Keep the bans, the clients and GeoJS's answers in state files (closes #17)

smallwebwaf now copies its state to bans.json, clients.json and
lookups.json in SWWAF_STATE_DIR, as "Persistent state" in SPEC.md
describes, and reads them back at start, so a restart lifts no ban and
gives no client a fresh allowance. Each client gains a history, and a
ban's notes count the netblock's requests. bans.json is written
SWWAF_STATE_WRITE_DELAY after a ban, and every file every
SWWAF_STATE_COUNTER_INTERVAL and at the stop. A ban read back is masked
to its netblock and refuses every client in it, whatever
SWWAF_BAN_SCOPE_V4_PREFIX is now. A file that does not parse, an unknown
version or an unwritable directory stops the start.

Deviation: no AS number or name, and no ban cause, reason or lifting yet.

Model: opus-5-5

Dockerfile

@@ -162,6 +162,11 @@ RUN groupadd --system --gid 65532 smallwebwaf \
         --gid smallwebwaf --no-create-home --shell /usr/sbin/nologin \
         smallwebwaf
 
+# The state files' directory, SWWAF_STATE_DIR by default, where a volume
+# is mounted to keep them across deploys. The run script gives it to the
+# smallwebwaf user at each start.
+RUN mkdir /var/lib/smallwebwaf
+
 # runsvinit starts runit's runsvdir on /etc/service, where Ubuntu's sv
 # looks too.
 COPY --chmod=755 share/smallwebwaf.run /etc/service/smallwebwaf/run

README.md

@@ -13,18 +13,19 @@ JSON log line for every request.
 
 Status: the first two milestones are built
 (https://git.eeqj.de/sneak/smallwebwaf/issues/13 and
-https://git.eeqj.de/sneak/smallwebwaf/issues/14), and so are three parts of
-milestone 3: the static lists and the bans that broken rate limits lead to,
-which come next in the build order, and the header size and the idle time as
-settings, which come last in it. `smallwebwaf` passes each request to the app
-and the app's answer back, unchanged, within its timeouts and size limits, works
-out each client's address, bans a client that sends too many requests, refuses a
-client that comes from a country you refuse or from a network you refuse, lets
-the networks you choose through, and writes a JSON log line for every request.
-It comes as the image the app's own image is built on. The rest of the design
-comes after that, in the order of the build order in [`SPEC.md`](SPEC.md). The
-survey of existing tools that led to the design is in
-[`EVALUATION.md`](EVALUATION.md).
+https://git.eeqj.de/sneak/smallwebwaf/issues/14), and so are four parts of
+milestone 3: the static lists, the bans that broken rate limits lead to and the
+JSON state files, which come next in the build order, and the header size and
+the idle time as settings, which come last in it. `smallwebwaf` passes each
+request to the app and the app's answer back, unchanged, within its timeouts and
+size limits, works out each client's address, bans a client that sends too many
+requests, refuses a client that comes from a country you refuse or from a
+network you refuse, lets the networks you choose through, keeps its bans, each
+client's counters and history, and GeoJS's answers in JSON files across
+restarts, and writes a JSON log line for every request. It comes as the image
+the app's own image is built on. The rest of the design comes after that, in the
+order of the build order in [`SPEC.md`](SPEC.md). The survey of existing tools
+that led to the design is in [`EVALUATION.md`](EVALUATION.md).
 
 ## Getting started
 
@@ -46,7 +47,8 @@ works.
 
 To work on the code, `make build` builds the binary alone, with Go installed,
 and `make run` builds and runs it, listening on port 8080 in front of an app at
-`SWWAF_UPSTREAM_URL`, by default `http://127.0.0.1:8081`.
+`SWWAF_UPSTREAM_URL`, by default `http://127.0.0.1:8081`, with its state files
+in `bin/state` unless `SWWAF_STATE_DIR` is set.
 
 ## What it does so far
 
@@ -77,8 +79,9 @@ and `make run` builds and runs it, listening on port 8080 in front of an app at
   bans the client. A client is one IPv4 address, or one IPv6 /64, since one
   abuser usually holds a whole /64. Each window is counted in two fixed buckets,
   the earlier one weighted by how much of it the window still covers. At most
-  20,000 clients are kept, the least recently seen dropped first, and only in
-  memory: a restart starts every client afresh.
+  20,000 clients are kept, the least recently seen dropped first, with their
+  history, and a restart gives no client a fresh allowance (see "State files"
+  below).
 - Bans a client that breaks a rate limit, as "Bans" in [`SPEC.md`](SPEC.md)
   describes: the first ban lasts an hour, and a limit broken again within a day
   of a ban ending bans for three times as long as that ban, so 1, 3, 9, 27 and
@@ -90,13 +93,13 @@ and `make run` builds and runs it, listening on port 8080 in front of an app at
   is not counted for the rate limits. A ban sets the client's counters back to
   zero. Each ban carries notes for deciding whether to lift it: the limit, its
   window and the requests counted in it, the request that broke it, the client's
-  country when it was looked up, how many requests the ban has refused, and how
-  many bans the netblock had before. At most `SWWAF_MAX_BANS` bans are kept,
-  past, active and permanent; past that, the earliest ban of the netblock that
-  has gone longest without a request is dropped first. Bans and their notes are
-  kept in memory only, so a restart lifts every ban, and nothing shows them yet:
-  `bans.json`, which shows them and lets you lift a ban, comes with the state
-  files (https://git.eeqj.de/sneak/smallwebwaf/issues/17).
+  country when it was looked up, the netblock's requests since it was first
+  seen, how many of them the ban has refused, and how many bans the netblock had
+  before. At most `SWWAF_MAX_BANS` bans are kept, past, active and permanent;
+  past that, the earliest ban of the netblock that has gone longest without a
+  request is dropped first. `bans.json` shows the bans and their notes, and a
+  restart lifts none (see "State files" below); lifting a ban by editing it
+  comes with https://git.eeqj.de/sneak/smallwebwaf/issues/68.
 - Refuses a request from a country you refuse with `SWWAF_BAN_RESPONSE`, as soon
   as the client's country is known and before its body is read; such a request
   is not counted for the rate limits. While one of the country lists below is
@@ -184,6 +187,13 @@ it, and the effective settings are logged at start.
 - `SWWAF_BAN_SCOPE_V4_PREFIX` (default `32`): the length of the netblock around
   an IPv4 client that a ban covers, such as `24` to ban the surrounding /24. An
   IPv6 ban covers the client's /64.
+- `SWWAF_STATE_DIR` (default `/var/lib/smallwebwaf`): the directory of the state
+  files, an absolute path. A directory `smallwebwaf` cannot write stops the
+  start.
+- `SWWAF_STATE_WRITE_DELAY` (default `10s`): how long after a ban is made
+  `bans.json` is written, with every ban made in between.
+- `SWWAF_STATE_COUNTER_INTERVAL` (default `15m`): how often every state file is
+  written.
 
 Durations are in Go's syntax, with `d` for days (`90s`, `15m`, `7d`). Sizes are
 bytes, with an optional `K`, `M` or `G`, which are powers of 1024 (`1K` is 1024
@@ -193,12 +203,13 @@ ISO 3166-1 assigns today, and `xk` for Kosovo, in either case (`de` and `DE` are
 the same); any other code, such as `nk` (North Korea is `kp`) or the withdrawn
 `su`, stops the start, and so does a code on both country lists. `off` switches
 a timeout, a size limit or a rate limit off;
-`SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES` and the ban settings cannot be off.
+`SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES`, the ban settings and the state settings
+cannot be off.
 
-Several limits are fixed rather than settings. At most 20,000 clients are kept
-for the rate limits, and an IPv6 client is counted by its /64. A new client
-waits at most a second for its country, and at most 100,000 answers from GeoJS
-are kept, for 7 days each.
+Several limits are fixed rather than settings. At most 20,000 clients are kept,
+with their counters and history, and an IPv6 client is counted by its /64. A new
+client waits at most a second for its country, and at most 100,000 answers from
+GeoJS are kept, for 7 days each.
 
 ## Request log
 
@@ -246,6 +257,44 @@ which it answers `431`, headers slower than `SWWAF_CLIENT_REQUEST_TIMEOUT`,
 whose connection it closes without an answer, and requests it cannot read at
 all, which it answers itself, mostly with `400`.
 
+## State files
+
+`smallwebwaf` keeps its state in memory and a copy of it in three JSON files in
+`SWWAF_STATE_DIR`, `/var/lib/smallwebwaf` by default, as "Persistent state" in
+[`SPEC.md`](SPEC.md) describes. Each has a top-level `version`, 1, and lists its
+entries by client address, with times in UTC.
+
+- `bans.json`: every ban with its notes, indented to be read; a permanent ban's
+  `expires` is `null`.
+- `clients.json`: each client's two buckets in the minute, the hour and the day,
+  and its history: when it was first and last seen, its country as last looked
+  up and when, its requests, how many were forwarded and how many refused, the
+  body bytes in each direction, its responses by status class and its offences
+  by kind. Each client is on a line of its own, so `grep` shows everything about
+  one.
+- `lookups.json`: GeoJS's answers, one to a line, with when GeoJS gave each and
+  when it was last used.
+
+`bans.json` is written `SWWAF_STATE_WRITE_DELAY` after a ban is made, with every
+ban made in between, and every file every `SWWAF_STATE_COUNTER_INTERVAL` and
+when `smallwebwaf` stops. Each write goes to a temporary file in the same
+directory, which then replaces the file, so a crash leaves the old file or the
+new one, whole. A write that fails is logged, and tried again at the next write.
+A hard kill loses what changed since the last write.
+
+At start the files are read back: each client keeps its counts, so a restart
+gives it no fresh allowance, and each ban keeps refusing every client in its
+netblock until it ends, even after `SWWAF_BAN_SCOPE_V4_PREFIX` has changed. A
+netblock whose address has bits past its length, such as `203.0.113.9/24`, is
+read as the netblock it is in, `203.0.113.0/24`. Buckets and answers whose time
+has passed are dropped. A missing file is empty state, as on a first start. A
+file that does not parse, or has another `version`, stops the start with a
+message naming the file, and the line and column where Go's JSON decoder gives
+them; so does a state directory `smallwebwaf` cannot write. An edit made while
+`smallwebwaf` runs is overwritten by its next write: taking it in comes with
+https://git.eeqj.de/sneak/smallwebwaf/issues/68. The AS number and AS name come
+with their lookup.
+
 ## Why
 
 Small self-hosted sites now receive a great deal of traffic nobody asked for:
@@ -347,9 +396,9 @@ goes through the candidates one by one.
   readable JSON files, written regularly and at every stop, so a restart loses
   nothing. Edit a file, or add a rule file, and the running `smallwebwaf` picks
   up the change. Nothing is read from disk while serving a request. The files
-  come in milestone 3 or later (see the build order in [`SPEC.md`](SPEC.md));
-  until then the rate counters, the bans and the GeoJS answers are kept in
-  memory only, and a restart loses them.
+  for the bans, the clients and the GeoJS answers are built (see "State files"
+  above); the others come with their features, and taking in an edit while
+  running comes with https://git.eeqj.de/sneak/smallwebwaf/issues/68.
 - Health checks, the metrics, and listing, adding and lifting bans or asking why
   a given address was refused, all on the one port every request uses: under
   `/_smallwebwaf/` on the app's own address, through traefik like any other
@@ -445,8 +494,9 @@ main "$@"
   the health check on `127.0.0.1`.
 - `smallwebwaf` keeps its state files in `/var/lib/smallwebwaf`. Mount a volume
   there to keep bans and client history when a deploy replaces the container;
-  without one, it still starts. The state files come in milestone 3 or later;
-  until then it writes nothing to disk and needs no volume.
+  without one, it still starts. At each start the `run` script of `smallwebwaf`
+  gives that directory and every file in it to the `smallwebwaf` user, so a host
+  directory mounted there needs no change of owner.
 - `docker stop` has runit stop both processes. `smallwebwaf` then stops taking
   requests and gives those in progress five seconds to finish.
 
@@ -484,11 +534,10 @@ client, for the request log, the metrics and the ban notes, and for the country
 lists and biased limits when you set them. It works with no setup: by default it
 asks the free GeoJS web service, which needs no account and no file. This means
 that, by default, the address of every new visitor is sent to GeoJS. Each answer
-is kept in memory for seven days, and many addresses are asked about in one
-request; writing the answers to disk, so that they survive a restart, comes in
-milestone 3 or later (see the build order in [`SPEC.md`](SPEC.md)). GeoJS
-publishes no rate limit but may block a caller it thinks asks too much; while it
-is not answering, new visitors count as coming from an unknown country, which
+is kept for seven days, in memory and in `lookups.json`, so that it survives a
+restart, and many addresses are asked about in one request. GeoJS publishes no
+rate limit but may block a caller it thinks asks too much; while it is not
+answering, new visitors count as coming from an unknown country, which
 `SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES` refuses.
 
 To keep your visitors' addresses on your own host, set
@@ -519,9 +568,10 @@ addresses are never sent to GeoJS.
 ## How the code is laid out
 
 - `cmd/smallwebwaf`: the binary, which only calls `internal/smallwebwaf`.
-- `internal/smallwebwaf`: the process: it reads the settings, listens, serves
-  requests until `SIGTERM` or `SIGINT`, and stops. Run as
-  `smallwebwaf healthcheck`, it is the image's health check instead.
+- `internal/smallwebwaf`: the process: it reads the settings and the state
+  files, listens, serves requests until `SIGTERM` or `SIGINT`, and 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, runs
   the checks, passes the request to the app and the answer back with the
@@ -534,8 +584,10 @@ addresses are never sent to GeoJS.
   long a new ban lasts, and which ban is dropped when `SWWAF_MAX_BANS` are held.
 - `internal/lookup`: looks up each client's country through GeoJS, and keeps the
   answers.
-- `internal/ratelimit`: counts each client's requests and tells when one takes
-  it over a rate limit.
+- `internal/ratelimit`: the table of clients: counts each client's requests,
+  tells when one takes it over a rate limit, and keeps each client's history.
+- `internal/state`: reads the state files at start, and writes them when they
+  are due and at the stop.
 - `internal/requestlog`: the lines on stdout: the request log line and the
   process's own messages.
 - `Dockerfile`: the lint and test phases, then the image, whose last stage
@@ -576,20 +628,24 @@ so that they run in minimal containers.
 - `script/install-precommit`: installs that hook; `make hooks` runs it.
 - `script/build`: builds `bin/smallwebwaf` on the host, with Go installed, for
   working on the code by hand; `make build` runs it.
-- `script/run`: builds `bin/smallwebwaf` with `script/build` and runs it;
-  `make run` runs it.
+- `script/run`: builds `bin/smallwebwaf` with `script/build` and runs it, with
+  its state files in `bin/state` unless `SWWAF_STATE_DIR` is set; `make run`
+  runs it.
 - `script/example-app`: builds the image and, on it, the example app in
-  `deploy/example-app`, runs it, and checks that the health check passes, that a
-  request reaches the app through `smallwebwaf`, and that `sv stop` and
-  `docker stop` stop it in order; then removes the container and both images. It
-  needs network access, for nixpkgs' binary cache, and `script/check` does not
-  run it; `make example-app` does.
+  `deploy/example-app`, runs it with a volume for the state files, and checks
+  that the health check passes, that a request reaches the app through
+  `smallwebwaf`, that a second request in a minute bans the client, that
+  `sv stop` and `docker stop` stop it in order, and that a new container on the
+  same volume still refuses the banned client; then removes the containers, the
+  volume and both images. It needs network access, for nixpkgs' binary cache,
+  and `script/check` does not run it; `make example-app` does.
 
 ## TODO
 
-- The rest of milestone 3, after the bans that broken rate limits lead to and up
-  to the metrics endpoint, and the rest of the design, in the order of the build
-  order in [`SPEC.md`](SPEC.md).
+- The rest of milestone 3, from taking in an admin's edits to the state files
+  (https://git.eeqj.de/sneak/smallwebwaf/issues/68) up to the metrics endpoint,
+  and the rest of the design, in the order of the build order in
+  [`SPEC.md`](SPEC.md).
 
 ## Documents
 

internal/bans/bans.go

@@ -1,6 +1,7 @@
 // Package bans is the ban ledger: the bans smallwebwaf makes on the
 // netblocks of clients that break a rate limit, with their notes, as the
-// "Bans" section of SPEC.md describes. The bans are kept in memory only.
+// "Bans" section of SPEC.md describes. The bans are kept in memory, and
+// written to bans.json and read from it by the state package.
 package bans
 
 import (
@@ -58,48 +59,65 @@ func (b Ban) ActiveAt(now time.Time) bool {
 	return b.Permanent() || now.Before(b.Expires)
 }
 
-// Notes are what an admin needs to decide whether to lift a ban.
+// Notes are what an admin needs to decide whether to lift a ban. The
+// JSON names are those of bans.json.
+//
+//nolint:tagliatelle // the state files use snake_case, as the request log does
 type Notes struct {
 	// Country is the client's country, when it was looked up.
-	Country string
+	Country string `json:"country"`
 	// Limit, Window and Count are the limit that was broken, its window,
 	// "minute", "hour" or "day", and the count reached: the client's
 	// requests in the window, the one that broke the limit included.
 	// These are the requests that counted toward the ban, and the window
 	// is the time over which they came.
-	Limit  int64
-	Window string
-	Count  float64
+	Limit  int64   `json:"limit"`
+	Window string  `json:"window"`
+	Count  float64 `json:"count"`
 	// Request is the request that broke the limit.
-	Request Request
-	// Refused is how many requests the ban has refused so far.
-	Refused int64
+	Request Request `json:"request"`
+	// Requests is how many requests the netblock has sent since it was
+	// first seen, and Refused how many of them the ban has refused so
+	// far. Both go up with each request the ban refuses.
+	Requests int64 `json:"requests"`
+	Refused  int64 `json:"refused"`
 	// EarlierBans is how many bans the netblock had before this one.
-	EarlierBans int
+	EarlierBans int `json:"earlier_bans"`
 }
 
 // Request is a request in a ban's notes. Each text is cut to 256 bytes.
+//
+//nolint:tagliatelle // the state files use snake_case, as the request log does
 type Request struct {
-	Time   time.Time
-	Method string
-	Host   string
+	Time   time.Time `json:"time"`
+	Method string    `json:"method"`
+	Host   string    `json:"host"`
 	// Path is the path with its query string.
-	Path string
+	Path string `json:"path"`
 	// Status is what the client was sent, 0 if nothing was.
-	Status    int
-	UserAgent string
+	Status    int    `json:"status"`
+	UserAgent string `json:"user_agent"`
 }
 
 // Ledger holds the bans. It is safe for concurrent use.
 type Ledger struct {
 	rules Rules
+	// changed receives a value when a ban is made, unless one is waiting
+	// already.
+	changed chan struct{}
 
 	mu sync.Mutex
-	// netblocks holds each banned netblock's bans, oldest first. Each
-	// request from a netblock makes it the most recently seen.
+	// netblocks holds each banned netblock's bans, oldest first. Check
+	// makes each netblock it finds the most recently seen.
 	netblocks *simplelru.LRU[netip.Prefix, *[]Ban]
 	// held is how many bans netblocks holds, at most rules.MaxBans.
 	held int
+	// v4Lengths and v6Lengths are the lengths of the IPv4 and IPv6
+	// netblocks that have been banned. Check looks for a ban at each of
+	// them, so that a ban read from bans.json refuses every client in its
+	// netblock even when it was made with another SWWAF_BAN_SCOPE_V4_PREFIX,
+	// or another length of an IPv6 client's netblock.
+	v4Lengths, v6Lengths []int
 }
 
 // New returns a Ledger with no ban yet.
@@ -111,31 +129,49 @@ func New(rules Rules) *Ledger {
 		panic(err) // NewLRU fails only for a size below one
 	}
 
-	return &Ledger{rules: rules, netblocks: netblocks}
+	return &Ledger{
+		rules:     rules,
+		changed:   make(chan struct{}, 1),
+		netblocks: netblocks,
+	}
 }
 
-// Check is called for each request from netblock, at now. It reports
-// whether a ban on netblock is active, and returns that ban, with the
-// request counted among those it refused.
-func (l *Ledger) Check(netblock netip.Prefix, now time.Time) (Ban, bool) {
+// Changed receives a value after a ban is made, so that bans.json can be
+// written. Several bans made before it is read leave one value.
+func (l *Ledger) Changed() <-chan struct{} {
+	return l.changed
+}
+
+// Check is called for each request from client, at now. It reports
+// whether a ban on a netblock client is in is active, and returns that
+// ban, with the request counted among those it refused.
+func (l *Ledger) Check(client netip.Addr, now time.Time) (Ban, bool) {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 
-	bans, found := l.netblocks.Get(netblock)
-	if !found {
-		return Ban{}, false
+	lengths := l.v6Lengths
+	if client.Is4() {
+		lengths = l.v4Lengths
 	}
 
-	// A ban is made only once the one before has ended, so only the last
-	// can be active.
-	last := &(*bans)[len(*bans)-1]
-	if !last.ActiveAt(now) {
-		return Ban{}, false
+	for _, length := range lengths {
+		bans, found := l.netblocks.Get(netip.PrefixFrom(client, length).Masked())
+		if !found {
+			continue
+		}
+
+		// A ban is made only once the one before has ended, so only the
+		// last can be active.
+		last := &(*bans)[len(*bans)-1]
+		if last.ActiveAt(now) {
+			last.Notes.Requests++
+			last.Notes.Refused++
+
+			return *last, true
+		}
 	}
 
-	last.Notes.Refused++
-
-	return *last, true
+	return Ban{}, false
 }
 
 // BanForLimit bans netblock at now for a broken limit, with notes, and
@@ -169,21 +205,13 @@ func (l *Ledger) BanForLimit(netblock netip.Prefix, now time.Time, notes Notes)
 		Expires:  l.expiry(last, now),
 		Notes:    notes,
 	}
+	l.add(ban)
 
-	if l.held == l.rules.MaxBans {
-		l.dropOne()
+	select {
+	case l.changed <- struct{}{}:
+	default: // a value is waiting already
 	}
 
-	// dropOne can have dropped netblock's last ban, and netblock with it.
-	bans, found = l.netblocks.Peek(netblock)
-	if !found {
-		bans = &[]Ban{}
-		l.netblocks.Add(netblock, bans)
-	}
-
-	*bans = append(*bans, ban)
-	l.held++
-
 	return ban
 }
 
@@ -201,6 +229,74 @@ func (l *Ledger) Bans(netblock netip.Prefix) []Ban {
 	return slices.Clone(*bans)
 }
 
+// Snapshot returns every ban held, sorted by netblock, and each
+// netblock's bans oldest first, as bans.json lists them.
+func (l *Ledger) Snapshot() []Ban {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	held := make([]Ban, 0, l.held)
+	for _, bans := range l.netblocks.Values() {
+		held = append(held, *bans...)
+	}
+
+	slices.SortStableFunc(held, func(a, b Ban) int {
+		return a.Netblock.Compare(b.Netblock)
+	})
+
+	return held
+}
+
+// Load puts bans read from bans.json into a ledger that holds none yet,
+// in the order they started, so that a netblock whose last ban started
+// latest counts as the most recently seen. Each netblock is masked to its
+// length, so that 203.0.113.9/24 is 203.0.113.0/24, and each text in the
+// notes is cut to 256 bytes. Past MaxBans the earliest bans are dropped,
+// as when they are made.
+func (l *Ledger) Load(bans []Ban) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	bans = slices.Clone(bans)
+	slices.SortStableFunc(bans, func(a, b Ban) int {
+		return a.Start.Compare(b.Start)
+	})
+
+	for _, ban := range bans {
+		ban.Netblock = ban.Netblock.Masked()
+		ban.Notes.Request = ban.Notes.Request.cut()
+		l.add(ban)
+	}
+}
+
+// add adds ban to its netblock's bans, after the last, and makes its
+// netblock the most recently seen. With MaxBans held, it drops one first.
+func (l *Ledger) add(ban Ban) {
+	if l.held == l.rules.MaxBans {
+		l.dropOne()
+	}
+
+	// dropOne can have dropped the netblock's last ban, and the netblock
+	// with it.
+	bans, found := l.netblocks.Get(ban.Netblock)
+	if !found {
+		bans = &[]Ban{}
+		l.netblocks.Add(ban.Netblock, bans)
+	}
+
+	*bans = append(*bans, ban)
+	l.held++
+
+	lengths := &l.v6Lengths
+	if ban.Netblock.Addr().Is4() {
+		lengths = &l.v4Lengths
+	}
+
+	if !slices.Contains(*lengths, ban.Netblock.Bits()) {
+		*lengths = append(*lengths, ban.Netblock.Bits())
+	}
+}
+
 // expiry returns when a ban for a broken limit made at now ends, or zero
 // when it is permanent. last is the netblock's last ban, which has ended,
 // or nil when it has none.

internal/bans/bans_test.go

@@ -39,7 +39,7 @@ func TestRepeatsTripleUntilPermanent(t *testing.T) {
 		t.Fatalf("sixth ban ends at %s, want a permanent one", ban.Expires)
 	}
 
-	_, banned := ledger.Check(netblock, now.Add(100*365*day))
+	_, banned := ledger.Check(netblock.Addr(), now.Add(100*365*day))
 	if !banned {
 		t.Error("a permanent ban ended")
 	}
@@ -135,28 +135,30 @@ func TestCheckRefusesWhileTheBanLastsAndCountsTheRefusals(t *testing.T) {
 
 	ledger := bans.New(defaultRules())
 	netblock := netip.MustParsePrefix("203.0.113.9/32")
-	ban := ledger.BanForLimit(netblock, midnight(), bans.Notes{})
+	ban := ledger.BanForLimit(netblock, midnight(), bans.Notes{Requests: 5})
 
 	for range 3 {
-		got, banned := ledger.Check(netblock, ban.Expires.Add(-time.Nanosecond))
+		got, banned := ledger.Check(netblock.Addr(), ban.Expires.Add(-time.Nanosecond))
 		if !banned || got.Start != ban.Start {
 			t.Fatalf("check during the ban gives %+v and %t", got, banned)
 		}
 	}
 
-	_, banned := ledger.Check(netip.MustParsePrefix("203.0.113.10/32"), midnight())
+	_, banned := ledger.Check(netip.MustParseAddr("203.0.113.10"), midnight())
 	if banned {
 		t.Error("another netblock is banned")
 	}
 
-	_, banned = ledger.Check(netblock, ban.Expires)
+	_, banned = ledger.Check(netblock.Addr(), ban.Expires)
 	if banned {
 		t.Error("the ban did not end")
 	}
 
-	refused := ledger.Bans(netblock)[0].Notes.Refused
-	if refused != 3 {
-		t.Errorf("the notes count %d refused requests, want 3", refused)
+	// The netblock's requests went from 5 to 8 with the three refused.
+	notes := ledger.Bans(netblock)[0].Notes
+	if notes.Refused != 3 || notes.Requests != 8 {
+		t.Errorf("the notes count %d refused requests of %d, want 3 of 8",
+			notes.Refused, notes.Requests)
 	}
 }
 
@@ -178,7 +180,7 @@ func TestMaxBansDropsTheEarliestBanOfTheNetblockSeenLongestAgo(t *testing.T) {
 
 	// A request from a makes b the netblock seen longest ago, and its ban
 	// goes to make room for d's.
-	ledger.Check(a, now)
+	ledger.Check(a.Addr(), now)
 	ledger.BanForLimit(d, now, bans.Notes{})
 	wantBans(t, ledger, map[netip.Prefix]int{a: 1, b: 0, c: 1, d: 1})
 
@@ -188,7 +190,7 @@ func TestMaxBansDropsTheEarliestBanOfTheNetblockSeenLongestAgo(t *testing.T) {
 
 	// With d seen since, a is seen longest ago, and its earlier ban goes
 	// first.
-	ledger.Check(d, first.Expires)
+	ledger.Check(d.Addr(), first.Expires)
 	ledger.BanForLimit(b, first.Expires, bans.Notes{})
 	wantBans(t, ledger, map[netip.Prefix]int{a: 1, b: 1, d: 1})
 

internal/bans/snapshot_test.go

@@ -0,0 +1,193 @@
+package bans_test
+
+import (
+	"net/netip"
+	"slices"
+	"strings"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/bans"
+)
+
+func TestChangedAfterABanIsMade(t *testing.T) {
+	t.Parallel()
+
+	ledger := bans.New(defaultRules())
+	netblock := netip.MustParsePrefix("203.0.113.9/32")
+
+	wantChanged(t, ledger, false)
+
+	ledger.BanForLimit(netblock, midnight(), bans.Notes{})
+	wantChanged(t, ledger, true)
+
+	// A limit broken during the ban makes no other, and a refusal changes
+	// only the counts in the notes, which wait for the interval's write.
+	ledger.BanForLimit(netblock, midnight().Add(time.Minute), bans.Notes{})
+	ledger.Check(netblock.Addr(), midnight().Add(time.Minute))
+	wantChanged(t, ledger, false)
+
+	// Two bans before the value is read leave one.
+	ledger.BanForLimit(netip.MustParsePrefix("203.0.113.10/32"), midnight(), bans.Notes{})
+	ledger.BanForLimit(netip.MustParsePrefix("203.0.113.11/32"), midnight(), bans.Notes{})
+	wantChanged(t, ledger, true)
+	wantChanged(t, ledger, false)
+}
+
+func TestSnapshotListsEveryBanByNetblock(t *testing.T) {
+	t.Parallel()
+
+	ledger := bans.New(defaultRules())
+	v6 := netip.MustParsePrefix("2001:db8::/64")
+	high := netip.MustParsePrefix("203.0.113.10/32")
+	low := netip.MustParsePrefix("203.0.113.9/32")
+
+	first := ledger.BanForLimit(v6, midnight(), bans.Notes{})
+	ledger.BanForLimit(high, midnight(), bans.Notes{})
+	ledger.BanForLimit(low, midnight(), bans.Notes{})
+	ledger.BanForLimit(v6, first.Expires, bans.Notes{})
+
+	snapshot := ledger.Snapshot()
+
+	got := make([]string, 0, len(snapshot))
+	for _, ban := range snapshot {
+		got = append(got, ban.Netblock.String()+" "+ban.Start.Format(time.Kitchen))
+	}
+
+	want := []string{
+		"203.0.113.9/32 12:00AM", "203.0.113.10/32 12:00AM",
+		"2001:db8::/64 12:00AM", "2001:db8::/64 1:00AM",
+	}
+	if !slices.Equal(got, want) {
+		t.Errorf("snapshot %v, want %v", got, want)
+	}
+}
+
+func TestLoadedBansCarryOn(t *testing.T) {
+	t.Parallel()
+
+	before := bans.New(defaultRules())
+	netblock := netip.MustParsePrefix("203.0.113.9/32")
+	ban := before.BanForLimit(netblock, midnight(), bans.Notes{Limit: 1})
+
+	// Loaded into a new ledger, as across a restart, the ban still refuses
+	// while it lasts, and once it has ended a broken limit bans for three
+	// times as long, with the loaded ban counted among the earlier ones.
+	after := bans.New(defaultRules())
+	after.Load(before.Snapshot())
+
+	_, banned := after.Check(netblock.Addr(), ban.Expires.Add(-time.Second))
+	if !banned {
+		t.Error("the loaded ban does not refuse")
+	}
+
+	again := after.BanForLimit(netblock, ban.Expires, bans.Notes{})
+	if again.Expires.Sub(again.Start) != 3*time.Hour || again.Notes.EarlierBans != 1 {
+		t.Errorf("the next ban lasts %s with %d earlier bans, want 3h and 1",
+			again.Expires.Sub(again.Start), again.Notes.EarlierBans)
+	}
+}
+
+func TestLoadedBanRefusesEveryClientInItsNetblock(t *testing.T) {
+	t.Parallel()
+
+	// Two entries as an admin might write them, with addresses not masked
+	// to their lengths, the IPv6 one shorter than the /64 an IPv6 client's
+	// ban covers, beside a ban the ledger makes on one IPv4 address.
+	ledger := bans.New(defaultRules())
+	ledger.Load([]bans.Ban{
+		{Netblock: netip.MustParsePrefix("203.0.113.9/24"), Start: midnight()},
+		{Netblock: netip.MustParsePrefix("2001:db8::1/48"), Start: midnight()},
+	})
+	ledger.BanForLimit(netip.MustParsePrefix("198.51.100.7/32"), midnight(), bans.Notes{})
+
+	for client, want := range map[string]bool{
+		"203.0.113.0":     true,
+		"203.0.113.200":   true,
+		"203.0.114.1":     false,
+		"2001:db8:0:5::1": true,
+		"2001:db8:1::1":   false,
+		"198.51.100.7":    true,
+		"198.51.100.8":    false,
+	} {
+		_, banned := ledger.Check(netip.MustParseAddr(client), midnight())
+		if banned != want {
+			t.Errorf("%s is refused: %t, want %t", client, banned, want)
+		}
+	}
+
+	// The loaded netblocks are written back masked.
+	snapshot := ledger.Snapshot()
+
+	got := make([]string, 0, len(snapshot))
+	for _, ban := range snapshot {
+		got = append(got, ban.Netblock.String())
+	}
+
+	want := []string{"198.51.100.7/32", "203.0.113.0/24", "2001:db8::/48"}
+	if !slices.Equal(got, want) {
+		t.Errorf("the ledger holds bans on %v, want %v", got, want)
+	}
+}
+
+func TestLoadKeepsAtMostMaxBansDroppingTheEarliest(t *testing.T) {
+	t.Parallel()
+
+	// bans.json lists the bans by netblock, not in the order they began.
+	later := bans.Ban{Netblock: netip.MustParsePrefix("203.0.113.1/32"), Start: midnight()}
+	earlier := bans.Ban{
+		Netblock: netip.MustParsePrefix("203.0.113.2/32"),
+		Start:    midnight().Add(-time.Hour),
+	}
+
+	rules := defaultRules()
+	rules.MaxBans = 1
+	ledger := bans.New(rules)
+	ledger.Load([]bans.Ban{later, earlier})
+
+	held := ledger.Snapshot()
+	if len(held) != 1 || held[0] != later {
+		t.Errorf("the ledger holds %+v, want only the ban that began later", held)
+	}
+}
+
+func TestLoadCutsTheTextsTo256Bytes(t *testing.T) {
+	t.Parallel()
+
+	long := strings.Repeat("a", 300)
+	ban := bans.Ban{
+		Netblock: netip.MustParsePrefix("203.0.113.9/32"),
+		Start:    midnight(),
+		Notes: bans.Notes{Request: bans.Request{
+			Method: long, Host: long, Path: long, UserAgent: long,
+		}},
+	}
+
+	ledger := bans.New(defaultRules())
+	ledger.Load([]bans.Ban{ban})
+
+	cut := long[:256]
+	want := bans.Request{Method: cut, Host: cut, Path: cut, UserAgent: cut}
+
+	got := ledger.Snapshot()[0].Notes.Request
+	if got != want {
+		t.Errorf("the notes keep %+v, want each text cut to 256 bytes", got)
+	}
+}
+
+// wantChanged checks whether the ledger's Changed has a value to read.
+func wantChanged(t *testing.T, ledger *bans.Ledger, want bool) {
+	t.Helper()
+
+	got := false
+
+	select {
+	case <-ledger.Changed():
+		got = true
+	default:
+	}
+
+	if got != want {
+		t.Errorf("Changed has a value: %t, want %t", got, want)
+	}
+}

internal/config/config.go

@@ -12,6 +12,7 @@ import (
 	"net/http"
 	"net/netip"
 	"net/url"
+	"path/filepath"
 	"slices"
 	"strconv"
 	"strings"
@@ -94,6 +95,14 @@ type Config struct {
 	// BanScopeV4Prefix is the length of the netblock around an IPv4
 	// client that a ban covers (SWWAF_BAN_SCOPE_V4_PREFIX).
 	BanScopeV4Prefix int
+	// StateDir is the directory of the state files, an absolute path
+	// (SWWAF_STATE_DIR). bans.json is written StateWriteDelay after a ban
+	// is made (SWWAF_STATE_WRITE_DELAY), and every state file every
+	// StateCounterInterval (SWWAF_STATE_COUNTER_INTERVAL). Neither can be
+	// off.
+	StateDir             string
+	StateWriteDelay      time.Duration
+	StateCounterInterval time.Duration
 
 	// settings are the values read, as given or by default, for the
 	// log line at start.
@@ -139,6 +148,8 @@ var (
 	errNotBanResponse = errors.New("is not 403, 429 or close")
 	errNotV4Prefix    = errors.New(
 		"is not the length of an IPv4 netblock, from 0 to 32, such as 24")
+	errNotAbsolutePath = errors.New(
+		"is not an absolute path, such as /var/lib/smallwebwaf")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -174,6 +185,9 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		MaxBanDuration:       env.durationNotOff("SWWAF_MAX_BAN_DURATION", "7d"),
 		MaxBans:              env.numberNotOff("SWWAF_MAX_BANS", "5000"),
 		BanScopeV4Prefix:     env.v4Prefix("SWWAF_BAN_SCOPE_V4_PREFIX", "32"),
+		StateDir:             env.absolutePath("SWWAF_STATE_DIR", "/var/lib/smallwebwaf"),
+		StateWriteDelay:      env.durationNotOff("SWWAF_STATE_WRITE_DELAY", "10s"),
+		StateCounterInterval: env.durationNotOff("SWWAF_STATE_COUNTER_INTERVAL", "15m"),
 	}
 
 	for _, country := range cfg.ExclusivelyAllowedCountries {
@@ -329,6 +343,16 @@ func (e *environment) v4Prefix(name, defaultValue string) int {
 	return length
 }
 
+// absolutePath reads a setting that is an absolute path.
+func (e *environment) absolutePath(name, defaultValue string) string {
+	path := e.value(name, defaultValue)
+	if !filepath.IsAbs(path) {
+		e.check(name, fmt.Errorf("%q %w", path, errNotAbsolutePath))
+	}
+
+	return path
+}
+
 // parseDuration reads a duration in Go's syntax, such as 90s or 15m, a
 // whole number of days such as 7d, or off.
 func parseDuration(value string) (time.Duration, error) {

internal/config/config_test.go

@@ -41,6 +41,9 @@ const (
 	maxBanDuration          = "SWWAF_MAX_BAN_DURATION"
 	maxBans                 = "SWWAF_MAX_BANS"
 	banScopeV4Prefix        = "SWWAF_BAN_SCOPE_V4_PREFIX"
+	stateDir                = "SWWAF_STATE_DIR"
+	stateWriteDelay         = "SWWAF_STATE_WRITE_DELAY"
+	stateCounterInterval    = "SWWAF_STATE_COUNTER_INTERVAL"
 )
 
 // off switches a timeout, a size limit or a rate limit off.
@@ -92,6 +95,9 @@ func TestDefaults(t *testing.T) {
 		MaxBanDuration:              7 * 24 * time.Hour,
 		MaxBans:                     5000,
 		BanScopeV4Prefix:            32,
+		StateDir:                    "/var/lib/smallwebwaf",
+		StateWriteDelay:             10 * time.Second,
+		StateCounterInterval:        15 * time.Minute,
 	})
 
 	if cfg.UpstreamURL.String() != "http://127.0.0.1:8081" {
@@ -136,6 +142,9 @@ func TestValuesAsSet(t *testing.T) {
 		maxBanDuration:          "30d",
 		maxBans:                 "100",
 		banScopeV4Prefix:        "24",
+		stateDir:                "/srv/waf-state",
+		stateWriteDelay:         "500ms",
+		stateCounterInterval:    "1h",
 	})
 
 	wantSettings(t, cfg, config.Config{
@@ -157,6 +166,9 @@ func TestValuesAsSet(t *testing.T) {
 		MaxBanDuration:              30 * 24 * time.Hour,
 		MaxBans:                     100,
 		BanScopeV4Prefix:            24,
+		StateDir:                    "/srv/waf-state",
+		StateWriteDelay:             500 * time.Millisecond,
+		StateCounterInterval:        time.Hour,
 	})
 
 	if cfg.UpstreamURL.String() != "https://app.internal:8443/" {
@@ -329,6 +341,9 @@ func TestInvalidValueStopsTheStart(t *testing.T) {
 		{maxBanDuration, off}, {maxBanDuration, "1w"},
 		{maxBans, off}, {maxBans, "0"}, {maxBans, "5K"},
 		{banScopeV4Prefix, "33"}, {banScopeV4Prefix, "-1"}, {banScopeV4Prefix, "/24"},
+		{stateDir, ""}, {stateDir, "state"}, {stateDir, "./var/lib/smallwebwaf"},
+		{stateWriteDelay, off}, {stateWriteDelay, "0s"},
+		{stateCounterInterval, off}, {stateCounterInterval, "15"},
 	} {
 		t.Run(tc.name+"="+tc.value, func(t *testing.T) {
 			t.Parallel()
@@ -389,6 +404,9 @@ func TestLogsEachSettingWithItsValue(t *testing.T) {
 		maxBanDuration:          "7d",
 		maxBans:                 "5000",
 		banScopeV4Prefix:        "32",
+		stateDir:                "/var/lib/smallwebwaf",
+		stateWriteDelay:         "10s",
+		stateCounterInterval:    "15m",
 	}
 	if !maps.Equal(line.Settings, want) {
 		t.Errorf("logged settings\n%v\nwant\n%v", line.Settings, want)
@@ -417,7 +435,7 @@ func wantSettings(t *testing.T, got *config.Config, want config.Config) {
 	wantBanSettings(t, got, want)
 }
 
-// wantBanSettings checks the settings for bans.
+// wantBanSettings checks the settings for bans and the state files.
 func wantBanSettings(t *testing.T, got *config.Config, want config.Config) {
 	t.Helper()
 
@@ -429,6 +447,12 @@ func wantBanSettings(t *testing.T, got *config.Config, want config.Config) {
 		got.BanScopeV4Prefix != want.BanScopeV4Prefix {
 		t.Errorf("ban settings\n%+v\nwant\n%+v", got, want)
 	}
+
+	if got.StateDir != want.StateDir ||
+		got.StateWriteDelay != want.StateWriteDelay ||
+		got.StateCounterInterval != want.StateCounterInterval {
+		t.Errorf("state settings\n%+v\nwant\n%+v", got, want)
+	}
 }
 
 // wantNetblocks checks a list of netblocks.

internal/lookup/lookup.go

@@ -1,6 +1,7 @@
 // Package lookup looks up each client's country through the GeoJS web
 // service, and keeps the answers in memory, for at most 100,000 clients
-// and for 7 days each.
+// and for 7 days each. The answers are written to lookups.json and read
+// from it by the state package.
 package lookup
 
 import (
@@ -12,6 +13,7 @@ import (
 	"log/slog"
 	"net/http"
 	"net/netip"
+	"slices"
 	"strings"
 	"sync"
 	"time"
@@ -76,7 +78,7 @@ type GeoJS struct {
 	httpClient *http.Client
 
 	mu      sync.Mutex
-	answers *simplelru.LRU[netip.Prefix, answer]
+	answers *simplelru.LRU[netip.Prefix, *Answer]
 	// waiting are the clients without an answer: those to ask GeoJS about,
 	// and those it is being asked about.
 	waiting map[netip.Prefix]*wait
@@ -88,11 +90,14 @@ type GeoJS struct {
 	retryAt    time.Time
 }
 
-// answer is what GeoJS said about a client: its country, "" when GeoJS
-// cannot place it, and when GeoJS said so.
-type answer struct {
-	country  string
-	received time.Time
+// Answer is what GeoJS said about a client, as lookups.json holds it: its
+// country, "" when GeoJS cannot place it, when GeoJS said so, and when
+// the answer was last used.
+type Answer struct {
+	Client   netip.Prefix `json:"client"`
+	Country  string       `json:"country"`
+	Answered time.Time    `json:"answered"`
+	Used     time.Time    `json:"used"`
 }
 
 // wait is a client waiting for its answer.
@@ -107,7 +112,7 @@ type wait struct {
 
 // New returns a GeoJS with no answer kept yet.
 func New(params Params) *GeoJS {
-	answers, err := simplelru.NewLRU[netip.Prefix, answer](maxAnswers, nil)
+	answers, err := simplelru.NewLRU[netip.Prefix, *Answer](maxAnswers, nil)
 	if err != nil {
 		panic(err) // NewLRU fails only for a size below one
 	}
@@ -164,6 +169,47 @@ func (g *GeoJS) Country(ctx context.Context, client netip.Prefix) string {
 	return country
 }
 
+// Snapshot returns every answer kept, sorted by client, as lookups.json
+// lists them.
+func (g *GeoJS) Snapshot() []Answer {
+	g.mu.Lock()
+
+	answers := make([]Answer, 0, g.answers.Len())
+	for _, kept := range g.answers.Values() {
+		answers = append(answers, *kept)
+	}
+
+	g.mu.Unlock()
+
+	slices.SortFunc(answers, func(a, b Answer) int {
+		return a.Client.Compare(b.Client)
+	})
+
+	return answers
+}
+
+// Load keeps answers read from lookups.json, in a GeoJS that keeps none
+// yet, in the order they were last used, so that the one used longest
+// ago is dropped first. Answers GeoJS gave keepFor ago or more are
+// dropped.
+func (g *GeoJS) Load(answers []Answer) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+
+	answers = slices.Clone(answers)
+	slices.SortStableFunc(answers, func(a, b Answer) int {
+		return a.Used.Compare(b.Used)
+	})
+
+	now := g.now()
+
+	for _, answer := range answers {
+		if now.Sub(answer.Answered) < keepFor {
+			g.answers.Add(answer.Client, &answer)
+		}
+	}
+}
+
 // answerOrWait returns client's kept answer if it has one. Otherwise it
 // puts the client among those waiting if there is room, has GeoJS asked
 // about them if it can be, and returns what to wait on for the answer, or
@@ -203,15 +249,19 @@ func (g *GeoJS) answerOrWait(
 	return "", w.asked
 }
 
-// kept returns client's answer, if one was received less than keepFor
-// ago.
+// kept returns client's answer, if GeoJS gave it less than keepFor ago,
+// and notes that it was used.
 func (g *GeoJS) kept(client netip.Prefix) (string, bool) {
+	now := g.now()
+
 	kept, found := g.answers.Get(client)
-	if !found || g.now().Sub(kept.received) >= keepFor {
+	if !found || now.Sub(kept.Answered) >= keepFor {
 		return "", false
 	}
 
-	return kept.country, true
+	kept.Used = now
+
+	return kept.Country, true
 }
 
 // ask starts asking GeoJS about the waiting clients, unless a request to
@@ -293,7 +343,9 @@ func (g *GeoJS) keep(
 			continue
 		}
 
-		g.answers.Add(client, answer{country: country, received: now})
+		g.answers.Add(client, &Answer{
+			Client: client, Country: country, Answered: now, Used: now,
+		})
 		close(g.waiting[client].asked)
 		delete(g.waiting, client)
 	}

internal/lookup/snapshot_test.go

@@ -0,0 +1,96 @@
+package lookup_test
+
+import (
+	"net/netip"
+	"slices"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/lookup"
+)
+
+func TestSnapshotHoldsEachAnswerAndWhenItWasLastUsed(t *testing.T) {
+	t.Parallel()
+
+	_, clock, g := start(t)
+	placed := netip.MustParsePrefix("203.0.113.9/32")
+	notPlaced := netip.MustParsePrefix(unplaced + "/32")
+	asked := clock.Now()
+
+	wantCountry(t, g, placed, germany)
+	wantCountry(t, g, notPlaced, "")
+
+	clock.advance(time.Hour)
+	wantCountry(t, g, placed, germany)
+
+	want := []lookup.Answer{
+		{Client: notPlaced, Country: "", Answered: asked, Used: asked},
+		{Client: placed, Country: germany, Answered: asked, Used: asked.Add(time.Hour)},
+	}
+	if got := g.Snapshot(); !slices.Equal(got, want) {
+		t.Errorf("snapshot\n%+v\nwant\n%+v", got, want)
+	}
+}
+
+func TestLoadedAnswersAreKeptFor7DaysFromWhenGeoJSGaveThem(t *testing.T) {
+	t.Parallel()
+
+	geojs, clock, g := start(t)
+	now := clock.Now()
+	kept := lookup.Answer{
+		Client:   netip.MustParsePrefix("203.0.113.9/32"),
+		Country:  "FR",
+		Answered: now.Add(-week + time.Second),
+		Used:     now.Add(-time.Hour),
+	}
+	stale := lookup.Answer{
+		Client:   netip.MustParsePrefix("203.0.113.10/32"),
+		Country:  "FR",
+		Answered: now.Add(-week),
+		Used:     now.Add(-time.Hour),
+	}
+
+	g.Load([]lookup.Answer{kept, stale})
+
+	if got := g.Snapshot(); !slices.Equal(got, []lookup.Answer{kept}) {
+		t.Errorf("kept %+v, want only the answer GeoJS gave less than 7 days ago", got)
+	}
+
+	wantCountry(t, g, kept.Client, "FR")
+	wantRequests(t, geojs, 0)
+}
+
+func TestLoadDropsTheAnswerUsedLongestAgoFirst(t *testing.T) {
+	t.Parallel()
+
+	const maxAnswers = 100000
+
+	_, clock, g := start(t)
+	now := clock.Now()
+
+	// lookups.json lists the answers by client. Here each was last used a
+	// second before the one listed before it, so the last listed is the
+	// one used longest ago, and the one dropped.
+	answers := make([]lookup.Answer, maxAnswers+1)
+	addr := netip.MustParseAddr("10.0.0.0")
+
+	for i := range answers {
+		answers[i] = lookup.Answer{
+			Client:   netip.PrefixFrom(addr, addr.BitLen()),
+			Country:  germany,
+			Answered: now,
+			Used:     now.Add(-time.Duration(i) * time.Second),
+		}
+		addr = addr.Next()
+	}
+
+	g.Load(answers)
+
+	got := g.Snapshot()
+	if len(got) != maxAnswers || got[0] != answers[0] ||
+		got[maxAnswers-1] != answers[maxAnswers-1] {
+		t.Errorf("%d answers kept, from %s to %s; want %d, from %s to %s",
+			len(got), got[0].Client, got[len(got)-1].Client, maxAnswers,
+			answers[0].Client, answers[maxAnswers-1].Client)
+	}
+}

internal/proxy/bans.go

@@ -14,10 +14,10 @@ func (rq *request) banResponse(action string) *refusal {
 	return &refusal{status: rq.h.config.BanResponse, action: action}
 }
 
-// banned reports whether a ban on the client's netblock refuses the
-// request at now, and notes for the log line when that ban ends.
+// banned reports whether a ban on a netblock the client is in refuses
+// the request at now, and notes for the log line when that ban ends.
 func (rq *request) banned(now time.Time) bool {
-	ban, banned := rq.h.ledger.Check(rq.netblock(), now)
+	ban, banned := rq.h.ledger.Check(rq.client, now)
 	if banned {
 		rq.line.BanExpires = banExpires(ban)
 	}
@@ -36,7 +36,8 @@ func (rq *request) limitBroken(now time.Time) bool {
 		return false
 	}
 
-	ban := rq.h.ledger.BanForLimit(rq.netblock(), now, bans.Notes{
+	netblock := rq.netblock()
+	ban := rq.h.ledger.BanForLimit(netblock, now, bans.Notes{
 		Country: rq.line.Country,
 		Limit:   hit.Limit,
 		Window:  hit.Window,
@@ -49,6 +50,8 @@ func (rq *request) limitBroken(now time.Time) bool {
 			Status:    rq.h.config.BanResponse,
 			UserAgent: rq.in.UserAgent(),
 		},
+		// The histories count this request only once it has ended.
+		Requests: rq.h.limiter.Requests(netblock) + 1,
 	})
 	rq.h.limiter.Reset(group)
 

internal/proxy/bans_test.go

@@ -133,7 +133,7 @@ func TestBanIsNotCountedAndResetsTheCounters(t *testing.T) {
 	s.get(client, http.StatusOK, requestlog.ActionForward)
 	s.get(client, http.StatusForbidden, requestlog.ActionRateLimited)
 
-	banned := proxy.LedgerOf(server).Bans(netip.MustParsePrefix(client + "/32"))
+	banned := server.Ledger.Bans(netip.MustParsePrefix(client + "/32"))
 	if len(banned) != 2 || banned[0].Notes.Refused != 3 {
 		t.Errorf("bans %+v, want two, the first with 3 requests refused", banned)
 	}
@@ -291,12 +291,15 @@ func TestBanNotes(t *testing.T) {
 				Status:    http.StatusForbidden,
 				UserAgent: userAgent,
 			},
+			// The one let through, the one that broke the limit and the two
+			// refused under the ban.
+			Requests:    4,
 			Refused:     2,
 			EarlierBans: 0,
 		},
 	}
 
-	ledger := proxy.LedgerOf(server)
+	ledger := server.Ledger
 
 	got := ledger.Bans(netblock)
 	if len(got) != 1 || got[0] != want {
@@ -361,7 +364,7 @@ func (c *clock) advance(d time.Duration) {
 // set to midnight, the start of a bucket in every window.
 func startWithClock(
 	t *testing.T, geojsURL string, env map[string]string,
-) (*sender, *clock, *http.Server) {
+) (*sender, *clock, *proxy.Server) {
 	t.Helper()
 
 	app := startApp(t, func(http.ResponseWriter, *http.Request) {})

internal/proxy/export_test.go

@@ -1,15 +0,0 @@
-package proxy
-
-import (
-	"net/http"
-
-	"sneak.berlin/go/smallwebwaf/internal/bans"
-)
-
-// LedgerOf returns the ban ledger of a server New returned, so that the
-// tests can read the bans' notes.
-func LedgerOf(server *http.Server) *bans.Ledger {
-	h, _ := server.Handler.(*handler)
-
-	return h.ledger
-}

internal/proxy/history_test.go

@@ -0,0 +1,103 @@
+package proxy_test
+
+import (
+	"io"
+	"net/http"
+	"net/netip"
+	"strings"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/proxy"
+	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
+	"sneak.berlin/go/smallwebwaf/internal/requestlog"
+)
+
+func TestHistoryKeepsEachRequestOfTheClient(t *testing.T) {
+	t.Parallel()
+
+	geojsURL, _ := startGeoJS(t)
+	s, clk, server := startWithClock(t, geojsURL, map[string]string{
+		rateLimitPerMinute: "2",
+		deniedCountries:    "kp",
+	})
+	start := clk.Now()
+
+	// Two let through, one over the limit, which bans the client, and one
+	// refused under that ban, for which the country is not looked up.
+	s.get(fromDE, http.StatusOK, requestlog.ActionForward)
+	clk.advance(time.Second)
+	s.get(fromDE, http.StatusOK, requestlog.ActionForward)
+	s.get(fromDE, http.StatusForbidden, requestlog.ActionRateLimited)
+	clk.advance(time.Second)
+	s.get(fromDE, http.StatusForbidden, requestlog.ActionBanned)
+
+	want := ratelimit.History{
+		FirstSeen: start,
+		LastSeen:  start.Add(2 * time.Second),
+		Country:   "DE",
+		LookedUp:  start.Add(time.Second),
+		Requests:  4,
+		Forwarded: 2,
+		Refused:   2,
+		// The app answers with no body, smallwebwaf with its status text.
+		ResponseBytes: 2 * int64(len("Forbidden\n")),
+		Responses:     ratelimit.Responses{Status2xx: 2, Status4xx: 2},
+		Offences:      ratelimit.Offences{Limit: 1},
+	}
+
+	got := historyOf(t, server, fromDE)
+	if got != want {
+		t.Errorf("history\n%+v\nwant\n%+v", got, want)
+	}
+}
+
+func TestHistoryCountsTheBodiesEachWay(t *testing.T) {
+	t.Parallel()
+
+	app := startApp(t, func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.Copy(io.Discard, r.Body)
+		_, _ = io.WriteString(w, "hello")
+	})
+	addr, out, server := startProxyWithClock(t, app.URL, "", time.Now, nil)
+
+	got := do(t, newRequest(t, http.MethodPost, addr, "/", strings.NewReader("abc")))
+	wantStatus(t, got, http.StatusOK)
+	out.requestLine(t)
+
+	history := historyOf(t, server, localhost)
+	if history.RequestBytes != 3 || history.ResponseBytes != 5 {
+		t.Errorf("history counts %d bytes in and %d out, want 3 and 5",
+			history.RequestBytes, history.ResponseBytes)
+	}
+}
+
+func TestHealthEndpointIsNotInTheHistory(t *testing.T) {
+	t.Parallel()
+
+	app := startApp(t, func(http.ResponseWriter, *http.Request) {})
+	addr, out, server := startProxyWithClock(t, app.URL, "", time.Now, nil)
+
+	wantStatus(t, get(t, addr, proxy.HealthPath), http.StatusOK)
+	out.requestLine(t)
+
+	if clients := server.Limiter.Snapshot(); len(clients) != 0 {
+		t.Errorf("the table holds %+v, want no client", clients)
+	}
+}
+
+// historyOf returns the history of the client at addr.
+func historyOf(t *testing.T, server *proxy.Server, addr string) ratelimit.History {
+	t.Helper()
+
+	client := netip.MustParsePrefix(addr + "/32")
+	for _, c := range server.Limiter.Snapshot() {
+		if c.Client == client {
+			return c.History
+		}
+	}
+
+	t.Fatalf("%s is not in the table", client)
+
+	return ratelimit.History{}
+}

internal/proxy/proxy.go

@@ -38,53 +38,70 @@ type Params struct {
 	// lookup.URL. GeoJS is asked only while a country list is set.
 	GeoJSURL string
 	// Now tells the time by which requests are counted for the rate
-	// limits and bans are made and run out, normally time.Now.
+	// limits, bans are made and run out, and GeoJS's answers are kept,
+	// normally time.Now in UTC, the time the state files give.
 	Now func() time.Time
 }
 
+// Server is the server smallwebwaf runs, with the parts of the proxy
+// whose state the state files keep.
+type Server struct {
+	*http.Server
+
+	Ledger  *bans.Ledger
+	Limiter *ratelimit.Limiter
+	GeoJS   *lookup.GeoJS
+}
+
 // New returns the server smallwebwaf runs: each request it reads passes
 // through the proxy. Go's server itself refuses a request line and
 // headers over SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES, with 431, closes a
 // connection idle for SWWAF_CLIENT_IDLE_TIMEOUT, and applies
 // SWWAF_CLIENT_REQUEST_TIMEOUT while the headers arrive; the proxy
 // applies the timeouts and size limits from then on.
-func New(params Params) *http.Server {
+func New(params Params) *Server {
 	errorLog := slog.NewLogLogger(params.ProcessLog.Handler(), slog.LevelWarn)
+	h := &handler{
+		config:     params.Config,
+		requestLog: params.RequestLog,
+		processLog: params.ProcessLog,
+		errorLog:   errorLog,
+		transport:  newTransport(),
+		now:        params.Now,
+		limiter: ratelimit.New(ratelimit.Limits{
+			PerMinute: params.Config.RateLimitPerMinute,
+			PerHour:   params.Config.RateLimitPerHour,
+			PerDay:    params.Config.RateLimitPerDay,
+		}),
+		ledger: bans.New(bans.Rules{
+			LimitBanDuration:     params.Config.LimitBanDuration,
+			LimitBanRepeatWindow: params.Config.LimitBanRepeatWindow,
+			MaxBanDuration:       params.Config.MaxBanDuration,
+			MaxBans:              params.Config.MaxBans,
+		}),
+		geojs: lookup.New(lookup.Params{
+			URL:        params.GeoJSURL,
+			Now:        params.Now,
+			ProcessLog: params.ProcessLog,
+		}),
+	}
 
-	return &http.Server{
-		Addr: params.Config.ListenAddr,
-		Handler: &handler{
-			config:     params.Config,
-			requestLog: params.RequestLog,
-			processLog: params.ProcessLog,
-			errorLog:   errorLog,
-			transport:  newTransport(),
-			now:        params.Now,
-			limiter: ratelimit.New(ratelimit.Limits{
-				PerMinute: params.Config.RateLimitPerMinute,
-				PerHour:   params.Config.RateLimitPerHour,
-				PerDay:    params.Config.RateLimitPerDay,
-			}),
-			ledger: bans.New(bans.Rules{
-				LimitBanDuration:     params.Config.LimitBanDuration,
-				LimitBanRepeatWindow: params.Config.LimitBanRepeatWindow,
-				MaxBanDuration:       params.Config.MaxBanDuration,
-				MaxBans:              params.Config.MaxBans,
-			}),
-			geojs: lookup.New(lookup.Params{
-				URL:        params.GeoJSURL,
-				Now:        time.Now,
-				ProcessLog: params.ProcessLog,
-			}),
+	return &Server{
+		Server: &http.Server{
+			Addr:              params.Config.ListenAddr,
+			Handler:           h,
+			ReadHeaderTimeout: params.Config.ClientRequestTimeout,
+			// Off is an IdleTimeout of 0, which Go's server replaces with
+			// ReadTimeout: no limit, as long as ReadTimeout stays unset.
+			IdleTimeout: params.Config.ClientIdleTimeout,
+			// Go's server reads 4 KiB past MaxHeaderBytes before it
+			// refuses, so the limit a client meets is the setting.
+			MaxHeaderBytes: int(params.Config.ClientRequestHeaderMaxBytes - 4<<10),
+			ErrorLog:       errorLog,
 		},
-		ReadHeaderTimeout: params.Config.ClientRequestTimeout,
-		// Off is an IdleTimeout of 0, which Go's server replaces with
-		// ReadTimeout: no limit, as long as ReadTimeout stays unset.
-		IdleTimeout: params.Config.ClientIdleTimeout,
-		// Go's server reads 4 KiB past MaxHeaderBytes before it refuses,
-		// so the limit a client meets is the setting.
-		MaxHeaderBytes: int(params.Config.ClientRequestHeaderMaxBytes - 4<<10),
-		ErrorLog:       errorLog,
+		Ledger:  h.ledger,
+		Limiter: h.limiter,
+		GeoJS:   h.geojs,
 	}
 }
 
@@ -130,6 +147,9 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Once the request has ended, before its log line is written.
+	defer rq.addToHistory()
+
 	refused := rq.check(r.Context())
 	if refused != nil {
 		rq.answer(*refused)

internal/proxy/proxy_test.go

@@ -200,7 +200,7 @@ func startProxyWithGeoJS(
 func startProxyWithClock(
 	t *testing.T, appURL, geojsURL string, now func() time.Time,
 	env map[string]string,
-) (string, *output, *http.Server) {
+) (string, *output, *proxy.Server) {
 	t.Helper()
 
 	settings := map[string]string{"SWWAF_UPSTREAM_URL": appURL}

internal/proxy/request.go

@@ -12,6 +12,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
@@ -318,6 +319,24 @@ func (rq *request) finish() {
 	}
 }
 
+// addToHistory adds the request, which has ended, to its client's
+// history.
+func (rq *request) addToHistory() {
+	var requestBytes int64
+	if rq.body != nil {
+		requestBytes = rq.body.bytes.Load()
+	}
+
+	rq.h.limiter.AddToHistory(clientGroup(rq.client), rq.h.now(), ratelimit.Request{
+		Country:       rq.line.Country,
+		Forwarded:     !rq.upstreamStart.IsZero(),
+		Status:        rq.out.status,
+		RequestBytes:  requestBytes,
+		ResponseBytes: rq.out.bytes,
+		BrokeLimit:    rq.line.Offence == requestlog.OffenceLimit,
+	})
+}
+
 // clientRequestDeadline is when the client must have sent its whole
 // request, or zero when SWWAF_CLIENT_REQUEST_TIMEOUT is off.
 func (rq *request) clientRequestDeadline() time.Time {

internal/ratelimit/history_test.go

@@ -0,0 +1,116 @@
+package ratelimit_test
+
+import (
+	"net/netip"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
+)
+
+func TestHistoryKeepsEveryRequest(t *testing.T) {
+	t.Parallel()
+
+	limiter := ratelimit.New(ratelimit.Limits{})
+	client := netip.MustParsePrefix("203.0.113.9/32")
+	start := midnight()
+
+	for i, r := range []ratelimit.Request{
+		{Country: "DE", Forwarded: true, Status: 200, RequestBytes: 10, ResponseBytes: 100},
+		{Forwarded: true, Status: 101},
+		{Forwarded: true, Status: 304, RequestBytes: 5},
+		{Country: "FR", Status: 403, ResponseBytes: 10, BrokeLimit: true},
+		{Forwarded: true, Status: 502, ResponseBytes: 12},
+		// Closed without an answer: refused, and no response.
+		{Status: 0},
+	} {
+		limiter.AddToHistory(client, start.Add(time.Duration(i)*time.Minute), r)
+	}
+
+	want := ratelimit.History{
+		FirstSeen:     start,
+		LastSeen:      start.Add(5 * time.Minute),
+		Country:       "FR",
+		LookedUp:      start.Add(3 * time.Minute),
+		Requests:      6,
+		Forwarded:     4,
+		Refused:       2,
+		RequestBytes:  15,
+		ResponseBytes: 122,
+		Responses: ratelimit.Responses{
+			Status1xx: 1, Status2xx: 1, Status3xx: 1, Status4xx: 1, Status5xx: 1,
+		},
+		Offences: ratelimit.Offences{Limit: 1},
+	}
+
+	got := historyOf(t, limiter, client)
+	if got != want {
+		t.Errorf("history\n%+v\nwant\n%+v", got, want)
+	}
+}
+
+func TestResetKeepsTheHistory(t *testing.T) {
+	t.Parallel()
+
+	limiter := ratelimit.New(ratelimit.Limits{PerMinute: limit})
+	client := netip.MustParsePrefix("203.0.113.9/32")
+	start := midnight()
+
+	for range limit {
+		wantCount(t, limiter, client, start, "")
+		limiter.AddToHistory(client, start, ratelimit.Request{Forwarded: true})
+	}
+
+	limiter.Reset(client)
+
+	if got := historyOf(t, limiter, client).Requests; got != limit {
+		t.Errorf("the history counts %d requests, want %d", got, limit)
+	}
+}
+
+func TestRequestsAddsUpTheClientsInsideTheNetblock(t *testing.T) {
+	t.Parallel()
+
+	limiter := ratelimit.New(ratelimit.Limits{})
+
+	for client, requests := range map[string]int{
+		"198.51.100.9/32":  2,
+		"198.51.100.10/32": 3,
+		"192.0.2.1/32":     5,
+		"2001:db8:5::/64":  7,
+	} {
+		for range requests {
+			limiter.AddToHistory(netip.MustParsePrefix(client), midnight(),
+				ratelimit.Request{})
+		}
+	}
+
+	for netblock, want := range map[string]int64{
+		"198.51.100.9/32": 2,
+		"198.51.100.0/24": 5,
+		"2001:db8:5::/64": 7,
+		"203.0.113.0/24":  0,
+	} {
+		got := limiter.Requests(netip.MustParsePrefix(netblock))
+		if got != want {
+			t.Errorf("%s has sent %d requests, want %d", netblock, got, want)
+		}
+	}
+}
+
+// historyOf returns client's history.
+func historyOf(
+	t *testing.T, limiter *ratelimit.Limiter, client netip.Prefix,
+) ratelimit.History {
+	t.Helper()
+
+	for _, c := range limiter.Snapshot() {
+		if c.Client == client {
+			return c.History
+		}
+	}
+
+	t.Fatalf("%s is not in the table", client)
+
+	return ratelimit.History{}
+}

internal/ratelimit/ratelimit.go

@@ -1,11 +1,15 @@
-// Package ratelimit counts each client's requests over a minute, an hour
-// and a day, as the "Counting method" section of SPEC.md describes, and
-// tells when a request takes a client over a rate limit. The counts are
-// kept in memory only, for at most 20,000 clients.
+// Package ratelimit keeps the table of clients: each client's requests
+// counted over a minute, an hour and a day, as the "Counting method"
+// section of SPEC.md describes, which tell when a request takes the client
+// over a rate limit, and each client's history since it was first seen.
+// At most 20,000 clients are kept, in memory, and written to clients.json
+// and read from it by the state package.
 package ratelimit
 
 import (
+	"net/http"
 	"net/netip"
+	"slices"
 	"sync"
 	"time"
 
@@ -13,7 +17,8 @@ import (
 )
 
 // maxClients is how many clients are kept. Past it, the least recently
-// seen client is dropped, and starts afresh if it comes back.
+// seen client is dropped, with its history, and starts afresh if it comes
+// back.
 const maxClients = 20000
 
 const day = 24 * time.Hour
@@ -26,20 +31,94 @@ type Limits struct {
 	PerDay    int64
 }
 
-// Limiter counts each client's requests against the limits. It is safe
-// for concurrent use.
+// Limiter counts each client's requests against the limits, and keeps
+// its history. It is safe for concurrent use.
 type Limiter struct {
+	// windows are the minute, the hour and the day, in the order of
+	// Client.buckets.
 	windows [3]window
 
-	mu sync.Mutex
-	// clients holds each client's buckets, one pair for each of windows,
-	// in the same order.
-	clients *simplelru.LRU[netip.Prefix, *[3]buckets]
+	mu      sync.Mutex
+	clients *simplelru.LRU[netip.Prefix, *Client]
+}
+
+// Client is a client in the table, as clients.json holds it: its buckets
+// in each window, and its history.
+type Client struct {
+	Client  netip.Prefix `json:"client"`
+	Minute  Buckets      `json:"minute"`
+	Hour    Buckets      `json:"hour"`
+	Day     Buckets      `json:"day"`
+	History History      `json:"history"`
+}
+
+// Buckets are a client's two buckets in one window: the requests in the
+// bucket under way, which began at Start, and in the bucket before it.
+type Buckets struct {
+	Start    time.Time `json:"start"`
+	Current  int64     `json:"current"`
+	Previous int64     `json:"previous"`
+}
+
+// History is what is known of a client since it was first seen.
+//
+//nolint:tagliatelle // the state files use snake_case, as the request log does
+type History struct {
+	FirstSeen time.Time `json:"first_seen"`
+	LastSeen  time.Time `json:"last_seen"`
+	// Country is the client's country as it was last looked up, and
+	// LookedUp when that was; both are empty while it never was.
+	Country  string    `json:"country,omitempty"`
+	LookedUp time.Time `json:"looked_up,omitzero"`
+	// Requests are all the client's requests: Forwarded those passed to
+	// the app, Refused those refused before anything reached it.
+	Requests  int64 `json:"requests"`
+	Forwarded int64 `json:"forwarded"`
+	Refused   int64 `json:"refused"`
+	// RequestBytes and ResponseBytes are the body bytes of its requests
+	// and of the responses it was sent.
+	RequestBytes  int64     `json:"request_bytes"`
+	ResponseBytes int64     `json:"response_bytes"`
+	Responses     Responses `json:"responses,omitzero"`
+	Offences      Offences  `json:"offences,omitzero"`
+}
+
+// Responses are the responses a client was sent, by status class;
+// Status5xx counts every status from 500 up.
+type Responses struct {
+	Status1xx int64 `json:"1xx,omitempty"`
+	Status2xx int64 `json:"2xx,omitempty"`
+	Status3xx int64 `json:"3xx,omitempty"`
+	Status4xx int64 `json:"4xx,omitempty"`
+	Status5xx int64 `json:"5xx,omitempty"`
+}
+
+// Offences are a client's offences, by kind.
+type Offences struct {
+	// Limit is its requests that broke a rate limit.
+	Limit int64 `json:"limit"`
+}
+
+// Request is what a client's history keeps of one of its requests.
+type Request struct {
+	// Country is the client's country, when the request looked it up.
+	Country string
+	// Forwarded is true for a request passed to the app, false for one
+	// refused before anything reached it.
+	Forwarded bool
+	// Status is what the client was sent, 0 if nothing was.
+	Status int
+	// RequestBytes and ResponseBytes are the body bytes of the request
+	// and of its response.
+	RequestBytes  int64
+	ResponseBytes int64
+	// BrokeLimit is true for a request that broke a rate limit.
+	BrokeLimit bool
 }
 
 // New returns a Limiter for limits, with no client counted yet.
 func New(limits Limits) *Limiter {
-	clients, err := simplelru.NewLRU[netip.Prefix, *[3]buckets](maxClients, nil)
+	clients, err := simplelru.NewLRU[netip.Prefix, *Client](maxClients, nil)
 	if err != nil {
 		panic(err) // NewLRU fails only for a size below one
 	}
@@ -73,16 +152,12 @@ func (l *Limiter) Count(client netip.Prefix, now time.Time) (Hit, bool) {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 
-	counts, seen := l.clients.Get(client)
-	if !seen {
-		counts = &[3]buckets{}
-		l.clients.Add(client, counts)
-	}
-
 	var hit Hit
 
-	for i, w := range l.windows {
-		requests := counts[i].add(now, w.length)
+	for i, b := range l.get(client).buckets() {
+		w := l.windows[i]
+
+		requests := b.add(now, w.length)
 		if hit.Window == "" && w.limit > 0 && requests > float64(w.limit) {
 			hit = Hit{Window: w.name, Limit: w.limit, Requests: requests}
 		}
@@ -91,12 +166,135 @@ func (l *Limiter) Count(client netip.Prefix, now time.Time) (Hit, bool) {
 	return hit, hit.Window != ""
 }
 
-// Reset sets client's counts in every window back to zero.
+// Reset sets client's counts in every window back to zero. Its history
+// keeps its totals.
 func (l *Limiter) Reset(client netip.Prefix) {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 
-	l.clients.Remove(client)
+	c, seen := l.clients.Peek(client)
+	if seen {
+		c.Minute, c.Hour, c.Day = Buckets{}, Buckets{}, Buckets{}
+	}
+}
+
+// AddToHistory adds r, a request from client at now, to the client's
+// history.
+func (l *Limiter) AddToHistory(client netip.Prefix, now time.Time, r Request) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	h := &l.get(client).History
+	if h.FirstSeen.IsZero() {
+		h.FirstSeen = now
+	}
+
+	h.LastSeen = now
+
+	if r.Country != "" {
+		h.Country = r.Country
+		h.LookedUp = now
+	}
+
+	h.Requests++
+	if r.Forwarded {
+		h.Forwarded++
+	} else {
+		h.Refused++
+	}
+
+	h.RequestBytes += r.RequestBytes
+	h.ResponseBytes += r.ResponseBytes
+	h.Responses.add(r.Status)
+
+	if r.BrokeLimit {
+		h.Offences.Limit++
+	}
+}
+
+// Requests returns how many requests the clients inside netblock have
+// sent, as their histories count them.
+func (l *Limiter) Requests(netblock netip.Prefix) int64 {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	// Most often the netblock is one client.
+	c, seen := l.clients.Peek(netblock)
+	if seen {
+		return c.History.Requests
+	}
+
+	var requests int64
+
+	for _, c := range l.clients.Values() {
+		if netblock.Overlaps(c.Client) {
+			requests += c.History.Requests
+		}
+	}
+
+	return requests
+}
+
+// Snapshot returns every client in the table, sorted by address, as
+// clients.json lists them.
+func (l *Limiter) Snapshot() []Client {
+	l.mu.Lock()
+
+	clients := make([]Client, 0, l.clients.Len())
+	for _, c := range l.clients.Values() {
+		clients = append(clients, *c)
+	}
+
+	l.mu.Unlock()
+
+	slices.SortFunc(clients, func(a, b Client) int {
+		return a.Client.Compare(b.Client)
+	})
+
+	return clients
+}
+
+// Load puts clients read from clients.json into a table that holds none
+// yet, in the order they were last seen, so that the least recently seen
+// is dropped first. Buckets whose time has passed at now are emptied.
+func (l *Limiter) Load(clients []Client, now time.Time) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	clients = slices.Clone(clients)
+	slices.SortStableFunc(clients, func(a, b Client) int {
+		return a.History.LastSeen.Compare(b.History.LastSeen)
+	})
+
+	for _, c := range clients {
+		for i, b := range c.buckets() {
+			// The window that ends at now covers neither bucket once it
+			// begins after the bucket under way has ended.
+			length := l.windows[i].length
+			if !now.Add(-length).Before(b.Start.Add(length)) {
+				*b = Buckets{}
+			}
+		}
+
+		l.clients.Add(c.Client, &c)
+	}
+}
+
+// get returns client's entry in the table, a new one if it has none, and
+// makes it the most recently seen.
+func (l *Limiter) get(client netip.Prefix) *Client {
+	c, seen := l.clients.Get(client)
+	if !seen {
+		c = &Client{Client: client}
+		l.clients.Add(client, c)
+	}
+
+	return c
+}
+
+// buckets returns c's buckets in the minute, the hour and the day.
+func (c *Client) buckets() [3]*Buckets {
+	return [3]*Buckets{&c.Minute, &c.Hour, &c.Day}
 }
 
 // window is a length of time over which requests are counted, and the
@@ -107,14 +305,6 @@ type window struct {
 	limit  int64
 }
 
-// buckets are a client's two buckets in one window: the requests in the
-// bucket under way, which began at start, and in the bucket before it.
-type buckets struct {
-	start    time.Time
-	current  int64
-	previous int64
-}
-
 // add counts a request at now in a window of length, and returns the
 // client's requests in the window that ends at now: those in the bucket
 // under way, and those in the bucket before it weighted by how much of
@@ -125,27 +315,44 @@ type buckets struct {
 // bucket. A request dated more than a second before it means the clock
 // was set back, and the buckets start afresh: otherwise the bucket before
 // would keep its full weight until the clock caught up.
-func (b *buckets) add(now time.Time, length time.Duration) float64 {
-	if now.Before(b.start.Add(-time.Second)) {
-		*b = buckets{}
+func (b *Buckets) add(now time.Time, length time.Duration) float64 {
+	if now.Before(b.Start.Add(-time.Second)) {
+		*b = Buckets{}
 	}
 
 	start := now.Truncate(length)
-	if start.After(b.start) {
-		if start.Equal(b.start.Add(length)) {
-			b.previous = b.current
+	if start.After(b.Start) {
+		if start.Equal(b.Start.Add(length)) {
+			b.Previous = b.Current
 		} else {
-			b.previous = 0
+			b.Previous = 0
 		}
 
-		b.start = start
-		b.current = 0
+		b.Start = start
+		b.Current = 0
 	}
 
-	b.current++
+	b.Current++
 
-	elapsed := max(now.Sub(b.start), 0)
+	elapsed := max(now.Sub(b.Start), 0)
 	covered := 1 - float64(elapsed)/float64(length)
 
-	return float64(b.previous)*covered + float64(b.current)
+	return float64(b.Previous)*covered + float64(b.Current)
+}
+
+// add counts a response with status in its class. A status of 0, for
+// nothing sent, is not a response.
+func (r *Responses) add(status int) {
+	switch {
+	case status >= http.StatusInternalServerError:
+		r.Status5xx++
+	case status >= http.StatusBadRequest:
+		r.Status4xx++
+	case status >= http.StatusMultipleChoices:
+		r.Status3xx++
+	case status >= http.StatusOK:
+		r.Status2xx++
+	case status >= http.StatusContinue:
+		r.Status1xx++
+	}
 }

internal/ratelimit/snapshot_test.go

@@ -0,0 +1,122 @@
+package ratelimit_test
+
+import (
+	"net/netip"
+	"slices"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
+)
+
+func TestSnapshotListsTheClientsByAddress(t *testing.T) {
+	t.Parallel()
+
+	want := []string{"192.0.2.1/32", "203.0.113.9/32", "203.0.113.10/32", "2001:db8::/64"}
+
+	limiter := ratelimit.New(ratelimit.Limits{})
+	for _, i := range []int{2, 3, 0, 1} {
+		limiter.Count(netip.MustParsePrefix(want[i]), midnight())
+	}
+
+	snapshot := limiter.Snapshot()
+
+	got := make([]string, 0, len(snapshot))
+	for _, c := range snapshot {
+		got = append(got, c.Client.String())
+	}
+
+	if !slices.Equal(got, want) {
+		t.Errorf("snapshot %v, want %v", got, want)
+	}
+
+	counted := ratelimit.Buckets{Start: midnight(), Current: 1}
+	if snapshot[0].Minute != counted || snapshot[0].Day != counted {
+		t.Errorf("buckets %+v and %+v, want %+v", snapshot[0].Minute, snapshot[0].Day,
+			counted)
+	}
+}
+
+func TestLoadedCountsCarryOn(t *testing.T) {
+	t.Parallel()
+
+	client := netip.MustParsePrefix("203.0.113.9/32")
+	start := midnight()
+
+	before := ratelimit.New(ratelimit.Limits{PerHour: limit})
+	for range limit {
+		wantCount(t, before, client, start, "")
+	}
+
+	// Loaded into a new limiter, as across a restart, the client has no
+	// fresh allowance.
+	later := start.Add(time.Minute)
+	after := ratelimit.New(ratelimit.Limits{PerHour: limit})
+	after.Load(before.Snapshot(), later)
+	wantCount(t, after, client, later, hour)
+}
+
+func TestLoadEmptiesBucketsWhoseTimeHasPassed(t *testing.T) {
+	t.Parallel()
+
+	client := netip.MustParsePrefix("203.0.113.9/32")
+	start := midnight()
+
+	limiter := ratelimit.New(ratelimit.Limits{})
+	limiter.Count(client, start)
+	limiter.AddToHistory(client, start, ratelimit.Request{Forwarded: true})
+
+	loaded := func(now time.Time) ratelimit.Client {
+		t.Helper()
+
+		after := ratelimit.New(ratelimit.Limits{})
+		after.Load(limiter.Snapshot(), now)
+
+		return after.Snapshot()[0]
+	}
+
+	// Two minutes on, the window that ends then covers neither of the
+	// minute's buckets, which are emptied; the hour's and the day's stay,
+	// and so does the history.
+	got := loaded(start.Add(2 * time.Minute))
+	if got.Minute != (ratelimit.Buckets{}) || got.Hour.Current != 1 ||
+		got.Day.Current != 1 || got.History.Requests != 1 {
+		t.Errorf("loaded two minutes on as %+v", got)
+	}
+
+	// A moment before, the window still covers some of the earlier one.
+	got = loaded(start.Add(2*time.Minute - time.Nanosecond))
+	if got.Minute.Current != 1 {
+		t.Errorf("loaded just under two minutes on with minute buckets %+v",
+			got.Minute)
+	}
+}
+
+func TestLoadDropsTheLeastRecentlySeenFirst(t *testing.T) {
+	t.Parallel()
+
+	const maxClients = 20000
+
+	// clients.json lists the clients by address. Here each was last seen
+	// a second before the one listed before it, so the last listed is the
+	// one seen longest ago, and the one dropped.
+	clients := make([]ratelimit.Client, maxClients+1)
+	addr := netip.MustParseAddr("10.0.0.0")
+
+	for i := range clients {
+		clients[i].Client = netip.PrefixFrom(addr, addr.BitLen())
+		clients[i].History.LastSeen = midnight().Add(-time.Duration(i) * time.Second)
+		addr = addr.Next()
+	}
+
+	limiter := ratelimit.New(ratelimit.Limits{})
+	limiter.Load(clients, midnight())
+
+	got := limiter.Snapshot()
+	if len(got) != maxClients || got[0].Client != clients[0].Client ||
+		got[maxClients-1].Client != clients[maxClients-1].Client {
+		t.Errorf("%d clients kept, from %s to %s; want %d, from %s to %s",
+			len(got), got[0].Client, got[len(got)-1].Client, maxClients,
+			clients[0].Client, clients[maxClients-1].Client)
+	}
+}

internal/smallwebwaf/healthcheck_test.go

@@ -24,12 +24,14 @@ func TestHealthCheck(t *testing.T) {
 
 	out := &output{}
 	exited := make(chan int, 1)
+	settings := map[string]string{
+		listenAddr:  localhost + ":0",
+		upstreamURL: app.URL,
+		stateDir:    t.TempDir(),
+	}
 
 	go func() {
-		exited <- run(ctx, map[string]string{
-			listenAddr:  localhost + ":0",
-			upstreamURL: app.URL,
-		}, out)
+		exited <- run(ctx, settings, out)
 	}()
 
 	addr, _ := out.line(t, "msg", "starting")["address"].(string)

internal/smallwebwaf/smallwebwaf.go

@@ -1,6 +1,6 @@
-// Package smallwebwaf runs the smallwebwaf process: it reads the settings,
-// serves requests until it is told to stop, and then stops in an orderly
-// way.
+// Package smallwebwaf runs the smallwebwaf process: it reads the settings
+// and the state files, serves requests until it is told to stop, and then
+// stops in an orderly way, writing the state files.
 package smallwebwaf
 
 import (
@@ -19,6 +19,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/lookup"
 	"sneak.berlin/go/smallwebwaf/internal/proxy"
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
+	"sneak.berlin/go/smallwebwaf/internal/state"
 )
 
 // shutdownTimeout is how long requests in progress may take to finish
@@ -55,8 +56,9 @@ func Main(version string) int {
 	})
 }
 
-// Run reads the settings, then serves requests until ctx is done. It
-// returns the process's exit status, 1 when smallwebwaf cannot start.
+// Run reads the settings and the state files, then serves requests until
+// ctx is done. It returns the process's exit status, 1 when smallwebwaf
+// cannot start.
 func Run(ctx context.Context, params Params) int {
 	processLog := requestlog.NewProcessLogger(params.Stdout)
 
@@ -67,6 +69,33 @@ func Run(ctx context.Context, params Params) int {
 		return 1
 	}
 
+	// The state files give times in UTC.
+	now := func() time.Time { return time.Now().UTC() }
+
+	server := proxy.New(proxy.Params{
+		Config:     cfg,
+		RequestLog: params.Stdout,
+		ProcessLog: processLog,
+		GeoJSURL:   lookup.URL,
+		Now:        now,
+	})
+
+	files, err := state.Load(state.Params{
+		Dir:             cfg.StateDir,
+		WriteDelay:      cfg.StateWriteDelay,
+		CounterInterval: cfg.StateCounterInterval,
+		Ledger:          server.Ledger,
+		Limiter:         server.Limiter,
+		GeoJS:           server.GeoJS,
+		Now:             now,
+		ProcessLog:      processLog,
+	})
+	if err != nil {
+		processLog.Error("cannot use the state files", "error", err.Error())
+
+		return 1
+	}
+
 	listener, err := (&net.ListenConfig{}).Listen(ctx, "tcp", cfg.ListenAddr)
 	if err != nil {
 		processLog.Error("cannot listen on SWWAF_LISTEN_ADDR",
@@ -75,27 +104,20 @@ func Run(ctx context.Context, params Params) int {
 		return 1
 	}
 
-	server := proxy.New(proxy.Params{
-		Config:     cfg,
-		RequestLog: params.Stdout,
-		ProcessLog: processLog,
-		GeoJSURL:   lookup.URL,
-		Now:        time.Now,
-	})
-
 	processLog.Info("starting",
 		"version", params.Version,
 		"address", listener.Addr().String(),
 		"settings", cfg)
 
-	return serve(ctx, server, listener, processLog)
+	return serve(ctx, server.Server, listener, files, processLog)
 }
 
-// serve serves requests on listener until ctx is done, then gives the
-// requests in progress shutdownTimeout to finish.
+// serve serves requests on listener, and writes the state files as they
+// are due, until ctx is done. Then it gives the requests in progress
+// shutdownTimeout to finish, and writes every state file.
 func serve(
 	ctx context.Context, server *http.Server, listener net.Listener,
-	processLog *slog.Logger,
+	files *state.Files, processLog *slog.Logger,
 ) int {
 	served := make(chan error, 1)
 
@@ -103,6 +125,16 @@ func serve(
 		served <- server.Serve(listener)
 	}()
 
+	writing, stopWriting := context.WithCancel(ctx)
+	defer stopWriting()
+
+	written := make(chan struct{})
+
+	go func() {
+		files.Run(writing)
+		close(written)
+	}()
+
 	select {
 	case err := <-served:
 		processLog.Error("serving failed", "error", err.Error())
@@ -132,6 +164,21 @@ func serve(
 		return 1
 	}
 
+	// Run's last write has ended, so nothing else writes the files. Every
+	// request has ended too, but for two kinds that Go's server does not
+	// wait for: one cut off because Shutdown timed out, and one whose
+	// connection switched protocols, such as a WebSocket. Such a request
+	// adds to its client's history only as it ends, which can be after
+	// this write, and then that request is missing from clients.json.
+	<-written
+
+	err = files.WriteAll()
+	if err != nil {
+		processLog.Error("writing the state files failed", "error", err.Error())
+
+		return 1
+	}
+
 	processLog.Info("stopped")
 
 	return 0

internal/smallwebwaf/smallwebwaf_test.go

@@ -8,6 +8,8 @@ import (
 	"net"
 	"net/http"
 	"net/http/httptest"
+	"os"
+	"path/filepath"
 	"strings"
 	"sync"
 	"testing"
@@ -24,9 +26,13 @@ const (
 	// testVersion is the version the tests give smallwebwaf.
 	testVersion = "test"
 	// localhost is where the tests listen.
-	localhost   = "127.0.0.1"
-	listenAddr  = "SWWAF_LISTEN_ADDR"
-	upstreamURL = "SWWAF_UPSTREAM_URL"
+	localhost       = "127.0.0.1"
+	listenAddr      = "SWWAF_LISTEN_ADDR"
+	upstreamURL     = "SWWAF_UPSTREAM_URL"
+	stateDir        = "SWWAF_STATE_DIR"
+	rateLimitPerDay = "SWWAF_RATE_LIMIT_PER_DAY"
+	// greeting is what the tests' app answers.
+	greeting = "hello from the app"
 )
 
 // output collects what smallwebwaf writes on stdout.
@@ -69,11 +75,19 @@ func (o *output) line(t *testing.T, key, value string) map[string]any {
 		time.Sleep(pollInterval)
 	}
 
-	t.Fatalf("no line with %s %q in the output:\n%s", key, value, o.buf.String())
+	t.Fatalf("no line with %s %q in the output:\n%s", key, value, o.text())
 
 	return nil
 }
 
+// text returns everything written so far.
+func (o *output) text() string {
+	o.mu.Lock()
+	defer o.mu.Unlock()
+
+	return o.buf.String()
+}
+
 // run runs smallwebwaf with the settings in env until ctx is done, and
 // returns its exit status.
 func run(ctx context.Context, env map[string]string, out *output) int {
@@ -121,7 +135,10 @@ func TestAddressInUseStopsTheStart(t *testing.T) {
 
 	out := &output{}
 
-	status := run(t.Context(), map[string]string{listenAddr: taken.Addr().String()}, out)
+	status := run(t.Context(), map[string]string{
+		listenAddr: taken.Addr().String(),
+		stateDir:   t.TempDir(),
+	}, out)
 	if status != 1 {
 		t.Errorf("exit status %d, want 1", status)
 	}
@@ -132,11 +149,8 @@ func TestAddressInUseStopsTheStart(t *testing.T) {
 func TestServesUntilToldToStop(t *testing.T) {
 	t.Parallel()
 
-	app := httptest.NewServer(http.HandlerFunc(
-		func(w http.ResponseWriter, _ *http.Request) {
-			_, _ = io.WriteString(w, "hello from the app")
-		}))
-	defer app.Close()
+	appURL := startApp(t)
+	dir := t.TempDir()
 
 	ctx, stop := context.WithCancel(t.Context())
 	out := &output{}
@@ -145,12 +159,13 @@ func TestServesUntilToldToStop(t *testing.T) {
 	go func() {
 		exited <- run(ctx, map[string]string{
 			listenAddr:  localhost + ":0",
-			upstreamURL: app.URL,
+			upstreamURL: appURL,
+			stateDir:    dir,
 		}, out)
 	}()
 
 	starting := out.line(t, "msg", "starting")
-	wantStartingLine(t, starting, app.URL)
+	wantStartingLine(t, starting, appURL, dir)
 
 	addr, _ := starting["address"].(string)
 	wantGreeting(t, "http://"+addr+"/")
@@ -170,15 +185,184 @@ func TestServesUntilToldToStop(t *testing.T) {
 	out.line(t, "msg", "stopped")
 }
 
+func TestStateKeptAcrossRestarts(t *testing.T) {
+	t.Parallel()
+
+	env := map[string]string{
+		listenAddr:      localhost + ":0",
+		upstreamURL:     startApp(t),
+		stateDir:        t.TempDir(),
+		rateLimitPerDay: "2",
+		// Neither comes due in the test: the files are written as
+		// smallwebwaf stops.
+		"SWWAF_STATE_WRITE_DELAY":      "1h",
+		"SWWAF_STATE_COUNTER_INTERVAL": "1h",
+	}
+
+	// The two requests a day allows, and a stop.
+	runUntilStopped(t, env, func(url string) {
+		wantGreeting(t, url)
+		wantGreeting(t, url)
+	})
+
+	// After a restart the client has no fresh allowance: its third
+	// request breaks the day limit, and bans it.
+	out := runUntilStopped(t, env, func(url string) {
+		wantRefused(t, url)
+	})
+	out.line(t, "action", "rate_limited")
+
+	// After another, the ban still refuses it.
+	out = runUntilStopped(t, env, func(url string) {
+		wantRefused(t, url)
+	})
+	out.line(t, "action", "banned")
+}
+
+func TestBanRefusesItsNetblockAfterARestartWithAnotherScope(t *testing.T) {
+	t.Parallel()
+
+	const scope = "SWWAF_BAN_SCOPE_V4_PREFIX"
+
+	env := map[string]string{
+		listenAddr:              localhost + ":0",
+		upstreamURL:             startApp(t),
+		stateDir:                t.TempDir(),
+		"SWWAF_TRUSTED_PROXIES": localhost + "/32",
+		rateLimitPerDay:         "1",
+		scope:                   "24",
+	}
+
+	// 203.0.113.9's second request breaks the day limit, and bans
+	// 203.0.113.0/24.
+	runUntilStopped(t, env, func(url string) {
+		wantStatus(t, url, "203.0.113.9", http.StatusOK)
+		wantStatus(t, url, "203.0.113.9", http.StatusForbidden)
+	})
+
+	// With each address a netblock of its own after a restart, that ban
+	// still refuses all of 203.0.113.0/24. 198.51.100.7 is banned alone.
+	env[scope] = "32"
+	runUntilStopped(t, env, func(url string) {
+		wantStatus(t, url, "203.0.113.200", http.StatusForbidden)
+		wantStatus(t, url, "203.0.114.1", http.StatusOK)
+		wantStatus(t, url, "198.51.100.7", http.StatusOK)
+		wantStatus(t, url, "198.51.100.7", http.StatusForbidden)
+	})
+
+	// With /24 netblocks again, that ban still refuses 198.51.100.7, and
+	// no other address.
+	env[scope] = "24"
+	runUntilStopped(t, env, func(url string) {
+		wantStatus(t, url, "198.51.100.7", http.StatusForbidden)
+		wantStatus(t, url, "198.51.100.8", http.StatusOK)
+	})
+}
+
+func TestStateFileThatDoesNotParseStopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+
+	err := os.WriteFile(filepath.Join(dir, "bans.json"), []byte("{\n"), 0o600)
+	if err != nil {
+		t.Fatalf("write bans.json: %v", err)
+	}
+
+	// The file ends at the newline that is the second byte of its first
+	// line.
+	wantStartRefused(t, dir, filepath.Join(dir, "bans.json")+", line 1, column 2: ")
+}
+
+func TestUnwritableStateDirStopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	wantStartRefused(t, filepath.Join(t.TempDir(), "missing"),
+		"SWWAF_STATE_DIR cannot be written: ")
+}
+
+// wantStartRefused runs smallwebwaf with its state files in dir, and
+// checks that it stops at start, with an error that starts with want. If
+// it starts instead, it is stopped after waitLimit.
+func wantStartRefused(t *testing.T, dir, want string) {
+	t.Helper()
+
+	ctx, stop := context.WithTimeout(t.Context(), waitLimit)
+	defer stop()
+
+	out := &output{}
+
+	status := run(ctx, map[string]string{listenAddr: localhost + ":0", stateDir: dir}, out)
+	if status != 1 {
+		t.Fatalf("exit status %d, want 1", status)
+	}
+
+	line := out.line(t, "msg", "cannot use the state files")
+	message, _ := line["error"].(string)
+
+	if !strings.HasPrefix(message, want) {
+		t.Errorf("start refused with %q, want an error starting %q", message, want)
+	}
+}
+
+// startApp starts an app that answers every request with greeting, and
+// returns its URL.
+func startApp(t *testing.T) string {
+	t.Helper()
+
+	app := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			_, _ = io.WriteString(w, greeting)
+		}))
+	t.Cleanup(app.Close)
+
+	return app.URL
+}
+
+// runUntilStopped runs smallwebwaf with the settings in env, has use send
+// it requests at url, then stops it as SIGTERM does, checks that it
+// stopped in order, and returns its output.
+func runUntilStopped(
+	t *testing.T, env map[string]string, use func(url string),
+) *output {
+	t.Helper()
+
+	ctx, stop := context.WithCancel(t.Context())
+	out := &output{}
+	exited := make(chan int, 1)
+
+	go func() {
+		exited <- run(ctx, env, out)
+	}()
+
+	addr, _ := out.line(t, "msg", "starting")["address"].(string)
+	use("http://" + addr + "/")
+	stop()
+
+	select {
+	case status := <-exited:
+		if status != 0 {
+			t.Fatalf("exit status %d, want 0; output:\n%s", status, out.text())
+		}
+	case <-time.After(waitLimit):
+		t.Fatal("still running after being told to stop")
+	}
+
+	return out
+}
+
 // wantStartingLine checks that the line at start gives the version and
 // every setting's value.
-func wantStartingLine(t *testing.T, line map[string]any, appURL string) {
+func wantStartingLine(t *testing.T, line map[string]any, appURL, dir string) {
 	t.Helper()
 
 	settings, _ := line["settings"].(map[string]any)
 	want := map[string]any{
 		listenAddr:                              localhost + ":0",
 		upstreamURL:                             appURL,
+		stateDir:                                dir,
+		"SWWAF_STATE_WRITE_DELAY":               "10s",
+		"SWWAF_STATE_COUNTER_INTERVAL":          "15m",
 		"SWWAF_TRUSTED_PROXIES":                 "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16",
 		"SWWAF_CLIENT_REQUEST_TIMEOUT":          "60s",
 		"SWWAF_CLIENT_REQUEST_HEADER_MAX_BYTES": "32K",
@@ -193,7 +377,7 @@ func wantStartingLine(t *testing.T, line map[string]any, appURL string) {
 		"SWWAF_DENY_NETS":                       "",
 		"SWWAF_RATE_LIMIT_PER_MINUTE":           "1000",
 		"SWWAF_RATE_LIMIT_PER_HOUR":             "10000",
-		"SWWAF_RATE_LIMIT_PER_DAY":              "50000",
+		rateLimitPerDay:                         "50000",
 		"SWWAF_DENIED_COUNTRIES":                "",
 		"SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES":   "",
 		"SWWAF_BAN_RESPONSE":                    "403",
@@ -236,7 +420,61 @@ func wantGreeting(t *testing.T, url string) {
 	body, err := io.ReadAll(res.Body)
 	_ = res.Body.Close()
 
-	if err != nil || string(body) != "hello from the app" {
+	if err != nil || string(body) != greeting {
 		t.Errorf("got %q (%v), want the app's answer", body, err)
 	}
 }
+
+// wantRefused checks that a request to url is refused with 403, the
+// default SWWAF_BAN_RESPONSE.
+func wantRefused(t *testing.T, url string) {
+	t.Helper()
+
+	req, err := http.NewRequestWithContext(t.Context(), http.MethodGet, url,
+		http.NoBody)
+	if err != nil {
+		t.Fatalf("new request: %v", err)
+	}
+
+	transport := &http.Transport{}
+	defer transport.CloseIdleConnections()
+
+	res, err := (&http.Client{Transport: transport}).Do(req)
+	if err != nil {
+		t.Fatalf("request: %v", err)
+	}
+
+	_ = res.Body.Close()
+
+	if res.StatusCode != http.StatusForbidden {
+		t.Errorf("status %d, want %d", res.StatusCode, http.StatusForbidden)
+	}
+}
+
+// wantStatus checks that a request to url from the client at from, as
+// X-Forwarded-For names it, is answered with status.
+func wantStatus(t *testing.T, url, from string, status int) {
+	t.Helper()
+
+	req, err := http.NewRequestWithContext(t.Context(), http.MethodGet, url,
+		http.NoBody)
+	if err != nil {
+		t.Fatalf("new request: %v", err)
+	}
+
+	req.Header.Set("X-Forwarded-For", from)
+
+	transport := &http.Transport{}
+	defer transport.CloseIdleConnections()
+
+	res, err := (&http.Client{Transport: transport}).Do(req)
+	if err != nil {
+		t.Fatalf("request: %v", err)
+	}
+
+	_ = res.Body.Close()
+
+	if res.StatusCode != status {
+		t.Errorf("request from %s: status %d, want %d", from, res.StatusCode, status)
+	}
+}

internal/state/state.go

@@ -0,0 +1,380 @@
+// Package state keeps smallwebwaf's state in JSON files in
+// SWWAF_STATE_DIR, as the "Persistent state" section of SPEC.md describes:
+// bans.json holds the bans, clients.json each client's counters and
+// history, and lookups.json GeoJS's answers. Load reads them at start, and
+// Run and WriteAll write them, each from a snapshot its part takes under
+// its own lock, so that no request waits on the disk.
+package state
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/fs"
+	"log/slog"
+	"net/netip"
+	"os"
+	"path/filepath"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/bans"
+	"sneak.berlin/go/smallwebwaf/internal/lookup"
+	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
+)
+
+// version is the version of the files' format, the only one read.
+const version = 1
+
+// fileMode lets the smallwebwaf user alone read and write the files, which
+// hold visitors' addresses.
+const fileMode = 0o600
+
+// The state files' names.
+const (
+	bansJSON    = "bans.json"
+	clientsJSON = "clients.json"
+	lookupsJSON = "lookups.json"
+)
+
+var errVersion = errors.New("unknown version")
+
+// Params are what Load needs.
+type Params struct {
+	// Dir is the directory of the state files (SWWAF_STATE_DIR).
+	Dir string
+	// WriteDelay is how long after a ban is made bans.json is written
+	// (SWWAF_STATE_WRITE_DELAY), and CounterInterval how often every file
+	// is (SWWAF_STATE_COUNTER_INTERVAL).
+	WriteDelay      time.Duration
+	CounterInterval time.Duration
+	// Ledger, Limiter and GeoJS hold the state.
+	Ledger  *bans.Ledger
+	Limiter *ratelimit.Limiter
+	GeoJS   *lookup.GeoJS
+	// Now tells the time by which the counters' buckets run out, normally
+	// time.Now in UTC.
+	Now func() time.Time
+	// ProcessLog receives what was read, and the writes that fail.
+	ProcessLog *slog.Logger
+}
+
+// Files are the state files of a running smallwebwaf.
+type Files struct {
+	params Params
+}
+
+// bansFile is bans.json, indented for an admin to read and edit.
+type bansFile struct {
+	Version int        `json:"version"`
+	Bans    []banEntry `json:"bans"`
+}
+
+// banEntry is a ban as bans.json holds it: a permanent ban's expires is
+// null.
+type banEntry struct {
+	Netblock netip.Prefix `json:"netblock"`
+	Start    time.Time    `json:"start"`
+	Expires  *time.Time   `json:"expires"`
+	Notes    bans.Notes   `json:"notes"`
+}
+
+// clientsFile is clients.json, with each client on a line of its own.
+type clientsFile struct {
+	Version int                `json:"version"`
+	Clients []ratelimit.Client `json:"clients"`
+}
+
+// lookupsFile is lookups.json, with each answer on a line of its own.
+type lookupsFile struct {
+	Version int             `json:"version"`
+	Lookups []lookup.Answer `json:"lookups"`
+}
+
+// Load checks that files can be written in Dir, and reads the state files
+// in it into the ledger, the limiter and GeoJS. A missing file is empty
+// state, as on a first start. A file that does not parse, or has an
+// unknown version, is an error that names the file and, where the JSON
+// decoder tells it, the line and column.
+func Load(params Params) (*Files, error) {
+	err := checkWritable(params.Dir)
+	if err != nil {
+		return nil, fmt.Errorf("SWWAF_STATE_DIR cannot be written: %w", err)
+	}
+
+	var (
+		bansIn    bansFile
+		clientsIn clientsFile
+		lookupsIn lookupsFile
+	)
+
+	err = errors.Join(
+		read(params.Dir, bansJSON, &bansIn),
+		read(params.Dir, clientsJSON, &clientsIn),
+		read(params.Dir, lookupsJSON, &lookupsIn),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	held := make([]bans.Ban, 0, len(bansIn.Bans))
+	for _, entry := range bansIn.Bans {
+		held = append(held, entry.ban())
+	}
+
+	params.Ledger.Load(held)
+	params.Limiter.Load(clientsIn.Clients, params.Now())
+	params.GeoJS.Load(lookupsIn.Lookups)
+
+	params.ProcessLog.Info("read the state files", "directory", params.Dir,
+		"bans", len(bansIn.Bans), "clients", len(clientsIn.Clients),
+		"lookups", len(lookupsIn.Lookups))
+
+	return &Files{params: params}, nil
+}
+
+// Run writes bans.json WriteDelay after a ban is made, with every ban
+// made in between, and every file every CounterInterval, until ctx is
+// done. A write that fails is logged, and the file is written again at
+// its next write.
+func (f *Files) Run(ctx context.Context) {
+	interval := time.NewTicker(f.params.CounterInterval)
+	defer interval.Stop()
+
+	var bansDue <-chan time.Time // nil while no ban waits to be written
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-f.params.Ledger.Changed():
+			if bansDue == nil {
+				bansDue = time.After(f.params.WriteDelay)
+			}
+		case <-bansDue:
+			bansDue = nil
+
+			f.logFailure(f.writeBans())
+		case <-interval.C:
+			f.logFailure(f.WriteAll())
+		}
+	}
+}
+
+// WriteAll writes every state file, as smallwebwaf stops. A file that
+// fails does not keep the others from being written.
+func (f *Files) WriteAll() error {
+	return errors.Join(f.writeBans(), f.writeClients(), f.writeLookups())
+}
+
+// logFailure logs a write that failed.
+func (f *Files) logFailure(err error) {
+	if err != nil {
+		f.params.ProcessLog.Error("writing the state files failed",
+			"error", err.Error())
+	}
+}
+
+// writeBans writes bans.json.
+func (f *Files) writeBans() error {
+	held := f.params.Ledger.Snapshot()
+
+	file := bansFile{Version: version, Bans: make([]banEntry, 0, len(held))}
+	for _, ban := range held {
+		file.Bans = append(file.Bans, newBanEntry(ban))
+	}
+
+	data, err := json.MarshalIndent(file, "", "    ")
+	if err != nil {
+		return fmt.Errorf("encode %s: %w", bansJSON, err)
+	}
+
+	return write(f.params.Dir, bansJSON, append(data, '\n'))
+}
+
+// writeClients writes clients.json.
+func (f *Files) writeClients() error {
+	data, err := encodeOnePerLine("clients", f.params.Limiter.Snapshot())
+	if err != nil {
+		return fmt.Errorf("encode %s: %w", clientsJSON, err)
+	}
+
+	return write(f.params.Dir, clientsJSON, data)
+}
+
+// writeLookups writes lookups.json.
+func (f *Files) writeLookups() error {
+	data, err := encodeOnePerLine("lookups", f.params.GeoJS.Snapshot())
+	if err != nil {
+		return fmt.Errorf("encode %s: %w", lookupsJSON, err)
+	}
+
+	return write(f.params.Dir, lookupsJSON, data)
+}
+
+// newBanEntry returns ban as bans.json holds it.
+func newBanEntry(ban bans.Ban) banEntry {
+	entry := banEntry{Netblock: ban.Netblock, Start: ban.Start, Notes: ban.Notes}
+	if !ban.Permanent() {
+		entry.Expires = &ban.Expires
+	}
+
+	return entry
+}
+
+// ban returns the ban an entry of bans.json holds.
+func (e banEntry) ban() bans.Ban {
+	ban := bans.Ban{Netblock: e.Netblock, Start: e.Start, Notes: e.Notes}
+	if e.Expires != nil {
+		ban.Expires = *e.Expires
+	}
+
+	return ban
+}
+
+// encodeOnePerLine encodes a state file whose entries, under key, are one
+// to a line, so that grep shows everything about one client.
+func encodeOnePerLine[E any](key string, entries []E) ([]byte, error) {
+	var b bytes.Buffer
+
+	fmt.Fprintf(&b, "{\n    \"version\": %d,\n    %q: [", version, key)
+
+	for i, entry := range entries {
+		line, err := json.Marshal(entry)
+		if err != nil {
+			return nil, err
+		}
+
+		if i > 0 {
+			b.WriteString(",")
+		}
+
+		b.WriteString("\n        ")
+		b.Write(line)
+	}
+
+	b.WriteString("\n    ]\n}\n")
+
+	return b.Bytes(), nil
+}
+
+// checkWritable makes a file in dir and removes it again.
+func checkWritable(dir string) error {
+	file, err := os.CreateTemp(dir, "write-check-*")
+	if err != nil {
+		return err
+	}
+
+	return errors.Join(file.Close(), os.Remove(file.Name()))
+}
+
+// read reads the state file name in dir into file, a pointer to that
+// file's struct. A missing file leaves file as it is.
+func read(dir, name string, file any) error {
+	path := filepath.Join(dir, name)
+
+	data, err := os.ReadFile(path) //nolint:gosec // a state file, in SWWAF_STATE_DIR
+	if errors.Is(err, fs.ErrNotExist) {
+		return nil
+	}
+
+	if err != nil {
+		return err
+	}
+
+	// The version is read first, so that a file of another version is
+	// refused for that, and not for an entry this version cannot read.
+	var header struct {
+		Version int `json:"version"`
+	}
+
+	err = json.Unmarshal(data, &header)
+	if err == nil && header.Version != version {
+		err = fmt.Errorf("%w %d, where this smallwebwaf reads version %d",
+			errVersion, header.Version, version)
+	}
+
+	if err == nil {
+		decoder := json.NewDecoder(bytes.NewReader(data))
+		// A field this version does not know is most likely misspelt, and
+		// its value would be lost without a word.
+		decoder.DisallowUnknownFields()
+		err = decoder.Decode(file)
+	}
+
+	if err != nil {
+		return fmt.Errorf("%s%s: %w", path, position(data, err), err)
+	}
+
+	return nil
+}
+
+// position returns where in data err was found, as ", line L, column C"
+// of the last byte the JSON decoder read, or "" when err does not tell.
+func position(data []byte, err error) string {
+	var (
+		syntaxErr *json.SyntaxError
+		typeErr   *json.UnmarshalTypeError
+		read      int64
+	)
+
+	switch {
+	case errors.As(err, &syntaxErr):
+		read = syntaxErr.Offset
+	case errors.As(err, &typeErr):
+		read = typeErr.Offset
+	default:
+		return ""
+	}
+
+	before := data[:max(min(read, int64(len(data)))-1, 0)]
+	line := bytes.Count(before, []byte("\n")) + 1
+	column := len(before) - bytes.LastIndexByte(before, '\n')
+
+	return fmt.Sprintf(", line %d, column %d", line, column)
+}
+
+// write writes data to the file name in dir so that a crash at any
+// moment leaves either the old file or the new one, whole: data goes to a
+// temporary file in the same directory, which is synced and renamed over
+// name, and then the directory is synced, so that the rename lasts.
+func write(dir, name string, data []byte) error {
+	path := filepath.Join(dir, name)
+	temporary := path + ".tmp"
+
+	err := writeSynced(temporary, data)
+	if err != nil {
+		_ = os.Remove(temporary)
+
+		return err
+	}
+
+	err = os.Rename(temporary, path)
+	if err != nil {
+		return err
+	}
+
+	directory, err := os.Open(dir) //nolint:gosec // SWWAF_STATE_DIR itself
+	if err != nil {
+		return err
+	}
+
+	return errors.Join(directory.Sync(), directory.Close())
+}
+
+// writeSynced writes data to the file at path, and syncs it to the disk.
+func writeSynced(path string, data []byte) error {
+	//nolint:gosec // a state file's temporary file, in SWWAF_STATE_DIR
+	file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, fileMode)
+	if err != nil {
+		return err
+	}
+
+	_, err = file.Write(data)
+	if err == nil {
+		err = file.Sync()
+	}
+
+	return errors.Join(err, file.Close())
+}

internal/state/state_test.go

@@ -0,0 +1,537 @@
+package state_test
+
+import (
+	"context"
+	"encoding/json"
+	"log/slog"
+	"net/netip"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+	"testing/synctest"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/bans"
+	"sneak.berlin/go/smallwebwaf/internal/lookup"
+	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
+	"sneak.berlin/go/smallwebwaf/internal/state"
+)
+
+const (
+	// The state files.
+	bansJSON    = "bans.json"
+	clientsJSON = "clients.json"
+	lookupsJSON = "lookups.json"
+)
+
+// permanentBansJSON is bans.json holding permanentBan.
+const permanentBansJSON = `{
+    "version": 1,
+    "bans": [
+        {
+            "netblock": "2001:db8::/64",
+            "start": "2026-10-06T00:00:00Z",
+            "expires": null,
+            "notes": {
+                "country": "DE",
+                "limit": 1000,
+                "window": "minute",
+                "count": 1000.5,
+                "request": {
+                    "time": "2026-10-06T00:00:00Z",
+                    "method": "GET",
+                    "host": "app.example",
+                    "path": "/repo?page=2",
+                    "status": 403,
+                    "user_agent": "scraper/1.0"
+                },
+                "requests": 1500,
+                "refused": 3,
+                "earlier_bans": 5
+            }
+        }
+    ]
+}
+`
+
+func TestFilesWrittenAndReadBack(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	before := newParams(dir)
+	fill(before)
+
+	files, err := state.Load(before)
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+
+	err = files.WriteAll()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	// Read into new parts, as at the next start, the files give back what
+	// was written.
+	after := newParams(dir)
+	load(t, after)
+
+	wantEqual(t, bansJSON, after.Ledger.Snapshot(), before.Ledger.Snapshot())
+	wantEqual(t, clientsJSON, after.Limiter.Snapshot(), before.Limiter.Snapshot())
+	wantEqual(t, lookupsJSON, after.GeoJS.Snapshot(), before.GeoJS.Snapshot())
+
+	// Each one-per-line file lists its entries by client, and nothing
+	// but the three files is left in the directory.
+	wantEntries(t, filepath.Join(dir, clientsJSON), "clients",
+		"192.0.2.1/32", "203.0.113.9/32", "2001:db8::/64")
+	wantEntries(t, filepath.Join(dir, lookupsJSON), "lookups",
+		"192.0.2.1/32", "203.0.113.9/32")
+	wantFiles(t, dir, bansJSON, clientsJSON, lookupsJSON)
+}
+
+func TestBansJSONIsIndentedWithNullForAPermanentBan(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	params := newParams(dir)
+	params.Ledger.Load([]bans.Ban{permanentBan()})
+
+	files := load(t, params)
+
+	err := files.WriteAll()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	got := readFile(t, filepath.Join(dir, bansJSON))
+	if got != permanentBansJSON {
+		t.Errorf("bans.json\n%s\nwant\n%s", got, permanentBansJSON)
+	}
+}
+
+func TestMissingFilesAreEmptyState(t *testing.T) {
+	t.Parallel()
+
+	params := newParams(t.TempDir())
+	load(t, params)
+
+	if len(params.Ledger.Snapshot()) != 0 || len(params.Limiter.Snapshot()) != 0 ||
+		len(params.GeoJS.Snapshot()) != 0 {
+		t.Error("state from no files")
+	}
+}
+
+func TestFileThatDoesNotParseStopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	for _, tc := range []struct {
+		name, file, content string
+		// want is what the error says after the file's path.
+		want string
+	}{
+		{
+			"a syntax error", bansJSON,
+			"{\n    \"version\": 1,\n    \"bans\": [\n" +
+				"        {\"netblock\": \"203.0.113.9/32\",}\n    ]\n}\n",
+			", line 4, column 39: invalid character '}'",
+		},
+		{
+			"a value of the wrong kind", clientsJSON,
+			"{\n    \"version\": 1,\n    \"clients\": [\n" +
+				"        {\"client\":\"203.0.113.9/32\",\"history\":{\"requests\":\"many\"}}\n" +
+				"    ]\n}\n",
+			", line 4, column ",
+		},
+		{
+			// Found at the newline that ends the file.
+			"a cut-off file", lookupsJSON,
+			"{\n    \"version\": 1,\n    \"lookups\": [\n",
+			", line 3, column 17: unexpected end of JSON input",
+		},
+		{
+			"an unknown field", lookupsJSON,
+			`{"version": 1, "lookups": [{"client": "203.0.113.9/32", "contry": "DE"}]}`,
+			`: json: unknown field "contry"`,
+		},
+		{
+			"a netblock that does not read", bansJSON,
+			`{"version": 1, "bans": [{"netblock": "203.0.113.300/32"}]}`,
+			`: netip.ParsePrefix("203.0.113.300/32")`,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			dir := t.TempDir()
+			path := filepath.Join(dir, tc.file)
+
+			err := os.WriteFile(path, []byte(tc.content), 0o600)
+			if err != nil {
+				t.Fatalf("write %s: %v", tc.file, err)
+			}
+
+			_, err = state.Load(newParams(dir))
+			if err == nil || !strings.HasPrefix(err.Error(), path+tc.want) {
+				t.Errorf("error %v, want one starting %s%s", err, path, tc.want)
+			}
+		})
+	}
+}
+
+func TestUnknownVersionStopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	for _, file := range []string{bansJSON, clientsJSON, lookupsJSON} {
+		for _, content := range []string{`{"version": 2}`, `{}`} {
+			t.Run(file+" "+content, func(t *testing.T) {
+				t.Parallel()
+
+				dir := t.TempDir()
+				path := filepath.Join(dir, file)
+
+				err := os.WriteFile(path, []byte(content), 0o600)
+				if err != nil {
+					t.Fatalf("write %s: %v", file, err)
+				}
+
+				_, err = state.Load(newParams(dir))
+				if err == nil || !strings.HasPrefix(err.Error(), path+": unknown version ") {
+					t.Errorf("error %v, want one naming %s and its version", err, path)
+				}
+			})
+		}
+	}
+}
+
+func TestUnwritableDirectoryStopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	notADirectory := filepath.Join(t.TempDir(), "file")
+
+	err := os.WriteFile(notADirectory, nil, 0o600)
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	for _, dir := range []string{
+		filepath.Join(t.TempDir(), "missing"),
+		notADirectory,
+	} {
+		const want = "SWWAF_STATE_DIR cannot be written: "
+
+		_, err := state.Load(newParams(dir))
+		if err == nil || !strings.HasPrefix(err.Error(), want) {
+			t.Errorf("state directory %s: error %v, want one starting %s", dir, err, want)
+		}
+	}
+}
+
+// The two tests below run Run in a synctest bubble, where time is a clock
+// of the test's own: time.Sleep moves it on at once, and synctest.Wait
+// returns once Run waits for its next write, so that every write due by
+// then is on disk.
+
+func TestBansWrittenOnceWriteDelayAfterABan(t *testing.T) {
+	t.Parallel()
+
+	synctest.Test(t, func(t *testing.T) {
+		dir := t.TempDir()
+		params := newParams(dir)
+		params.WriteDelay = 10 * time.Second
+		run(t, load(t, params))
+
+		// A second ban, made while the first waits to be written, puts the
+		// write off no further, and is written with it.
+		first := params.Ledger.BanForLimit(netip.MustParsePrefix("203.0.113.9/32"),
+			midnight(), bans.Notes{})
+
+		time.Sleep(5 * time.Second)
+
+		second := params.Ledger.BanForLimit(netip.MustParsePrefix("203.0.113.10/32"),
+			midnight(), bans.Notes{})
+
+		time.Sleep(5*time.Second - time.Nanosecond)
+		synctest.Wait()
+		wantFiles(t, dir)
+
+		time.Sleep(time.Nanosecond)
+		synctest.Wait()
+		wantFiles(t, dir, bansJSON)
+
+		read := newParams(dir)
+		load(t, read)
+
+		want := []bans.Ban{first, second}
+		if got := read.Ledger.Snapshot(); !slices.Equal(got, want) {
+			t.Errorf("bans.json holds %+v, want %+v", got, want)
+		}
+
+		// That write was the only one: bans.json is not written again for
+		// the second ban. The other files wait for the interval, an hour
+		// away.
+		removeFiles(t, dir, bansJSON)
+		time.Sleep(params.WriteDelay)
+		synctest.Wait()
+		wantFiles(t, dir)
+	})
+}
+
+func TestEveryFileWrittenEveryCounterInterval(t *testing.T) {
+	t.Parallel()
+
+	synctest.Test(t, func(t *testing.T) {
+		dir := t.TempDir()
+		params := newParams(dir)
+		params.CounterInterval = time.Minute
+		run(t, load(t, params))
+
+		// The files are removed once written, so that each interval shows
+		// them written again.
+		for range 3 {
+			time.Sleep(time.Minute - time.Nanosecond)
+			synctest.Wait()
+			wantFiles(t, dir)
+
+			time.Sleep(time.Nanosecond)
+			synctest.Wait()
+			wantFiles(t, dir, bansJSON, clientsJSON, lookupsJSON)
+			removeFiles(t, dir, bansJSON, clientsJSON, lookupsJSON)
+		}
+	})
+}
+
+func TestFailedWriteLeavesTheFileAsItWas(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	params := newParams(dir)
+	params.Ledger.Load([]bans.Ban{permanentBan()})
+	files := load(t, params)
+
+	err := files.WriteAll()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	// A directory in the way of bans.json's temporary file fails its
+	// next write, but not the others'.
+	err = os.Mkdir(filepath.Join(dir, bansJSON+".tmp"), 0o700)
+	if err != nil {
+		t.Fatalf("mkdir: %v", err)
+	}
+
+	params.Ledger.BanForLimit(netip.MustParsePrefix("203.0.113.9/32"), midnight(),
+		bans.Notes{})
+	params.Limiter.Count(netip.MustParsePrefix("203.0.113.9/32"), midnight())
+
+	err = files.WriteAll()
+	if err == nil || !strings.Contains(err.Error(), bansJSON+".tmp") {
+		t.Errorf("error %v, want one naming bans.json's temporary file", err)
+	}
+
+	got := readFile(t, filepath.Join(dir, bansJSON))
+	if got != permanentBansJSON {
+		t.Errorf("bans.json is now\n%s\nwant it as it was", got)
+	}
+
+	read := newParams(dir)
+	load(t, read)
+
+	if len(read.Limiter.Snapshot()) != 1 {
+		t.Error("clients.json was not written")
+	}
+}
+
+// midnight is the time of the tests' clock.
+func midnight() time.Time {
+	return time.Date(2026, 10, 6, 0, 0, 0, 0, time.UTC)
+}
+
+// newParams returns Params for the state files in dir, with parts that
+// hold nothing yet. GeoJS is never asked.
+func newParams(dir string) state.Params {
+	discard := slog.New(slog.DiscardHandler)
+
+	return state.Params{
+		Dir:             dir,
+		WriteDelay:      time.Hour,
+		CounterInterval: time.Hour,
+		Ledger: bans.New(bans.Rules{
+			LimitBanDuration:     time.Hour,
+			LimitBanRepeatWindow: 24 * time.Hour,
+			MaxBanDuration:       7 * 24 * time.Hour,
+			MaxBans:              5000,
+		}),
+		Limiter:    ratelimit.New(ratelimit.Limits{}),
+		GeoJS:      lookup.New(lookup.Params{Now: midnight, ProcessLog: discard}),
+		Now:        midnight,
+		ProcessLog: discard,
+	}
+}
+
+// fill puts a ban that ends and one that does not, clients with counts
+// and histories, and GeoJS answers into the parts of params.
+func fill(params state.Params) {
+	now := midnight()
+	client := netip.MustParsePrefix("203.0.113.9/32")
+
+	params.Ledger.Load([]bans.Ban{permanentBan()})
+	params.Ledger.BanForLimit(client, now, bans.Notes{Country: "DE", Limit: 1})
+
+	for _, c := range []string{"2001:db8::/64", "203.0.113.9/32", "192.0.2.1/32"} {
+		params.Limiter.Count(netip.MustParsePrefix(c), now)
+	}
+
+	params.Limiter.AddToHistory(client, now, ratelimit.Request{
+		Country: "DE", Forwarded: true, Status: 200, RequestBytes: 3, ResponseBytes: 5,
+	})
+
+	params.GeoJS.Load([]lookup.Answer{
+		{Client: client, Country: "DE", Answered: now.Add(-time.Hour), Used: now},
+		{
+			Client:   netip.MustParsePrefix("192.0.2.1/32"),
+			Answered: now.Add(-time.Hour), Used: now.Add(-time.Minute),
+		},
+	})
+}
+
+// permanentBan is the ban permanentBansJSON holds.
+func permanentBan() bans.Ban {
+	return bans.Ban{
+		Netblock: netip.MustParsePrefix("2001:db8::/64"),
+		Start:    midnight(),
+		Notes: bans.Notes{
+			Country: "DE",
+			Limit:   1000,
+			Window:  "minute",
+			Count:   1000.5,
+			Request: bans.Request{
+				Time:      midnight(),
+				Method:    "GET",
+				Host:      "app.example",
+				Path:      "/repo?page=2",
+				Status:    403,
+				UserAgent: "scraper/1.0",
+			},
+			Requests:    1500,
+			Refused:     3,
+			EarlierBans: 5,
+		},
+	}
+}
+
+// load reads the state files into the parts of params.
+func load(t *testing.T, params state.Params) *state.Files {
+	t.Helper()
+
+	files, err := state.Load(params)
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+
+	return files
+}
+
+// run runs files' writes until the test ends.
+func run(t *testing.T, files *state.Files) {
+	t.Helper()
+
+	ctx, stop := context.WithCancel(t.Context())
+	stopped := make(chan struct{})
+
+	go func() {
+		files.Run(ctx)
+		close(stopped)
+	}()
+
+	t.Cleanup(func() {
+		stop()
+		<-stopped
+	})
+}
+
+// wantEqual checks that the entries read back from file are those
+// written.
+func wantEqual[E comparable](t *testing.T, file string, got, want []E) {
+	t.Helper()
+
+	if !slices.Equal(got, want) {
+		t.Errorf("%s read back\n%+v\nwant\n%+v", file, got, want)
+	}
+}
+
+// readFile returns what the file at path holds.
+func readFile(t *testing.T, path string) string {
+	t.Helper()
+
+	data, err := os.ReadFile(path) //nolint:gosec // a file the test wrote
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+
+	return string(data)
+}
+
+// removeFiles removes the named files from dir.
+func removeFiles(t *testing.T, dir string, names ...string) {
+	t.Helper()
+
+	for _, name := range names {
+		err := os.Remove(filepath.Join(dir, name))
+		if err != nil {
+			t.Fatalf("remove: %v", err)
+		}
+	}
+}
+
+// wantFiles checks the names of the files in dir.
+func wantFiles(t *testing.T, dir string, want ...string) {
+	t.Helper()
+
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("read %s: %v", dir, err)
+	}
+
+	got := make([]string, 0, len(entries))
+	for _, entry := range entries {
+		got = append(got, entry.Name())
+	}
+
+	if !slices.Equal(got, want) {
+		t.Errorf("%s holds %v, want %v", dir, got, want)
+	}
+}
+
+// wantEntries checks that the file at path has its version, then its
+// entries under key, each on a line of its own, for the clients want
+// names in that order.
+func wantEntries(t *testing.T, path, key string, want ...string) {
+	t.Helper()
+
+	data := readFile(t, path)
+	lines := strings.Split(strings.TrimSuffix(data, "\n"), "\n")
+	head := []string{"{", `    "version": 1,`, `    "` + key + `": [`}
+	tail := []string{"    ]", "}"}
+
+	if len(lines) != len(head)+len(want)+len(tail) ||
+		!slices.Equal(lines[:len(head)], head) ||
+		!slices.Equal(lines[len(lines)-len(tail):], tail) {
+		t.Fatalf("%s is\n%s", path, data)
+	}
+
+	for i, client := range want {
+		line := strings.TrimSuffix(lines[len(head)+i], ",")
+
+		var entry struct {
+			Client string `json:"client"`
+		}
+
+		err := json.Unmarshal([]byte(line), &entry)
+		if err != nil || entry.Client != client {
+			t.Errorf("entry %d of %s is %s (%v), want %s's", i, path, line, err, client)
+		}
+	}
+}

script/example-app

@@ -1,12 +1,14 @@
 #!/bin/sh
 # script/example-app: build the image, and on it the example app in
-# deploy/example-app, then run the app's container and check that the
-# health check passes, that a request is served through smallwebwaf,
-# that `sv stop` stops smallwebwaf in order, and that `docker stop`
-# stops the container without having to kill it. The container and both
-# images are removed however the script ends. Building the app needs
-# network access, for nixpkgs' binary cache. script/check does not run
-# this.
+# deploy/example-app, then run the app's container with a volume for the
+# state files and check that the health check passes, that a request is
+# served through smallwebwaf, that a second one in a minute bans the
+# client, that `sv stop` stops smallwebwaf in order, that `docker stop`
+# stops the container without having to kill it, and that a new
+# container on the same volume still refuses the banned client. The
+# containers, the volume and both images are removed however the script
+# ends. Building the app needs network access, for nixpkgs' binary cache.
+# script/check does not run this.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -18,9 +20,11 @@ NAME="$("$SCRIPT_DIR/projectname")-example-$$"
 IMAGE="$NAME-base"
 APP_IMAGE="$NAME-app"
 CONTAINER="$NAME"
+VOLUME="$NAME-state"
 
 cleanup() {
     docker rm --force "$CONTAINER" >/dev/null 2>&1 || true
+    docker volume rm --force "$VOLUME" >/dev/null 2>&1 || true
     docker rmi --force "$APP_IMAGE" "$IMAGE" >/dev/null 2>&1 || true
 }
 
@@ -53,6 +57,26 @@ logged() {
     docker logs "$CONTAINER" 2>&1 | grep -qF "$1"
 }
 
+# start_container: run the app's container, with the state files on the
+# volume and a rate limit of one request a minute, and wait until it is
+# healthy.
+start_container() {
+    docker run --detach --name "$CONTAINER" --publish 127.0.0.1::8080 \
+        --volume "$VOLUME:/var/lib/smallwebwaf" \
+        --env SWWAF_RATE_LIMIT_PER_MINUTE=1 \
+        "$APP_IMAGE" >/dev/null
+    wait_for "the health check did not pass" healthy
+    address="$(docker port "$CONTAINER" 8080/tcp)"
+}
+
+# refused: a request to the container gets 403, SWWAF_BAN_RESPONSE's
+# default.
+refused() {
+    code="$(curl --silent --output /dev/null --write-out '%{http_code}' \
+        --max-time 10 "http://$address/")" || true
+    [ "$code" = 403 ]
+}
+
 main() {
     cd "$ROOT"
     trap cleanup EXIT
@@ -62,18 +86,20 @@ main() {
     docker build --no-cache --build-arg SMALLWEBWAF_IMAGE="$IMAGE" \
         -t "$APP_IMAGE" deploy/example-app
 
-    docker run --detach --name "$CONTAINER" --publish 127.0.0.1::8080 \
-        "$APP_IMAGE" >/dev/null
-    wait_for "the health check did not pass" healthy
+    docker volume create "$VOLUME" >/dev/null
+    start_container
     echo "example-app: the health check passes"
 
-    address="$(docker port "$CONTAINER" 8080/tcp)"
     page="$(curl --fail --silent --show-error --max-time 10 "http://$address/")" ||
         fail "no answer on port 8080"
     [ "$page" = "hello from the example app" ] || fail "port 8080 answered $page"
     wait_for "smallwebwaf logged no request it forwarded" logged '"action":"forward"'
     echo "example-app: smallwebwaf passes a request to the app and its answer back"
 
+    refused || fail "a second request in a minute was not refused"
+    wait_for "smallwebwaf logged no ban" logged '"action":"rate_limited"'
+    echo "example-app: a second request in a minute bans the client"
+
     docker exec "$CONTAINER" sv stop smallwebwaf >/dev/null ||
         fail "sv stop smallwebwaf failed"
     wait_for "smallwebwaf did not stop in order" logged '"msg":"stopped"'
@@ -83,6 +109,13 @@ main() {
     status="$(docker inspect --format '{{.State.ExitCode}}' "$CONTAINER")"
     [ "$status" = 0 ] || fail "docker stop left exit status $status"
     echo "example-app: docker stop stops the container in order"
+
+    docker rm "$CONTAINER" >/dev/null
+    start_container
+    refused || fail "the new container let the banned client through"
+    wait_for "smallwebwaf logged no request refused under the ban" \
+        logged '"action":"banned"'
+    echo "example-app: a new container on the same volume keeps the ban"
 }
 
 main "$@"

script/run

@@ -1,6 +1,7 @@
 #!/bin/sh
 # script/run: build bin/smallwebwaf with script/build and run it, with
-# the settings in the environment.
+# the settings in the environment. Unless SWWAF_STATE_DIR is set, the
+# state files go in bin/state, beside the binary.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -8,6 +9,11 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     "$SCRIPT_DIR/build"
+    if [ -z "${SWWAF_STATE_DIR+set}" ]; then
+        SWWAF_STATE_DIR="$ROOT/bin/state"
+        export SWWAF_STATE_DIR
+        mkdir -p "$SWWAF_STATE_DIR"
+    fi
     exec "$ROOT/bin/smallwebwaf"
 }
 

share/smallwebwaf.run

@@ -2,10 +2,14 @@
 set -euo pipefail
 
 # runit's run script for smallwebwaf, run again whenever smallwebwaf
-# exits; the wait spaces out the restarts. exec, so that the signal
-# `sv stop` sends reaches smallwebwaf itself.
+# exits; the wait spaces out the restarts. The state directory and every
+# file in it are given to the smallwebwaf user, so that a volume mounted
+# there needs no change of owner; chown -R changes a symbolic link itself,
+# never what it points to. exec, so that the signal `sv stop` sends
+# reaches smallwebwaf itself.
 main() {
     sleep 1
+    chown -R smallwebwaf:smallwebwaf "${SWWAF_STATE_DIR:-/var/lib/smallwebwaf}"
     exec chpst -u smallwebwaf:smallwebwaf /usr/local/bin/smallwebwaf
 }