metrics: resolve per-host inifinite cardinality

Signed-off-by: Mohammed Al Sahaf <msaa1990@gmail.com>

modules/caddyhttp/app.go

@@ -198,6 +198,8 @@ func (app *App) Provision(ctx caddy.Context) error {
 	if app.Metrics != nil {
 		app.Metrics.init = sync.Once{}
 		app.Metrics.httpMetrics = &httpMetrics{}
+		// Scan config for allowed hosts to prevent cardinality explosion
+		app.Metrics.scanConfigForHosts(app)
 	}
 	// prepare each server
 	oldContext := ctx.Context

modules/caddyhttp/metrics.go

@@ -17,14 +17,60 @@ import (
 
 // Metrics configures metrics observations.
 // EXPERIMENTAL and subject to change or removal.
+//
+// Example configuration:
+//
+//	{
+//		"apps": {
+//			"http": {
+//				"metrics": {
+//					"per_host": true,
+//					"allow_catch_all_hosts": false
+//				},
+//				"servers": {
+//					"srv0": {
+//						"routes": [{
+//							"match": [{"host": ["example.com", "www.example.com"]}],
+//							"handle": [{"handler": "static_response", "body": "Hello"}]
+//						}]
+//					}
+//				}
+//			}
+//		}
+//	}
+//
+// In this configuration:
+// - Requests to example.com and www.example.com get individual host labels
+// - All other hosts (e.g., attacker.com) are aggregated under "_other" label
+// - This prevents unlimited cardinality from arbitrary Host headers
 type Metrics struct {
 	// Enable per-host metrics. Enabling this option may
 	// incur high-memory consumption, depending on the number of hosts
 	// managed by Caddy.
+	//
+	// CARDINALITY PROTECTION: To prevent unbounded cardinality attacks,
+	// only explicitly configured hosts (via host matchers) are allowed
+	// by default. Other hosts are aggregated under the "_other" label.
+	// See AllowCatchAllHosts to change this behavior.
 	PerHost bool `json:"per_host,omitempty"`
 
-	init        sync.Once
-	httpMetrics *httpMetrics `json:"-"`
+	// Allow metrics for catch-all hosts (hosts without explicit configuration).
+	// When false (default), only hosts explicitly configured via host matchers
+	// will get individual metrics labels. All other hosts will be aggregated
+	// under the "_other" label to prevent cardinality explosion.
+	//
+	// This is automatically enabled for HTTPS servers (since certificates provide
+	// some protection against unbounded cardinality), but disabled for HTTP servers
+	// by default to prevent cardinality attacks from arbitrary Host headers.
+	//
+	// Set to true to allow all hosts to get individual metrics (NOT RECOMMENDED
+	// for production environments exposed to the internet).
+	AllowCatchAllHosts bool `json:"allow_catch_all_hosts,omitempty"`
+
+	init           sync.Once
+	httpMetrics    *httpMetrics
+	allowedHosts   map[string]struct{}
+	hasHTTPSServer bool
 }
 
 type httpMetrics struct {
@@ -101,6 +147,63 @@ func initHTTPMetrics(ctx caddy.Context, metrics *Metrics) {
 	}, httpLabels)
 }
 
+// scanConfigForHosts scans the HTTP app configuration to build a set of allowed hosts
+// for metrics collection, similar to how auto-HTTPS scans for domain names.
+func (m *Metrics) scanConfigForHosts(app *App) {
+	if !m.PerHost {
+		return
+	}
+
+	m.allowedHosts = make(map[string]struct{})
+	m.hasHTTPSServer = false
+
+	for _, srv := range app.Servers {
+		// Check if this server has TLS enabled
+		serverHasTLS := len(srv.TLSConnPolicies) > 0
+		if serverHasTLS {
+			m.hasHTTPSServer = true
+		}
+
+		// Collect hosts from route matchers
+		for _, route := range srv.Routes {
+			for _, matcherSet := range route.MatcherSets {
+				for _, matcher := range matcherSet {
+					if hm, ok := matcher.(*MatchHost); ok {
+						for _, host := range *hm {
+							// Only allow non-fuzzy hosts to prevent unbounded cardinality
+							if !hm.fuzzy(host) {
+								m.allowedHosts[strings.ToLower(host)] = struct{}{}
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+}
+
+// shouldAllowHostMetrics determines if metrics should be collected for the given host.
+// This implements the cardinality protection by only allowing metrics for:
+// 1. Explicitly configured hosts
+// 2. Catch-all requests on HTTPS servers (if AllowCatchAllHosts is true or auto-enabled)
+// 3. Catch-all requests on HTTP servers only if explicitly allowed
+func (m *Metrics) shouldAllowHostMetrics(host string, isHTTPS bool) bool {
+	if !m.PerHost {
+		return true // host won't be used in labels anyway
+	}
+
+	normalizedHost := strings.ToLower(host)
+
+	// Always allow explicitly configured hosts
+	if _, exists := m.allowedHosts[normalizedHost]; exists {
+		return true
+	}
+
+	// For catch-all requests (not in allowed hosts)
+	allowCatchAll := m.AllowCatchAllHosts || (isHTTPS && m.hasHTTPSServer)
+	return allowCatchAll
+}
+
 // serverNameFromContext extracts the current server name from the context.
 // Returns "UNKNOWN" if none is available (should probably never happen).
 func serverNameFromContext(ctx context.Context) string {
@@ -133,9 +236,19 @@ func (h *metricsInstrumentedHandler) ServeHTTP(w http.ResponseWriter, r *http.Re
 	// of a panic
 	statusLabels := prometheus.Labels{"server": server, "handler": h.handler, "method": method, "code": ""}
 
+	// Determine if this is an HTTPS request
+	isHTTPS := r.TLS != nil
+
 	if h.metrics.PerHost {
-		labels["host"] = strings.ToLower(r.Host)
-		statusLabels["host"] = strings.ToLower(r.Host)
+		// Apply cardinality protection for host metrics
+		if h.metrics.shouldAllowHostMetrics(r.Host, isHTTPS) {
+			labels["host"] = strings.ToLower(r.Host)
+			statusLabels["host"] = strings.ToLower(r.Host)
+		} else {
+			// Use a catch-all label for unallowed hosts to prevent cardinality explosion
+			labels["host"] = "_other"
+			statusLabels["host"] = "_other"
+		}
 	}
 
 	inFlight := h.metrics.httpMetrics.requestInFlight.With(labels)

modules/caddyhttp/metrics_test.go

@@ -2,6 +2,7 @@ package caddyhttp
 
 import (
 	"context"
+	"crypto/tls"
 	"errors"
 	"net/http"
 	"net/http/httptest"
@@ -206,9 +207,11 @@ func TestMetricsInstrumentedHandler(t *testing.T) {
 func TestMetricsInstrumentedHandlerPerHost(t *testing.T) {
 	ctx, _ := caddy.NewContext(caddy.Context{Context: context.Background()})
 	metrics := &Metrics{
-		PerHost:     true,
-		init:        sync.Once{},
-		httpMetrics: &httpMetrics{},
+		PerHost:            true,
+		AllowCatchAllHosts: true, // Allow all hosts for testing
+		init:               sync.Once{},
+		httpMetrics:        &httpMetrics{},
+		allowedHosts:       make(map[string]struct{}),
 	}
 	handlerErr := errors.New("oh noes")
 	response := []byte("hello world!")
@@ -379,6 +382,112 @@ func TestMetricsInstrumentedHandlerPerHost(t *testing.T) {
 	}
 }
 
+func TestMetricsCardinalityProtection(t *testing.T) {
+	ctx, _ := caddy.NewContext(caddy.Context{Context: context.Background()})
+
+	// Test 1: Without AllowCatchAllHosts, arbitrary hosts should be mapped to "_other"
+	metrics := &Metrics{
+		PerHost:            true,
+		AllowCatchAllHosts: false, // Default - should map unknown hosts to "_other"
+		init:               sync.Once{},
+		httpMetrics:        &httpMetrics{},
+		allowedHosts:       make(map[string]struct{}),
+	}
+
+	// Add one allowed host
+	metrics.allowedHosts["allowed.com"] = struct{}{}
+
+	mh := middlewareHandlerFunc(func(w http.ResponseWriter, r *http.Request, h Handler) error {
+		w.Write([]byte("hello"))
+		return nil
+	})
+
+	ih := newMetricsInstrumentedHandler(ctx, "test", mh, metrics)
+
+	// Test request to allowed host
+	r1 := httptest.NewRequest("GET", "http://allowed.com/", nil)
+	r1.Host = "allowed.com"
+	w1 := httptest.NewRecorder()
+	ih.ServeHTTP(w1, r1, HandlerFunc(func(w http.ResponseWriter, r *http.Request) error { return nil }))
+
+	// Test request to unknown host (should be mapped to "_other")
+	r2 := httptest.NewRequest("GET", "http://attacker.com/", nil)
+	r2.Host = "attacker.com"
+	w2 := httptest.NewRecorder()
+	ih.ServeHTTP(w2, r2, HandlerFunc(func(w http.ResponseWriter, r *http.Request) error { return nil }))
+
+	// Test request to another unknown host (should also be mapped to "_other")
+	r3 := httptest.NewRequest("GET", "http://evil.com/", nil)
+	r3.Host = "evil.com"
+	w3 := httptest.NewRecorder()
+	ih.ServeHTTP(w3, r3, HandlerFunc(func(w http.ResponseWriter, r *http.Request) error { return nil }))
+
+	// Check that metrics contain:
+	// - One entry for "allowed.com"
+	// - One entry for "_other" (aggregating attacker.com and evil.com)
+	expected := `
+	# HELP caddy_http_requests_total Counter of HTTP(S) requests made.
+	# TYPE caddy_http_requests_total counter
+	caddy_http_requests_total{handler="test",host="_other",server="UNKNOWN"} 2
+	caddy_http_requests_total{handler="test",host="allowed.com",server="UNKNOWN"} 1
+	`
+
+	if err := testutil.GatherAndCompare(ctx.GetMetricsRegistry(), strings.NewReader(expected),
+		"caddy_http_requests_total",
+	); err != nil {
+		t.Errorf("Cardinality protection test failed: %s", err)
+	}
+}
+
+func TestMetricsHTTPSCatchAll(t *testing.T) {
+	ctx, _ := caddy.NewContext(caddy.Context{Context: context.Background()})
+
+	// Test that HTTPS requests allow catch-all even when AllowCatchAllHosts is false
+	metrics := &Metrics{
+		PerHost:            true,
+		AllowCatchAllHosts: false,
+		hasHTTPSServer:     true, // Simulate having HTTPS servers
+		init:               sync.Once{},
+		httpMetrics:        &httpMetrics{},
+		allowedHosts:       make(map[string]struct{}), // Empty - no explicitly allowed hosts
+	}
+
+	mh := middlewareHandlerFunc(func(w http.ResponseWriter, r *http.Request, h Handler) error {
+		w.Write([]byte("hello"))
+		return nil
+	})
+
+	ih := newMetricsInstrumentedHandler(ctx, "test", mh, metrics)
+
+	// Test HTTPS request (should be allowed even though not in allowedHosts)
+	r1 := httptest.NewRequest("GET", "https://unknown.com/", nil)
+	r1.Host = "unknown.com"
+	r1.TLS = &tls.ConnectionState{} // Mark as TLS/HTTPS
+	w1 := httptest.NewRecorder()
+	ih.ServeHTTP(w1, r1, HandlerFunc(func(w http.ResponseWriter, r *http.Request) error { return nil }))
+
+	// Test HTTP request (should be mapped to "_other")
+	r2 := httptest.NewRequest("GET", "http://unknown.com/", nil)
+	r2.Host = "unknown.com"
+	// No TLS field = HTTP request
+	w2 := httptest.NewRecorder()
+	ih.ServeHTTP(w2, r2, HandlerFunc(func(w http.ResponseWriter, r *http.Request) error { return nil }))
+
+	// Check that HTTPS request gets real host, HTTP gets "_other"
+	expected := `
+	# HELP caddy_http_requests_total Counter of HTTP(S) requests made.
+	# TYPE caddy_http_requests_total counter
+	caddy_http_requests_total{handler="test",host="_other",server="UNKNOWN"} 1
+	caddy_http_requests_total{handler="test",host="unknown.com",server="UNKNOWN"} 1
+	`
+
+	if err := testutil.GatherAndCompare(ctx.GetMetricsRegistry(), strings.NewReader(expected),
+		"caddy_http_requests_total",
+	); err != nil {
+		t.Errorf("HTTPS catch-all test failed: %s", err)
+	}
+}
+
 type middlewareHandlerFunc func(http.ResponseWriter, *http.Request, Handler) error
 
 func (f middlewareHandlerFunc) ServeHTTP(w http.ResponseWriter, r *http.Request, h Handler) error {