DNS blocklists asked in the background, verdicts kept (closes #104)

Work in progress.

Model: opus-5-5

internal/alerts/alerts.go

@@ -46,10 +46,11 @@ const (
 	EventAnomaly = "anomaly"
 	// EventWAFBlock comes with the Core Rule Set; nothing raises it yet.
 	EventWAFBlock = "waf_block"
-	// EventReputationHit is a request whose client a blocklist lists.
+	// EventReputationHit is a request whose client a blocklist or a DNSBL
+	// zone lists.
 	EventReputationHit = "reputation_hit"
-	// EventSourceFailure is GeoJS failing or refusing smallwebwaf, or a
-	// fetch of a list failing.
+	// EventSourceFailure is GeoJS failing or refusing smallwebwaf, a fetch
+	// of a list failing, or a query to a DNSBL zone failing or refused.
 	EventSourceFailure = "source_failure"
 	// EventFileError is a rule file or state file edited while smallwebwaf
 	// runs that does not parse, a replacement of the lookup database that

internal/config/config.go

@@ -152,6 +152,21 @@ type Config struct {
 	BlocklistRefresh      time.Duration
 	BlocklistAction       string
 	BlocklistLimitPercent int64
+	// DNSBLZones are the DNSBL zones clients are asked about
+	// (SWWAF_DNSBL_ZONES), through DNSBLResolver (SWWAF_DNSBL_RESOLVER), or
+	// the host's resolver while that is the zero AddrPort.
+	// ReputationAction is what is done with a client a zone's verdict lists
+	// (SWWAF_REPUTATION_ACTION): deny, limit or log; for limit,
+	// ReputationLimitPercent is the percentage of every limit it gets. A
+	// verdict is used for ReputationCacheTTL after it was fetched
+	// (SWWAF_REPUTATION_CACHE_TTL), and a query may take ReputationTimeout
+	// (SWWAF_REPUTATION_TIMEOUT). Neither can be off.
+	DNSBLZones             []string
+	DNSBLResolver          netip.AddrPort
+	ReputationAction       string
+	ReputationLimitPercent int64
+	ReputationCacheTTL     time.Duration
+	ReputationTimeout      time.Duration
 	// BanResponse is the status a refused client is answered with, 403
 	// or 429, or 0 to close the connection without an answer
 	// (SWWAF_BAN_RESPONSE). It answers a banned client, a request that
@@ -366,6 +381,11 @@ var (
 	errNotAnHourOrMore = errors.New("is not a duration of 1h or more, such as 24h")
 	errNotAction       = errors.New(
 		"is not deny, limit:<percent> such as limit:25, or log")
+	errNotZone     = errors.New("is not a DNS zone such as dnsbl.dronebl.org")
+	errZoneTooLong = errors.New("is longer than 189 characters, too long for the " +
+		"names IPv6 clients are asked about by")
+	errNotResolver = errors.New("is not an IP address with an optional port, " +
+		"such as 192.0.2.53 or [2001:db8::53]:5353")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -418,6 +438,10 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		ASNLimitPercentURL:   env.listURL("SWWAF_ASN_LIMIT_PERCENT_URL"),
 		BlocklistURLs:        env.listURLs("SWWAF_BLOCKLIST_URLS"),
 		BlocklistRefresh:     env.refresh("SWWAF_BLOCKLIST_REFRESH", "24h"),
+		DNSBLZones:           env.zones("SWWAF_DNSBL_ZONES"),
+		DNSBLResolver:        env.resolver("SWWAF_DNSBL_RESOLVER"),
+		ReputationCacheTTL:   env.durationNotOff("SWWAF_REPUTATION_CACHE_TTL", "24h"),
+		ReputationTimeout:    env.durationNotOff("SWWAF_REPUTATION_TIMEOUT", "2s"),
 		BanResponse:          env.banResponse("SWWAF_BAN_RESPONSE", "403"),
 		LimitBanDuration:     env.durationNotOff("SWWAF_LIMIT_BAN_DURATION", "1h"),
 		LimitBanRepeatWindow: env.durationNotOff("SWWAF_LIMIT_BAN_REPEAT_WINDOW", "24h"),
@@ -462,6 +486,8 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		cfg.InstanceName, cfg.LogRemoteURL != nil)
 	cfg.BlocklistAction, cfg.BlocklistLimitPercent = env.action(
 		"SWWAF_BLOCKLIST_ACTION", "deny")
+	cfg.ReputationAction, cfg.ReputationLimitPercent = env.action(
+		"SWWAF_REPUTATION_ACTION", "limit:25")
 
 	env.checkInstanceNameForNtfy(cfg.InstanceName, cfg.AlertNtfyURL != nil)
 	env.checkLookupDBPath(cfg)
@@ -733,9 +759,9 @@ func (e *environment) refresh(name, defaultValue string) time.Duration {
 	return duration
 }
 
-// action reads a setting that is what is done with a client a list names:
-// deny, log, or limit:<percent>, which it returns as limit and the
-// percentage.
+// action reads a setting that is what is done with a client a blocklist
+// or a DNSBL zone lists: deny, log, or limit:<percent>, which it returns
+// as limit and the percentage.
 func (e *environment) action(name, defaultValue string) (string, int64) {
 	value := e.value(name, defaultValue)
 	if value == "deny" || value == "log" {
@@ -752,6 +778,24 @@ func (e *environment) action(name, defaultValue string) (string, int64) {
 	return "limit", percent
 }
 
+// zones reads the setting that is the list of DNSBL zones. It is empty by
+// default.
+func (e *environment) zones(name string) []string {
+	zones, err := parseZones(e.value(name, ""))
+	e.check(name, err)
+
+	return zones
+}
+
+// resolver reads the setting that is the resolver the DNSBL zones are
+// asked through, the zero AddrPort while it is unset or empty.
+func (e *environment) resolver(name string) netip.AddrPort {
+	resolver, err := parseResolver(e.value(name, ""))
+	e.check(name, err)
+
+	return resolver
+}
+
 // lookupSource reads the setting that is where clients are looked up:
 // geojs, file, or off.
 func (e *environment) lookupSource(name, defaultValue string) string {
@@ -1697,6 +1741,85 @@ func parseListURLs(value string) ([]string, error) {
 	return urls, nil
 }
 
+const (
+	// maxZoneLength is the most characters a DNSBL zone may have: 253, the
+	// most a DNS name may have, less the 64 that come before the zone in
+	// the name an IPv6 client is asked about by, its 32 hex digits each
+	// followed by a dot.
+	maxZoneLength = 189
+	// maxLabelLength is the most characters a label of a DNS name may have.
+	maxLabelLength = 63
+	// labelChars are the characters a label of a DNS zone may hold.
+	labelChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-"
+	// dnsPort is the port a resolver is asked on when SWWAF_DNSBL_RESOLVER
+	// gives none.
+	dnsPort = 53
+)
+
+// parseZones reads a comma-separated list of DNSBL zones, each a DNS name
+// such as dnsbl.dronebl.org: labels separated by dots, each of 1 to 63
+// letters, digits and hyphens, neither starting nor ending with a hyphen,
+// and at most maxZoneLength characters in all. Go's resolver takes any
+// other name for one that does not exist, so that the zone would list no
+// client. A zone listed twice is an error.
+func parseZones(value string) ([]string, error) {
+	zones, err := parseList(value)
+	if err != nil {
+		return nil, err
+	}
+
+	for i, zone := range zones {
+		switch {
+		case len(zone) > maxZoneLength:
+			return nil, fmt.Errorf("%q %w", zone, errZoneTooLong)
+		case !isZone(zone):
+			return nil, fmt.Errorf("%q %w", zone, errNotZone)
+		case slices.Contains(zones[:i], zone):
+			return nil, fmt.Errorf("%q %w", zone, errListedTwice)
+		}
+	}
+
+	return zones, nil
+}
+
+// isZone reports whether each label of zone is as parseZones takes it.
+func isZone(zone string) bool {
+	for label := range strings.SplitSeq(zone, ".") {
+		badChar := strings.ContainsFunc(label, func(char rune) bool {
+			return !strings.ContainsRune(labelChars, char)
+		})
+
+		if label == "" || len(label) > maxLabelLength || badChar ||
+			strings.HasPrefix(label, "-") || strings.HasSuffix(label, "-") {
+			return false
+		}
+	}
+
+	return true
+}
+
+// parseResolver reads the resolver the DNSBL zones are asked through: an
+// IP address with a port from 1 to 65535, such as 192.0.2.53:5353 or
+// [2001:db8::53]:5353, or without one, such as 192.0.2.53 or 2001:db8::53,
+// for port 53. An empty value is none, the zero AddrPort.
+func parseResolver(value string) (netip.AddrPort, error) {
+	if value == "" {
+		return netip.AddrPort{}, nil
+	}
+
+	resolver, err := netip.ParseAddrPort(value)
+	if err == nil && resolver.Port() != 0 {
+		return resolver, nil
+	}
+
+	addr, err := netip.ParseAddr(value)
+	if err != nil {
+		return netip.AddrPort{}, fmt.Errorf("%q %w", value, errNotResolver)
+	}
+
+	return netip.AddrPortFrom(addr, dnsPort), nil
+}
+
 // parseWebhookHeaders reads a comma-separated list of headers, each its
 // name, :, and its value, and returns them, and how the log shows them,
 // with each value as ********. An error names the item by its place in

internal/config/config_test.go

@@ -61,6 +61,11 @@ const (
 	blocklistURLs           = "SWWAF_BLOCKLIST_URLS"
 	blocklistRefresh        = "SWWAF_BLOCKLIST_REFRESH"
 	blocklistAction         = "SWWAF_BLOCKLIST_ACTION"
+	dnsblZones              = "SWWAF_DNSBL_ZONES"
+	dnsblResolver           = "SWWAF_DNSBL_RESOLVER"
+	reputationAction        = "SWWAF_REPUTATION_ACTION"
+	reputationCacheTTL      = "SWWAF_REPUTATION_CACHE_TTL"
+	reputationTimeout       = "SWWAF_REPUTATION_TIMEOUT"
 	banResponse             = "SWWAF_BAN_RESPONSE"
 	limitBanDuration        = "SWWAF_LIMIT_BAN_DURATION"
 	limitBanRepeatWindow    = "SWWAF_LIMIT_BAN_REPEAT_WINDOW"
@@ -152,6 +157,9 @@ const (
 	defaultAlertCooldown = "15m"
 )
 
+// defaultReputationCacheTTL is the default of SWWAF_REPUTATION_CACHE_TTL.
+const defaultReputationCacheTTL = "24h"
+
 // defaultLogRequestHeaders is the default of SWWAF_LOG_REQUEST_HEADERS.
 const defaultLogRequestHeaders = "accept,accept-language,accept-encoding," +
 	"content-type,origin,range"
@@ -1322,6 +1330,130 @@ func TestASNLimitPercentURLThatIsABlocklistStopsTheStart(t *testing.T) {
 	}
 }
 
+// dronebl is a DNSBL zone, and spamhaus one of Spamhaus's, a name
+// containing the key of its keyed query service.
+const (
+	dronebl  = "dnsbl.dronebl.org"
+	spamhaus = "abcdefghijklmnopqrstuvwxyz.xbl.dq.spamhaus.net"
+)
+
+func TestDNSBLSettingsAsSet(t *testing.T) {
+	t.Parallel()
+
+	cfg := fromEnvironment(t, environment{})
+	if len(cfg.DNSBLZones) != 0 || cfg.DNSBLResolver.IsValid() ||
+		cfg.ReputationAction != actionLimit || cfg.ReputationLimitPercent != 25 ||
+		cfg.ReputationCacheTTL != 24*time.Hour || cfg.ReputationTimeout != 2*time.Second {
+		t.Errorf("by default, the zones %v, the resolver %s, the action %s:%d, the TTL %s "+
+			"and the timeout %s, want no zone, no resolver, limit:25, 24h and 2s",
+			cfg.DNSBLZones,
+			cfg.DNSBLResolver, cfg.ReputationAction, cfg.ReputationLimitPercent,
+			cfg.ReputationCacheTTL, cfg.ReputationTimeout)
+	}
+
+	// The longest zone allowed, of 189 characters, with labels of 63, the
+	// longest allowed.
+	longest := strings.Repeat("a", 63) + "." + strings.Repeat("b", 63) + "." +
+		strings.Repeat("c", 61)
+
+	for _, tc := range []struct {
+		zones, resolver, action string
+		// want are the zones, resolver, action and percent Config gives.
+		want         []string
+		wantResolver string
+		wantAction   string
+		wantPercent  int64
+	}{
+		{
+			dronebl + ", " + spamhaus, "192.0.2.53", actionDeny,
+			[]string{dronebl, spamhaus}, "192.0.2.53:53", actionDeny, 0,
+		},
+		{
+			longest, "192.0.2.53:5353", actionLog,
+			[]string{longest}, "192.0.2.53:5353", actionLog, 0,
+		},
+		{
+			"Dnsbl-1.Example", "2001:db8::53", "limit:10",
+			[]string{"Dnsbl-1.Example"}, "[2001:db8::53]:53", actionLimit, 10,
+		},
+		{
+			dronebl, "[2001:db8::53]:5353", "limit:0",
+			[]string{dronebl}, "[2001:db8::53]:5353", actionLimit, 0,
+		},
+	} {
+		cfg := fromEnvironment(t, environment{
+			dnsblZones: tc.zones, dnsblResolver: tc.resolver, reputationAction: tc.action,
+			reputationCacheTTL: "12h", reputationTimeout: "3s",
+		})
+
+		if !slices.Equal(cfg.DNSBLZones, tc.want) ||
+			cfg.DNSBLResolver.String() != tc.wantResolver ||
+			cfg.ReputationAction != tc.wantAction ||
+			cfg.ReputationLimitPercent != tc.wantPercent ||
+			cfg.ReputationCacheTTL != 12*time.Hour ||
+			cfg.ReputationTimeout != 3*time.Second {
+			t.Errorf("%s=%s, %s=%s and %s=%s gave %v, %s, %s:%d, %s and %s", dnsblZones,
+				tc.zones, dnsblResolver, tc.resolver, reputationAction, tc.action,
+				cfg.DNSBLZones, cfg.DNSBLResolver, cfg.ReputationAction,
+				cfg.ReputationLimitPercent, cfg.ReputationCacheTTL, cfg.ReputationTimeout)
+		}
+	}
+}
+
+func TestInvalidDNSBLSettingStopsTheStartSayingWhatIsWrong(t *testing.T) {
+	t.Parallel()
+
+	const (
+		notZone     = " is not a DNS zone such as dnsbl.dronebl.org"
+		notResolver = " is not an IP address with an optional port, such as 192.0.2.53 " +
+			"or [2001:db8::53]:5353"
+		notAboveZero = " is not a duration above zero, such as 1h or 7d"
+	)
+
+	label64 := strings.Repeat("a", 64) + ".example"
+	tooLong := strings.Repeat("a", 63) + "." + strings.Repeat("b", 63) + "." +
+		strings.Repeat("c", 62)
+
+	for _, tc := range []struct{ name, value, want string }{
+		{dnsblZones, "dnsbl..example", `"dnsbl..example"` + notZone},
+		{dnsblZones, "dnsbl.example.", `"dnsbl.example."` + notZone},
+		{dnsblZones, "-dnsbl.example", `"-dnsbl.example"` + notZone},
+		{dnsblZones, "dnsbl-.example", `"dnsbl-.example"` + notZone},
+		{dnsblZones, "dns_bl.example", `"dns_bl.example"` + notZone},
+		{dnsblZones, label64, `"` + label64 + `"` + notZone},
+		{
+			dnsblZones, tooLong,
+			`"` + tooLong + `" is longer than 189 characters, too long for the names ` +
+				`IPv6 clients are asked about by`,
+		},
+		{
+			dnsblZones, dronebl + "," + spamhaus + "," + dronebl,
+			`"` + dronebl + `" is listed twice`,
+		},
+		{dnsblResolver, "resolver.example", `"resolver.example"` + notResolver},
+		{dnsblResolver, "192.0.2.53:0", `"192.0.2.53:0"` + notResolver},
+		{dnsblResolver, "192.0.2.53:65536", `"192.0.2.53:65536"` + notResolver},
+		{dnsblResolver, "[2001:db8::53]", `"[2001:db8::53]"` + notResolver},
+		{
+			reputationAction, "ban",
+			`"ban" is not deny, limit:<percent> such as limit:25, or log`,
+		},
+		{reputationCacheTTL, off, `"off"` + notAboveZero},
+		{reputationTimeout, "0s", `"0s"` + notAboveZero},
+	} {
+		t.Run(tc.name+"="+tc.value, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := config.FromEnvironment(environment{tc.name: tc.value}.lookupEnv)
+
+			want := tc.name + ": " + tc.want
+			if err == nil || err.Error() != want {
+				t.Errorf("error %v, want %s", err, want)
+			}
+		})
+	}
+}
+
 func TestSizesAndOff(t *testing.T) {
 	t.Parallel()
 
@@ -1704,6 +1836,7 @@ func writeFile(t *testing.T, contents string) string {
 	return path
 }
 
+//nolint:funlen // one line for each setting, a list that grows with them
 func TestLogsEachSettingWithItsValue(t *testing.T) {
 	t.Parallel()
 
@@ -1749,6 +1882,11 @@ func TestLogsEachSettingWithItsValue(t *testing.T) {
 		blocklistURLs:           "",
 		blocklistRefresh:        "24h",
 		blocklistAction:         actionDeny,
+		dnsblZones:              "",
+		dnsblResolver:           "",
+		reputationAction:        "limit:25",
+		reputationCacheTTL:      defaultReputationCacheTTL,
+		reputationTimeout:       "2s",
 		banResponse:             "403",
 		limitBanDuration:        "1h",
 		limitBanRepeatWindow:    "24h",

internal/metrics/metrics.go

@@ -256,24 +256,53 @@ func (m *Metrics) AddLookupFile(lastRead func() time.Time, readFailures func() i
 	)
 }
 
-// AddReputation adds the metrics of the lists fetched from URLs, by
-// source, each list's URL: the requests whose client a blocklist lists,
-// which ReputationHit counts, and, read from lists as the metrics are
-// asked for, the fetches that failed and when the copy in use was fetched.
-// It is called once, before ReputationHit.
-func (m *Metrics) AddReputation(lists *reputation.Lists) {
+// AddReputation adds the metrics of the lists fetched from URLs and of the
+// DNSBL zones, by source, each list's URL or each zone: the requests whose
+// client a blocklist or a zone's verdict lists, which ReputationHit
+// counts, and, read from lists and dnsbl as the metrics are asked for, for
+// a list, the fetches that failed and when the copy in use was fetched,
+// and for a zone, the queries made and those that failed. It is called
+// once, before ReputationHit.
+func (m *Metrics) AddReputation(lists *reputation.Lists, dnsbl *reputation.DNSBL) {
+	const (
+		sourceLabel  = "source"
+		failuresHelp = "Fetches of the list, or queries to the DNSBL zone, that failed."
+	)
+
 	m.reputationHits = counterVec("smallwebwaf_reputation_hits_total",
-		"Requests whose client a blocklist lists, by the blocklist's URL.",
-		[]string{"source"})
+		"Requests whose client a blocklist or a DNSBL zone lists, by the "+
+			"blocklist's URL or the zone.",
+		[]string{sourceLabel})
 	m.registry.MustRegister(m.reputationHits)
 
+	for _, zone := range dnsbl.Zones() {
+		source := prometheus.Labels{sourceLabel: zone}
+
+		m.registry.MustRegister(
+			prometheus.NewCounterFunc(prometheus.CounterOpts{
+				Name:        "smallwebwaf_reputation_queries_total",
+				Help:        "Queries to the DNSBL zone.",
+				ConstLabels: source,
+			}, func() float64 {
+				return float64(dnsbl.Queries(zone))
+			}),
+			prometheus.NewCounterFunc(prometheus.CounterOpts{
+				Name:        "smallwebwaf_reputation_failures_total",
+				Help:        failuresHelp,
+				ConstLabels: source,
+			}, func() float64 {
+				return float64(dnsbl.Failures(zone))
+			}),
+		)
+	}
+
 	for _, listURL := range lists.URLs() {
-		source := prometheus.Labels{"source": listURL}
+		source := prometheus.Labels{sourceLabel: listURL}
 
 		m.registry.MustRegister(
 			prometheus.NewCounterFunc(prometheus.CounterOpts{
 				Name:        "smallwebwaf_reputation_failures_total",
-				Help:        "Fetches of the list that failed.",
+				Help:        failuresHelp,
 				ConstLabels: source,
 			}, func() float64 {
 				return float64(lists.Failures(listURL))
@@ -295,8 +324,8 @@ func (m *Metrics) AddReputation(lists *reputation.Lists) {
 	}
 }
 
-// ReputationHit counts a request whose client the blocklist at source, its
-// URL, lists.
+// ReputationHit counts a request whose client source lists: a blocklist,
+// by its URL, or a DNSBL zone.
 func (m *Metrics) ReputationHit(source string) {
 	m.reputationHits.WithLabelValues(source).Inc()
 }

internal/proxy/biased.go

@@ -28,16 +28,18 @@ func biasedThresholdsSet(cfg *config.Config) bool {
 
 // limitPercentages returns the client's limit percentages, for the rate
 // limits and for the byte limits, by its AS number and country as looked
-// up, each "" when unknown, and the blocklists that list it. Each is the
-// lowest of those the settings give it, the first of them in the order
-// below when several are lowest: the percentage SWWAF_ASN_LIMIT_PERCENT
-// gives its AS number, the one the file SWWAF_ASN_LIMIT_PERCENT_URL names
-// gives it, the one SWWAF_COUNTRY_LIMIT_PERCENT gives its country, for a
-// client without a country, SWWAF_UNKNOWN_LIMIT_PERCENT, and for a client
-// a blocklist lists, the percentage of SWWAF_BLOCKLIST_ACTION while it is
-// limit. For the byte limits, SWWAF_ASN_BYTES_PERCENT and
-// SWWAF_COUNTRY_BYTES_PERCENT take the place of the first three for an AS
-// number or a country they list.
+// up, each "" when unknown, and the blocklists and DNSBL zones that list
+// it. Each is the lowest of those the settings give it, the first of them
+// in the order below when several are lowest: the percentage
+// SWWAF_ASN_LIMIT_PERCENT gives its AS number, the one the file
+// SWWAF_ASN_LIMIT_PERCENT_URL names gives it, the one
+// SWWAF_COUNTRY_LIMIT_PERCENT gives its country, for a client without a
+// country, SWWAF_UNKNOWN_LIMIT_PERCENT, for a client a blocklist lists,
+// the percentage of SWWAF_BLOCKLIST_ACTION while it is limit, and for a
+// client a DNSBL zone's verdict lists, the percentage of
+// SWWAF_REPUTATION_ACTION while it is limit. For the byte limits,
+// SWWAF_ASN_BYTES_PERCENT and SWWAF_COUNTRY_BYTES_PERCENT take the place
+// of the first three for an AS number or a country they list.
 func (rq *request) limitPercentages() (percentage, percentage) {
 	cfg := rq.h.config
 	asn, country := rq.line.ASN, rq.line.Country
@@ -52,9 +54,14 @@ func (rq *request) limitPercentages() (percentage, percentage) {
 		fetched = percentage{percent, "SWWAF_ASN_LIMIT_PERCENT_URL"}
 	}
 
-	listed := percentage{percent: whole}
-	if len(rq.line.Reputation) > 0 && cfg.BlocklistAction == "limit" {
-		listed = percentage{cfg.BlocklistLimitPercent, "SWWAF_BLOCKLIST_ACTION"}
+	blocklisted := percentage{percent: whole}
+	if rq.blocklisted && cfg.BlocklistAction == "limit" {
+		blocklisted = percentage{cfg.BlocklistLimitPercent, "SWWAF_BLOCKLIST_ACTION"}
+	}
+
+	dnsblListed := percentage{percent: whole}
+	if rq.dnsblListed && cfg.ReputationAction == "limit" {
+		dnsblListed = percentage{cfg.ReputationLimitPercent, "SWWAF_REPUTATION_ACTION"}
 	}
 
 	asnRequests := lowest(given(cfg.ASNLimitPercent, asn, "SWWAF_ASN_LIMIT_PERCENT"),
@@ -71,8 +78,8 @@ func (rq *request) limitPercentages() (percentage, percentage) {
 		countryBytes = given(cfg.CountryBytesPercent, country, "SWWAF_COUNTRY_BYTES_PERCENT")
 	}
 
-	return lowest(asnRequests, countryRequests, unknown, listed),
-		lowest(asnBytes, countryBytes, unknown, listed)
+	return lowest(asnRequests, countryRequests, unknown, blocklisted, dnsblListed),
+		lowest(asnBytes, countryBytes, unknown, blocklisted, dnsblListed)
 }
 
 // given returns the percentage percents, the setting named setting, gives

internal/proxy/proxy.go

@@ -71,15 +71,15 @@ type Params struct {
 	Rules *rules.Files
 	// Alerts receive the alert for each ban the proxy makes or makes
 	// permanent, for each count over an anomaly threshold, for each request
-	// whose client a blocklist lists, and for GeoJS failing or a fetch of a
-	// list failing.
+	// whose client a blocklist or a DNSBL zone lists, and for GeoJS failing,
+	// a fetch of a list failing or a query to a DNSBL zone failing.
 	Alerts *alerts.Queue
 }
 
 // Server is the server smallwebwaf runs, with the parts of the proxy
 // whose state the state files keep, the lookup database, nil unless
 // SWWAF_LOOKUP_SOURCE is file, the lists fetched from URLs, which its Run
-// fetches, and the metrics.
+// fetches, the DNSBL zones' verdicts, and the metrics.
 type Server struct {
 	*http.Server
 
@@ -89,6 +89,7 @@ type Server struct {
 	Anomalies  *anomaly.Counters
 	LookupFile *lookup.File
 	Lists      *reputation.Lists
+	DNSBL      *reputation.DNSBL
 	Metrics    *metrics.Metrics
 }
 
@@ -101,6 +102,7 @@ type Server struct {
 func New(params Params) *Server {
 	errorLog := slog.NewLogLogger(params.ProcessLog.Handler(), slog.LevelWarn)
 	m := metrics.New(params.Config.MetricsTopN, params.Config.InstanceName)
+	lists, dnsbl := newReputation(params)
 	h := &handler{
 		config:     params.Config,
 		requestLog: params.RequestLog,
@@ -136,13 +138,10 @@ func New(params Params) *Server {
 			Alerts:         params.Alerts,
 		}),
 		lookupFile: params.LookupFile,
-		lists: reputation.New(reputation.Params{
-			BlocklistURLs: params.Config.BlocklistURLs, Refresh: params.Config.BlocklistRefresh,
-			ASNLimitPercentURL: params.Config.ASNLimitPercentURL, Now: params.Now,
-			ProcessLog: params.ProcessLog, Alerts: params.Alerts,
-		}),
-		rules:  params.Rules,
-		alerts: params.Alerts,
+		lists:      lists,
+		dnsbl:      dnsbl,
+		rules:      params.Rules,
+		alerts:     params.Alerts,
 	}
 	h.geojs = lookup.New(lookup.Params{
 		URL:     params.GeoJSURL,
@@ -160,7 +159,7 @@ func New(params Params) *Server {
 	})
 	m.AddBansAndClients(h.ledger, h.limiter, params.Now)
 	m.AddRules(params.Rules)
-	m.AddReputation(h.lists)
+	m.AddReputation(h.lists, h.dnsbl)
 
 	return &Server{
 		Server: &http.Server{
@@ -181,10 +180,30 @@ func New(params Params) *Server {
 		Anomalies:  h.anomalies,
 		LookupFile: h.lookupFile,
 		Lists:      h.lists,
+		DNSBL:      h.dnsbl,
 		Metrics:    m,
 	}
 }
 
+// newReputation returns the lists fetched from URLs and the DNSBL zones'
+// verdicts, as the settings in params name them, with none fetched or
+// asked for yet.
+func newReputation(params Params) (*reputation.Lists, *reputation.DNSBL) {
+	cfg := params.Config
+	lists := reputation.New(reputation.Params{
+		BlocklistURLs: cfg.BlocklistURLs, Refresh: cfg.BlocklistRefresh,
+		ASNLimitPercentURL: cfg.ASNLimitPercentURL, Now: params.Now,
+		ProcessLog: params.ProcessLog, Alerts: params.Alerts,
+	})
+	dnsbl := reputation.NewDNSBL(reputation.DNSBLParams{
+		Zones: cfg.DNSBLZones, Resolver: cfg.DNSBLResolver, CacheTTL: cfg.ReputationCacheTTL,
+		Timeout: cfg.ReputationTimeout, Now: params.Now, ProcessLog: params.ProcessLog,
+		Alerts: params.Alerts,
+	})
+
+	return lists, dnsbl
+}
+
 // handler is the proxy. It holds what every request shares; what belongs
 // to one request is in a request.
 type handler struct {
@@ -201,6 +220,7 @@ type handler struct {
 	anomalies  *anomaly.Counters
 	lookupFile *lookup.File
 	lists      *reputation.Lists
+	dnsbl      *reputation.DNSBL
 	rules      *rules.Files
 	alerts     *alerts.Queue
 }

internal/proxy/reputation.go

@@ -1,21 +1,47 @@
 package proxy
 
 import (
+	"context"
+
 	"sneak.berlin/go/smallwebwaf/internal/alerts"
 )
 
-// blocklistDenied notes in the log line the URLs of the blocklists that
-// list the client, counts each of them in the metrics and raises a
-// reputation_hit alert for it, and reports whether SWWAF_BLOCKLIST_ACTION,
-// being deny, refuses the request. Being limit, it lowers the client's
-// limits instead (see limitPercentages), and being log, it does nothing
-// more.
+// blocklistDenied notes the blocklists that list the client, as
+// noteListed does, and reports whether SWWAF_BLOCKLIST_ACTION, being deny,
+// refuses the request. Being limit, it lowers the client's limits instead
+// (see limitPercentages), and being log, it does nothing more.
 func (rq *request) blocklistDenied() bool {
 	listedBy := rq.h.lists.ListedBy(rq.client)
-	rq.line.Reputation = listedBy
+	rq.blocklisted = len(listedBy) > 0
+	rq.noteListed(listedBy, "listed by a blocklist")
 
-	for _, listURL := range listedBy {
-		rq.h.metrics.ReputationHit(listURL)
+	return rq.blocklisted && rq.h.config.BlocklistAction == "deny"
+}
+
+// dnsblDenied notes the DNSBL zones whose verdict lists the client, as
+// noteListed does, and reports whether SWWAF_REPUTATION_ACTION, being
+// deny, refuses the request. Being limit, it lowers the client's limits
+// instead (see limitPercentages), and being log, it does nothing more. A
+// zone without a verdict on the client is asked about it in the
+// background, and the request does not wait for the answer. ctx is the
+// request's own context.
+func (rq *request) dnsblDenied(ctx context.Context) bool {
+	listedBy := rq.h.dnsbl.ListedBy(ctx, rq.client)
+	rq.dnsblListed = len(listedBy) > 0
+	rq.noteListed(listedBy, "listed by a DNSBL zone")
+
+	return rq.dnsblListed && rq.h.config.ReputationAction == "deny"
+}
+
+// noteListed adds sources, the URLs of the blocklists or the DNSBL zones
+// that list the client, to the log line's reputation, counts each of them
+// in the metrics, and raises a reputation_hit alert, with reason, for
+// each.
+func (rq *request) noteListed(sources []string, reason string) {
+	rq.line.Reputation = append(rq.line.Reputation, sources...)
+
+	for _, source := range sources {
+		rq.h.metrics.ReputationHit(source)
 		rq.h.alerts.Raise(alerts.Alert{
 			Event:    alerts.EventReputationHit,
 			Client:   rq.client,
@@ -23,10 +49,8 @@ func (rq *request) blocklistDenied() bool {
 			ASN:      rq.line.ASN,
 			ASName:   rq.line.ASName,
 			Country:  rq.line.Country,
-			Reason:   "listed by a blocklist",
-			Detail:   map[string]any{"source": listURL},
+			Reason:   reason,
+			Detail:   map[string]any{"source": source},
 		})
 	}
-
-	return len(listedBy) > 0 && rq.h.config.BlocklistAction == "deny"
 }

internal/proxy/reputation_test.go

@@ -21,11 +21,14 @@ const (
 	asnLimitPercentURL = "SWWAF_ASN_LIMIT_PERCENT_URL"
 )
 
-// The actions of SWWAF_BLOCKLIST_ACTION but limit, which has a
-// percentage.
+// The actions of SWWAF_BLOCKLIST_ACTION and SWWAF_REPUTATION_ACTION:
+// limitHalf gives a listed client half of every limit, and limitQuarter a
+// quarter.
 const (
-	actionDeny = "deny"
-	actionLog  = "log"
+	actionDeny   = "deny"
+	actionLog    = "log"
+	limitHalf    = "limit:50"
+	limitQuarter = "limit:25"
 )
 
 // The lists these tests name, which are never fetched: each test puts in
@@ -55,7 +58,7 @@ func TestEachBlocklistActionForAListedAddressAndAListedNetblock(t *testing.T) {
 		},
 		{
 			// Half of 4 requests a minute: the third breaks the limit.
-			"limit:50", []int{http.StatusOK, http.StatusOK, http.StatusForbidden},
+			limitHalf, []int{http.StatusOK, http.StatusOK, http.StatusForbidden},
 			[]string{forward, forward, requestlog.ActionRateLimited},
 			"50 from " + blocklistAction,
 		},
@@ -151,10 +154,10 @@ func TestBlocklistLimitTakesPartInTheLowestPercentageOfEveryLimit(t *testing.T)
 		// percentText gives them, and limitHit its limit_hit.
 		want, limitHit string
 	}{
-		{"limit:50", asnDEQuarter, "25 from " + asnLimitPercent, minuteBytes},
-		{"limit:25", asnDEHalf, "25 from " + blocklistAction, minuteBytes},
+		{limitHalf, asnDEQuarter, "25 from " + asnLimitPercent, minuteBytes},
+		{limitQuarter, asnDEHalf, "25 from " + blocklistAction, minuteBytes},
 		// The AS number's, the first of two alike.
-		{"limit:25", asnDEQuarter, "25 from " + asnLimitPercent, minuteBytes},
+		{limitQuarter, asnDEQuarter, "25 from " + asnLimitPercent, minuteBytes},
 		{actionLog, asnDE + ":100", none, ""},
 	} {
 		t.Run(tc.action+" "+tc.asnPercent, func(t *testing.T) {
@@ -299,11 +302,279 @@ func TestEachBlocklistThatListsAClientRaisesAnAlertOncePerCooldownAndIsCounted(
 	}
 }
 
+// The DNSBL settings.
+const (
+	dnsblZones       = "SWWAF_DNSBL_ZONES"
+	dnsblResolver    = "SWWAF_DNSBL_RESOLVER"
+	reputationAction = "SWWAF_REPUTATION_ACTION"
+)
+
+// The DNSBL zones these tests name, which are never asked about the
+// clients the tests send requests from: each test puts in the verdicts it
+// needs, as reputation.json would at start. A query a test does start is
+// sent to noResolver, where nothing listens, so that none leaves the host.
+const (
+	dnsblZone  = "dnsbl.example"
+	otherZone  = "other.example"
+	noResolver = "127.0.0.1:9"
+)
+
+func TestEachReputationActionForAClientADNSBLZoneLists(t *testing.T) {
+	t.Parallel()
+
+	forward, denied := requestlog.ActionForward, requestlog.ActionDenied
+
+	for _, tc := range []struct {
+		action string
+		// statuses and actions are those of a listed client's three
+		// requests, and percent their limit_percent, as percentText gives it.
+		statuses []int
+		actions  []string
+		percent  string
+	}{
+		{
+			actionDeny, []int{http.StatusForbidden, http.StatusForbidden, http.StatusForbidden},
+			[]string{denied, denied, denied}, none,
+		},
+		{
+			// Half of 4 requests a minute: the third breaks the limit.
+			limitHalf, []int{http.StatusOK, http.StatusOK, http.StatusForbidden},
+			[]string{forward, forward, requestlog.ActionRateLimited},
+			"50 from " + reputationAction,
+		},
+		{
+			actionLog, []int{http.StatusOK, http.StatusOK, http.StatusOK},
+			[]string{forward, forward, forward}, none,
+		},
+	} {
+		t.Run(tc.action, func(t *testing.T) {
+			t.Parallel()
+
+			s, server, _ := startWithLookups(t, map[string]string{
+				rateLimitPerMinute: fourAMinute, dnsblZones: dnsblZone + "," + otherZone,
+				dnsblResolver: noResolver, reputationAction: tc.action,
+			})
+			listedBy := map[string][]string{
+				fromDE: {dnsblZone, otherZone}, fromKP: {otherZone}, unplaced: nil,
+			}
+			loadVerdicts(server, listedBy)
+
+			for _, from := range []string{fromDE, fromKP} {
+				for i := range 3 {
+					line := s.get(from, tc.statuses[i], tc.actions[i])
+					// In the order SWWAF_DNSBL_ZONES names them.
+					wantReputation(t, line, listedBy[from]...)
+					wantPercent(t, "limit_percent", line.LimitPercent,
+						line.LimitPercentSetting, tc.percent)
+
+					// A request refused for the verdict is not counted.
+					counted := line.fields["counts"] != nil
+					if counted != (tc.actions[i] != denied) {
+						t.Errorf("request from %s counted %t, logged %s", from, counted,
+							tc.actions[i])
+					}
+				}
+			}
+
+			// A client no zone lists has the whole limit.
+			for range 3 {
+				line := s.get(unplaced, http.StatusOK, forward)
+				wantReputation(t, line)
+				wantPercent(t, "limit_percent", line.LimitPercent, line.LimitPercentSetting,
+					none)
+			}
+
+			// A refusal for the verdict makes no ban, and every client had its
+			// verdicts, so no zone was asked.
+			if held := server.Ledger.Snapshot(); tc.action == actionDeny && len(held) != 0 {
+				t.Errorf("bans %+v, want none", held)
+			}
+
+			if queries := server.DNSBL.Queries(dnsblZone); queries != 0 {
+				t.Errorf("%d queries, want none", queries)
+			}
+		})
+	}
+}
+
+func TestDNSBLZonesComeAfterTheBlocklistsAndSkipAllowNets(t *testing.T) {
+	t.Parallel()
+
+	s, server, queue := startWithLookups(t, map[string]string{
+		blocklistURLs: dropURL, dnsblZones: dnsblZone, dnsblResolver: noResolver,
+		reputationAction: actionDeny, allowNets: fromDE,
+	})
+	loadLists(t, server, map[string][]string{dropURL: {fromKP}})
+	loadVerdicts(server, map[string][]string{fromKP: {dnsblZone}, fromDE: {dnsblZone}})
+
+	// The blocklist refuses fromKP before its verdict is looked at, and
+	// fromDE, in SWWAF_ALLOW_NETS, is not checked at all: neither is noted
+	// for the zone, nor alerted, nor asked about.
+	wantReputation(t, s.get(fromKP, http.StatusForbidden, requestlog.ActionDenied),
+		dropURL)
+	wantReputation(t, s.get(fromDE, http.StatusOK, requestlog.ActionForward))
+
+	waiting := queue.Snapshot().Waiting[alerts.DestinationWebhook]
+	if len(waiting) != 1 || waiting[0].Detail["source"] != dropURL {
+		t.Errorf("alerts waiting %+v, want the blocklist's reputation_hit alone", waiting)
+	}
+
+	if queries := server.DNSBL.Queries(dnsblZone); queries != 0 {
+		t.Errorf("%d queries, want none", queries)
+	}
+}
+
+func TestReputationLimitTakesPartInTheLowestPercentageOfEveryLimit(t *testing.T) {
+	t.Parallel()
+
+	for _, tc := range []struct {
+		blocklistAction, reputationAction string
+		// want is the request's limit_percent and bytes_percent, as
+		// percentText gives them.
+		want string
+	}{
+		{limitHalf, limitQuarter, "25 from " + reputationAction},
+		{limitQuarter, limitHalf, "25 from " + blocklistAction},
+		// The blocklist's, the first of two alike.
+		{limitQuarter, limitQuarter, "25 from " + blocklistAction},
+		{actionLog, limitQuarter, "25 from " + reputationAction},
+		{actionLog, actionLog, none},
+	} {
+		t.Run(tc.blocklistAction+" "+tc.reputationAction, func(t *testing.T) {
+			t.Parallel()
+
+			s, server, _ := startWithLookups(t, map[string]string{
+				blocklistURLs: dropURL, blocklistAction: tc.blocklistAction,
+				dnsblZones: dnsblZone, dnsblResolver: noResolver,
+				reputationAction: tc.reputationAction,
+			})
+			loadLists(t, server, map[string][]string{dropURL: {fromDE}})
+			loadVerdicts(server, map[string][]string{fromDE: {dnsblZone}})
+
+			// Named by the blocklist, then by the zone.
+			line := s.get(fromDE, http.StatusOK, requestlog.ActionForward)
+			wantReputation(t, line, dropURL, dnsblZone)
+			wantPercent(t, "limit_percent", line.LimitPercent, line.LimitPercentSetting,
+				tc.want)
+			wantPercent(t, "bytes_percent", line.BytesPercent, line.BytesPercentSetting,
+				tc.want)
+		})
+	}
+}
+
+func TestEachZoneThatListsAClientRaisesAnAlertOncePerCooldownAndIsCounted(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	s, clk, server, queue := startWithLookupsAndClock(t, map[string]string{
+		dnsblZones: dnsblZone + "," + otherZone, dnsblResolver: noResolver,
+		reputationAction: actionLog, metricsToken: token,
+	})
+	loadVerdicts(server, map[string][]string{
+		fromDE: {dnsblZone, otherZone}, unplaced: nil,
+	})
+
+	// The second request's alerts are repeats, which the cooldown holds
+	// back.
+	for range 2 {
+		wantReputation(t, s.get(fromDE, http.StatusOK, requestlog.ActionForward),
+			dnsblZone, otherZone)
+	}
+
+	hit := func(zone string) alerts.Alert {
+		return alerts.Alert{
+			Instance: alertInstance,
+			Time:     clk.Now(),
+			Event:    alerts.EventReputationHit,
+			Client:   netip.MustParseAddr(fromDE),
+			Netblock: netip.MustParsePrefix(fromDE + "/32"),
+			ASN:      asnDE,
+			ASName:   asNameDE,
+			Country:  "DE",
+			Reason:   "listed by a DNSBL zone",
+			Detail:   map[string]any{"source": zone},
+		}
+	}
+	wantAlerts(t, queue, hit(dnsblZone), hit(otherZone))
+
+	if queue.Suppressed() != 2 {
+		t.Errorf("%d alerts held back, want the second request's 2", queue.Suppressed())
+	}
+
+	// Each zone's hits, and its queries and their failures, none, since
+	// every client had its verdicts.
+	metrics := s.scrape(unplaced)
+
+	for _, zone := range []string{dnsblZone, otherZone} {
+		labels := `{instance="` + alertInstance + `",source="` + zone + `"}`
+
+		wantMetric(t, metrics, "smallwebwaf_reputation_hits_total"+labels, 2)
+		wantMetric(t, metrics, "smallwebwaf_reputation_queries_total"+labels, 0)
+		wantMetric(t, metrics, "smallwebwaf_reputation_failures_total"+labels, 0)
+	}
+}
+
+func TestRequestFromAClientWithoutAVerdictHasTheZoneAskedAboutIt(t *testing.T) {
+	t.Parallel()
+
+	s, server, _ := startWithLookups(t, map[string]string{
+		dnsblZones: dnsblZone + "," + otherZone, dnsblResolver: noResolver,
+	})
+	server.DNSBL.Load([]reputation.Verdict{{
+		Zone: otherZone, Client: netip.MustParseAddr(fromDE), Listed: true,
+		Fetched: verdictsFetched(),
+	}})
+
+	// The verdict of the other zone is used, and dnsbl.example, which has
+	// none, is asked about the client in the background, once: the second
+	// request finds the query under way, or the zone left alone after it
+	// failed, since nothing answers at noResolver.
+	for range 2 {
+		wantReputation(t, s.get(fromDE, http.StatusOK, requestlog.ActionForward), otherZone)
+	}
+
+	if queries := server.DNSBL.Queries(dnsblZone); queries != 1 {
+		t.Errorf("%d queries to %s, want 1", queries, dnsblZone)
+	}
+
+	if queries := server.DNSBL.Queries(otherZone); queries != 0 {
+		t.Errorf("%d queries to %s, want none", queries, otherZone)
+	}
+}
+
 // listsFetched is when loadLists has the copies fetched.
 func listsFetched() time.Time {
 	return time.Date(2026, 10, 5, 0, 0, 0, 0, time.UTC)
 }
 
+// verdictsFetched is when loadVerdicts has the verdicts fetched: half a
+// day before the time the tests' clock is set to, so that they are in use
+// until half a day later.
+func verdictsFetched() time.Time {
+	return time.Date(2026, 10, 5, 12, 0, 0, 0, time.UTC)
+}
+
+// loadVerdicts puts into server's DNSBL, for each client listedBy names,
+// a verdict of each zone SWWAF_DNSBL_ZONES names, fetched at
+// verdictsFetched, as reputation.json would at start: one that lists the
+// client from each zone listedBy gives for it, and one that does not from
+// each other zone.
+func loadVerdicts(server *proxy.Server, listedBy map[string][]string) {
+	verdicts := make([]reputation.Verdict, 0, len(listedBy)*len(server.DNSBL.Zones()))
+
+	for client, zones := range listedBy {
+		for _, zone := range server.DNSBL.Zones() {
+			verdicts = append(verdicts, reputation.Verdict{
+				Zone: zone, Client: netip.MustParseAddr(client),
+				Listed: slices.Contains(zones, zone), Fetched: verdictsFetched(),
+			})
+		}
+	}
+
+	server.DNSBL.Load(verdicts)
+}
+
 // loadLists puts copies of lists into server's lists, by URL, each with
 // its lines, fetched at listsFetched, as reputation.json would at start.
 func loadLists(t *testing.T, server *proxy.Server, copies map[string][]string) {

internal/proxy/request.go

@@ -63,7 +63,10 @@ type request struct {
 	// limits and for the byte limits.
 	counted                    bool
 	limitPercent, bytesPercent percentage
-	start                      time.Time
+	// blocklisted is true once a blocklist is found to list the client,
+	// and dnsblListed once a DNSBL zone's verdict is.
+	blocklisted, dnsblListed bool
+	start                    time.Time
 	// checked is when the checks were done, and upstreamStart when the
 	// request was handed to the app.
 	checked       time.Time
@@ -213,14 +216,14 @@ func (rq *request) check(ctx context.Context) *refusal {
 // client in SWWAF_ALLOW_NETS skips them, and is not looked up. For any
 // other client, SWWAF_DENY_NETS comes first, then a ban on its netblock,
 // so that a client either refuses is not looked up, then the lookup of
-// its AS number and country, then the country lists, and then the
-// blocklists; a request any of them refuses is not counted for the rate
-// limits. Then come the rate limits, unless the client is in
-// SWWAF_RATE_LIMIT_EXEMPT_NETS or the request's path is exempt under
-// SWWAF_RATE_LIMIT_EXEMPT_PATHS, so that every other request is counted,
-// each of them by the client's limit percentages, and last the rule
-// files. A request exempt from the rate limits is exempt from the byte
-// limits too. ctx is the request's own context.
+// its AS number and country, then the country lists, then the blocklists,
+// and then the DNSBL zones' verdicts; a request any of them refuses is not
+// counted for the rate limits. Then come the rate limits, unless the
+// client is in SWWAF_RATE_LIMIT_EXEMPT_NETS or the request's path is
+// exempt under SWWAF_RATE_LIMIT_EXEMPT_PATHS, so that every other request
+// is counted, each of them by the client's limit percentages, and last the
+// rule files. A request exempt from the rate limits is exempt from the
+// byte limits too. ctx is the request's own context.
 func (rq *request) checkClient(ctx context.Context) string {
 	cfg := rq.h.config
 	if isInside(rq.client, cfg.AllowNets) {
@@ -247,6 +250,10 @@ func (rq *request) checkClient(ctx context.Context) string {
 		return requestlog.ActionDenied
 	}
 
+	if rq.dnsblDenied(ctx) {
+		return requestlog.ActionDenied
+	}
+
 	rq.counted = !isInside(rq.client, cfg.RateLimitExemptNets) &&
 		!pathExempt(rq.in.URL, cfg.RateLimitExemptPaths)
 	if rq.counted {

internal/reputation/dnsbl.go

@@ -0,0 +1,332 @@
+package reputation
+
+import (
+	"cmp"
+	"context"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net"
+	"net/netip"
+	"slices"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/hashicorp/golang-lru/v2/simplelru"
+	"sneak.berlin/go/smallwebwaf/internal/alerts"
+)
+
+const (
+	// maxVerdicts is how many verdicts are kept. Past it, the one fetched
+	// longest ago is dropped.
+	maxVerdicts = 100000
+	// maxQueries is how many queries may be under way at once. Past it, a
+	// zone is not asked about a client until the client's next request, so
+	// that a swarm of new addresses cannot fill the memory.
+	maxQueries = 1000
+	// failureDelay is how long a zone is not asked again after a query to
+	// it fails, so that a zone refusing queries is not asked on every
+	// request.
+	failureDelay = time.Minute
+)
+
+var (
+	errAsk        = errors.New("ask the zone")
+	errRefused    = errors.New("the zone refused the query")
+	errNotListing = errors.New("the answer is outside 127.0.0.0/8")
+)
+
+// Verdict is what a zone said about a client, as reputation.json holds
+// it: the zone, the client's address, whether the zone lists it, and when
+// the zone answered.
+type Verdict struct {
+	Zone    string     `json:"zone"`
+	Client  netip.Addr `json:"client"`
+	Listed  bool       `json:"listed"`
+	Fetched time.Time  `json:"fetched"`
+}
+
+// DNSBLParams are what NewDNSBL needs.
+type DNSBLParams struct {
+	// Zones are the DNSBL zones clients are asked about in
+	// (SWWAF_DNSBL_ZONES).
+	Zones []string
+	// Resolver is the resolver they are asked through
+	// (SWWAF_DNSBL_RESOLVER), or, while it is the zero AddrPort, the
+	// host's, as /etc/resolv.conf names it.
+	Resolver netip.AddrPort
+	// CacheTTL is how long a verdict is used after it was fetched
+	// (SWWAF_REPUTATION_CACHE_TTL), and Timeout how long a query may take
+	// (SWWAF_REPUTATION_TIMEOUT).
+	CacheTTL time.Duration
+	Timeout  time.Duration
+	// Now tells the time, normally time.Now in UTC.
+	Now func() time.Time
+	// ProcessLog receives each query that fails, and why.
+	ProcessLog *slog.Logger
+	// Alerts receive a source_failure alert for each query that fails.
+	Alerts *alerts.Queue
+}
+
+// DNSBL asks the DNSBL zones about clients, in the background, and keeps
+// their verdicts. It is safe for concurrent use.
+type DNSBL struct {
+	params   DNSBLParams
+	resolver *net.Resolver
+
+	mu sync.Mutex
+	// verdicts are by query. Each is added as it is fetched and never moved
+	// up, so that the one fetched longest ago is the first dropped.
+	verdicts *simplelru.LRU[query, Verdict]
+	// asking are the queries under way.
+	asking map[query]bool
+	// queries and failures count, by zone, the queries made and those that
+	// failed, and retryAt is when a zone whose last query failed may be
+	// asked again.
+	queries  map[string]int
+	failures map[string]int
+	retryAt  map[string]time.Time
+}
+
+// query is a client's address, to ask a zone about.
+type query struct {
+	zone   string
+	client netip.Addr
+}
+
+// NewDNSBL returns a DNSBL with no verdict yet.
+func NewDNSBL(params DNSBLParams) *DNSBL {
+	verdicts, err := simplelru.NewLRU[query, Verdict](maxVerdicts, nil)
+	if err != nil {
+		panic(err) // NewLRU fails only for a size below one
+	}
+
+	resolver := &net.Resolver{}
+	if params.Resolver.IsValid() {
+		// Dial is used by Go's own resolver alone.
+		resolver.PreferGo = true
+		resolver.Dial = func(ctx context.Context, network, _ string) (net.Conn, error) {
+			var dialer net.Dialer
+
+			return dialer.DialContext(ctx, network, params.Resolver.String())
+		}
+	}
+
+	return &DNSBL{
+		params:   params,
+		resolver: resolver,
+		verdicts: verdicts,
+		asking:   map[query]bool{},
+		queries:  map[string]int{},
+		failures: map[string]int{},
+		retryAt:  map[string]time.Time{},
+	}
+}
+
+// Zones returns the zones, in the order SWWAF_DNSBL_ZONES names them.
+func (d *DNSBL) Zones() []string {
+	return slices.Clone(d.params.Zones)
+}
+
+// ListedBy returns the zones whose verdict on addr, a client's address,
+// lists it, in the order SWWAF_DNSBL_ZONES names them. A verdict is used
+// until CacheTTL has passed since it was fetched. Each zone without one is
+// asked about addr in the background, unless a query about addr to it is
+// under way, the zone is left alone after a failure, or maxQueries are
+// under way; ListedBy never waits for a query. ctx is the context of the
+// client's request, and a query goes on after the request ends.
+func (d *DNSBL) ListedBy(ctx context.Context, addr netip.Addr) []string {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	now := d.params.Now()
+
+	var listedBy []string
+
+	for _, zone := range d.params.Zones {
+		q := query{zone: zone, client: addr}
+
+		kept, found := d.verdicts.Peek(q)
+
+		switch {
+		case found && now.Sub(kept.Fetched) < d.params.CacheTTL:
+			if kept.Listed {
+				listedBy = append(listedBy, zone)
+			}
+		case !d.asking[q] && !now.Before(d.retryAt[zone]) && len(d.asking) < maxQueries:
+			d.asking[q] = true
+			d.queries[zone]++
+
+			go d.ask(context.WithoutCancel(ctx), q)
+		}
+	}
+
+	return listedBy
+}
+
+// Queries returns how many queries were made to zone.
+func (d *DNSBL) Queries(zone string) int {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	return d.queries[zone]
+}
+
+// Failures returns how many queries to zone failed.
+func (d *DNSBL) Failures(zone string) int {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	return d.failures[zone]
+}
+
+// Snapshot returns every verdict still in use, sorted by client, then by
+// zone, as reputation.json lists them.
+func (d *DNSBL) Snapshot() []Verdict {
+	d.mu.Lock()
+
+	now := d.params.Now()
+	verdicts := make([]Verdict, 0, d.verdicts.Len())
+
+	for _, kept := range d.verdicts.Values() {
+		if now.Sub(kept.Fetched) < d.params.CacheTTL {
+			verdicts = append(verdicts, kept)
+		}
+	}
+
+	d.mu.Unlock()
+
+	slices.SortFunc(verdicts, func(a, b Verdict) int {
+		return cmp.Or(a.Client.Compare(b.Client), strings.Compare(a.Zone, b.Zone))
+	})
+
+	return verdicts
+}
+
+// Load keeps verdicts, read from reputation.json, in place of those it
+// keeps. A verdict of a zone SWWAF_DNSBL_ZONES does not name, or one
+// fetched CacheTTL ago or more, is dropped, and past maxVerdicts, those
+// fetched longest ago.
+func (d *DNSBL) Load(verdicts []Verdict) {
+	verdicts = slices.Clone(verdicts)
+	slices.SortStableFunc(verdicts, func(a, b Verdict) int {
+		return a.Fetched.Compare(b.Fetched)
+	})
+
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	d.verdicts.Purge()
+
+	now := d.params.Now()
+
+	for _, kept := range verdicts {
+		if slices.Contains(d.params.Zones, kept.Zone) &&
+			now.Sub(kept.Fetched) < d.params.CacheTTL {
+			d.verdicts.Add(query{zone: kept.Zone, client: kept.Client}, kept)
+		}
+	}
+}
+
+// ask asks q's zone about q's client, keeps the verdict, and notes the
+// query as no longer under way. A query that fails gives no verdict: it
+// is counted, logged and raised as a source_failure alert, and the zone is
+// not asked again for failureDelay.
+func (d *DNSBL) ask(ctx context.Context, q query) {
+	listed, err := d.lookUp(ctx, q)
+	now := d.params.Now()
+
+	d.mu.Lock()
+
+	delete(d.asking, q)
+
+	if err == nil {
+		d.verdicts.Add(q, Verdict{
+			Zone: q.zone, Client: q.client, Listed: listed, Fetched: now,
+		})
+	} else {
+		d.failures[q.zone]++
+		d.retryAt[q.zone] = now.Add(failureDelay)
+	}
+
+	d.mu.Unlock()
+
+	if err != nil {
+		const failed = "asking a DNSBL zone failed"
+
+		// Raised before it is logged, so that the alert is there once the
+		// log line is.
+		d.params.Alerts.Raise(alerts.Alert{
+			Event:  alerts.EventSourceFailure,
+			Reason: failed,
+			Detail: map[string]any{"source": q.zone, "error": err.Error()},
+		})
+		d.params.ProcessLog.Warn(failed, "zone", q.zone, "error", err.Error())
+	}
+}
+
+// lookUp asks q's zone about q's client through the resolver, and returns
+// whether the zone lists it, as readAnswer reads the answer. No such name
+// is a client the zone does not list. A query not answered within Timeout
+// fails.
+func (d *DNSBL) lookUp(ctx context.Context, q query) (bool, error) {
+	ctx, cancel := context.WithTimeout(ctx, d.params.Timeout)
+	defer cancel()
+
+	answer, err := d.resolver.LookupNetIP(ctx, "ip4", queryName(q.zone, q.client))
+
+	var dnsErr *net.DNSError
+
+	switch {
+	case err == nil:
+		return readAnswer(answer)
+	case errors.As(err, &dnsErr) && dnsErr.IsNotFound:
+		return false, nil
+	case errors.As(err, &dnsErr):
+		// The error names the name asked about, which holds the client's
+		// address, which is not to be logged: only what went wrong is kept.
+		return false, fmt.Errorf("%w: %s", errAsk, dnsErr.Err)
+	default:
+		return false, fmt.Errorf("%w: %w", errAsk, err)
+	}
+}
+
+// queryName returns the name a zone is asked about addr by, as RFC 5782
+// builds it: the four numbers of an IPv4 address, or the 32 hex digits of
+// an IPv6 address, in reverse order, each followed by a dot, then the zone
+// and a dot, which makes it a full name, to which the resolver adds no
+// search domain of /etc/resolv.conf.
+func queryName(zone string, addr netip.Addr) string {
+	parts := strings.Split(addr.String(), ".")
+	if addr.Is6() {
+		parts = strings.Split(hex.EncodeToString(addr.AsSlice()), "")
+	}
+
+	slices.Reverse(parts)
+
+	return strings.Join(parts, ".") + "." + zone + "."
+}
+
+// readAnswer reads the addresses a zone answered with. An address in
+// 127.0.0.0/8 lists the client, as RFC 5782 has zones answer, but one in
+// 127.255.255.0/24 is how Spamhaus refuses a query, such as one sent
+// through a public resolver or one past its limit, and is a failure. So is
+// an address outside 127.0.0.0/8, such as a resolver gives that answers
+// even for names that do not exist.
+func readAnswer(answer []netip.Addr) (bool, error) {
+	listing := netip.MustParsePrefix("127.0.0.0/8")
+	refusal := netip.MustParsePrefix("127.255.255.0/24")
+
+	for _, addr := range answer {
+		switch {
+		case refusal.Contains(addr):
+			return false, fmt.Errorf("%w: %s", errRefused, addr)
+		case !listing.Contains(addr):
+			return false, fmt.Errorf("%w: %s", errNotListing, addr)
+		}
+	}
+
+	return len(answer) > 0, nil
+}

internal/reputation/dnsbl_test.go

@@ -0,0 +1,675 @@
+package reputation_test
+
+import (
+	"bytes"
+	"context"
+	"encoding/binary"
+	"errors"
+	"io"
+	"log/slog"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"reflect"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+	"testing/synctest"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/alerts"
+	"sneak.berlin/go/smallwebwaf/internal/metrics"
+	"sneak.berlin/go/smallwebwaf/internal/reputation"
+)
+
+// The tests of the DNSBL zones run in synctest bubbles, as those of the
+// lists do, and the resolver the zones are asked through is a stand-in
+// reached through an in-memory connection, net.Pipe's, for the same
+// reason. They run one at a time, none in parallel with another test of
+// this package: Go's resolver counts the queries under way in one
+// sync.WaitGroup for the whole process, and the process fails when
+// queries from two bubbles, or from a bubble and from outside one, are
+// under way at once. TestMain has the resolver make its configuration,
+// which it makes on its first query, outside every bubble, since the
+// configuration holds a channel, which the bubble it was made in would
+// keep to itself.
+
+const (
+	// zone and otherZone are the DNSBL zones the tests name.
+	zone      = "dnsbl.example"
+	otherZone = "other.example"
+	// cacheTTL is the tests' SWWAF_REPUTATION_CACHE_TTL, and timeout their
+	// SWWAF_REPUTATION_TIMEOUT: a second, the least time /etc/resolv.conf
+	// can have Go's resolver wait for one server, so that it is the
+	// DNSBL's own timeout that ends a query, whatever that file says.
+	cacheTTL = 24 * time.Hour
+	timeout  = time.Second
+	// listed and unlisted are clients zone is asked about by the names
+	// listedName and unlistedName, and most tests have zone list the first
+	// alone, by answering with listing.
+	listed       = "192.0.2.99"
+	unlisted     = "192.0.2.100"
+	listedName   = "99.2.0.192." + zone + "."
+	unlistedName = "100.2.0.192." + zone + "."
+	listing      = "127.0.0.2"
+)
+
+// The DNS response codes the stand-in answers with, besides no error.
+const (
+	serverFailure = 2
+	noSuchName    = 3
+	refused       = 5
+)
+
+var errNoNetwork = errors.New("the test dials nothing")
+
+func TestMain(m *testing.M) {
+	// A query that fails at once, as nothing is dialled for it.
+	resolver := &net.Resolver{
+		PreferGo: true,
+		Dial: func(context.Context, string, string) (net.Conn, error) {
+			return nil, errNoNetwork
+		},
+	}
+	_, _ = resolver.LookupNetIP(context.Background(), "ip4", "warm-up.invalid.")
+
+	m.Run()
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestZonesListOrNotClientsByTheirIPv4AndIPv6Addresses(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		// The addresses of the examples of RFC 5782, and the names it
+		// gives for them.
+		const (
+			v4 = "192.0.2.99"
+			v6 = "2001:db8:1:2:3:4:567:89ab"
+			// v6Name is the hex digits of v6, in reverse order.
+			v6Name = "b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2."
+		)
+
+		resolver := &resolverStandIn{answers: map[string]answer{
+			"99.2.0.192." + zone + ".":      {addrs: []string{listing}},
+			v6Name + otherZone + ".":        {addrs: []string{"127.0.0.4", "127.0.0.10"}},
+			"99.2.0.192." + otherZone + ".": {},
+		}}
+		dnsbl := newDNSBL(resolver, dnsblParams(zone, otherZone))
+
+		// Neither client has a verdict yet, so neither is listed, and each
+		// zone is asked about each.
+		wantZones(t, dnsbl, v4)
+		wantZones(t, dnsbl, v6)
+		synctest.Wait()
+
+		wantZones(t, dnsbl, v4, zone)
+		wantZones(t, dnsbl, v6, otherZone)
+		wantAsked(t, resolver,
+			"99.2.0.192."+zone+".", "99.2.0.192."+otherZone+".",
+			v6Name+zone+".", v6Name+otherZone+".")
+	})
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestListedByNeverWaitsForAQuery(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		dnsbl := newDNSBL(&resolverStandIn{hanging: true}, dnsblParams(zone))
+		began := time.Now()
+
+		// The second, while the first's query is under way, starts none.
+		wantZones(t, dnsbl, listed)
+		wantZones(t, dnsbl, listed)
+
+		if waited := time.Since(began); waited != 0 {
+			t.Errorf("waited %s for the query, want no wait", waited)
+		}
+
+		synctest.Wait()
+		wantQueries(t, dnsbl, 1, 0)
+		waitForTheResolver()
+	})
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestVerdictUsedUntilTheCacheTTLHasPassedSinceItWasFetched(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		resolver := &resolverStandIn{answers: map[string]answer{
+			listedName: {addrs: []string{listing}},
+		}}
+		dnsbl := newDNSBL(resolver, dnsblParams(zone))
+
+		wantZones(t, dnsbl, listed)
+		wantZones(t, dnsbl, unlisted)
+		synctest.Wait()
+
+		// The zone lists the other client from now on, but the verdicts
+		// kept are used, and the zone is not asked again, until the TTL
+		// has passed.
+		resolver.set(listedName, answer{rcode: noSuchName})
+		resolver.set(unlistedName, answer{addrs: []string{listing}})
+		time.Sleep(cacheTTL - time.Nanosecond)
+
+		wantZones(t, dnsbl, listed, zone)
+		wantZones(t, dnsbl, unlisted)
+		synctest.Wait()
+		wantQueries(t, dnsbl, 2, 0)
+
+		// Then neither verdict is used, and both clients are asked about
+		// again.
+		time.Sleep(time.Nanosecond)
+		wantZones(t, dnsbl, listed)
+		wantZones(t, dnsbl, unlisted)
+		synctest.Wait()
+
+		wantQueries(t, dnsbl, 4, 0)
+		wantZones(t, dnsbl, listed)
+		wantZones(t, dnsbl, unlisted, zone)
+	})
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestQueryNotAnsweredWithinTheTimeoutFails(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		queue := newQueue()
+		p := dnsblParams(zone)
+		p.Alerts = queue
+		dnsbl := newDNSBL(&resolverStandIn{hanging: true}, p)
+
+		wantZones(t, dnsbl, listed)
+		time.Sleep(timeout - time.Nanosecond)
+		synctest.Wait()
+		wantQueries(t, dnsbl, 1, 0)
+
+		time.Sleep(time.Nanosecond)
+		synctest.Wait()
+		wantQueries(t, dnsbl, 1, 1)
+
+		if got := waiting(queue); len(got) != 1 ||
+			got[0].Detail["error"] != "ask the zone: i/o timeout" {
+			t.Errorf("alerts waiting %+v, want the timeout's", got)
+		}
+
+		if verdicts := dnsbl.Snapshot(); len(verdicts) != 0 {
+			t.Errorf("verdicts %+v, want none", verdicts)
+		}
+
+		waitForTheResolver()
+	})
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestZoneThatFailsOrRefusesGivesNoVerdictAndIsLeftAloneForAMinute(t *testing.T) {
+	for _, tc := range []struct {
+		name   string
+		answer answer
+		error  string
+	}{
+		{
+			"a server failure", answer{rcode: serverFailure},
+			"ask the zone: server misbehaving",
+		},
+		{"a refusal", answer{rcode: refused}, "ask the zone: server misbehaving"},
+		{
+			"an answer in 127.255.255.0/24, with which Spamhaus refuses a query",
+			answer{addrs: []string{"127.255.255.254"}},
+			"the zone refused the query: 127.255.255.254",
+		},
+		{
+			"an answer outside 127.0.0.0/8, as for a name that does not exist",
+			answer{addrs: []string{"192.0.2.1"}},
+			"the answer is outside 127.0.0.0/8: 192.0.2.1",
+		},
+	} {
+		//nolint:paralleltest // one at a time, as the comment at the top of this file says
+		t.Run(tc.name, func(t *testing.T) {
+			synctest.Test(t, func(t *testing.T) {
+				var log bytes.Buffer
+
+				queue := newQueue()
+				p := dnsblParams(zone)
+				p.Alerts = queue
+				p.ProcessLog = slog.New(slog.NewJSONHandler(&log, nil))
+				dnsbl := newDNSBL(&resolverStandIn{answers: map[string]answer{
+					listedName: tc.answer,
+				}}, p)
+
+				// The failure gives no verdict, and the zone is not asked
+				// again within a minute of it.
+				wantZones(t, dnsbl, listed)
+				synctest.Wait()
+				time.Sleep(time.Minute - time.Nanosecond)
+				wantZones(t, dnsbl, listed)
+				synctest.Wait()
+				wantQueries(t, dnsbl, 1, 1)
+
+				time.Sleep(time.Nanosecond)
+				wantZones(t, dnsbl, listed)
+				synctest.Wait()
+				wantQueries(t, dnsbl, 2, 2)
+
+				if verdicts := dnsbl.Snapshot(); len(verdicts) != 0 {
+					t.Errorf("verdicts %+v, want none", verdicts)
+				}
+
+				// One alert for the first failure; the cooldown holds back
+				// the second.
+				wantAlert(t, queue, alerts.Alert{
+					Time:   time.Now().Add(-time.Minute),
+					Event:  alerts.EventSourceFailure,
+					Reason: "asking a DNSBL zone failed",
+					Detail: map[string]any{"source": zone, "error": tc.error},
+				})
+
+				if !strings.Contains(log.String(), `"msg":"asking a DNSBL zone failed",`+
+					`"zone":"`+zone+`","error":"`+tc.error) {
+					t.Errorf("logged\n%s\nwant the failures", log.String())
+				}
+			})
+		})
+	}
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestAtMost1000QueriesUnderWay(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		dnsbl := newDNSBL(&resolverStandIn{hanging: true}, dnsblParams(zone))
+
+		client := netip.MustParseAddr("198.18.0.0")
+		for range 1001 {
+			dnsbl.ListedBy(t.Context(), client)
+			client = client.Next()
+		}
+
+		synctest.Wait()
+		wantQueries(t, dnsbl, 1000, 0)
+		waitForTheResolver()
+	})
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestMetricsCountEachZonesQueriesAndThoseThatFailed(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		resolver := &resolverStandIn{answers: map[string]answer{
+			listedName:                      {addrs: []string{listing}},
+			"99.2.0.192." + otherZone + ".": {rcode: serverFailure},
+		}}
+		dnsbl := newDNSBL(resolver, dnsblParams(zone, otherZone))
+		m := metrics.New(1, "app")
+		m.AddReputation(reputation.New(params()), dnsbl)
+
+		wantZones(t, dnsbl, listed)
+		synctest.Wait()
+
+		scraped := httptest.NewRecorder()
+		m.ServeHTTP(scraped, httptest.NewRequestWithContext(t.Context(), http.MethodGet,
+			"/", http.NoBody))
+
+		for series, want := range map[string]string{
+			"queries_total" + `{instance="app",source="` + zone + `"}`:       "1",
+			"failures_total" + `{instance="app",source="` + zone + `"}`:      "0",
+			"queries_total" + `{instance="app",source="` + otherZone + `"}`:  "1",
+			"failures_total" + `{instance="app",source="` + otherZone + `"}`: "1",
+		} {
+			line := "\nsmallwebwaf_reputation_" + series + " " + want + "\n"
+			if !strings.Contains(scraped.Body.String(), line) {
+				t.Errorf("metrics\n%s\nwant%s", scraped.Body.String(), line)
+			}
+		}
+	})
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestVerdictsKeptAcrossARestart(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		resolver := &resolverStandIn{answers: map[string]answer{
+			listedName: {addrs: []string{listing}},
+		}}
+		dnsbl := newDNSBL(resolver, dnsblParams(zone))
+		fetched := time.Now()
+
+		wantZones(t, dnsbl, unlisted)
+		wantZones(t, dnsbl, listed)
+		synctest.Wait()
+
+		kept := dnsbl.Snapshot()
+
+		want := []reputation.Verdict{
+			{Zone: zone, Client: netip.MustParseAddr(listed), Listed: true, Fetched: fetched},
+			{Zone: zone, Client: netip.MustParseAddr(unlisted), Fetched: fetched},
+		}
+		if !reflect.DeepEqual(kept, want) {
+			t.Errorf("verdicts %+v, want %+v", kept, want)
+		}
+
+		// Restarted an hour later with what reputation.json keeps, it uses
+		// the verdicts, and asks the zone nothing, until the TTL has passed
+		// since they were fetched.
+		time.Sleep(time.Hour)
+
+		restarted := &resolverStandIn{}
+		again := newDNSBL(restarted, dnsblParams(zone))
+		again.Load(kept)
+
+		wantZones(t, again, listed, zone)
+		wantZones(t, again, unlisted)
+		synctest.Wait()
+		wantAsked(t, restarted)
+
+		time.Sleep(cacheTTL - time.Hour)
+		wantZones(t, again, listed)
+		synctest.Wait()
+		wantAsked(t, restarted, listedName)
+	})
+}
+
+func TestLoadKeepsTheNewestVerdictsOfTheZonesNamedWithinTheirTTL(t *testing.T) {
+	t.Parallel()
+
+	now := time.Date(2026, 10, 7, 0, 0, 0, 0, time.UTC)
+	p := dnsblParams(zone)
+	p.Now = func() time.Time { return now }
+	dnsbl := reputation.NewDNSBL(p)
+
+	// Of a zone not named, and fetched a TTL ago.
+	notNamed := reputation.Verdict{
+		Zone: otherZone, Client: netip.MustParseAddr(listed), Listed: true, Fetched: now,
+	}
+	stale := reputation.Verdict{
+		Zone: zone, Client: netip.MustParseAddr(listed), Listed: true,
+		Fetched: now.Add(-cacheTTL),
+	}
+	// Then 100,001 verdicts, each fetched a millisecond after the one
+	// before, the last now: the first is one too many.
+	const count = 100001
+
+	verdicts := make([]reputation.Verdict, 0, 2+count)
+	verdicts = append(verdicts, notNamed, stale)
+
+	client := netip.MustParseAddr("198.18.0.0")
+	for i := range count {
+		verdicts = append(verdicts, reputation.Verdict{
+			Zone: zone, Client: client,
+			Fetched: now.Add(time.Duration(i-count+1) * time.Millisecond),
+		})
+		client = client.Next()
+	}
+
+	dnsbl.Load(verdicts)
+
+	got := dnsbl.Snapshot()
+	if len(got) != count-1 || slices.Contains(got, verdicts[2]) ||
+		!slices.Contains(got, verdicts[3]) || !slices.Contains(got, verdicts[count+1]) {
+		t.Errorf("%d verdicts kept, want all but the one fetched first", len(got))
+	}
+
+	if slices.Contains(got, notNamed) || slices.Contains(got, stale) {
+		t.Error("the verdict of a zone not named, or past its TTL, was kept")
+	}
+}
+
+//nolint:paralleltest // one at a time, as the comment at the top of this file says
+func TestQueriesGoToTheResolverSWWAFDNSBLResolverNames(t *testing.T) {
+	resolver := &resolverStandIn{answers: map[string]answer{
+		listedName: {addrs: []string{listing}},
+	}}
+
+	conn, err := (&net.ListenConfig{}).ListenPacket(t.Context(), "udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+
+	served := make(chan struct{})
+
+	go func() {
+		resolver.serveUDP(conn)
+		close(served)
+	}()
+
+	t.Cleanup(func() {
+		_ = conn.Close()
+
+		<-served
+	})
+
+	p := dnsblParams(zone)
+	p.Resolver = netip.MustParseAddrPort(conn.LocalAddr().String())
+	// On the real clock: the stand-in answers at once, so only a test
+	// process held up for a whole minute would see the query fail.
+	p.Timeout = time.Minute
+
+	isListed, err := reputation.NewDNSBL(p).LookUp(zone, netip.MustParseAddr(listed))
+	if err != nil || !isListed {
+		t.Errorf("listed %t (%v), want true", isListed, err)
+	}
+
+	wantAsked(t, resolver, listedName)
+}
+
+// resolverStandIn is a stand-in for the resolver the zones are asked
+// through. It answers each query by the name asked about, as answers
+// gives, with no such name for a name answers does not give, and not at
+// all while hanging. It notes each name asked about.
+type resolverStandIn struct {
+	mu      sync.Mutex
+	answers map[string]answer
+	hanging bool
+	names   []string
+}
+
+// answer is how the stand-in answers a name: with an A record of each of
+// addrs, or with the response code rcode, unless it is 0, for no error.
+type answer struct {
+	addrs []string
+	rcode uint16
+}
+
+// What the stand-in reads of a query, and writes in its reply.
+const (
+	// headerLength is the length of a DNS message's header, which the
+	// question follows: its id, its flags, and how many questions,
+	// answers and other records it holds, two bytes each.
+	headerLength = 12
+	// typeAndClass is the length of the type and the class that end a
+	// question, after its name.
+	typeAndClass = 4
+	// replyFlags mark a reply to a query that asked for recursion, which
+	// is available, with no error. The response code goes in their last
+	// four bits.
+	replyFlags = 0x8180
+	// maxMessage is the longest query read over UDP.
+	maxMessage = 1232
+)
+
+// set has the stand-in answer name with given.
+func (s *resolverStandIn) set(name string, given answer) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.answers[name] = given
+}
+
+// dial connects Go's resolver to the stand-in through an in-memory
+// connection, on which it sends each query, and reads each reply, after
+// its length, as over TCP.
+func (s *resolverStandIn) dial(context.Context, string, string) (net.Conn, error) {
+	client, server := net.Pipe()
+
+	go s.serve(server)
+
+	return client, nil
+}
+
+// serve answers the queries that come on conn until the resolver closes
+// it.
+func (s *resolverStandIn) serve(conn net.Conn) {
+	defer func() {
+		_ = conn.Close()
+	}()
+
+	for {
+		var length [2]byte
+
+		_, err := io.ReadFull(conn, length[:])
+		if err != nil {
+			return
+		}
+
+		message := make([]byte, binary.BigEndian.Uint16(length[:]))
+
+		_, err = io.ReadFull(conn, message)
+		if err != nil {
+			return
+		}
+
+		reply, answered := s.reply(message)
+		if !answered {
+			continue // the resolver gives up, and closes conn
+		}
+
+		//nolint:gosec // a reply of a few dozen bytes
+		_, err = conn.Write(append(binary.BigEndian.AppendUint16(nil, uint16(len(reply))),
+			reply...))
+		if err != nil {
+			return
+		}
+	}
+}
+
+// serveUDP answers the queries that come on conn, each in a datagram, as
+// a resolver does, until conn is closed.
+func (s *resolverStandIn) serveUDP(conn net.PacketConn) {
+	message := make([]byte, maxMessage)
+
+	for {
+		n, from, err := conn.ReadFrom(message)
+		if err != nil {
+			return
+		}
+
+		reply, answered := s.reply(message[:n])
+		if answered {
+			_, _ = conn.WriteTo(reply, from)
+		}
+	}
+}
+
+// reply returns the stand-in's reply to message, a query, and false for
+// none, while it hangs. It notes the name asked about.
+func (s *resolverStandIn) reply(message []byte) ([]byte, bool) {
+	// The name is labels, each after its length, ended by a length of 0.
+	var labels []string
+
+	end := headerLength
+	for message[end] != 0 {
+		length := int(message[end])
+		labels = append(labels, string(message[end+1:end+1+length]))
+		end += 1 + length
+	}
+
+	end += 1 + typeAndClass
+	name := strings.Join(labels, ".") + "."
+
+	s.mu.Lock()
+	s.names = append(s.names, name)
+	given, found := s.answers[name]
+	hanging := s.hanging
+	s.mu.Unlock()
+
+	if hanging {
+		return nil, false
+	}
+
+	if !found {
+		given = answer{rcode: noSuchName}
+	}
+
+	// The query's id, the flags, one question, the answers, and no other
+	// records, then the question, as asked.
+	reply := slices.Clone(message[:2])
+	reply = binary.BigEndian.AppendUint16(reply, replyFlags|given.rcode)
+	reply = binary.BigEndian.AppendUint16(reply, 1)
+	//nolint:gosec // a handful of answers
+	reply = binary.BigEndian.AppendUint16(reply, uint16(len(given.addrs)))
+	reply = append(reply, 0, 0, 0, 0)
+	reply = append(reply, message[headerLength:end]...)
+
+	// An A record starts with the name asked about, by a pointer to it in
+	// the question, then its type, A, its class, IN, how long it may be
+	// kept, 60 seconds, and the length of its address, 4 bytes.
+	record := []byte{0xc0, headerLength, 0, 1, 0, 1, 0, 0, 0, 60, 0, 4}
+
+	for _, addr := range given.addrs {
+		reply = append(reply, record...)
+		reply = append(reply, netip.MustParseAddr(addr).AsSlice()...)
+	}
+
+	return reply, true
+}
+
+// dnsblParams returns the DNSBLParams of zones, with the tests' cache TTL
+// and timeout, by the bubble's clock, with alerts to a queue that sends
+// none.
+func dnsblParams(zones ...string) reputation.DNSBLParams {
+	return reputation.DNSBLParams{
+		Zones:      zones,
+		CacheTTL:   cacheTTL,
+		Timeout:    timeout,
+		Now:        time.Now,
+		ProcessLog: slog.New(slog.DiscardHandler),
+		Alerts:     newQueue(),
+	}
+}
+
+// waitForTheResolver waits, on the bubble's clock, an hour, until Go's
+// resolver has given up on every stand-in that does not answer: it waits
+// for a server as long as /etc/resolv.conf has it wait, a few seconds,
+// even after the query was given up, and a bubble cannot end before it.
+func waitForTheResolver() {
+	time.Sleep(time.Hour)
+}
+
+// newDNSBL returns the DNSBL of p, asking resolver.
+func newDNSBL(resolver *resolverStandIn, p reputation.DNSBLParams) *reputation.DNSBL {
+	dnsbl := reputation.NewDNSBL(p)
+	dnsbl.SetDial(resolver.dial)
+
+	return dnsbl
+}
+
+// wantZones checks the zones whose verdict dnsbl says lists client, as a
+// request from client finds them.
+func wantZones(t *testing.T, dnsbl *reputation.DNSBL, client string, want ...string) {
+	t.Helper()
+
+	got := dnsbl.ListedBy(t.Context(), netip.MustParseAddr(client))
+	if !slices.Equal(got, want) {
+		t.Errorf("%s is listed by %v, want %v", client, got, want)
+	}
+}
+
+// wantQueries checks how many queries dnsbl made to zone, and how many of
+// them failed.
+func wantQueries(t *testing.T, dnsbl *reputation.DNSBL, queries, failures int) {
+	t.Helper()
+
+	if dnsbl.Queries(zone) != queries || dnsbl.Failures(zone) != failures {
+		t.Errorf("%d queries and %d failures, want %d and %d", dnsbl.Queries(zone),
+			dnsbl.Failures(zone), queries, failures)
+	}
+}
+
+// wantAsked checks the names the stand-in was asked about, in any order.
+func wantAsked(t *testing.T, resolver *resolverStandIn, want ...string) {
+	t.Helper()
+
+	resolver.mu.Lock()
+	got := slices.Sorted(slices.Values(resolver.names))
+	resolver.mu.Unlock()
+
+	slices.Sort(want)
+
+	if !slices.Equal(got, want) {
+		t.Errorf("asked about %v, want %v", got, want)
+	}
+}

internal/reputation/export_test.go

@@ -1,9 +1,27 @@
 package reputation
 
-import "net/http"
+import (
+	"context"
+	"net"
+	"net/http"
+	"net/netip"
+)
 
 // SetTransport has l's fetches go through transport instead of the
 // network.
 func (l *Lists) SetTransport(transport http.RoundTripper) {
 	l.httpClient.Transport = transport
 }
+
+// SetDial has d's queries go through dial instead of the network.
+func (d *DNSBL) SetDial(
+	dial func(ctx context.Context, network, address string) (net.Conn, error),
+) {
+	d.resolver = &net.Resolver{PreferGo: true, Dial: dial}
+}
+
+// LookUp asks zone about addr at once, as a query in the background does,
+// and returns whether zone lists addr.
+func (d *DNSBL) LookUp(zone string, addr netip.Addr) (bool, error) {
+	return d.lookUp(context.Background(), query{zone: zone, client: addr})
+}

internal/reputation/reputation.go

@@ -2,8 +2,10 @@
 // blocklists of SWWAF_BLOCKLIST_URLS, and the file of AS:percent lines
 // SWWAF_ASN_LIMIT_PERCENT_URL names. It keeps the last good copy of each,
 // whole, comment lines included, which is used while a fetch fails, and
-// when each was last tried, which the state package writes to
-// reputation.json and reads from it, so that a restart keeps them too.
+// when each was last tried. It also asks the DNSBL zones of
+// SWWAF_DNSBL_ZONES about clients, and keeps their verdicts. The state
+// package writes all of these to reputation.json and reads them from it,
+// so that a restart keeps them too.
 package reputation
 
 import (

internal/requestlog/requestlog.go

@@ -35,8 +35,8 @@ const (
 	// rule.
 	ActionRuleBlocked = "rule_blocked"
 	// ActionDenied is a request refused because its client is in
-	// SWWAF_DENY_NETS, or in a blocklist while SWWAF_BLOCKLIST_ACTION is
-	// deny.
+	// SWWAF_DENY_NETS, in a blocklist while SWWAF_BLOCKLIST_ACTION is deny,
+	// or listed by a DNSBL zone while SWWAF_REPUTATION_ACTION is deny.
 	ActionDenied = "denied"
 	// ActionCountryDenied is a request refused for its client's country.
 	ActionCountryDenied = "country_denied"
@@ -138,7 +138,8 @@ type Line struct {
 	// Counts names its count: minute, hour or day for a rate limit, and
 	// minute_bytes, hour_bytes or day_bytes for a byte limit.
 	LimitHit string `json:"limit_hit,omitempty"`
-	// Reputation are the URLs of the blocklists that list the client.
+	// Reputation are the URLs of the blocklists that list the client, then
+	// the DNSBL zones whose verdict lists it.
 	Reputation []string `json:"reputation,omitempty"`
 	// Offence is the offence the request was held as, OffenceLimit.
 	Offence string `json:"offence,omitempty"`

internal/smallwebwaf/smallwebwaf.go

@@ -201,6 +201,7 @@ func loadStateFiles(
 		Limiter:         server.Limiter,
 		GeoJS:           server.GeoJS,
 		Lists:           server.Lists,
+		DNSBL:           server.DNSBL,
 		Alerts:          alertQueue,
 		Anomalies:       server.Anomalies,
 		Now:             now,

internal/state/state.go

@@ -2,7 +2,8 @@
 // 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, lookups.json GeoJS's answers, reputation.json the last try and
-// last good copy of each list fetched from a URL, and alerts.json the
+// last good copy of each list fetched from a URL and the DNSBL zones'
+// verdicts, and alerts.json the
 // cooldowns, the hour under way, the alerts waiting for each destination
 // and the anomaly counters. Load
 // reads them at start, Watch takes in an admin's edit of one while
@@ -76,13 +77,14 @@ type Params struct {
 	// is (SWWAF_STATE_COUNTER_INTERVAL).
 	WriteDelay      time.Duration
 	CounterInterval time.Duration
-	// Ledger, Limiter, GeoJS, Lists, Alerts and Anomalies hold the state.
-	// Alerts also receive a file_error alert for an edit set aside, and for
-	// a write that fails while smallwebwaf runs.
+	// Ledger, Limiter, GeoJS, Lists, DNSBL, Alerts and Anomalies hold the
+	// state. Alerts also receive a file_error alert for an edit set aside,
+	// and for a write that fails while smallwebwaf runs.
 	Ledger    *bans.Ledger
 	Limiter   *ratelimit.Limiter
 	GeoJS     *lookup.GeoJS
 	Lists     *reputation.Lists
+	DNSBL     *reputation.DNSBL
 	Alerts    *alerts.Queue
 	Anomalies *anomaly.Counters
 	// Now tells the time by which the counters' buckets run out, normally
@@ -145,8 +147,9 @@ type lookupsFile struct {
 // reputationFile is reputation.json, indented for an admin to read and
 // edit, so that each line of a list's copy is on a line of its own.
 type reputationFile struct {
-	Version int               `json:"version"`
-	Lists   []reputation.List `json:"lists"`
+	Version  int                  `json:"version"`
+	Lists    []reputation.List    `json:"lists"`
+	Verdicts []reputation.Verdict `json:"verdicts"`
 }
 
 // alertsFile is alerts.json, indented for an admin to read and edit.
@@ -432,6 +435,7 @@ func (f *Files) takeIn(name string, data []byte, edit bool) (int, error) {
 			return 0, fmt.Errorf("%s: %w", path, err)
 		}
 
+		f.params.DNSBL.Load(file.Verdicts)
 		entries = len(file.Lists)
 	case alertsJSON:
 		waiting, err := f.takeInAlerts(path, data)
@@ -567,6 +571,7 @@ func (f *Files) encode(name string) ([]byte, error) {
 	case reputationJSON:
 		return encodeIndented(reputationFile{
 			Version: version, Lists: f.params.Lists.Snapshot(),
+			Verdicts: f.params.DNSBL.Snapshot(),
 		})
 	default: // alerts.json
 		held := f.params.Alerts.Snapshot()
@@ -729,8 +734,12 @@ func (f *lookupsFile) check(data []byte) error {
 // check refuses a list without its URL, which would name no list, or the
 // time it was last tried, which would have it fetched at once, and a copy
 // of it without the time it was fetched, or without its lines, which hold
-// the list.
-func (f *reputationFile) check([]byte) error {
+// the list. It refuses a verdict without its zone or its client, which
+// would be about no one, whether the zone lists the client, or the time
+// it was fetched, which would drop it. A verdict's listed is false for a
+// client the zone does not list, which Verdicts cannot tell from a
+// missing one, so each listed is read again as written.
+func (f *reputationFile) check(data []byte) error {
 	for i, kept := range f.Lists {
 		switch {
 		case kept.URL == "":
@@ -744,6 +753,30 @@ func (f *reputationFile) check([]byte) error {
 		}
 	}
 
+	var written struct {
+		Verdicts []struct {
+			Listed *bool `json:"listed"`
+		} `json:"verdicts"`
+	}
+
+	err := json.Unmarshal(data, &written)
+	if err != nil {
+		return err
+	}
+
+	for i, verdict := range f.Verdicts {
+		switch {
+		case verdict.Zone == "":
+			return fmt.Errorf("verdicts %w", missing(i, "zone"))
+		case !verdict.Client.IsValid():
+			return fmt.Errorf("verdicts %w", missing(i, "client"))
+		case written.Verdicts[i].Listed == nil:
+			return fmt.Errorf("verdicts %w", missing(i, "listed"))
+		case verdict.Fetched.IsZero():
+			return fmt.Errorf("verdicts %w", missing(i, "fetched"))
+		}
+	}
+
 	return nil
 }
 

internal/state/state_test.go

@@ -38,9 +38,11 @@ const (
 	lookupsJSON    = "lookups.json"
 	reputationJSON = "reputation.json"
 	alertsJSON     = "alerts.json"
-	// blocklistURL and torURL are the blocklists the tests' lists name.
+	// blocklistURL and torURL are the blocklists the tests' lists name, and
+	// dnsblZone the DNSBL zone of the tests' verdicts.
 	blocklistURL = "https://lists.example/drop.txt"
 	torURL       = "https://lists.example/tor.txt"
+	dnsblZone    = "dnsbl.example"
 	// The AS number and AS name the tests' clients are looked up in.
 	asn    = "AS64496"
 	asName = "Example Net"
@@ -210,7 +212,8 @@ const filledAlertsJSON = `{
 `
 
 // filledReputationJSON is reputation.json holding the blocklists' last
-// tries and the copy of one, with its comment line, as fill puts them in.
+// tries and the copy of one, with its comment line, and two verdicts of a
+// DNSBL zone, as fill puts them in.
 const filledReputationJSON = `{
     "version": 1,
     "lists": [
@@ -228,6 +231,20 @@ const filledReputationJSON = `{
             "url": "https://lists.example/tor.txt",
             "tried": "2026-10-06T00:00:00Z"
         }
+    ],
+    "verdicts": [
+        {
+            "zone": "dnsbl.example",
+            "client": "203.0.113.9",
+            "listed": true,
+            "fetched": "2026-10-05T23:00:00Z"
+        },
+        {
+            "zone": "dnsbl.example",
+            "client": "2001:db8::1",
+            "listed": false,
+            "fetched": "2026-10-05T22:00:00Z"
+        }
     ]
 }
 `
@@ -263,6 +280,8 @@ func TestFilesWrittenAndReadBack(t *testing.T) {
 		t.Errorf("%s read back\n%+v\nwant\n%+v", reputationJSON, got, want)
 	}
 
+	wantEqual(t, reputationJSON, after.DNSBL.Snapshot(), before.DNSBL.Snapshot())
+
 	if got, want := after.Alerts.Snapshot(), before.Alerts.Snapshot(); !reflect.DeepEqual(
 		got, want) {
 		t.Errorf("%s read back\n%+v\nwant\n%+v", alertsJSON, got, want)
@@ -422,8 +441,8 @@ func TestMissingFilesAreEmptyState(t *testing.T) {
 	held := params.Alerts.Snapshot()
 	if len(params.Ledger.Snapshot()) != 0 || len(params.Limiter.Snapshot()) != 0 ||
 		len(params.GeoJS.Snapshot()) != 0 || len(params.Lists.Snapshot()) != 0 ||
-		len(held.Cooldowns) != 0 || len(held.Waiting[alerts.DestinationWebhook]) != 0 ||
-		held.Hour.Sent != 0 {
+		len(params.DNSBL.Snapshot()) != 0 || len(held.Cooldowns) != 0 ||
+		len(held.Waiting[alerts.DestinationWebhook]) != 0 || held.Hour.Sent != 0 {
 		t.Error("state from no files")
 	}
 }
@@ -592,6 +611,11 @@ func TestReputationJSONEntryWithoutAFieldItNeedsStopsTheStart(t *testing.T) {
 		drop    = `"url": "` + blocklistURL + `", `
 		tried   = `"tried": "2026-10-06T00:00:00Z", `
 		fetched = `"fetched": "2026-10-06T00:00:00Z"`
+		// verdictZone, verdictClient and listed start a verdict, which
+		// fetched ends.
+		verdictZone   = `"zone": "` + dnsblZone + `", `
+		verdictClient = `"client": "198.51.100.7", `
+		listed        = `"listed": false, `
 	)
 
 	for _, tc := range []struct {
@@ -621,6 +645,30 @@ func TestReputationJSONEntryWithoutAFieldItNeedsStopsTheStart(t *testing.T) {
 				`{"url": "` + torURL + `", ` + tried + fetched + `}]}`,
 			`: entry 2 has no "lines"`,
 		},
+		{
+			"a verdict without its zone",
+			`{"version": 1, "verdicts": [{` + verdictClient + listed + fetched + `}]}`,
+			`: verdicts entry 1 has no "zone"`,
+		},
+		{
+			"a verdict without its client",
+			`{"version": 1, "verdicts": [{` + verdictZone + listed + fetched + `}]}`,
+			`: verdicts entry 1 has no "client"`,
+		},
+		{
+			// A client the zone does not list has a listed of false, which is
+			// not having none.
+			"a verdict without whether the zone lists the client",
+			`{"version": 1, "verdicts": [{` + verdictZone + verdictClient + listed +
+				fetched + `}, {` + verdictZone + verdictClient + fetched + `}]}`,
+			`: verdicts entry 2 has no "listed"`,
+		},
+		{
+			"a verdict without the time it was fetched",
+			`{"version": 1, "verdicts": [{` + verdictZone + verdictClient +
+				`"listed": true}]}`,
+			`: verdicts entry 1 has no "fetched"`,
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
@@ -1133,7 +1181,8 @@ func TestEditOfEachFileTakenIn(t *testing.T) {
 
 	edit(t, dir, reputationJSON, `{"version": 1, "lists": [{"url": "`+blocklistURL+`", `+
 		`"tried": "2026-10-06T00:00:00Z", "fetched": "2026-10-06T00:00:00Z", `+
-		`"lines": ["198.51.100.7"]}]}`)
+		`"lines": ["198.51.100.7"]}], "verdicts": [{"zone": "`+dnsblZone+`", `+
+		`"client": "198.51.100.7", "listed": true, "fetched": "2026-10-06T00:00:00Z"}]}`)
 	wantTakenIn(t, lines, dir, reputationJSON)
 
 	listedBy := params.Lists.ListedBy(client.Addr())
@@ -1142,6 +1191,10 @@ func TestEditOfEachFileTakenIn(t *testing.T) {
 			client.Addr(), listedBy)
 	}
 
+	wantEqual(t, reputationJSON, params.DNSBL.Snapshot(), []reputation.Verdict{{
+		Zone: dnsblZone, Client: client.Addr(), Listed: true, Fetched: midnight(),
+	}})
+
 	// A netblock with bits past its length is read as the netblock it is
 	// in.
 	edit(t, dir, alertsJSON, `{"version": 1, "cooldowns": [{"event": "ban", `+
@@ -1558,6 +1611,10 @@ func newParams(dir string) state.Params {
 			BlocklistURLs: []string{blocklistURL, torURL}, Refresh: 24 * time.Hour,
 			Now: midnight, ProcessLog: discard, Alerts: queue,
 		}),
+		DNSBL: reputation.NewDNSBL(reputation.DNSBLParams{
+			Zones: []string{dnsblZone}, CacheTTL: 24 * time.Hour, Timeout: time.Second,
+			Now: midnight, ProcessLog: discard, Alerts: queue,
+		}),
 		Alerts: queue,
 		Anomalies: anomaly.New(anomaly.Params{
 			Net:            anomaly.Thresholds{RequestsPerMinute: 1000},
@@ -1582,9 +1639,10 @@ func office() netip.Prefix {
 
 // fill puts a permanent ban an admin made, a ban for a broken limit and
 // one for a clear sign of attack, clients with counts and histories,
-// GeoJS answers, the blocklists' last tries and the copy of one, as
-// filledReputationJSON holds them, and alerts and anomaly counters, as
-// filledAlertsJSON holds them, into the parts of params.
+// GeoJS answers, the blocklists' last tries and the copy of one, and two
+// verdicts of a DNSBL zone, as filledReputationJSON holds them, and alerts
+// and anomaly counters, as filledAlertsJSON holds them, into the parts of
+// params.
 func fill(params state.Params) {
 	now := midnight()
 	client := netip.MustParsePrefix("203.0.113.9/32")
@@ -1631,6 +1689,14 @@ func fill(params state.Params) {
 		panic(err) // the copy reads
 	}
 
+	params.DNSBL.Load([]reputation.Verdict{
+		{
+			Zone: dnsblZone, Client: netip.MustParseAddr("2001:db8::1"),
+			Fetched: now.Add(-2 * time.Hour),
+		},
+		{Zone: dnsblZone, Client: client.Addr(), Listed: true, Fetched: now.Add(-time.Hour)},
+	})
+
 	// An alert waiting, a repeat of it the cooldown holds back, another
 	// alert waiting, and one past the two an hour, for the hour's summary.
 	ban := alerts.Alert{