Anomaly thresholds: alerts for unusual traffic, nothing refused (closes #101)

SWWAF_ANOMALY_CLIENT_*, _NET_*, _ASN_*, _TOTAL_* and SWWAF_WATCH_* with
SWWAF_WATCH_NETS: requests and bytes per minute and per hour, each off by
default; with all off, nothing is counted. Otherwise every request but the
health check is counted, allow-listed and exempt ones included; a count
over its threshold raises an anomaly alert, with a cooldown per scope. At
most 20,000 counters, kept in alerts.json. A per-AS-number threshold with
lookups off, or a malformed SWWAF_WATCH_NETS, stops the start. A cooldown
that has run out is dropped as the hour ends, whatever it held back; the
hour's summary gives its repeats.

Judgement call: refused requests are counted too.
Judgement call: per-client counters are kept in alerts.json, which SPEC.md does not list.
Judgement call: a request counts for an AS number only if the lookup answered before it ended.

Model: opus-5-5

internal/proxy/proxy.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"sneak.berlin/go/smallwebwaf/internal/alerts"
+	"sneak.berlin/go/smallwebwaf/internal/anomaly"
 	"sneak.berlin/go/smallwebwaf/internal/bans"
 	"sneak.berlin/go/smallwebwaf/internal/config"
 	"sneak.berlin/go/smallwebwaf/internal/lookup"
@@ -68,7 +69,8 @@ type Params struct {
 	// against.
 	Rules *rules.Files
 	// Alerts receive the alert for each ban the proxy makes or makes
-	// permanent, and for GeoJS failing.
+	// permanent, for each count over an anomaly threshold, and for GeoJS
+	// failing.
 	Alerts *alerts.Queue
 }
 
@@ -81,6 +83,7 @@ type Server struct {
 	Ledger     *bans.Ledger
 	Limiter    *ratelimit.Limiter
 	GeoJS      *lookup.GeoJS
+	Anomalies  *anomaly.Counters
 	LookupFile *lookup.File
 	Metrics    *metrics.Metrics
 }
@@ -117,6 +120,17 @@ func New(params Params) *Server {
 			AttackBanDuration:    params.Config.AttackBanDuration,
 			MaxBans:              params.Config.MaxBans,
 		}),
+		anomalies: anomaly.New(anomaly.Params{
+			Client:         params.Config.AnomalyClient,
+			Net:            params.Config.AnomalyNet,
+			ASN:            params.Config.AnomalyASN,
+			Total:          params.Config.AnomalyTotal,
+			Watch:          params.Config.AnomalyWatch,
+			NetV4Prefix:    params.Config.AnomalyNetV4Prefix,
+			NetV6Prefix:    params.Config.AnomalyNetV6Prefix,
+			NamedNetblocks: params.Config.WatchNets,
+			Alerts:         params.Alerts,
+		}),
 		lookupFile: params.LookupFile,
 		rules:      params.Rules,
 		alerts:     params.Alerts,
@@ -154,6 +168,7 @@ func New(params Params) *Server {
 		Ledger:     h.ledger,
 		Limiter:    h.limiter,
 		GeoJS:      h.geojs,
+		Anomalies:  h.anomalies,
 		LookupFile: h.lookupFile,
 		Metrics:    m,
 	}
@@ -172,6 +187,7 @@ type handler struct {
 	limiter    *ratelimit.Limiter
 	ledger     *bans.Ledger
 	geojs      *lookup.GeoJS
+	anomalies  *anomaly.Counters
 	lookupFile *lookup.File
 	rules      *rules.Files
 	alerts     *alerts.Queue
@@ -211,6 +227,7 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	// Once the request has ended, before its log line is written.
 	defer rq.addToHistory()
+	defer rq.countAnomalies()
 
 	refused := rq.check(r.Context())
 	rq.checked = time.Now()