens on port 8080, on every address. traefik sends the app's
  requests there. `smallwebwaf` passes them 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`. The health check, the metrics and the ban management are on the
  same port, under `/_smallwebwaf/`. A request for that prefix never reaches the
  app.
- The app sees what it would see from traefik directly: the same `Host`, the
  same `X-Forwarded-Proto`, and `X-Forwarded-For` with traefik's address added
  at the end. The app's TCP peer is `smallwebwaf` on `127.0.0.1`. So the app
  must trust `127.0.0.1` and `::1` for forwarded headers, besides the private
  address ranges; left at the usual default, it sees every visitor as
  `127.0.0.1`. Each request also carries its id in `X-Request-ID`. Any
  `X-Client-ASN` or `X-Client-Country` header a client sent is removed.
- Both processes start with the container's environment variables. That is why
  every setting of `smallwebwaf` starts with `SWWAF_`. A token given as an
  environment variable is readable by the app. A token given as a file that only
  the `smallwebwaf` user can read is not (see "Settings given as files" in
  [`docs/settings.md`](docs/settings.md)).
- The state files live in `/var/lib/smallwebwaf`. Without a volume there, they
  survive a restart of the container and are lost when a deploy replaces it. The
  `run` script gives that directory and every file in it to the `smallwebwaf`
  user at each start, so a host directory mounted there needs no change of
  owner.
- The image's health check passes while `smallwebwaf` answers
  `GET /_smallwebwaf/healthz` and the app accepts connections. traefik sends a
  container no requests until it is healthy.
- When `smallwebwaf` or the app exits, runit starts it again a second later. The
  other process keeps running. While either is down, the client gets `502`. A
  `smallwebwaf` that stops at start, as on an invalid setting, is started again
  every second until the cause is fixed, and logs its message each time.
- `docker stop` has runit send `SIGTERM` to both processes at once.
  `smallwebwaf` gives the requests in progress five seconds to finish and writes
  its state files. To let them finish, the app must keep answering for five
  seconds after `SIGTERM`.

"Deployment" in [`SPEC.md`](SPEC.md) has the rest: moving `smallwebwaf` to
another port, the health check's timing, and the forwarded headers.

## What happens to a request

Two terms recur below. A clear sign of attack is a request that matches a rule
whose action is `ban`, or asks for a trap path (steps 9 and 10). An offence is a
request that counts against its client in its history. It is one of:

- a clear sign of attack;
- a broken limit;
- a refusal by a rule, by the Core Rule Set or for a missing or wrong token.

`smallwebwaf` handles each request in this order. The first step that refuses it
ends the handling. Nothing of a refused request reaches the app.

1. `GET /_smallwebwaf/healthz` is answered `200` with `ok` at once, before any
   check and without asking the app.
2. The client is worked out. A netblock is a range of addresses, written as an
   address and a prefix length: `203.0.113.0/24`. `SWWAF_TRUSTED_PROXIES` lists
   the netblocks traefik comes from, by default the private address ranges. For
   a TCP peer inside them, `X-Forwarded-For` is read from the right, and the
   first address outside those netblocks is the client. A peer outside them is
   the client itself. An IPv6 client is its group: the whole netblock around its
   address, a /64 by default; `SWWAF_IPV6_GROUP_PREFIX` sets its length.
3. A client in `SWWAF_ALLOW_NETS` skips steps 4 to 11 and the byte limits, but
   still needs a token at step 12. A client in `SWWAF_DENY_NETS` is refused.
4. A client under a ban is refused. If the ban was made for a clear sign of
   attack, this request makes it permanent.
5. The client's AS number and country are looked up, unless
   `SWWAF_LOOKUP_SOURCE` is `off`. An AS number names the network an address
   belongs to, such as `AS64496`.
6. The country lists are checked. A client from a country in
   `SWWAF_DENIED_COUNTRIES` is refused. When
   `SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES` is set, a client from any other country
   is refused, and so is a client without a country. Such a refusal is not an
   offence.
7. The reputation sources are checked: the blocklists, the CrowdSec decision
   list, the DNS blocklists and AbuseIPDB. A source the client is on refuses it,
   lowers its limits, or only notes it, as `SWWAF_BLOCKLIST_ACTION` and
   `SWWAF_REPUTATION_ACTION` say. No request waits for a query.
8. Rate limiting counts the request, unless the client or the path is exempt. A
   request that takes the client over a limit is refused and bans the client.
9. A request for a path in `SWWAF_TRAP_PATHS` is refused and bans the client, as
   a clear sign of attack.
10. The rule files are checked. A `log` rule notes the match. A `block` rule
    refuses the request. A `ban` rule refuses it and bans the client, as a clear
    sign of attack.
11. The Core Rule Set inspects the request's method, URL and headers, and the
    body too once `SWWAF_WAF_BODY_LIMIT` is set. A request it takes for an
    attack is refused while `SWWAF_WAF_MODE` is `block`, the default, and let
    through while it is `detect`. It bans no one by itself.
12. A request under `/_smallwebwaf/` is answered here by `smallwebwaf`, after
    its token is checked. Any other request is passed to the app. The app's
    answer is streamed back within the size and time limits.
13. Once the answer has ended, the request's bytes are counted for the byte
    limits, and its refusal, if any, for the error burst. The client's history
    and the anomaly counters are updated. More than
    `SWWAF_ERROR_BURST_THRESHOLD` refusals of one client in a minute ban it.

In `observe` mode (`SWWAF_MODE=observe`), `smallwebwaf` makes no ban of its own.
It refuses a request only for a size or time limit, or for a missing or wrong
token. A request it would otherwise have refused is passed to the app, and its
log line says what `enforce` mode would have done. Observe mode is for trying a
configuration before enforcing it.

What a refused visitor sees:

- `SWWAF_BAN_RESPONSE`, by default `403` with the plain text `Forbidden`. It can
  be `429` instead, or `close`, which closes the connection without an answer.
  It answers:
    - a ban or a broken rate limit;
    - a `ban` rule or a trap path;
    - `SWWAF_DENY_NETS` and the country lists;
    - a reputation source whose action is `deny`.
- `403` for a `block` rule and the Core Rule Set.
- `413`, `408`, `504` or `502` for a size or time limit, by which side passed
  it.
- `401` from an endpoint of `smallwebwaf`'s own, for a request without its
  token.

## Settings you are likely to touch

Every setting is an environment variable whose name starts with `SWWAF_`. Every
setting has a default, so none has to be set. Settings are read once, at start;
to change one, restart the container. A setting set to a value it cannot take
stops the start with a log line naming the variable and what is wrong.
[`docs/settings.md`](docs/settings.md) describes every setting, its default and
the values it takes. The ones most deployments touch:

- `SWWAF_MODE=observe`: log what would have been refused, refuse almost nothing,
  ban no one. For trying a configuration.
- `SWWAF_ALLOW_NETS`: netblocks that skip every check, such as your monitoring.
- `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.
- `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` or through the
  ban endpoint (see [`docs/bans.md`](docs/bans.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.

## Developing and testing

This repository adheres to the
[Scripts to Rule Them All](https://github.com/github/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 tests and the linter
run in Docker, as phases of the `Dockerfile`, and never on the host.

- `make bootstrap`: installs what the other scripts need on the host: `make`,
  `git` and `curl`; Go, for `gofmt`; node, yarn and prettier. `make setup` also
  installs the git pre-commit hook, which runs `make check`.
- `make check`: `make test` (tidy `go.mod` and `go.sum`, then the tests),
  `make lint` (golangci-lint) and `make fmt-check`. `make fmt` formats the Go
  code with `gofmt` and the Markdown with prettier.
- `make docker`: builds the image, whose build runs the tests and the linter
  first. `make tidy` rewrites `go.mod` and `go.sum` in the image's `tidy` stage.
- `make build`: builds `bin/smallwebwaf` with the Go on your host. `make run`
  builds it 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 example-app`: builds and checks the example app, then removes the
  containers, volumes and images it made. Needs network access.
- `make 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`](docs/gitea.md)
  records the results.
- `script/cibuild`: what CI runs: `script/bootstrap`, `script/check`, then the
  image build.

"Architecture" in [`SPEC.md`](SPEC.md) lists the packages and what each does.

## TODO

- Everything `SPEC.md` describes is built.
- Two points `SPEC.md` defers until there is experience to go on: sharing bans
  between the instances of several apps, and sending the log over RELP.

## Documents

- [`SPEC.md`](SPEC.md): the design.
- [`EVALUATION.md`](EVALUATION.md): what already exists, what each tool covers
  and misses, and why none was adopted.
- [`REPO_POLICIES.md`](REPO_POLICIES.md): the policies this repository follows.
- [`docs/settings.md`](docs/settings.md): every setting, its default and the
  values it takes.
- [`docs/bans.md`](docs/bans.md): how bans are made, how long they last, and how
  to lift or make one.
- [`docs/rules.md`](docs/rules.md): the rule files.
- [`docs/core-rule-set.md`](docs/core-rule-set.md): what the Core Rule Set
  inspects and how it scores a request.
- [`docs/admin-endpoints.md`](docs/admin-endpoints.md): the endpoints under
  `/_smallwebwaf/`.
- [`docs/request-log.md`](docs/request-log.md): the request log's fields, and
  sending the log to a syslog server.
- [`docs/alerts.md`](docs/alerts.md): the alert events, their contents and
  destinations.
- [`docs/metrics.md`](docs/metrics.md): the metrics.
- [`docs/state-files.md`](docs/state-files.md): the state files and editing
  them.
- [`docs/lookup.md`](docs/lookup.md): GeoJS and the lookup database.
- [`docs/reputation.md`](docs/reputation.md): blocklists, DNS blocklists,
  AbuseIPDB and CrowdSec.
- [`docs/image.md`](docs/image.md): what the image holds and how it is pinned.
- [`docs/gitea.md`](docs/gitea.md): `smallwebwaf` in front of gitea.

## License

MIT. See [`LICENSE`](LICENSE).

## Author

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