smallwebwaf/SPEC.md

smallwebwaf specification

smallwebwaf is a web application firewall and reverse proxy for one app. It runs inside the app's container, between traefik and the app. This document says what it must do and why, each requirement once. Everything in it is built, except the last section, "Planned, not built". README.md says how to run it. EVALUATION.md says why no existing tool was adopted. The reference documents under docs/, which this document links to, give the settings, the formats and the lists; every fact lives in one of these files and is linked from the others.

Purpose

• It protects one self-hosted app on the open internet from scrapers, scanners and other abusers, from the app's first request, with no setting at all. Every setting has a default chosen for a service facing the internet in 2026. A setting that needs an alert destination, an account key or a token stays off until the operator supplies it.
• It limits each client's requests and bytes per minute, hour and day. It bounds the size and duration of every request and answer. It detects attacks.
• It bans abusers: briefly at first, for seven days on a clear sign of attack, and permanently when they keep at it.
• It looks up each client's AS number and country. It can refuse whole countries and netblocks, lower its limits for listed AS numbers and countries, consult reputation sources, and send alerts.
• Everything is set by environment variables whose names start with SWWAF_, since smallwebwaf shares its container, and so its environment, with the app. The only other inputs are files: the rule files, the state files, a lookup database, and a setting given as a file.

Non-goals

• TLS, certificates and routing by host name. Those are traefik's job.
• More than one app per smallwebwaf. Each app's image carries its own.
• Browser challenges such as captchas or proof of work. If wanted, chain Anubis between smallwebwaf and the app.
• Defence against floods that saturate the host's network link. That needs help upstream of the host.
• A web console or a configuration file.

Architecture

• One statically linked Go binary. It writes only to its state directory. It ships in the image that is the base of the app's own image (see "Deployment").
• No database of any kind. Nothing is read from disk while serving a request.
• Packages:
  • cmd/smallwebwaf: the binary, which only calls internal/smallwebwaf.
  • internal/smallwebwaf: the process. It reads the settings, the rule files, the lookup database and the state files, listens, and serves until SIGTERM or SIGINT. Then it stops, writing the state files. Run as smallwebwaf healthcheck, it is the image's health check instead.
  • internal/config: reads the settings, the one place they are read.
  • internal/proxy: what happens to each request. It works out the client and runs the checks in check. It passes the request to the app with the standard library's httputil.ReverseProxy, within the timeouts and size limits, and answers its own endpoints in answerAdmin. Once the request has ended, it counts bytes, refusals and anomalies, and writes the log line.
  • internal/ratelimit: the table of clients. It counts requests, bytes and refusals, and keeps each client's history.
  • internal/bans: the bans. It holds each netblock's bans with their notes, decides how long a new ban lasts, and picks the ban to drop past SWWAF_MAX_BANS.
  • internal/rules: reads the rule files, and again as they change, and tells which rules a request matches.
  • internal/waf: the Core Rule Set with the six changes, as Coraza directives.
  • internal/lookup: looks clients up through GeoJS or in the lookup database.
  • internal/reputation: fetches the lists, asks the DNS blocklists and AbuseIPDB in the background, and keeps their answers.
  • internal/anomaly: the anomaly counters and their alerts.
  • internal/alerts: holds back repeats and alerts past the hourly limit, and sends the rest to the webhook, Slack and ntfy, each from a queue of its own.
  • internal/state: reads the state files at start, takes in edits while running, and writes the files when due and at the stop.
  • internal/requestlog and internal/remotelog: the lines on stdout, and sending them to the syslog server.
  • internal/metrics: the metrics, in the Prometheus text format.
  • Dockerfile: the lint and test phases, then the image. deploy/example-app is an app built on the image, which script/example-app checks.
• Libraries, checked against the owner's Go dependency defaults:
  • the standard library for the proxy, HTTP clients, DNS, logging (log/slog) and the state files (encoding/json, os.Rename);
  • github.com/corazawaf/coraza/v3 and github.com/corazawaf/coraza-coreruleset/v4 at v4.25.0, which carries the Core Rule Set 4.25.0. The changes smallwebwaf makes to it, and the default SWWAF_WAF_DISABLED_RULES, are built for that version. They are checked again before the version changes, since rule ids, and what each rule matches, change between versions;
  • github.com/hashicorp/golang-lru/v2 for the bounded tables;
  • github.com/oschwald/maxminddb-golang/v2 to read the lookup database;
  • github.com/fsnotify/fsnotify to notice edited files;
  • github.com/prometheus/client_golang for the metrics;
  • github.com/maxmind/mmdbwriter, in the tests, to write lookup databases;
  • remote log sending written in this project, since the standard library's log/syslog is frozen and writes only the older syslog format.

What happens to a request

The steps run in this order. The first step that produces a final answer ends the handling. The size and time limits apply to the whole exchange. Each step names the section that holds its rules.

1. GET /_smallwebwaf/healthz is answered at once, before any check.

2. The client is identified.

   • If the TCP peer is inside SWWAF_TRUSTED_PROXIES, X-Forwarded-For is walked from the right, and the first address outside those netblocks is the client. If every address is inside them, the leftmost is, so that a visitor on a private network is known by its own address. With no header, the peer is the client.
   • If the peer is outside SWWAF_TRUSTED_PROXIES, it is the client, and the header is ignored.
   • An IPv6 client is its group: the netblock of SWWAF_IPV6_GROUP_PREFIX around its address, a /64 by default, since one abuser usually controls a whole /64. A client is therefore one IPv4 address or one IPv6 group.

3. The static lists. A client in SWWAF_ALLOW_NETS skips steps 4 to 12 and the byte limits. It is still counted for the anomaly thresholds, and still needs a token at step 13. A client in SWWAF_DENY_NETS is refused; a client in both lists is let through.

4. Bans. A client under a ban is refused (see "Bans").

5. The lookup, unless SWWAF_LOOKUP_SOURCE is off (see "Lookup of AS number and country"). The request goes on at once. It waits for the first GeoJS answer, up to SWWAF_LOOKUP_TIMEOUT, only when one of these needs it:

   • a country list;
   • a biased threshold;
   • SWWAF_ADD_LOOKUP_HEADERS.

   A request that ends before the answer comes is logged without it.

6. The country lists (see "Country lists").

7. The reputation sources (see "Reputation"). A client on a fetched blocklist gets SWWAF_BLOCKLIST_ACTION. A client on the CrowdSec decision list is banned. A client listed by a DNS blocklist, or scored at least SWWAF_ABUSEIPDB_MIN_SCORE by AbuseIPDB, gets SWWAF_REPUTATION_ACTION. With no cached verdict, a query is queued in the background and the request goes on, so a first request is never delayed by a reputation query.

8. The client's limit percentage (see "Biased thresholds").

9. Rate limiting (see "Request rate limits, per client"). A request over a limit times the percentage is refused and bans the client.

10. The trap paths (see "Attack detection"). A request for one is refused and bans the client.

11. The rule files (see "Rule files"). Matching stops at the first rule that refuses.

12. The Core Rule Set (see "Attack detection"). A match is refused with 403 while SWWAF_WAF_MODE is block, and only logged and alerted while it is detect.

13. A request under /_smallwebwaf/ is answered here (see "Admin endpoints").

14. The request is forwarded to SWWAF_UPSTREAM_URL, streaming, with X-Forwarded-For added, and X-Client-ASN and X-Client-Country when enabled.

15. After the response:

    • The response bytes, the request bytes or both are added to the byte counters, as SWWAF_BYTES_COUNT says. A client whose total passes a byte limit times the percentage has broken that limit and is banned. The response in progress is not cut off.
    • The client's refusals are counted for the error burst (see "Attack detection").
    • The client's history and the anomaly counters are updated, and the alert thresholds are evaluated.

In SWWAF_MODE=observe, every refusal above is logged as what would have happened, nothing is refused, and no ban is made. The size and time limits and the token check still apply.

Counting method

• Each window (minute, hour, day) uses two adjacent fixed buckets per client. The earlier bucket is weighted by how much of it still overlaps the sliding window. This costs a few integers per client per window and avoids the burst at bucket boundaries that a single fixed bucket allows.
• Counters are read and updated in memory. They are written to clients.json and loaded again at start (see "Persistent state"), so a restart does not hand every client a fresh allowance. Buckets whose time has passed are discarded on load.
• Memory is bounded. When the table of clients holds SWWAF_MAX_TRACKED_CLIENTS, the least recently seen client is dropped first, with its history. The bans, the GeoJS answers, the reputation verdicts and the anomaly counters have bounds of their own; docs/settings.md gives the fixed ones under "Fixed limits".

Bans

Until a ban ends, every request from its netblock is refused with SWWAF_BAN_RESPONSE. The netblock is the client: its IPv4 address, widened to SWWAF_BAN_SCOPE_V4_PREFIX, or its IPv6 group. At the defaults a ban covers one address or one /64, so a permanent ban does not reach neighbours who did nothing. Requests refused under a ban are not counted against limits. They are counted in the client's history and in the ban's notes.

An offence is a request smallwebwaf holds against the client. It is one of:

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

Offences are counted by kind in the client's history. The first two kinds lead to a ban, each by its own rule:

• A clear sign of attack: a request that matches a rule whose action is ban, or asks for a path in SWWAF_TRAP_PATHS. Examples are a probe for a .env file or a .git directory, and a known scanner's user agent.
  • The first one bans the netblock for SWWAF_ATTACK_BAN_DURATION (default 7d).
  • Any further request from the netblock while that ban lasts shows it is malicious: the ban becomes permanent.
  • Once that ban has run out, the netblock is served like any other. Its next clear sign of attack bans it permanently at once.
• A broken limit: a request over a rate limit, a response that takes the client's byte total past a byte limit, or more than SWWAF_ERROR_BURST_THRESHOLD refusals within a minute.
  • The first such ban lasts SWWAF_LIMIT_BAN_DURATION (default 1h). So does one that comes more than SWWAF_LIMIT_BAN_REPEAT_WINDOW (default 24h) after the last such ban ended.
  • Breaking a limit again within that window makes the new ban three times as long as the last one: one hour, then 3, 9, 27 and 81 hours.
  • A ban that would be longer than SWWAF_MAX_BAN_DURATION (default 7d) is permanent instead. At the defaults, that is the sixth ban in a row.
  • Each such ban sets the client's counters for every limit back to zero. Once it ends, only new requests can break a limit again. The client's history keeps its totals.

A Core Rule Set match refuses only the request it matched, without a ban, because the Core Rule Set has false positives. A block rule does the same. Those refusals count toward the error burst, so a client that keeps setting them off is banned under the second rule.

A client on the CrowdSec decision list is banned until the decision ends, with the cause crowdsec. Only a client that sends a request is banned, so a long list does not fill bans.json. Such a ban is never made permanent. It does not make the netblock's next ban for a broken limit longer.

These refuse each request they cover, record nothing in bans.json, and count no offence:

• SWWAF_DENY_NETS;
• a blocklist or reputation hit whose action is deny;
• the country lists.

Every ban carries notes with what an admin needs to decide whether to lift it, drawn from what smallwebwaf already knows; nothing is looked up to fill them. docs/bans.md lists the notes and says how a ban is made, kept or lifted, by editing bans.json or through the ban endpoints. A ban lifted before it ends is kept, marked as lifted, and does not count toward a longer ban. Deleting its entry from bans.json forgets it entirely.

A permanent ban does not run out. It ends when an admin lifts it, or when smallwebwaf drops it from a full table, as below. When smallwebwaf holds SWWAF_MAX_BANS bans of its own, it drops the earliest ban of one netblock: the netblock that has gone longest without a request. A past, active or permanent ban is dropped alike. A scanner whose ban was dropped that way is refused and banned again by its next probe. A ban an admin made, whose cause is admin, is never dropped and does not count toward SWWAF_MAX_BANS, so such bans can never fill the table. An admin who wants to keep a ban smallwebwaf made sets its cause to admin.

Settings

Conventions:

• Lists are comma separated. Netblocks are CIDR; a bare address means /32 or /128. Durations use Go's syntax plus d for days (90s, 15m, 24h, 7d). Sizes take K, M and G, powers of 1024. Countries are two-letter ISO codes in either case. A code that is not a country, such as nk, stops the start with a message naming it.

• These settings take off, which switches them off:

  • SWWAF_LOOKUP_SOURCE and SWWAF_WAF_MODE;
  • the five timeouts under "Size and time limits";
  • SWWAF_REQUEST_MAX_BYTES and SWWAF_RESPONSE_MAX_BYTES;
  • the three rate limits and the three byte limits;
  • SWWAF_WAF_ANOMALY_THRESHOLD, SWWAF_WAF_BODY_LIMIT and SWWAF_ERROR_BURST_THRESHOLD;
  • SWWAF_ALERT_COOLDOWN and SWWAF_ALERT_MAX_PER_HOUR;
  • every anomaly threshold.

  Every other setting refuses off.

• A list set to an empty value is an empty list, and replaces the default.

• Any setting can be read from a file instead, for secrets and long lists: SWWAF_ADMIN_TOKEN_FILE names the file that holds SWWAF_ADMIN_TOKEN. SWWAF_LOG_REMOTE_TLS_CA_FILE already holds a file name, so it has no _FILE form.

• Settings, files included, are read once at start. Changing one means restarting the container. The files smallwebwaf watches while it runs are its state files, its rule files and the lookup database.

• An invalid setting stops the process at start with a message naming the variable. At start the settings in effect are logged, with secrets masked.

docs/settings.md lists every setting with its default and the values it takes, in the groups below. What each group must do, and why:

Core

• One listener, SWWAF_LISTEN_ADDR, for the requests smallwebwaf forwards to SWWAF_UPSTREAM_URL and for its own endpoints (see "Ports").
• SWWAF_INSTANCE_NAME is in every log line, metric and alert, so that instances can be told apart. The default, the container's host name, changes at every redeploy; a name set by the operator survives it.
• SWWAF_MODE=observe logs and alerts on every decision while refusing nothing, so that a configuration can be tried before it is enforced.
• X-Forwarded-For is believed only from SWWAF_TRUSTED_PROXIES, by default the private address ranges, so a client that connects directly cannot claim another address. Where other containers reach smallwebwaf directly, or visitors on a private network reach traefik, they can name another address in the header; the setting, narrowed to traefik's own network, closes that gap.
• SWWAF_IPV6_GROUP_PREFIX, SWWAF_MAX_TRACKED_CLIENTS and SWWAF_MAX_BANS set the group size and the bounds under "Counting method" and "Bans".

Tokens

• SWWAF_ADMIN_TOKEN switches on the ban endpoints and /_smallwebwaf/clients/<ip>; SWWAF_METRICS_TOKEN switches on the metrics, which would otherwise be open to anyone on the internet. The two are separate, so a scraper that holds the metrics token cannot manage bans.
• A token is a long random value, since the endpoints can be reached from the internet. One shorter than 32 characters stops the start. The app starts with the same environment variables as smallwebwaf, so it can read a token set as an environment variable. The file form keeps a token out of the app's reach (see "Tokens" under "Deployment").

State files

• The state files live in SWWAF_STATE_DIR, a directory of their own beside the app's data, belonging to the smallwebwaf user.
• bans.json is written SWWAF_STATE_WRITE_DELAY after a ban is made, lifted or made permanent, so a burst of changes becomes one write. Every file is written every SWWAF_STATE_COUNTER_INTERVAL, and at an orderly stop.

Logging

• The request log on stdout is always on and has no switch. The process's own messages are JSON lines on stdout too, marked "type":"process"; SWWAF_LOG_LEVEL filters only those.
• SWWAF_LOG_REQUEST_HEADERS names the request headers to record. Authorization, Cookie and Set-Cookie values are never logged, only whether they were present.
• SWWAF_LOG_REMOTE_URL sends every log line to a syslog server as well, over UDP, TCP or TLS. Lines wait in a buffer of SWWAF_LOG_REMOTE_BUFFER lines; when it is full, the oldest are dropped and counted.

Metrics

• SWWAF_METRICS_TOP_N bounds the series by AS number and by country: that many of the busiest each, the rest summed as other, so the label set stays bounded.

Static lists

• SWWAF_ALLOW_NETS bypasses everything (monitoring, the owner's own networks). SWWAF_RATE_LIMIT_EXEMPT_NETS bypasses the request and byte limits only; the error burst, attack detection and bans still apply. SWWAF_DENY_NETS is always refused.
• Many clients share one address (mobile carriers, offices, Tor), and so share its limits and its bans. The defaults sit well above one person's use, and the first two lists take known shared addresses out.

Request rate limits, per client

Breaking one bans the client, so the defaults sit several times above what one busy person produces. A browser loading a heavy page makes a few hundred requests, a git clone a handful, and several people often share one address. A machine that talks to the app all day, such as a gitea Actions runner, can still pass the day limit. Its address belongs in SWWAF_RATE_LIMIT_EXEMPT_NETS. SWWAF_RATE_LIMIT_EXEMPT_PATHS takes path prefixes out of the count, such as static assets or health checks.

Byte limits, per client

A response's bytes are counted when it ends, so every default sits above the largest response allowed, and no single download breaks one. SWWAF_BYTES_COUNT says whether request bytes, response bytes or both are counted.

Size and time limits

These apply per request, in both directions. The client-facing timeouts apply between the client and smallwebwaf, the app-facing ones between smallwebwaf and the app, since a slow client and a slow app are separate problems. There is one size limit for request bodies and one for response bodies. Bodies pass through unchanged, so a limit on each side would bound the same bytes, and the lower one would always decide. Bodies stream straight through, so a request body reaches the app while the client is still sending it.

• SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES must be more than 4K and cannot be off. Go's HTTP server always has such a limit, and reads 4 KiB past the one it is given before it refuses.
• SWWAF_CLIENT_IDLE_TIMEOUT is longer by default than the 90 seconds after which traefik, by default, closes an unused connection. So traefik closes first, and never sends a request on a connection smallwebwaf is closing.

What the client gets when a limit is passed:

• Go's HTTP server reads a request's line and headers before smallwebwaf sees the request. It answers 431 to headers over SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES, and a client too slow to send them gets an error answer, mostly 400, or none. Either way the server closes the connection, nothing reaches the app, and the request log gets no line.
• A request that announces a body larger than its limit is refused before anything reaches the app.
• A limit passed after the headers have been read, but before the response has started, is answered by smallwebwaf:
  • 413 for a request body that is too large;
  • 408 for a client too slow to send its body;
  • 502 for a response that is too large;
  • 504 for an app that is too slow.
• A request body streams through, so each side can hold up the other. A request timeout that runs out while the body is still on its way answers 408 if smallwebwaf was waiting for the client at that moment, and 504 if it was waiting for the app.
• Once the response has started it can only be cut off, and the connection is closed.
• A WebSocket connection leaves these limits behind once it is upgraded. It stays open until either side closes it.

Lookup of AS number and country

On by default through GeoJS, which needs no account or file, so that country lookup works with no setup. A local database file, the free IPinfo Lite database, is the alternative for a service that keeps its visitors' addresses on its own host; the operator downloads and mounts it, and smallwebwaf never fetches it and holds no account or token for it. docs/lookup.md says what each source sends where, how long answers are kept, and when a request waits.

• Only one source is used. The start stops, naming both settings, when file is set without SWWAF_LOOKUP_DB_PATH, or SWWAF_LOOKUP_DB_PATH with any other source. The start also stops when the source is off and a setting needs lookups: a country list, a biased threshold, SWWAF_ADD_LOOKUP_HEADERS or an anomaly threshold per AS number.
• GeoJS is told the address of every new visitor, whether or not a setting uses the answer, since the request log, the metrics and the ban notes carry it. Private, loopback and link-local addresses, which no source can place, are never sent. Addresses are asked about in batches, one request at a time, and answers are kept for days, which keeps the number of requests to GeoJS low. Answers are kept apart from the table of clients, so that an answer outlasts a client dropped from that table.
• A client the lookup cannot place has an unknown country and AS number. The exclusive country list refuses it, and the biased thresholds give it SWWAF_UNKNOWN_LIMIT_PERCENT. Nothing else treats it differently.
• SWWAF_LOOKUP_TIMEOUT bounds both how long a visitor's request waits for its client's first answer, when a setting needs it, and how long a request to GeoJS may take. A client whose answer has not come in time counts as unknown until it comes, and its later requests do not wait.
• GeoJS publishes no rate limit, but its terms forbid "an excessive amount of API requests", judged by GeoJS alone, and let it block a caller. While GeoJS is slow, down or refusing smallwebwaf, new clients count as unknown, smallwebwaf keeps asking with backoff, and one source_failure alert is sent per cooldown. A service that cannot accept this, or that every new visitor's address goes to a third party, uses the file.
• A replaced database file is read again; one that cannot be read is logged, sent as one file_error alert, and ignored, the file in use staying in use. The directory that holds the file is mounted, not the file itself (see "Rule files" for why).

Country lists

Both are empty by default: the defaults judge a client by what it does, not by where it comes from. Clients in SWWAF_ALLOW_NETS are not checked.

• SWWAF_DENIED_COUNTRIES refuses every request from a listed country.
• SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES lets through only clients placed in a listed country. Every request from any other country is refused. So is every request from a client the lookup cannot place, such as internal monitoring; those that should reach the app go in SWWAF_ALLOW_NETS. A list that let unplaced clients through would let every new client in whenever GeoJS stops answering. The cost: while GeoJS fails, this list refuses every new client. No other failure of GeoJS, a reputation source or an alert destination comes this close to taking the service down.
• Both may be set. SWWAF_DENIED_COUNTRIES then adds nothing, since the exclusive list already refuses every other country. A code on both lists stops the start with a message naming it.
• A refused request is answered with SWWAF_BAN_RESPONSE, as soon as the client's address has been looked up, before its body is read. It skips the reputation checks, the limits, the rule files and the Core Rule Set. It is logged with the action country_denied, and counted in the metrics.

Biased thresholds

The lists are empty by default, for the same reason as the country lists. The request log and the metrics show which AS numbers and countries a service's abuse comes from.

• Clients in a listed AS number or country get that percentage of every request and byte limit (SWWAF_ASN_LIMIT_PERCENT, SWWAF_COUNTRY_LIMIT_PERCENT), or of the byte limits alone (SWWAF_ASN_BYTES_PERCENT, SWWAF_COUNTRY_BYTES_PERCENT). 0 is a zero allowance: the client's first request breaks a limit and bans it.
• SWWAF_ASN_LIMIT_PERCENT_URL names a text file of AS:percent lines, so one abuse-source list can be shared by every smallwebwaf in the fleet. It is fetched and refreshed like the blocklists.
• A client's limit percentage is the lowest of those that apply to it: its AS number's, its country's, or a reputation hit's limit:<percent>. It is 100 if none applies. Clients given a lower percentage are banned after fewer requests, first for a while and then, for repeated abuse, permanently.
• No budget is shared by a whole country or AS number. One abuser could use it up and lock out everyone else there: a denial of service nobody chose, and the one this tool exists to prevent. Refusing a whole country is left to the operator's choice, through the country lists.

Attack detection

By default the Core Rule Set reads the method, the URL with its query string, and the headers of every request. That is where automated attacks show: injection in query strings, path traversal, attacks carried in headers, scanners' user agents. It reads no request body. On a code forge, the example deployment, bodies carry what people write and publish, such as issue text and files saved in the web editor. The Core Rule Set takes the shell commands and code in them for attacks. The default gives up refusing an attack carried in a body; the rule files, the limits and the bans still apply to the client that sends it, and SWWAF_WAF_BODY_LIMIT switches body inspection on. docs/core-rule-set.md says what the Core Rule Set reads and how it scores a request.

• SWWAF_WAF_PARANOIA_LEVEL and SWWAF_WAF_ANOMALY_THRESHOLD are the Core Rule Set's own two tuning values, at its own defaults. A request whose score reaches the threshold is a match. With the threshold off, nothing is a match, and scores are still logged.

• SWWAF_WAF_DISABLED_RULES switches off rule ids when an app trips a false positive. The default switches off six rules:

  • 920340, 920420 and 920640 refuse a request for the type of its body, when the type is missing or not on the Core Rule Set's short list;
  • 920440 refuses a request for its file extension, such as .sh or .sql;
  • 930130 and 930140 refuse a file or directory name in the path, such as .git/, .gitignore, Dockerfile, package.json or an editor's settings directory.

  In front of a code forge these refuse git over HTTP, container image and package uploads, and views of ordinary files in a repository. At the site root, where no app serves such files, the default rule file bans the common probes these rules caught (see "Rule files"). A list you give replaces the default, so name these six ids in it to keep them off.

• SWWAF_WAF_BODY_LIMIT switches body inspection on, up to that size. Form data and multipart bodies are read up to the size, the rest streaming on without being held in memory; JSON and XML bodies are read only when no larger than the size, since those cannot be read in part. Any other body is not read: the Core Rule Set would read it as form data, where binary content such as a git push trips rules written for text. Body inspection suits apps whose forms carry no code. docs/gitea.md records what it refuses in front of gitea.

• SWWAF_TRAP_PATHS names paths the app never serves and only scanners ask for, such as /wp-login.php in front of gitea. A request for one is a clear sign of attack. This is the short form of a path rule with the ban action, for deployments that mount no rule files.

• SWWAF_ERROR_BURST_THRESHOLD bounds the refusals per client per minute after a rule file match, a trap path, a Core Rule Set match, or a missing or wrong token. More than this breaks a limit (see "Bans"). A client trying one attack or token after another is refused many times a minute; a person rarely more than a few times. Answers the app gives are not counted, since they do not tell a scanner from an ordinary client. In front of gitea, registry and git clients get 404 and 401 by design, hundreds of times a minute.

• A Core Rule Set engine error on a request forwards the request, and is logged and counted.

smallwebwaf changes the Core Rule Set 4.25.0 in six ways, since in front of gitea it would otherwise refuse ordinary requests. The changes hold in front of every app, and no setting undoes them:

1. PUT, PATCH and DELETE are allowed methods besides GET, HEAD, POST and OPTIONS. APIs, container image pushes and package uploads use them. Other methods stay refused.

2. The request header Expect is allowed, since curl and git send it before some large uploads. Content-Encoding is allowed on a request whose body the Core Rule Set does not read, which by default is every request. git compresses a fetch request over 1 KiB and says so in that header. On a body the Core Rule Set does read, the header stays refused, since a compressed body cannot be inspected. The other headers the Core Rule Set refuses, such as Proxy, stay refused.

3. The redirect_uri parameter is not checked for a URL naming an IP address or localhost (931100, 934110). Git Credential Manager, git-credential-oauth and tea, which gitea registers for OAuth sign-in, ask to be sent back to http://127.0.0.1 on the user's own machine. The server never fetches that address.

4. Ten query parameters are not checked against the Core Rule Set's lists of system files (930120), shell paths (932160) and command names (932260):

   • path, files, skip-to and sub_path;
   • ref, sha, branch and workflow;
   • artifactName and redirect_to.

   gitea puts file paths, branch and workflow names, and the page to return to after signing in into them. Any name on those lists can be an ordinary file or branch, such as .gitignore or docker-build. In these parameters, anything only these three rules catch reaches the app, commands such as |cat /etc/passwd included. Path traversal, SQL and script injection, and PHP, Java and Node.js code are still refused there. Every other parameter keeps all three rules. Parameter names are matched without regard to case, as Coraza matches them. An app that uses one of these parameters as a file on the server, or passes it to a shell, must check the value itself. A rule file can refuse it instead.

5. Two cookies are left out, and Referer keeps fewer rules:

   • The Core Rule Set reads the request without the gitea_flash and redirect_to cookies. gitea puts a message naming what was just done into gitea_flash. It puts the page to return to after signing in into redirect_to. The Core Rule Set takes file names in them, such as package.json, for attacks. A browser sends such a cookie with every request until gitea replaces it. gitea cannot replace it while those requests are refused, so one refusal would keep the browser out of the whole site. gitea shows the message with any script removed and returns only to its own pages. Every other cookie is read in full. An app that reads a cookie of either name must check it itself.
   • Referer is not checked for a Unix command given without arguments (932340) or for Java starting a process (944110). Referer names the page a request comes from, on everything that page loads and on every link followed from it. The Core Rule Set takes some page addresses for attacks, such as a search for env, or OpenJDK's Runtime.java, whose path holds both runtime and java.. All of those requests would be refused. Referer keeps the other rules. An app that passes Referer to a shell must check it itself.

6. Responses are not inspected. A raw file from a repository, such as a shell script, looks to the response rules like source code leaking from the server.

Ban response and scope

SWWAF_BAN_RESPONSE is 403 by default, which makes a ban easy to recognise when debugging; 429 and close are the alternatives. Behind traefik, close does not leave the client unanswered: traefik answers 502, as it does whenever its backend drops a connection. SWWAF_BAN_SCOPE_V4_PREFIX widens an IPv4 ban to the surrounding netblock. The four ban lengths are under "Bans".

Reputation

No source is on by default. A source acts on a list kept elsewhere; the defaults judge a client only by what it does here. AbuseIPDB and CrowdSec also need an account or an engine of the operator's own. The list best suited to be on by default, Spamhaus DROP, must not be fetched more than once an hour, and Spamhaus may block an address that does. Each smallwebwaf fetches its own copy, so several apps on one host, sharing one address, would often fetch less than an hour apart. docs/reputation.md says how each source is asked and what is done with the answer.

• Blocklists (SWWAF_BLOCKLIST_URLS) are text files of addresses and netblocks, such as the Spamhaus DROP list. Spamhaus's terms for it, on its DROP page:

  • automated downloads must be at least one hour apart, and once a day is more than enough in most cases;
  • a product that uses it must credit The Spamhaus Project and keep the list's date and copyright lines with the data.

  So SWWAF_BLOCKLIST_REFRESH is never less than 1h, and the last good copy is kept whole, comment lines included, on failure and across restarts.

• SWWAF_BLOCKLIST_ACTION is deny by default, which refuses every request from a listed address and suits lists of networks that send nothing legitimate, such as DROP. limit:<percent> gives listed clients that percentage of every limit, and suits a list of addresses shared with ordinary visitors, such as Tor exits. log only notes the hit.

• DNS blocklists (SWWAF_DNSBL_ZONES) meant for mail list residential ranges and would block ordinary visitors, so they should not be used. Spamhaus zones need their keyed query service, given as a zone name containing the key. SWWAF_DNSBL_RESOLVER names a resolver, since public resolvers are refused by several list operators.

• AbuseIPDB is asked only about clients that have already committed one offence, so the daily budget (SWWAF_ABUSEIPDB_DAILY_BUDGET, under the free tier's 1000 checks a day) is spent on suspects.

• A CrowdSec engine on the host (SWWAF_CROWDSEC_LAPI_URL) has its decision list pulled on a schedule. It is kept like a fetched blocklist, and a client on it is banned (see "Bans"). This is how a fleet-wide blocklist can arrive without smallwebwaf depending on CrowdSec.

• SWWAF_REPUTATION_ACTION, for DNS blocklist and AbuseIPDB hits, is limit:25 by default. Such verdicts are less certain than a blocklist, so a listed client gets a quarter of every limit rather than a refusal.

• Verdicts and scores are kept for SWWAF_REPUTATION_CACHE_TTL, with a fixed bound on their number. A source that is down or over quota gives no verdict. The service continues, with one source_failure alert per cooldown.

Alerting

• Alerts go to a webhook as JSON (docs/alerts.md gives the object), and to Slack and ntfy as formatted messages. SWWAF_ALERT_EVENTS chooses the event types sent.
• SWWAF_ALERT_COOLDOWN: the same event is not repeated within this time for the same netblock, source or file. For an anomaly, it is the same thing counted that must match: one client, netblock, AS number or named netblock, or the whole service. A count of suppressed repeats is included in the next one.
• SWWAF_ALERT_MAX_PER_HOUR: beyond this, alerts are rolled into one summary per hour, so a wide attack cannot flood the channel.
• A destination that is down is retried with backoff from a bounded queue, the oldest alert dropped first, and the drops counted in the metrics.

Anomaly thresholds

These only alert; nothing is blocked. None is set by default, and a threshold left unset sends no alert. An alert needs a destination only the operator can supply, and what counts as unusual depends on each service's normal traffic, which the metrics show. They are what reveals a widely distributed scraper, thousands of addresses at low rates each, which the per-client limits do not see; a low SWWAF_ASN_LIMIT_PERCENT for its AS number is the response.

• Requests and bytes are counted per minute and per hour, for each client, each surrounding netblock, each AS number, the whole service, and each named netblock in SWWAF_WATCH_NETS.
• Anomaly counting includes clients in SWWAF_ALLOW_NETS and SWWAF_RATE_LIMIT_EXEMPT_NETS, since an exempt client misbehaving is worth knowing about. At most 20,000 counters are kept, in memory and in alerts.json.

Rule files

smallwebwaf reads every *.rules file in SWWAF_RULES_DIR and checks each request against them, unless the client is in SWWAF_ALLOW_NETS. docs/rules.md gives the format, the targets and actions, the errors, and the default file.

• The files are plain text meant to be edited by hand, so that a new scanning pattern seen in the request log becomes a rule in one line. smallwebwaf watches the directory: a file edited, added or removed there takes effect while it runs.
• A rule's regex is Go's RE2 syntax. It has no backreferences or lookaround; in exchange matching time is linear in the input, so no rule can be made to stall the proxy.
• path and uri are matched both as received and once percent-decoded, so an encoded probe cannot slip past. Request bodies are not available to rule files; reading them is the Core Rule Set's job.
• block refuses the request without a ban; ban makes the request a clear sign of attack (see "Bans"). ban is for requests no real visitor sends, anchored at the site root. A file of the same name deeper in a site can be ordinary content, such as a file in a gitea repository. A visitor banned by mistake is let back in by lifting the ban.
• At start, a file that does not parse, a regex that does not compile, or a duplicate id stops the process with a message naming the file and line. So does a SWWAF_RULES_DIR that does not exist. While running, the same faults leave the rules as they were, and the log and one file_error alert name the file and line. An empty directory is not an error.
• The image ships a default file, kept short and limited to patterns that are wrong for every app, with its path rules anchored at the site root. Anything app-specific belongs in a file the app's Dockerfile adds or the deployer mounts. Rules meant to be edited while smallwebwaf runs belong in a mounted directory, not a mounted file. Many editors save a file by replacing it, and a container keeps seeing the old one when the file alone is mounted. The lookup database is mounted the same way, for the same reason.

Persistent state

All state lives in memory, and the files in SWWAF_STATE_DIR hold a copy of all of it, so an orderly stop loses nothing. The one exception is log lines still waiting to be sent to SWWAF_LOG_REMOTE_URL, which stdout has already carried. The files are meant for people as well: an admin can read or edit them at any time, and the running smallwebwaf takes the edit in. docs/state-files.md lists the files, what each holds, their sizes, and how an edit is taken in.

• Each file holds one kind of state, as JSON with:
  • a top-level version number;
  • entries sorted by client address;
  • times in RFC 3339 UTC;
  • durations and sizes as plain numbers, with the unit in the field name.
• bans.json is indented, so that a person can open it in an editor, find an address, and remove or add an entry. clients.json and lookups.json put each entry on one line, which halves their size and lets grep show everything about one client.
• Clients that were never banned are kept too, so every client's history survives a restart.
• Each file is rewritten whole, so each has a bound given by its setting. Writing a file of these sizes takes well under a second, in the background, so rewriting whole files needs nothing cleverer.
• A write serialises from a snapshot taken under the lock, so requests are not held up. It goes to a temporary file in the same directory, which is synced and renamed over the real name. A crash at any point leaves either the old complete file or the new complete file, never a partial one. Before writing a file, smallwebwaf takes in any edit made to it on disk, so an admin's edit is never overwritten.
• At start, a file that does not parse, or has an unknown version, stops the process with a message naming the file and position. Starting with empty state would silently forgive every repeat offender. Since writes are atomic, a broken file can only come from a hand edit, which the editor should hear about. A missing file means empty state, and is normal on first run.
• While running, an edit that parses replaces what smallwebwaf held for that file. An edit that does not parse never stops the running smallwebwaf. It keeps the state it has and sets the edited file aside as <name>.bad. It writes the file again from memory, and names the file and the position of the error in the log and in one file_error alert.
• A write that fails leaves the state correct in memory. It is logged, counted in the metrics and sent as one file_error alert per cooldown, and retried at the next write.
• A hard kill loses the changes since the last write. Losing the volume loses ban history, not service.

Request log

Every request gets one JSON object on one line on stdout, refused ones included, apart from those Go's HTTP server ends before smallwebwaf sees them (see "Size and time limits"). docs/request-log.md lists the fields and describes the syslog sending. Bodies are never logged. Query strings are logged as received; an app that carries secrets in query strings needs that fixed in the app. The process's own messages share the stream as JSON lines with "type":"process"; request lines carry "type":"request".

When SWWAF_LOG_REMOTE_URL is set, the same lines are also sent there as RFC 5424 syslog records. So a deployment can stop depending on docker's log handling while docker logs keeps working. Sending happens on its own goroutine from a bounded buffer: an unreachable or slow server never delays a request and never stops stdout. UDP gives no delivery signal at all and is offered only for compatibility.

Metrics endpoint

GET /_smallwebwaf/metrics answers in the Prometheus text format, for a request carrying SWWAF_METRICS_TOKEN. docs/metrics.md lists the metrics. No metric carries a client IP address as a label. Per-address questions are answered by the request log and GET /_smallwebwaf/clients/<ip>.

Admin endpoints

A request whose path starts with /_smallwebwaf/ is for smallwebwaf itself: it answers it and never passes it to the app. The prefix carries the tool's name, so it takes no path an app uses. Admins and scrapers reach these endpoints through traefik, like any other request. docs/admin-endpoints.md lists the endpoints and what each answers.

• GET /_smallwebwaf/healthz answers 200 with ok to anyone, without a token, while smallwebwaf is running. It does not ask the app; the container's health check does. It is answered before any check, so a health checker never needs an exemption and is never refused, for example by SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES.
• Every other endpoint needs its token, sent as Authorization: Bearer <token> (see "Tokens" under "Settings"). While a token is unset, the endpoints that need it answer 404, as does any other path under the prefix.
• Apart from the health check, these requests go through every check that any other request goes through. They are answered at the point where another request would be forwarded to the app: a banned or refused client stays refused, and each request counts toward the client's limits. A missing or wrong token is answered 401, in SWWAF_MODE=observe too, and counts toward the error burst. A client guessing tokens is therefore banned once it passes SWWAF_ERROR_BURST_THRESHOLD guesses in a minute. A client in SWWAF_ALLOW_NETS skips the checks but still needs the token.

Deployment

smallwebwaf runs inside the app's own container: the app's Dockerfile builds FROM the smallwebwaf image. Each app remains one container. An app is hardened by changing its base image. It can add packages from nixpkgs.

The image holds:

• Ubuntu 26.04 LTS, with ca-certificates, nix-bin and runit from Ubuntu;
• nixpkgs, fixed at one release;
• runsvinit as the entrypoint;
• the smallwebwaf binary, a user of its own (uid and gid 65532), and its runit service;
• its state and rules directories;
• EXPOSE 8080 and the health check below.

Every input to the image is pinned by digest or hash, so that an app built on it gets the same packages each time. docs/image.md says how each part is pinned, and why.

Every run script, the app's included, is a bash script. It starts with #!/usr/bin/env bash and set -euo pipefail, and puts its code in a main function, called on its last line, as the owner's code style guide asks. Ubuntu ships bash.

Beyond its FROM line, the app's Dockerfile adds:

• its binary;
• any packages it needs, from nixpkgs, with nix-env -iA nixpkgs.<name>;
• its runit service: a directory under /etc/service named after the app, never smallwebwaf. Its run script waits one second (sleep 1) and then starts the app with chpst, listening where SWWAF_UPSTREAM_URL points (see "Ports"). The app runs as a user of its own, which the Dockerfile creates with useradd. That user is neither root nor smallwebwaf, so that the app cannot touch the state files or the process of smallwebwaf.

It sets no ENTRYPOINT or USER of its own. The container must start runsvinit, as root, so that it can start each service as its own user. "Getting started" in README.md shows such a Dockerfile and run script.

The two processes

• runsvinit, runsvdir and each service's runsv run as root. Each run script starts as root and hands over to its program's user with chpst. Both services start with the container's environment variables and write to the container's output, which docker logs shows.
• When smallwebwaf or the app exits, runit starts it again a second later. The other process keeps running, and the container does not stop. While smallwebwaf is down, traefik answers 502. While the app is down, smallwebwaf answers 502, which is not a client offence.
• Only four things stop smallwebwaf, all at start:
  • invalid configuration;
  • a lookup database that is missing or unreadable;
  • a rule file or state file that does not parse;
  • an unwritable SWWAF_STATE_DIR.
• A smallwebwaf that stops at start is started again every second, logging its message each time, until the cause is fixed; the health check fails meanwhile. Once running, a failing GeoJS, reputation source or alert destination, or a broken edit, never takes the protected service down.
• docker stop sends SIGTERM to runsvinit, which has runit stop both services at once, each with SIGTERM, and exits once they have stopped. smallwebwaf stops taking requests, gives those in progress five seconds to finish, and writes its state files. An app that exits at once on SIGTERM ends those requests: the client gets 502, or a response cut short. To let them finish, the app must keep answering for five seconds after SIGTERM.
• The container's root filesystem stays writable: runit writes each service's status into its directory under /etc/service.

The health check

The image's HEALTHCHECK runs smallwebwaf healthcheck. It passes while smallwebwaf answers GET /_smallwebwaf/healthz on 127.0.0.1, at the port in SWWAF_LISTEN_ADDR, and the app accepts connections at SWWAF_UPSTREAM_URL. So the container shows as healthy only while both processes are up. traefik sends a container no requests until it shows as healthy. So the check runs every second from the container's start until it first passes, for up to a minute, and every 30 seconds after that. An app with a health check of its own can replace the image's HEALTHCHECK with one that checks both.

Ports

smallwebwaf listens on no port but the one in SWWAF_LISTEN_ADDR, 8080 by default, which traefik routes to. Its own endpoints are on that listener, and it forwards every other request to the app at SWWAF_UPSTREAM_URL, 127.0.0.1:8081 by default. Another port may be set: the health check follows the setting, traefik's port label (traefik.http.services.<name>.loadbalancer.server.port) must name it, and the app must leave it free. The address part of SWWAF_LISTEN_ADDR stays empty (:9000, never 127.0.0.1:9000), so that traefik reaches smallwebwaf on the container's address and the health check on 127.0.0.1. The app listens on that loopback address only. An app that listens on every address can be reached around smallwebwaf by anything that reaches the container.

State

smallwebwaf keeps its state files in SWWAF_STATE_DIR, beside the app's own data directories. Without a volume there, the files survive a restart of the container and are lost when a deploy replaces it. A volume mounted at /var/lib/smallwebwaf, named or a host directory, keeps them across deploys. The run script makes it and every file in it belong to the smallwebwaf user, so a host directory mounted there needs no change of owner. The volume is separate from the app's, holds a few tens of MiB at most with the defaults, and needs no backup beyond the host's own. The image declares no volume, since every app image built on it would inherit it.

Tokens

A token given as a file (SWWAF_ADMIN_TOKEN_FILE, SWWAF_METRICS_TOKEN_FILE) is out of the app's reach only while the smallwebwaf user alone can read the file. The operator makes the file on the host, owned by uid 65532, the smallwebwaf user, with mode 0400, and mounts the directory that holds it into the container read-only. The container sees the same owner and mode.

Forwarded headers

The app's TCP peer is smallwebwaf on 127.0.0.1, and the X-Forwarded-For the app receives ends with traefik's address, which smallwebwaf adds after the visitor's. The app must therefore trust 127.0.0.1 and ::1 for forwarded headers, besides the private address ranges traefik is on. The usual default for a trusted-proxy setting, the private ranges alone, does not include loopback: an app left at it ignores the header and sees every visitor as 127.0.0.1. A list given replaces that default, so the app's own trusted-proxy setting names them all: 10.0.0.0/8,172.16.0.0/12,192.168.0.0/16,127.0.0.1/32,::1/128.

upaas

An app deployed by upaas takes this shape with no change to upaas. upaas builds the app's Dockerfile and runs the one container as it runs any app. The traefik labels route to port 8080, and any SWWAF_ settings go with the app's environment variables. The volume for /var/lib/smallwebwaf and the directory of any token file go beside the app's own volumes. gitea is built on the image like any other app; docs/gitea.md records what the defaults do in front of it, and script/gitea-requests checks those notes.

Planned, not built

Everything above is built. These two points are specified as later work, and nothing in the code does them yet:

• Planned: sharing bans between the instances of several apps, by running one CrowdSec engine per host that every instance reports its bans to. It is reconsidered once real ban volumes are known. Until then each instance keeps its own bans, and can read a CrowdSec engine's decisions as described under "Reputation".
• Planned: sending the log over RELP, the reliable syslog transport. It is revisited against the available Go packages. Until then the remote log goes over UDP, TCP or TLS as described under "Request log".