smallwebwaf/docs/settings.md

# Settings

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 that is set to a value it cannot take stops the start. The process
writes one line, `"type":"process"` with `"msg":"invalid setting"`, naming the
variable and what is wrong with the value. It exits with status 1, and runit
starts it again every second until the value is fixed. The entries below say
which values each setting takes. Any other value stops the start. At start, the
settings in effect are logged on one line. Tokens, keys and the paths of
webhooks are shown as `********`.

The forms of values:

- A duration is Go's syntax, with `d` for days: `90s`, `15m`, `24h`, `7d`. Zero
  and negative durations are refused.
- A size is a number of bytes with an optional `K`, `M` or `G`, which are powers
  of 1024: `1K` is 1024 bytes, `512K`, `100M`, `5G`.
- A count is a whole number of requests, such as `1000`.
- `off` switches a setting off, where its entry below says it takes `off`.
- A list is comma separated, with spaces around items ignored. An empty item is
  refused. A list set but empty is an empty list, and replaces the default.
- A netblock is an address and a prefix length, such as `10.0.0.0/8`. A bare
  address stands for itself alone.
- A country is a two-letter code of ISO 3166-1, in either case: `de` and `DE`
  are the same. `xk` stands for Kosovo. A code that is not a country is refused,
  such as `nk` (North Korea is `kp`) or the withdrawn `su`.
- An AS number is `AS` and the number, in either case, such as `AS64496`.
- A percentage is a whole number from 0 to 100.
- A path prefix starts with `/`, such as `/assets/`.

## Where it listens and what it protects

- `SWWAF_LISTEN_ADDR` (default `:8080`): where `smallwebwaf` listens, an
  optional host and a port. Keep the host empty: `:9000`, never
  `127.0.0.1:9000`. The app must not listen on that port. The health check
  follows the setting, and traefik's label must name the port.
- `SWWAF_UPSTREAM_URL` (default `http://127.0.0.1:8081`): the app. An `http` or
  `https` URL with a host and an optional port, and nothing more: no path, user
  or query.
- `SWWAF_INSTANCE_NAME` (default: the container's host name, the first 12
  characters of the container's id): the `instance` name in every log line,
  alert and metric. Set it to a name that survives deploys and tells instances
  apart, such as `fsn1app1/gitea`. It must be valid UTF-8. While
  `SWWAF_ALERT_NTFY_URL` is set, it may hold no control character. While
  `SWWAF_LOG_REMOTE_URL` is set and `SWWAF_LOG_REMOTE_APP_NAME` is not, it must
  be 1 to 48 printable ASCII characters without a space.
- `SWWAF_MODE` (default `enforce`): `enforce`, or `observe` to ban no one and
  log what `enforce` would have done. Observe mode still refuses a request over
  a size or time limit, and one without the right token.
- `SWWAF_TRUSTED_PROXIES` (default `10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`,
  the private address ranges): the netblocks whose `X-Forwarded-For` is
  believed, normally where traefik comes from. Set but empty, it trusts nothing:
  every TCP peer is then the client.
- `SWWAF_IPV6_GROUP_PREFIX` (default `64`): the length of the netblock that is
  one IPv6 client, from 32 to 128. After a change, each IPv6 client starts
  afresh. Existing bans keep refusing their netblocks until they end.
- `SWWAF_MAX_TRACKED_CLIENTS` (default `20000`): the most clients kept in memory
  and in `clients.json`, a whole number above zero. Past it, the least recently
  seen client is dropped first.

## Time and size limits

Each request has limits on both sides: between the client and `smallwebwaf`, and
between `smallwebwaf` and the app. Bodies stream through in both directions,
except the part the Core Rule Set reads while `SWWAF_WAF_BODY_LIMIT` is set.

- `SWWAF_CLIENT_REQUEST_TIMEOUT` (default `60s`): how long a client may take to
  send its request line and headers, and then, from the end of the headers, its
  body. A duration or `off`.
- `SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES` (default `32K`): the largest request
  line and headers. A size over `4K`. It cannot be `off`: Go's HTTP server
  always has such a limit.
- `SWWAF_CLIENT_IDLE_TIMEOUT` (default `120s`): how long a kept-open connection
  may wait for its next request before it is closed. A duration or `off`. The
  default is longer than the 90 seconds after which traefik closes an unused
  connection, so traefik closes first.
- `SWWAF_CLIENT_RESPONSE_TIMEOUT` (default `30m`): how long the answer may take
  to reach the client, from the end of the request to the last byte. A duration
  or `off`.
- `SWWAF_UPSTREAM_REQUEST_TIMEOUT` (default `60s`): how long connecting to the
  app and sending it the whole request may take. A duration or `off`.
- `SWWAF_UPSTREAM_RESPONSE_TIMEOUT` (default `30m`): how long the app may take
  to send its whole answer. A duration or `off`.
- `SWWAF_REQUEST_MAX_BYTES` (default `100M`): the largest request body. A size
  or `off`. A request that announces a larger body is refused before anything
  reaches the app.
- `SWWAF_RESPONSE_MAX_BYTES` (default `5G`): the largest response body. A size
  or `off`.

In front of a code forge, a clone is a response and a push is a request. At the
defaults, an upload over 100 MiB or slower than 60 seconds is cut off. A forge
that takes large uploads needs `SWWAF_REQUEST_MAX_BYTES`,
`SWWAF_CLIENT_REQUEST_TIMEOUT` and `SWWAF_UPSTREAM_REQUEST_TIMEOUT` raised to
fit.

What the client gets when a limit is passed is under "Size and time limits" in
[`SPEC.md`](../SPEC.md).

## Netblock lists

- `SWWAF_ALLOW_NETS` (default empty): netblocks whose clients skip every check,
  such as your monitoring or your own network. The size and time limits still
  apply to them. They are not looked up. They still need a token for the admin
  endpoints.
- `SWWAF_RATE_LIMIT_EXEMPT_NETS` (default empty): netblocks whose clients are
  exempt from rate limiting and the byte limits, such as a machine that talks to
  the app all day. Everything else still applies to them.
- `SWWAF_DENY_NETS` (default empty): netblocks whose clients are always refused.
  A client in both lists is let through.

## Rate limiting and byte limits

- `SWWAF_RATE_LIMIT_PER_MINUTE` (default `1000`), `SWWAF_RATE_LIMIT_PER_HOUR`
  (default `10000`) and `SWWAF_RATE_LIMIT_PER_DAY` (default `50000`): the most
  requests a client may make in each window. A count or `off`. The defaults are
  several times what one busy person produces. A browser loading a heavy page
  makes a few hundred requests, and several people often share one address.
- `SWWAF_RATE_LIMIT_EXEMPT_PATHS` (default empty): path prefixes exempt from
  rate limiting, such as `/assets/`. Their bytes are not counted either. The
  path is matched exactly as sent, without the query, and is not
  percent-decoded. `/assets/` matches `/assets/app.js`. It does not match
  `/Assets/app.js`, `/%61ssets/app.js` or `/static/assets/app.js`. There are no
  wildcards. A path holding `..`, a backslash or an encoded slash (`%2F`) is
  never exempt, since the app may read it as a path outside the prefix.
- `SWWAF_BYTES_LIMIT_PER_MINUTE` (default `10G`), `SWWAF_BYTES_LIMIT_PER_HOUR`
  (default `20G`) and `SWWAF_BYTES_LIMIT_PER_DAY` (default `50G`): the most
  bytes a client may have counted in each window. A size or `off`. Bytes are
  counted once the answer has ended, so a byte limit never cuts an answer short.
  The client's next request is refused instead. Each default is above the
  largest request and answer together, so at the defaults no single download
  breaks a limit.
- `SWWAF_BYTES_COUNT` (default `both`): which body bytes the byte limits count:
  `response`, `request` or `both`. For a WebSocket, what it carried each way
  counts once it closes.

## Looking up the AS number and country

- `SWWAF_LOOKUP_SOURCE` (default `geojs`): `geojs`, `file` or `off`. `geojs`
  asks the GeoJS web service, which is then told the address of every new
  visitor. `file` looks clients up in the IPinfo Lite database file at
  `SWWAF_LOOKUP_DB_PATH`, and sends nothing off the host. `off` looks up no
  client. With `off`, a setting that needs lookups stops the start:
    - a country list;
    - `SWWAF_ADD_LOOKUP_HEADERS`;
    - an entry in `SWWAF_ASN_LIMIT_PERCENT`, `SWWAF_COUNTRY_LIMIT_PERCENT`,
      `SWWAF_ASN_BYTES_PERCENT` or `SWWAF_COUNTRY_BYTES_PERCENT`;
    - `SWWAF_UNKNOWN_LIMIT_PERCENT` below 100;
    - `SWWAF_ASN_LIMIT_PERCENT_URL`;
    - an anomaly threshold per AS number.
- `SWWAF_LOOKUP_DB_PATH` (default empty): the IPinfo Lite database file, in its
  `.mmdb` form, for `SWWAF_LOOKUP_SOURCE=file`. `file` without it stops the
  start, and so does setting it with any other source. A file that is missing or
  cannot be read stops the start too.
- `SWWAF_LOOKUP_TIMEOUT` (default `1s`): how long a request waits for its
  client's first answer from GeoJS when a setting needs it, and how long a
  request to GeoJS may take. A duration; it cannot be `off`.
- `SWWAF_ADD_LOOKUP_HEADERS` (default `false`): `true` passes the app the
  client's AS number in `X-Client-ASN` and its country in `X-Client-Country`. A
  header whose value is unknown is left out. Whatever this setting says, any
  such header the client sent is removed.

See [`lookup.md`](lookup.md) for what each source sends where, and how long
answers are kept.

## Country lists

- `SWWAF_DENIED_COUNTRIES` (default empty): countries whose clients are refused,
  such as `cn,ru,kp`.
- `SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES` (default empty): when set, the only
  countries whose clients get through, such as `us,de`. A client whose country
  cannot be found is refused too. Otherwise every new client would be let in
  whenever GeoJS stops answering. A client on a private address has no country,
  so it is refused unless it is in `SWWAF_ALLOW_NETS`.

A country on both lists stops the start. What a refused request gets, and why
the lists are empty by default, is under "Country lists" in
[`SPEC.md`](../SPEC.md).

## Lower limits by AS number and country

Clients of the AS numbers and countries listed here get a percentage of every
rate limit and byte limit. A lowered limit is rounded down. "Biased thresholds"
in [`SPEC.md`](../SPEC.md) says why the lists are empty by default and why no
budget is shared by a whole AS number or country.

- `SWWAF_ASN_LIMIT_PERCENT` (default empty): a list of AS numbers, each with `:`
  and a percentage, such as `AS14061:50,AS16276:50,AS45102:25`. An AS number
  listed twice is refused.
- `SWWAF_COUNTRY_LIMIT_PERCENT` (default empty): the same by country, such as
  `cn:25,ru:50`.
- `SWWAF_ASN_BYTES_PERCENT` and `SWWAF_COUNTRY_BYTES_PERCENT` (default empty):
  the same for the byte limits alone. For the AS numbers and countries they
  list, they take the place of the two settings above.
- `SWWAF_UNKNOWN_LIMIT_PERCENT` (default `100`): the percentage of every limit
  for a client without a country:
    - one the lookup cannot place;
    - one on a private address;
    - one whose answer from GeoJS has not come in time.
- `SWWAF_ASN_LIMIT_PERCENT_URL` (default unset): the `http` or `https` URL of a
  text file of such entries, one to a line, such as `AS14061:50`. One list can
  then serve several instances. It is fetched and kept as a blocklist is. Where
  it and `SWWAF_ASN_LIMIT_PERCENT` both give a percentage, the lower applies. A
  URL that is also in `SWWAF_BLOCKLIST_URLS` stops the start.

A client to which several percentages apply gets the lowest. The log line of
each counted request, and the notes of each ban, say which setting lowered the
limit.

## Reputation sources

None is on by default. A reputation source judges a client by what others saw it
do. The defaults judge it by what it does to your service.

- `SWWAF_BLOCKLIST_URLS` (default empty): blocklists to fetch, such as
  `https://www.spamhaus.org/drop/drop.txt`. Each is an `http` or `https` URL
  without a user or a fragment. A URL listed twice is refused.
- `SWWAF_BLOCKLIST_REFRESH` (default `24h`): how long a list waits after its
  last fetch, or failed try, before it is fetched again. A duration of `1h` or
  more; it cannot be `off`. The Spamhaus lists may be fetched no more than once
  an hour.
- `SWWAF_BLOCKLIST_ACTION` (default `deny`): what is done with a client on a
  blocklist. `deny` refuses its requests. `limit:<percent>`, such as `limit:25`,
  gives it that percentage of every limit. `log` only notes the list in the log
  line and raises an alert.
- `SWWAF_DNSBL_ZONES` (default empty): the DNS blocklists to ask about each
  client, such as `dnsbl.dronebl.org`. A DNS blocklist answers a DNS query for a
  name made from an address, and so says whether the address is on it. A zone is
  a DNS name of at most 189 characters. A Spamhaus zone carries its key, as in
  `<key>.xbl.dq.spamhaus.net`, and is shown with the key masked. A zone listed
  twice is refused, whatever its case or key. Do not name a zone meant for mail:
  such a zone holds the residential addresses ordinary visitors come from.
- `SWWAF_DNSBL_RESOLVER` (default unset): the resolver to ask the zones through,
  such as `192.0.2.53` or `[2001:db8::53]:5353`. An IP address with an optional
  port, 53 when none is given. Unset, the host's resolver is used. Several zones
  refuse queries that come through a public resolver.
- `SWWAF_ABUSEIPDB_KEY` (default unset): the key of your AbuseIPDB account.
  While it is set, a client that has committed an offence is checked. A control
  character in it is refused.
- `SWWAF_ABUSEIPDB_MIN_SCORE` (default `75`): the least abuse confidence score,
  from 0 to 100, that is a hit.
- `SWWAF_ABUSEIPDB_DAILY_BUDGET` (default `900`): the most checks made in a day,
  in UTC, a whole number above zero. AbuseIPDB's free accounts may make 1,000.
- `SWWAF_CROWDSEC_LAPI_URL` (default unset): the local API of a CrowdSec engine
  you run, such as `http://172.17.0.1:8080`. An `http` or `https` URL without a
  user or a fragment. Its decision list is at `v1/decisions` under that URL, and
  is fetched every minute. Set without the key, it stops the start. So does a
  decision list URL that is also in `SWWAF_BLOCKLIST_URLS`, or that is
  `SWWAF_ASN_LIMIT_PERCENT_URL`.
- `SWWAF_CROWDSEC_LAPI_KEY` (default unset): the key the engine gave for
  `smallwebwaf`, as `cscli bouncers add smallwebwaf` prints it. Set without the
  URL, it stops the start.
- `SWWAF_REPUTATION_ACTION` (default `limit:25`): what is done with a client on
  a DNS blocklist, or one whose AbuseIPDB score is a hit. `deny`,
  `limit:<percent>` or `log`, as for `SWWAF_BLOCKLIST_ACTION`. Such a verdict is
  less certain than the Spamhaus DROP list, so by default the client gets a
  quarter of every limit.
- `SWWAF_REPUTATION_CACHE_TTL` (default `24h`): how long a zone's verdict or
  AbuseIPDB's score is used. A duration; it cannot be `off`.
- `SWWAF_REPUTATION_TIMEOUT` (default `2s`): how long a query to a zone or a
  check with AbuseIPDB may take. A duration; it cannot be `off`.

See [`reputation.md`](reputation.md) for how each source is asked and what it
does with the answer.

## Attack detection

- `SWWAF_RULES_DIR` (default `/etc/smallwebwaf/rules.d`): the directory of the
  rule files (see [`rules.md`](rules.md)). A directory that cannot be read stops
  the start. An empty one is no error.
- `SWWAF_RULES_ENABLED` (default `true`): `false` reads no rule file.
- `SWWAF_TRAP_PATHS` (default empty): paths the app never serves and only
  scanners ask for, such as `/wp-login.php,/xmlrpc.php` in front of a code
  forge. A request for one bans the client, as a `ban` rule does. Each starts
  with `/` and holds no `?`. A trap path is matched whole, without the query,
  against the path as sent and once percent-decoded. `/wp-login.php` matches
  `/wp-login.php?x=1` and `/%77p-login.php`. It does not match `/wp-login.php/`
  or `/blog/wp-login.php`.
- `SWWAF_WAF_MODE` (default `block`): what the Core Rule Set does with a request
  it takes for an attack. `block` refuses it with `403`. `detect` lets it
  through, logged and alerted. `off` inspects no request.
- `SWWAF_WAF_PARANOIA_LEVEL` (default `1`): the Core Rule Set's paranoia level,
  from 1 to 4. Each level up runs more rules. Those find more attacks, and take
  more ordinary requests for attacks.
- `SWWAF_WAF_ANOMALY_THRESHOLD` (default `5`): the score at which a request is a
  match. Each rule that matches adds to the request's score, and a rule rated
  critical adds 5. So by default one such rule is enough. A whole number above
  zero, or `off`. With `off` no request is a match, and scores are still logged.
- `SWWAF_WAF_DISABLED_RULES` (default
  `920340,920420,920440,920640,930130,930140`): the ids of Core Rule Set rules
  to switch off. The log line names the rules a request matched, so a rule that
  refuses your app's ordinary requests can be found there. "Attack detection" in
  [`SPEC.md`](../SPEC.md) says what the six default ids refuse and why they are
  off. A list you give replaces the default, so name these six ids in it to keep
  them off. An id must be a whole number above zero. An id from 900000 to 900999
  is refused: those are the rules that set the Core Rule Set up, and
  `smallwebwaf`'s own.
- `SWWAF_WAF_EXEMPT_PATHS` (default empty): path prefixes the Core Rule Set does
  not inspect, matched as `SWWAF_RATE_LIMIT_EXEMPT_PATHS` is.
- `SWWAF_WAF_BODY_LIMIT` (default `off`): `off` reads no request body. A size,
  at most `1G`, switches body inspection on. The Core Rule Set then reads a form
  data or multipart body up to that size, and the rest streams on to the app. It
  reads a JSON or XML body no larger than that size, since those cannot be read
  in part. Any other body, such as a git push, reaches the app uninspected. See
  [`core-rule-set.md`](core-rule-set.md).
- `SWWAF_ERROR_BURST_THRESHOLD` (default `30`): the most refusals of one client
  in a minute for a rule match, a trap path, a Core Rule Set match or a failed
  token check. One more bans the client, as a broken rate limit does. A count or
  `off`. The app's own `401` and `404` answers are not counted.

## How long bans last and what they cover

- `SWWAF_BAN_RESPONSE` (default `403`): how a refused client is answered: `403`,
  `429`, or `close` to close the connection without an answer. A `block` rule
  and the Core Rule Set always answer `403`.
- `SWWAF_LIMIT_BAN_DURATION` (default `1h`): the ban for a first broken rate
  limit, byte limit or error burst. A duration; it cannot be `off`.
- `SWWAF_LIMIT_BAN_REPEAT_WINDOW` (default `24h`): a client that breaks a limit
  again within this time after its ban ended is banned three times as long. A
  duration; it cannot be `off`.
- `SWWAF_MAX_BAN_DURATION` (default `7d`): a ban for a broken limit that would
  be longer is permanent instead. A duration; it cannot be `off`.
- `SWWAF_ATTACK_BAN_DURATION` (default `7d`): the ban for a first clear sign of
  attack. A duration; it cannot be `off`.
- `SWWAF_MAX_BANS` (default `5000`): the most bans `smallwebwaf` made that are
  kept, whether past, active or permanent. A whole number above zero. Bans whose
  cause is `admin` are kept besides and never dropped (see
  [`bans.md`](bans.md)).
- `SWWAF_BAN_SCOPE_V4_PREFIX` (default `32`): the length of the netblock around
  an IPv4 client that a ban covers, from 0 to 32. `24` bans the surrounding /24.
  An IPv6 ban covers the client's group.

## Where and how often the state files are written

- `SWWAF_STATE_DIR` (default `/var/lib/smallwebwaf`): the directory of the state
  files, an absolute path. A directory that does not exist or cannot be written
  stops the start.
- `SWWAF_STATE_WRITE_DELAY` (default `10s`): how long `bans.json` waits to be
  written after a ban is made, lifted or made permanent. Every change in between
  goes into the same write. A duration; it cannot be `off`.
- `SWWAF_STATE_COUNTER_INTERVAL` (default `15m`): how often every state file is
  written. A duration; it cannot be `off`.

## Logging

- `SWWAF_LOG_REQUEST_HEADERS` (default
  `accept,accept-language,accept-encoding,content-type,origin,range`): the
  request headers to record in the log line, in either case. `Authorization`,
  `Cookie` and `Set-Cookie` are never logged, even if listed. `Host` and
  `Transfer-Encoding` are refused: Go's HTTP server takes them out of the
  request. The host is the field `host`.
- `SWWAF_LOG_LEVEL` (default `info`): the least severe of `smallwebwaf`'s own
  messages that are written: `debug`, `info`, `warn` or `error`. No message is
  at `debug`. It holds back no request log line. `warn` and `error` hold back
  the line of settings at start.
- `SWWAF_LOG_REMOTE_URL` (default unset): a syslog server every line on stdout
  is also sent to, such as `syslog+tls://logs.example:6514`. The forms are
  `syslog+udp://`, `syslog+tcp://` and `syslog+tls://`, each with a host and a
  port and nothing more.
- `SWWAF_LOG_REMOTE_TLS_CA_FILE` (default unset): a file of PEM certificates
  that a `syslog+tls` server's certificate must chain to, instead of the host's
  own. A file that cannot be read, or holds no certificate, stops the start.
- `SWWAF_LOG_REMOTE_BUFFER` (default `10000`): the most lines held while they
  wait to be sent, a whole number above zero.
- `SWWAF_LOG_REMOTE_FACILITY` (default `local0`): the syslog facility: `kern`,
  `user`, `mail`, `daemon`, `auth`, `syslog`, `lpr`, `news`, `uucp`, `cron`,
  `authpriv`, `ftp`, or `local0` to `local7`.
- `SWWAF_LOG_REMOTE_APP_NAME` (default: `SWWAF_INSTANCE_NAME`): the app name in
  each syslog record, 1 to 48 printable ASCII characters without a space.

## Tokens and metrics

- `SWWAF_ADMIN_TOKEN` (default unset): the token for the ban endpoints and
  `/_smallwebwaf/clients/<ip>`. A long random value of at least 32 characters.
  Unset, those endpoints are off. Give it as a file to keep it from the app (see
  "Settings given as files" below).
- `SWWAF_METRICS_TOKEN` (default unset): the token a scraper sends for
  `/_smallwebwaf/metrics`, at least 32 characters. Unset, the metrics are off.
  It is separate from the admin token, so a scraper cannot manage bans.
- `SWWAF_METRICS_TOP_N` (default `50`): how many AS numbers and how many
  countries get series of their own in the metrics. The rest count as `other`. A
  whole number above zero.

## Alert destinations

- `SWWAF_ALERT_WEBHOOK_URL` (default unset): the webhook that receives each
  alert as a JSON object. An `http` or `https` URL without a user or a fragment.
  The settings logged at start show its path and query as `********`, since many
  webhooks carry their secret there.
- `SWWAF_ALERT_WEBHOOK_HEADERS` (default empty): headers sent with each post,
  such as `Authorization:Bearer 0123456789abcdef`. A list of name, `:` and
  value. A value cannot hold a comma.
- `SWWAF_ALERT_SLACK_WEBHOOK_URL` (default unset): the Slack incoming webhook
  that receives each alert as a message. It is checked and masked as the webhook
  URL is.
- `SWWAF_ALERT_NTFY_URL` (default unset): the ntfy topic that receives each
  alert, as the topic's full URL, such as `https://ntfy.sh/my-alerts`. It is
  checked and masked as the webhook URL is.
- `SWWAF_ALERT_NTFY_TOKEN` (default unset): an ntfy access token, sent as
  `Authorization: Bearer <token>`. A control character in it is refused.
- `SWWAF_ALERT_EVENTS` (default
  `ban,permanent_ban,waf_block,anomaly,reputation_hit,source_failure,file_error`):
  the events alerts are sent for. Any other name is refused.
- `SWWAF_ALERT_COOLDOWN` (default `15m`): how long a repeat of an alert is held
  back. A duration or `off`.
- `SWWAF_ALERT_MAX_PER_HOUR` (default `60`): the most alerts sent in an hour.
  The rest are rolled into one summary. A whole number above zero, or `off`.

See [`alerts.md`](alerts.md) for the events and what each destination gets.

## Anomaly thresholds

These only alert. They refuse and ban nothing. Each scope has four thresholds:
the most requests and the most bytes counted in a minute and in an hour.
Requests are counts, bytes are sizes. Each is `off` by default. What is unusual
depends on each service's normal traffic, which the metrics show. The four
thresholds per client are:

- `SWWAF_ANOMALY_CLIENT_REQUESTS_PER_MINUTE`;
- `SWWAF_ANOMALY_CLIENT_REQUESTS_PER_HOUR`;
- `SWWAF_ANOMALY_CLIENT_BYTES_PER_MINUTE`;
- `SWWAF_ANOMALY_CLIENT_BYTES_PER_HOUR`.

The other scopes' names have another prefix before the same four endings, such
as `REQUESTS_PER_MINUTE`:

- `SWWAF_ANOMALY_NET_...`: per netblock around a client. Two settings give the
  netblock's length:
    - `SWWAF_ANOMALY_NET_V4_PREFIX` (default `24`, from 0 to 32) for an IPv4
      client;
    - `SWWAF_ANOMALY_NET_V6_PREFIX` (default `48`, from 0 to 128) for an IPv6
      client.
- `SWWAF_ANOMALY_ASN_...`: per AS number. Needs lookups on.
- `SWWAF_ANOMALY_TOTAL_...`: the whole service.
- `SWWAF_WATCH_...`: each netblock in `SWWAF_WATCH_NETS`, as a whole.
  `SWWAF_WATCH_NETS` (default empty) is a list of name, `=` and netblock, such
  as `office=203.0.113.0/24,scraper-x=198.51.100.0/22`. An item without `=` or a
  name, or with a bad netblock, is refused. So is a name used twice.

Every request but the health check is counted, whatever was done with it,
clients in `SWWAF_ALLOW_NETS` included. At most 20,000 counters are kept.
`alerts.json` keeps them across a restart.

## Settings given as files

Any setting may be given as a file holding its value. Add `_FILE` to the
setting's name: `SWWAF_METRICS_TOKEN_FILE` names the file that holds
`SWWAF_METRICS_TOKEN`. The file's contents are the value, less one newline at
the end. They are checked as the value itself would be. Setting both forms stops
the start, and so does naming a file that cannot be read.
`SWWAF_LOG_REMOTE_TLS_CA_FILE` already holds a file name, so it has no `_FILE`
form.

A token given as a file is out of the app's reach only while the `smallwebwaf`
user alone can read the file. Make it on the host, owned by uid 65532 with mode
`0400`:

```sh
mkdir -p /srv/app/tokens
openssl rand -hex 32 > /srv/app/tokens/metrics
chown 65532:65532 /srv/app/tokens/metrics
chmod 0400 /srv/app/tokens/metrics
```

Mount its directory into the container read-only, and name the file:

```sh
docker run \
    -v /srv/app/tokens:/etc/smallwebwaf/tokens:ro \
    -e SWWAF_METRICS_TOKEN_FILE=/etc/smallwebwaf/tokens/metrics \
    ...
```

## Fixed limits

A few limits are fixed rather than settings:

- at most 100,000 GeoJS answers are kept, for 7 days each;
- at most 100,000 DNS blocklist verdicts and 100,000 AbuseIPDB scores are kept;
- at most 1,000 DNS blocklist queries are under way at once;
- the CrowdSec decision list is fetched again a minute after the last fetch.