feat(edge): community IP blocklist (Spamhaus DROP) at the gateway

Refuse a client whose IP is in a curated CIDR feed with 403 in the same
abuseGuard, before the fail2ban ban. Prod-only (keys by real client IP), off by
default.

- ratelimit.Blocklist: a sorted-range IPv4 matcher (binary search) with an
  allowlist checked first; ParseDROP reads the feed; ApplyRefresh keeps the
  last-good feed on a transient fetch failure and drops it fail-open once stale
  (better to under-block than block a legitimate client on a frozen feed). A
  separate static CIDR set, not the per-IP fail2ban store.
- gateway: a refresher goroutine re-fetches every few hours (bounded fetch + size
  cap); config GATEWAY_BLOCKLIST_{ENABLED,URL,ALLOW,REFRESH,MAX_STALENESS}.
  IPv6 is not matched (a v6 client is still covered by fail2ban / the honeypot).
- observability: gateway_blocklist_blocked_total + entries/age gauges; a Grafana
  alert warns before the feed is dropped; service-overview panels.
- deploy: compose env + write-prod-env.sh + prod-deploy/rollback wiring (opt-in
  via PROD_ vars).
- docs (ARCHITECTURE, deploy/README); unit tests (match, allowlist, IPv6 skip,
  parser, fault-tolerance) + a 403 abuse-guard integration test.

gateway/internal/connectsrv/abuse_test.go

@@ -2,6 +2,7 @@ package connectsrv_test
 
 import (
 	"context"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -24,7 +25,7 @@ const honeypotHeader = "X-Scrabble-Honeypot"
 
 // guardedEdge wires an edge with an explicit banlist and honeytoken over a fake
 // backend, returning the front URL, a Connect client and a cleanup func.
-func guardedEdge(t *testing.T, bl *ratelimit.Banlist, honeytoken string, limits config.RateLimitConfig, backendHandler http.HandlerFunc) (string, edgev1connect.GatewayClient, func()) {
+func guardedEdge(t *testing.T, bl *ratelimit.Banlist, block *ratelimit.Blocklist, honeytoken string, limits config.RateLimitConfig, backendHandler http.HandlerFunc) (string, edgev1connect.GatewayClient, func()) {
 	t.Helper()
 	backendSrv := httptest.NewServer(backendHandler)
 	backend, err := backendclient.New(backendSrv.URL, "localhost:9090", 2*time.Second)
@@ -36,6 +37,7 @@ func guardedEdge(t *testing.T, bl *ratelimit.Banlist, honeytoken string, limits
 		Sessions:   session.NewCache(backend, time.Minute, 100),
 		Limiter:    ratelimit.New(),
 		Banlist:    bl,
+		Blocklist:  block,
 		Honeytoken: honeytoken,
 		Hub:        push.NewHub(0),
 		RateLimit:  limits,
@@ -69,7 +71,7 @@ func enabledBanlist(threshold int) *ratelimit.Banlist {
 func TestAbuseGuardBlocksBannedIP(t *testing.T) {
 	bl := enabledBanlist(100)
 	bl.BanNow("127.0.0.1", ratelimit.ReasonTripwire)
-	url, _, cleanup := guardedEdge(t, bl, "", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {})
+	url, _, cleanup := guardedEdge(t, bl, nil, "", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {})
 	defer cleanup()
 
 	resp, err := noRedirect().Get(url + "/")
@@ -86,7 +88,7 @@ func TestAbuseGuardBlocksBannedIP(t *testing.T) {
 // and bans the client IP, so the next request is blocked.
 func TestHoneypotHeaderTrips(t *testing.T) {
 	bl := enabledBanlist(100)
-	url, _, cleanup := guardedEdge(t, bl, "", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {
+	url, _, cleanup := guardedEdge(t, bl, nil, "", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {
 		t.Error("backend must not be called for a honeypot hit")
 	})
 	defer cleanup()
@@ -118,7 +120,7 @@ func TestHoneypotHeaderTrips(t *testing.T) {
 // banlist still 404s the decoy (detection/logging) but bans nothing.
 func TestHoneypotDetectsWithoutBanWhenDisabled(t *testing.T) {
 	bl := ratelimit.NewBanlist(ratelimit.BanConfig{}) // disabled
-	url, _, cleanup := guardedEdge(t, bl, "", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {})
+	url, _, cleanup := guardedEdge(t, bl, nil, "", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {})
 	defer cleanup()
 
 	req, _ := http.NewRequest(http.MethodGet, url+"/.env", nil)
@@ -150,7 +152,7 @@ func TestPublicRejectionStrikesBan(t *testing.T) {
 	bl := enabledBanlist(1)
 	limits := config.DefaultRateLimit()
 	limits.PublicPerMinute, limits.PublicBurst = 1, 1
-	_, client, cleanup := guardedEdge(t, bl, "", limits, func(w http.ResponseWriter, r *http.Request) {
+	_, client, cleanup := guardedEdge(t, bl, nil, "", limits, func(w http.ResponseWriter, r *http.Request) {
 		_, _ = w.Write([]byte(`{"token":"tok","user_id":"u-1","is_guest":true,"display_name":"Guest"}`))
 	})
 	defer cleanup()
@@ -173,7 +175,7 @@ func TestUserRejectionDoesNotBan(t *testing.T) {
 	bl := enabledBanlist(1)
 	limits := config.DefaultRateLimit()
 	limits.UserPerMinute, limits.UserBurst = 1, 1
-	_, client, cleanup := guardedEdge(t, bl, "", limits, func(w http.ResponseWriter, r *http.Request) {
+	_, client, cleanup := guardedEdge(t, bl, nil, "", limits, func(w http.ResponseWriter, r *http.Request) {
 		switch r.URL.Path {
 		case "/api/v1/internal/sessions/resolve":
 			_, _ = w.Write([]byte(`{"user_id":"u-1","is_guest":false}`))
@@ -202,7 +204,7 @@ func TestUserRejectionDoesNotBan(t *testing.T) {
 // caller and returns the ordinary invalid-session error without a backend call.
 func TestHoneytokenBansAndRejects(t *testing.T) {
 	bl := enabledBanlist(100)
-	_, client, cleanup := guardedEdge(t, bl, "s3cr3t-trap", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {
+	_, client, cleanup := guardedEdge(t, bl, nil, "s3cr3t-trap", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {
 		t.Error("backend must not be called for the honeytoken")
 	})
 	defer cleanup()
@@ -217,3 +219,25 @@ func TestHoneytokenBansAndRejects(t *testing.T) {
 		t.Fatal("the honeytoken must ban the caller")
 	}
 }
+
+// TestAbuseGuardBlocksBlocklistedIP verifies a client IP in the community blocklist feed is refused
+// with 403 at the HTTP layer, before any handler runs.
+func TestAbuseGuardBlocksBlocklistedIP(t *testing.T) {
+	block := ratelimit.NewBlocklist(true, nil)
+	_, feed, err := net.ParseCIDR("127.0.0.0/8")
+	if err != nil {
+		t.Fatal(err)
+	}
+	block.SetCIDRs([]*net.IPNet{feed}, time.Now())
+	url, _, cleanup := guardedEdge(t, nil, block, "", config.DefaultRateLimit(), func(w http.ResponseWriter, r *http.Request) {})
+	defer cleanup()
+
+	resp, err := noRedirect().Get(url + "/")
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	_ = resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("blocklisted GET / = %d, want 403", resp.StatusCode)
+	}
+}

gateway/internal/connectsrv/metrics.go

@@ -7,6 +7,8 @@ import (
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/metric"
 	"go.opentelemetry.io/otel/metric/noop"
+
+	"scrabble/gateway/internal/ratelimit"
 )
 
 // meterName scopes the gateway edge's OpenTelemetry instruments.
@@ -27,6 +29,7 @@ type serverMetrics struct {
 	edge        metric.Float64Histogram
 	rateLimited metric.Int64Counter
 	banned      metric.Int64Counter
+	blocklisted metric.Int64Counter
 	active      *activeUsers
 	// Client-reported local move-preview adoption (see localEvalMetricsHandler).
 	localColdStart metric.Int64Counter
@@ -40,7 +43,7 @@ type serverMetrics struct {
 // falling back to a no-op histogram on the (rare) construction error. The
 // active_users gauge is registered as an observable callback over the in-memory
 // tracker.
-func newServerMetrics(meter metric.Meter) *serverMetrics {
+func newServerMetrics(meter metric.Meter, bl *ratelimit.Blocklist) *serverMetrics {
 	if meter == nil {
 		meter = noop.NewMeterProvider().Meter(meterName)
 	}
@@ -68,6 +71,8 @@ func newServerMetrics(meter metric.Meter) *serverMetrics {
 	}
 	m := &serverMetrics{
 		edge: h, rateLimited: c, banned: b, active: newActiveUsers(),
+		blocklisted: counterOf(meter, "gateway_blocklist_blocked_total",
+			"Requests refused at the edge by the community IP blocklist (Spamhaus DROP)."),
 		localColdStart: counterOf(meter, "local_eval_cold_start_total", "App cold starts reported by clients — the denominator for local-move-preview adoption."),
 		localDictLoad:  counterOf(meter, "local_eval_dict_load_total", "Client dictionary loads for the local move preview, by result (fetched, cache_hit or miss)."),
 		localPreview:   counterOf(meter, "local_eval_preview_total", "Client move previews, by path (local on-device, or network fallback)."),
@@ -90,6 +95,25 @@ func newServerMetrics(meter metric.Meter) *serverMetrics {
 			return nil
 		}, gauge)
 	}
+
+	// Community blocklist status: the feed size and how stale it is, for the dashboard and the
+	// "feed not refreshing" alert. Age is observed only once a feed has loaded, so a disabled or
+	// never-fetched blocklist reports no age (and entries 0).
+	if bl != nil {
+		entries, e1 := meter.Int64ObservableGauge("gateway_blocklist_entries",
+			metric.WithDescription("CIDR ranges in the active community IP blocklist feed."))
+		age, e2 := meter.Float64ObservableGauge("gateway_blocklist_age_seconds",
+			metric.WithDescription("Seconds since the community IP blocklist feed was last successfully fetched (absent until the first fetch)."))
+		if e1 == nil && e2 == nil {
+			_, _ = meter.RegisterCallback(func(_ context.Context, o metric.Observer) error {
+				o.ObserveInt64(entries, int64(bl.Len()))
+				if last := bl.LastFetch(); !last.IsZero() {
+					o.ObserveFloat64(age, time.Since(last).Seconds())
+				}
+				return nil
+			}, entries, age)
+		}
+	}
 	return m
 }
 
@@ -118,6 +142,11 @@ func (m *serverMetrics) recordBan(ctx context.Context, reason string) {
 	m.banned.Add(ctx, 1, metric.WithAttributes(attribute.String("reason", reason)))
 }
 
+// recordBlocklistBlock counts one request refused by the community IP blocklist.
+func (m *serverMetrics) recordBlocklistBlock(ctx context.Context) {
+	m.blocklisted.Add(ctx, 1)
+}
+
 // recordUnsupportedEngine counts one client turned away by the unsupported-engine boot screen,
 // labelled by reason and Chromium major. The caller passes both already reduced to bounded label
 // sets (see normalizeUnsupported) so a spoofed beacon cannot explode the metric cardinality.

gateway/internal/connectsrv/metrics_test.go

@@ -16,7 +16,7 @@ func TestEdgeMetric(t *testing.T) {
 	ctx := context.Background()
 	reader := sdkmetric.NewManualReader()
 	meter := sdkmetric.NewMeterProvider(sdkmetric.WithReader(reader)).Meter("test")
-	m := newServerMetrics(meter)
+	m := newServerMetrics(meter, nil)
 
 	m.recordEdge(ctx, "game.submit_play", "ok", time.Now().Add(-time.Millisecond))
 	m.recordEdge(ctx, "game.submit_play", "ok", time.Now().Add(-time.Millisecond))
@@ -74,7 +74,7 @@ func TestRateLimitedMetric(t *testing.T) {
 	ctx := context.Background()
 	reader := sdkmetric.NewManualReader()
 	meter := sdkmetric.NewMeterProvider(sdkmetric.WithReader(reader)).Meter("test")
-	m := newServerMetrics(meter)
+	m := newServerMetrics(meter, nil)
 
 	m.recordRateLimited(ctx, "user")
 	m.recordRateLimited(ctx, "user")
@@ -112,7 +112,7 @@ func TestBannedMetric(t *testing.T) {
 	ctx := context.Background()
 	reader := sdkmetric.NewManualReader()
 	meter := sdkmetric.NewMeterProvider(sdkmetric.WithReader(reader)).Meter("test")
-	m := newServerMetrics(meter)
+	m := newServerMetrics(meter, nil)
 
 	m.recordBan(ctx, "tripwire")
 	m.recordBan(ctx, "tripwire")
@@ -150,7 +150,7 @@ func TestUnsupportedEngineMetric(t *testing.T) {
 	ctx := context.Background()
 	reader := sdkmetric.NewManualReader()
 	meter := sdkmetric.NewMeterProvider(sdkmetric.WithReader(reader)).Meter("test")
-	m := newServerMetrics(meter)
+	m := newServerMetrics(meter, nil)
 
 	m.recordUnsupportedEngine(ctx, "no_bigint", "66")
 	m.recordUnsupportedEngine(ctx, "no_bigint", "66")

gateway/internal/connectsrv/server.go

@@ -77,6 +77,7 @@ type Server struct {
 	limiter     *ratelimit.Limiter
 	tracker     *ratelimit.Tracker
 	banlist     *ratelimit.Banlist
+	blocklist   *ratelimit.Blocklist
 	honeytoken  string
 	vkAppSecret string
 	hub         *push.Hub
@@ -109,6 +110,9 @@ type Deps struct {
 	// Banlist enforces temporary IP bans on the hot path; nil selects a disabled
 	// (inert) banlist.
 	Banlist *ratelimit.Banlist
+	// Blocklist enforces the community IP blocklist on the hot path; nil selects a
+	// disabled (inert) blocklist.
+	Blocklist *ratelimit.Blocklist
 	// Honeytoken, when non-empty, is the planted bearer value whose presentation
 	// bans the caller and raises a high-severity alarm.
 	Honeytoken string
@@ -148,6 +152,10 @@ func NewServer(d Deps) *Server {
 	if banlist == nil {
 		banlist = ratelimit.NewBanlist(ratelimit.BanConfig{})
 	}
+	blocklist := d.Blocklist
+	if blocklist == nil {
+		blocklist = ratelimit.NewBlocklist(false, nil)
+	}
 	rl := d.RateLimit
 	if rl == (config.RateLimitConfig{}) {
 		rl = config.DefaultRateLimit()
@@ -160,12 +168,13 @@ func NewServer(d Deps) *Server {
 		limiter:      limiter,
 		tracker:      tracker,
 		banlist:      banlist,
+		blocklist:    blocklist,
 		honeytoken:   d.Honeytoken,
 		hub:          d.Hub,
 		heartbeat:    d.Heartbeat,
 		log:          log,
 		adminProxy:   d.AdminProxy,
-		metrics:      newServerMetrics(d.Meter),
+		metrics:      newServerMetrics(d.Meter, blocklist),
 		maxBodyBytes: maxBody,
 		publicPolicy: ratelimit.PerMinute(rl.PublicPerMinute, rl.PublicBurst),
 		userPolicy:   ratelimit.PerMinute(rl.UserPerMinute, rl.UserBurst),
@@ -255,6 +264,13 @@ func (s *Server) abuseGuard(next http.Handler) http.Handler {
 			http.Error(w, "banned", http.StatusTooManyRequests)
 			return
 		}
+		// The community IP blocklist (Spamhaus DROP): a known-bad source is refused before any work.
+		// Counted, not logged per request (a blocked scanner hammers). Inert on a disabled blocklist.
+		if s.blocklist.Blocked(ip) {
+			s.metrics.recordBlocklistBlock(r.Context())
+			http.Error(w, "forbidden", http.StatusForbidden)
+			return
+		}
 		if r.Header.Get(honeypotHeader) != "" {
 			s.log.Warn("honeypot tripwire",
 				zap.String("path", r.URL.Path),