Add Prometheus metrics for connections, firewall, L7, and bytes transferred

Instrument mc-proxy with prometheus/client_golang. New internal/metrics/ package defines counters, gauges, and histograms for connection totals, active connections, firewall blocks by reason, backend dial latency, bytes transferred, L7 HTTP status codes, and L7 policy blocks. Optional [metrics] config section starts a scrape endpoint. Firewall gains BlockedWithReason() to report block cause. L7 handler wraps ResponseWriter to record status codes per hostname. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
2026-03-25 18:05:25 -07:00
parent 42c7fffc3e
commit ffc31f7d55
16 changed files with 439 additions and 32 deletions
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -710,9 +710,6 @@ Items are listed roughly in priority order:
 | Item | Description |
 |------|-------------|
 | **ACME integration** | Automatic certificate provisioning via Let's Encrypt for L7 routes, removing the need for manual cert management. |
-| **L7 policies** | User-agent blocking, header-based routing, request rate limiting per endpoint. Requires L7 mode. |
 | **MCP integration** | Wire the gRPC admin API into the Metacircular Control Plane for centralized management. |
 | **Connection pooling** | Pool backend connections for L7 routes to reduce connection setup overhead under high request volume. |
-| **Per-listener connection limits** | Cap maximum concurrent connections per listener. |
-| **Metrics** | Prometheus-compatible metrics: connections per listener, firewall blocks by rule, backend dial latency, active connections, HTTP status code distributions. |
 | **Metacrypt key storage** | Store L7 TLS private keys in metacrypt rather than on the filesystem. |
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -49,6 +49,7 @@ go test ./internal/sni -run TestExtract
 - `internal/l7/` — L7 TLS termination, `prefixConn`, HTTP/2 reverse proxy with h2c backend transport
 - `internal/server/` — orchestrates listeners → PROXY protocol → firewall → SNI → route → L4/L7 dispatch; per-listener state with connection tracking
 - `internal/grpcserver/` — gRPC admin API: route/firewall CRUD, status, write-through to DB
+- `internal/metrics/` — Prometheus metric definitions and HTTP server; optional `[metrics]` config section
 - `proto/mc_proxy/v1/` — protobuf definitions; `gen/mc_proxy/v1/` has generated code

 ## Signals
--- a/PROGRESS.md
+++ b/PROGRESS.md
@@ -71,20 +71,21 @@ proceeds. Each item is marked:

 ## Phase 8: Prometheus Metrics

- [ ] 8.1 Dependency: add `prometheus/client_golang`
- [ ] 8.2 Config: `Metrics` section (`addr`, `path`)
- [ ] 8.3 Package: `internal/metrics/` definitions and HTTP server
- [ ] 8.4 Instrumentation: connections, firewall, dial latency, bytes, HTTP status, policy blocks
- [ ] 8.5 Firewall: `BlockedWithReason()` method
- [ ] 8.6 L7: status recording on ResponseWriter
- [ ] 8.7 Startup: conditionally start metrics server
- [ ] 8.8 Tests: metric sanity, server endpoint, `BlockedWithReason`
+- [x] 8.1 Dependency: add `prometheus/client_golang`
+- [x] 8.2 Config: `Metrics` section (`addr`, `path`)
+- [x] 8.3 Package: `internal/metrics/` definitions and HTTP server
+- [x] 8.4 Instrumentation: connections, firewall, dial latency, bytes, HTTP status, policy blocks
+- [x] 8.5 Firewall: `BlockedWithReason()` method
+- [x] 8.6 L7: status recording on ResponseWriter
+- [x] 8.7 Startup: conditionally start metrics server
+- [x] 8.8 Tests: metric sanity, server endpoint, `BlockedWithReason`

 ---

 ## Current State

-Phases 1-6 complete. Per-listener connection limits are implemented and
-tested. L7 policies and Prometheus metrics are next.
+Phases 1-8 complete. Prometheus metrics are instrumented across
+connections, firewall, dial latency, bytes transferred, L7 HTTP status
+codes, and L7 policy blocks.

-`go vet` and `go test` pass across all 13 packages.
+`go vet` and `go test` pass across all 14 packages.
--- a/cmd/mc-proxy/server.go
+++ b/cmd/mc-proxy/server.go
@@ -15,6 +15,7 @@ import (
 	"git.wntrmute.dev/kyle/mc-proxy/internal/db"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/firewall"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/grpcserver"
+	"git.wntrmute.dev/kyle/mc-proxy/internal/metrics"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/server"
 )

@@ -108,6 +109,16 @@ func serverCmd() *cobra.Command {
 				}
 			}()

+			// Start Prometheus metrics server if configured.
+			if cfg.Metrics.Addr != "" {
+				logger.Info("metrics server listening", "addr", cfg.Metrics.Addr, "path", cfg.Metrics.Path)
+				go func() {
+					if err := metrics.ListenAndServe(ctx, cfg.Metrics.Addr, cfg.Metrics.Path); err != nil {
+						logger.Error("metrics server error", "error", err)
+					}
+				}()
+			}
+
 			logger.Info("mc-proxy starting", "version", version)
 			return srv.Run(ctx)
 		},
--- a/go.mod
+++ b/go.mod
@@ -14,14 +14,22 @@ require (
 replace git.wntrmute.dev/kyle/mcdsl => /home/kyle/src/metacircular/mcdsl

 require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/pelletier/go-toml/v2 v2.3.0 // indirect
+	github.com/prometheus/client_golang v1.23.2 // indirect
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.66.1 // indirect
+	github.com/prometheus/procfs v0.16.1 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/spf13/pflag v1.0.9 // indirect
+	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/text v0.32.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 // indirect
--- a/go.sum
+++ b/go.sum
@@ -1,3 +1,5 @@
+github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
+github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
 github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
 github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=
@@ -23,6 +25,8 @@ github.com/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2
 github.com/inconshreveable/mousetrap v1.1.0/go.mod h1:vpF70FUmC8bwa3OWnCshd2FqLfsEA9PFc4w1p2J65bw=
 github.com/mattn/go-isatty v0.0.20 h1:xfD0iDuEKnDkl03q4limB+vH+GxLEtL/jb4xVJSWWEY=
 github.com/mattn/go-isatty v0.0.20/go.mod h1:W+V8PltTTMOvKvAeJH7IuucS94S2C6jfK/D7dTCTo3Y=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 h1:C3w9PqII01/Oq1c1nUAm88MOHcQC9l5mIlSMApZMrHA=
+github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822/go.mod h1:+n7T8mK8HuQTcFwEeznm/DIxMOiR9yIdICNftLE1DvQ=
 github.com/ncruces/go-strftime v1.0.0 h1:HMFp8mLCTPp341M/ZnA4qaf7ZlsbTc+miZjCLOFAw7w=
 github.com/ncruces/go-strftime v1.0.0/go.mod h1:Fwc5htZGVVkseilnfgOVb9mKy6w1naJmn9CehxcKcls=
 github.com/oschwald/maxminddb-golang v1.13.1 h1:G3wwjdN9JmIK2o/ermkHM+98oX5fS+k5MbwsmL4MRQE=
@@ -31,6 +35,14 @@ github.com/pelletier/go-toml/v2 v2.3.0 h1:k59bC/lIZREW0/iVaQR8nDHxVq8OVlIzYCOJf4
 github.com/pelletier/go-toml/v2 v2.3.0/go.mod h1:2gIqNv+qfxSVS7cM2xJQKtLSTLUE9V8t9Stt+h56mCY=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/prometheus/client_golang v1.23.2 h1:Je96obch5RDVy3FDMndoUsjAhG5Edi49h0RJWRi/o0o=
+github.com/prometheus/client_golang v1.23.2/go.mod h1:Tb1a6LWHB3/SPIzCoaDXI4I8UHKeFTEQ1YCr+0Gyqmg=
+github.com/prometheus/client_model v0.6.2 h1:oBsgwpGs7iVziMvrGhE53c/GrLUsZdHnqNwqPLxwZyk=
+github.com/prometheus/client_model v0.6.2/go.mod h1:y3m2F6Gdpfy6Ut/GBsUqTWZqCUvMVzSfMLjcu6wAwpE=
+github.com/prometheus/common v0.66.1 h1:h5E0h5/Y8niHc5DlaLlWLArTQI7tMrsfQjHV+d9ZoGs=
+github.com/prometheus/common v0.66.1/go.mod h1:gcaUsgf3KfRSwHY4dIMXLPV0K/Wg1oZ8+SbZk/HH/dA=
+github.com/prometheus/procfs v0.16.1 h1:hZ15bTNuirocR6u0JZ6BAHHmwS1p8B4P6MRqxtzMyRg=
+github.com/prometheus/procfs v0.16.1/go.mod h1:teAbpZRB1iIAJYREa1LsoWUXykVXA1KlTmWl8x/U+Is=
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94icq4NjY3clb7Lk8O1qJ8BdBEF8z0ibU0rE=
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
 github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
@@ -52,6 +64,8 @@ go.opentelemetry.io/otel/sdk/metric v1.39.0 h1:cXMVVFVgsIf2YL6QkRF4Urbr/aMInf+2W
 go.opentelemetry.io/otel/sdk/metric v1.39.0/go.mod h1:xq9HEVH7qeX69/JnwEfp6fVq5wosJsY1mt4lLfYdVew=
 go.opentelemetry.io/otel/trace v1.39.0 h1:2d2vfpEDmCJ5zVYz7ijaJdOF59xLomrvj7bjt6/qCJI=
 go.opentelemetry.io/otel/trace v1.39.0/go.mod h1:88w4/PnZSazkGzz/w84VHpQafiU4EtqqlVdxWy+rNOA=
+go.yaml.in/yaml/v2 v2.4.2 h1:DzmwEr2rDGHl7lsFgAHxmNz/1NlQ7xLIrlN2h5d1eGI=
+go.yaml.in/yaml/v2 v2.4.2/go.mod h1:081UH+NErpNdqlCXm3TtEran0rJZGxAYx9hb/ELlsPU=
 go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
 golang.org/x/mod v0.33.0 h1:tHFzIWbBifEmbwtGz65eaWyGiGZatSrT9prnU8DbVL8=
 golang.org/x/mod v0.33.0/go.mod h1:swjeQEj+6r7fODbD2cqrnje9PnziFuw4bmLbBZFrQ5w=
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -22,9 +22,16 @@ type Config struct {
 	GRPC      GRPC       `toml:"grpc"`
 	Firewall  Firewall   `toml:"firewall"`
 	Proxy     Proxy      `toml:"proxy"`
+	Metrics   Metrics    `toml:"metrics"`
 	Log       Log        `toml:"log"`
 }

+// Metrics holds the Prometheus metrics endpoint configuration.
+type Metrics struct {
+	Addr string `toml:"addr"` // e.g. "127.0.0.1:9090"
+	Path string `toml:"path"` // e.g. "/metrics" (default)
+}
+
 // Database holds the database configuration.
 type Database struct {
 	Path string `toml:"path"`
@@ -215,6 +222,10 @@ func (c *Config) validate() error {
 		}
 	}

+	if c.Metrics.Addr != "" && c.Metrics.Path != "" && !strings.HasPrefix(c.Metrics.Path, "/") {
+		return fmt.Errorf("metrics.path must start with \"/\"")
+	}
+
 	if c.Proxy.ConnectTimeout.Duration < 0 {
 		return fmt.Errorf("proxy.connect_timeout must not be negative")
 	}
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -541,3 +541,54 @@ proxy_protocol = true
 		t.Fatal("expected send_proxy_protocol = true")
 	}
 }
+
+func TestLoadMetricsConfig(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.toml")
+
+	data := `
+[database]
+path = "/tmp/test.db"
+
+[metrics]
+addr = "127.0.0.1:9090"
+path = "/metrics"
+`
+	if err := os.WriteFile(path, []byte(data), 0600); err != nil {
+		t.Fatalf("write config: %v", err)
+	}
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if cfg.Metrics.Addr != "127.0.0.1:9090" {
+		t.Fatalf("got metrics.addr %q, want %q", cfg.Metrics.Addr, "127.0.0.1:9090")
+	}
+	if cfg.Metrics.Path != "/metrics" {
+		t.Fatalf("got metrics.path %q, want %q", cfg.Metrics.Path, "/metrics")
+	}
+}
+
+func TestValidateMetricsInvalidPath(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.toml")
+
+	data := `
+[database]
+path = "/tmp/test.db"
+
+[metrics]
+addr = "127.0.0.1:9090"
+path = "no-slash"
+`
+	if err := os.WriteFile(path, []byte(data), 0600); err != nil {
+		t.Fatalf("write config: %v", err)
+	}
+
+	_, err := Load(path)
+	if err == nil {
+		t.Fatal("expected error for metrics.path without leading slash")
+	}
+}
--- a/internal/firewall/firewall.go
+++ b/internal/firewall/firewall.go
@@ -71,18 +71,25 @@ func New(geoIPPath string, ips, cidrs, countries []string, rateLimit int64, rate

 // Blocked returns true if the given address should be blocked.
 func (f *Firewall) Blocked(addr netip.Addr) bool {
+	blocked, _ := f.BlockedWithReason(addr)
+	return blocked
+}
+
+// BlockedWithReason returns whether the address is blocked and the reason.
+// Possible reasons: "ip", "cidr", "country", "rate_limit", or "" if not blocked.
+func (f *Firewall) BlockedWithReason(addr netip.Addr) (bool, string) {
 	addr = addr.Unmap()

 	f.mu.RLock()
 	defer f.mu.RUnlock()

 	if _, ok := f.blockedIPs[addr]; ok {
-		return true
+		return true, "ip"
 	}

 	for _, prefix := range f.blockedCIDRs {
 		if prefix.Contains(addr) {
-			return true
+			return true, "cidr"
 		}
 	}

@@ -90,7 +97,7 @@ func (f *Firewall) Blocked(addr netip.Addr) bool {
 		var record geoIPRecord
 		if err := f.geoDB.Lookup(addr.AsSlice(), &record); err == nil {
 			if _, ok := f.blockedCountries[record.Country.ISOCode]; ok {
-				return true
+				return true, "country"
 			}
 		}
 	}
@@ -98,10 +105,10 @@ func (f *Firewall) Blocked(addr netip.Addr) bool {
 	// Rate limiting is checked after blocklist — no point tracking state
 	// for already-blocked IPs.
 	if f.rl != nil && !f.rl.Allow(addr) {
-		return true
+		return true, "rate_limit"
 	}

-	return false
+	return false, ""
 }

 // AddIP adds an IP address to the blocklist.
--- a/internal/firewall/firewall_test.go
+++ b/internal/firewall/firewall_test.go
@@ -170,6 +170,47 @@ func TestRateLimitBlocklistFirst(t *testing.T) {
 	}
 }

+func TestBlockedWithReason(t *testing.T) {
+	fw, err := New("", []string{"10.0.0.1"}, []string{"192.168.0.0/16"}, nil, 2, time.Minute)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	defer fw.Close()
+
+	tests := []struct {
+		addr       string
+		wantBlock  bool
+		wantReason string
+	}{
+		{"10.0.0.1", true, "ip"},
+		{"192.168.1.1", true, "cidr"},
+		{"172.16.0.1", false, ""},
+	}
+
+	for _, tt := range tests {
+		addr := netip.MustParseAddr(tt.addr)
+		blocked, reason := fw.BlockedWithReason(addr)
+		if blocked != tt.wantBlock {
+			t.Fatalf("BlockedWithReason(%s) blocked = %v, want %v", tt.addr, blocked, tt.wantBlock)
+		}
+		if reason != tt.wantReason {
+			t.Fatalf("BlockedWithReason(%s) reason = %q, want %q", tt.addr, reason, tt.wantReason)
+		}
+	}
+
+	// Test rate limit reason: use a fresh IP that will exceed the limit.
+	rlAddr := netip.MustParseAddr("10.10.10.10")
+	fw.BlockedWithReason(rlAddr)                    // 1
+	fw.BlockedWithReason(rlAddr)                    // 2
+	blocked, reason := fw.BlockedWithReason(rlAddr) // 3 — should be blocked
+	if !blocked {
+		t.Fatal("expected rate limit block")
+	}
+	if reason != "rate_limit" {
+		t.Fatalf("reason = %q, want %q", reason, "rate_limit")
+	}
+}
+
 func TestRuntimeMutation(t *testing.T) {
 	fw, err := New("", nil, nil, nil, 0, 0)
 	if err != nil {
--- a/internal/l7/policy.go
+++ b/internal/l7/policy.go
@@ -3,6 +3,8 @@ package l7
 import (
 	"net/http"
 	"strings"
+
+	"git.wntrmute.dev/kyle/mc-proxy/internal/metrics"
 )

 // PolicyRule defines an L7 blocking policy.
@@ -14,7 +16,7 @@ type PolicyRule struct {
 // PolicyMiddleware returns an http.Handler that evaluates L7 policies
 // before delegating to next. Returns HTTP 403 if any policy blocks.
 // If policies is empty, returns next unchanged.
-func PolicyMiddleware(policies []PolicyRule, next http.Handler) http.Handler {
+func PolicyMiddleware(policies []PolicyRule, hostname string, next http.Handler) http.Handler {
 	if len(policies) == 0 {
 		return next
 	}
@@ -23,11 +25,13 @@ func PolicyMiddleware(policies []PolicyRule, next http.Handler) http.Handler {
 			switch p.Type {
 			case "block_user_agent":
 				if strings.Contains(r.UserAgent(), p.Value) {
+					metrics.L7PolicyBlocksTotal.WithLabelValues(hostname, "block_user_agent").Inc()
 					w.WriteHeader(http.StatusForbidden)
 					return
 				}
 			case "require_header":
 				if r.Header.Get(p.Value) == "" {
+					metrics.L7PolicyBlocksTotal.WithLabelValues(hostname, "require_header").Inc()
 					w.WriteHeader(http.StatusForbidden)
 					return
 				}
--- a/internal/l7/policy_test.go
+++ b/internal/l7/policy_test.go
@@ -13,7 +13,7 @@ func TestPolicyMiddlewareNoPolicies(t *testing.T) {
 		w.WriteHeader(200)
 	})

-	handler := PolicyMiddleware(nil, next)
+	handler := PolicyMiddleware(nil, "test.example.com", next)

 	req := httptest.NewRequest("GET", "/", nil)
 	w := httptest.NewRecorder()
@@ -35,7 +35,7 @@ func TestPolicyBlockUserAgentMatch(t *testing.T) {
 	policies := []PolicyRule{
 		{Type: "block_user_agent", Value: "BadBot"},
 	}
-	handler := PolicyMiddleware(policies, next)
+	handler := PolicyMiddleware(policies, "test.example.com", next)

 	req := httptest.NewRequest("GET", "/", nil)
 	req.Header.Set("User-Agent", "Mozilla/5.0 BadBot/1.0")
@@ -57,7 +57,7 @@ func TestPolicyBlockUserAgentNoMatch(t *testing.T) {
 	policies := []PolicyRule{
 		{Type: "block_user_agent", Value: "BadBot"},
 	}
-	handler := PolicyMiddleware(policies, next)
+	handler := PolicyMiddleware(policies, "test.example.com", next)

 	req := httptest.NewRequest("GET", "/", nil)
 	req.Header.Set("User-Agent", "Mozilla/5.0 GoodBrowser/1.0")
@@ -82,7 +82,7 @@ func TestPolicyRequireHeaderPresent(t *testing.T) {
 	policies := []PolicyRule{
 		{Type: "require_header", Value: "X-API-Key"},
 	}
-	handler := PolicyMiddleware(policies, next)
+	handler := PolicyMiddleware(policies, "test.example.com", next)

 	req := httptest.NewRequest("GET", "/", nil)
 	req.Header.Set("X-API-Key", "secret")
@@ -105,7 +105,7 @@ func TestPolicyRequireHeaderAbsent(t *testing.T) {
 	policies := []PolicyRule{
 		{Type: "require_header", Value: "X-API-Key"},
 	}
-	handler := PolicyMiddleware(policies, next)
+	handler := PolicyMiddleware(policies, "test.example.com", next)

 	req := httptest.NewRequest("GET", "/", nil)
 	w := httptest.NewRecorder()
@@ -125,7 +125,7 @@ func TestPolicyMultipleRules(t *testing.T) {
 		{Type: "block_user_agent", Value: "BadBot"},
 		{Type: "require_header", Value: "X-Token"},
 	}
-	handler := PolicyMiddleware(policies, next)
+	handler := PolicyMiddleware(policies, "test.example.com", next)

 	// Blocked by UA even though header is present.
 	req := httptest.NewRequest("GET", "/", nil)
--- a/internal/l7/serve.go
+++ b/internal/l7/serve.go
@@ -12,14 +12,17 @@ import (
 	"net/http/httputil"
 	"net/netip"
 	"net/url"
+	"strconv"
 	"time"

+	"git.wntrmute.dev/kyle/mc-proxy/internal/metrics"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/proxyproto"
 	"golang.org/x/net/http2"
 )

 // RouteConfig holds the L7 route parameters needed by the l7 package.
 type RouteConfig struct {
+	Hostname          string
 	Backend           string
 	TLSCert           string
 	TLSKey            string
@@ -29,6 +32,21 @@ type RouteConfig struct {
 	Policies          []PolicyRule
 }

+// statusRecorder wraps http.ResponseWriter to capture the status code.
+type statusRecorder struct {
+	http.ResponseWriter
+	status int
+}
+
+func (sr *statusRecorder) WriteHeader(code int) {
+	sr.status = code
+	sr.ResponseWriter.WriteHeader(code)
+}
+
+func (sr *statusRecorder) Unwrap() http.ResponseWriter {
+	return sr.ResponseWriter
+}
+
 // contextKey is an unexported type for context keys in this package.
 type contextKey int

@@ -75,12 +93,14 @@ func Serve(ctx context.Context, conn net.Conn, peeked []byte, route RouteConfig,
 		return fmt.Errorf("creating reverse proxy: %w", err)
 	}

-	// Build handler chain: context injection → L7 policies → reverse proxy.
+	// Build handler chain: context injection → metrics → L7 policies → reverse proxy.
 	var inner http.Handler = rp
-	inner = PolicyMiddleware(route.Policies, inner)
+	inner = PolicyMiddleware(route.Policies, route.Hostname, inner)
 	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		r = r.WithContext(context.WithValue(r.Context(), clientAddrKey, clientAddr))
-		inner.ServeHTTP(w, r)
+		sr := &statusRecorder{ResponseWriter: w, status: http.StatusOK}
+		inner.ServeHTTP(sr, r)
+		metrics.L7ResponsesTotal.WithLabelValues(route.Hostname, strconv.Itoa(sr.status)).Inc()
 	})

 	// Serve HTTP on the TLS connection. Use HTTP/2 if negotiated,
--- a/internal/metrics/metrics.go
+++ b/internal/metrics/metrics.go
@@ -0,0 +1,95 @@
+// Package metrics defines Prometheus metrics for mc-proxy and provides
+// an HTTP server for the /metrics endpoint.
+package metrics
+
+import (
+	"context"
+	"errors"
+	"net"
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+var (
+	// ConnectionsTotal counts connections accepted per listener and mode.
+	ConnectionsTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "mcproxy",
+		Name:      "connections_total",
+		Help:      "Total connections accepted.",
+	}, []string{"listener", "mode"})
+
+	// ConnectionsActive tracks currently active connections per listener.
+	ConnectionsActive = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "mcproxy",
+		Name:      "connections_active",
+		Help:      "Currently active connections.",
+	}, []string{"listener"})
+
+	// FirewallBlockedTotal counts firewall blocks by reason.
+	FirewallBlockedTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "mcproxy",
+		Name:      "firewall_blocked_total",
+		Help:      "Total connections blocked by the firewall.",
+	}, []string{"reason"})
+
+	// BackendDialDuration observes backend dial latency in seconds.
+	BackendDialDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: "mcproxy",
+		Name:      "backend_dial_duration_seconds",
+		Help:      "Backend dial latency in seconds.",
+		Buckets:   []float64{0.001, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5},
+	}, []string{"backend"})
+
+	// TransferredBytesTotal counts bytes transferred by direction and hostname.
+	TransferredBytesTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "mcproxy",
+		Name:      "transferred_bytes_total",
+		Help:      "Total bytes transferred.",
+	}, []string{"direction", "hostname"})
+
+	// L7ResponsesTotal counts L7 HTTP responses by hostname and status code.
+	L7ResponsesTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "mcproxy",
+		Name:      "l7_responses_total",
+		Help:      "Total L7 HTTP responses.",
+	}, []string{"hostname", "code"})
+
+	// L7PolicyBlocksTotal counts L7 policy blocks by hostname and policy type.
+	L7PolicyBlocksTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "mcproxy",
+		Name:      "l7_policy_blocks_total",
+		Help:      "Total L7 policy blocks.",
+	}, []string{"hostname", "policy_type"})
+)
+
+// ListenAndServe starts a Prometheus metrics HTTP server. It blocks until
+// ctx is cancelled, then shuts down gracefully.
+func ListenAndServe(ctx context.Context, addr, path string) error {
+	if path == "" {
+		path = "/metrics"
+	}
+
+	mux := http.NewServeMux()
+	mux.Handle(path, promhttp.Handler())
+
+	ln, err := net.Listen("tcp", addr)
+	if err != nil {
+		return err
+	}
+
+	srv := &http.Server{Handler: mux}
+
+	go func() {
+		<-ctx.Done()
+		_ = srv.Close()
+	}()
+
+	err = srv.Serve(ln)
+	if errors.Is(err, http.ErrServerClosed) {
+		return nil
+	}
+	return err
+}
--- a/internal/metrics/metrics_test.go
+++ b/internal/metrics/metrics_test.go
@@ -0,0 +1,121 @@
+package metrics
+
+import (
+	"context"
+	"io"
+	"net"
+	"net/http"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestListenAndServeShutdown(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	errCh := make(chan error, 1)
+	go func() {
+		errCh <- ListenAndServe(ctx, "127.0.0.1:0", "/metrics")
+	}()
+
+	time.Sleep(50 * time.Millisecond)
+	cancel()
+
+	select {
+	case err := <-errCh:
+		if err != nil {
+			t.Fatalf("ListenAndServe returned error: %v", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("ListenAndServe did not return after context cancel")
+	}
+}
+
+func TestMetricsEndpoint(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	addr := ln.Addr().String()
+	_ = ln.Close()
+
+	// Increment counters so they appear in output.
+	ConnectionsTotal.WithLabelValues("127.0.0.1:4430", "l4").Inc()
+	FirewallBlockedTotal.WithLabelValues("ip").Inc()
+	ConnectionsActive.WithLabelValues("127.0.0.1:4430").Set(1)
+
+	go func() { _ = ListenAndServe(ctx, addr, "/metrics") }()
+	time.Sleep(100 * time.Millisecond)
+
+	resp, err := http.Get("http://" + addr + "/metrics")
+	if err != nil {
+		t.Fatalf("GET /metrics: %v", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != 200 {
+		t.Fatalf("status = %d, want 200", resp.StatusCode)
+	}
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("reading body: %v", err)
+	}
+
+	text := string(body)
+	for _, want := range []string{
+		"mcproxy_connections_total",
+		"mcproxy_firewall_blocked_total",
+		"mcproxy_connections_active",
+	} {
+		if !strings.Contains(text, want) {
+			t.Errorf("response missing %s", want)
+		}
+	}
+}
+
+func TestMetricsDefaultPath(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	addr := ln.Addr().String()
+	_ = ln.Close()
+
+	go func() { _ = ListenAndServe(ctx, addr, "") }()
+	time.Sleep(100 * time.Millisecond)
+
+	resp, err := http.Get("http://" + addr + "/metrics")
+	if err != nil {
+		t.Fatalf("GET /metrics: %v", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != 200 {
+		t.Fatalf("status = %d, want 200", resp.StatusCode)
+	}
+}
+
+func TestMetricsSanity(t *testing.T) {
+	// Verify all metric vars can be used without panicking.
+	ConnectionsTotal.WithLabelValues("test:443", "l4").Inc()
+	ConnectionsActive.WithLabelValues("test:443").Set(5)
+	FirewallBlockedTotal.WithLabelValues("ip").Inc()
+	FirewallBlockedTotal.WithLabelValues("cidr").Inc()
+	FirewallBlockedTotal.WithLabelValues("country").Inc()
+	FirewallBlockedTotal.WithLabelValues("rate_limit").Inc()
+	BackendDialDuration.WithLabelValues("127.0.0.1:8080").Observe(0.005)
+	TransferredBytesTotal.WithLabelValues("client_to_backend", "example.com").Add(1024)
+	TransferredBytesTotal.WithLabelValues("backend_to_client", "example.com").Add(2048)
+	L7ResponsesTotal.WithLabelValues("example.com", "200").Inc()
+	L7ResponsesTotal.WithLabelValues("example.com", "502").Inc()
+	L7PolicyBlocksTotal.WithLabelValues("example.com", "block_user_agent").Inc()
+	L7PolicyBlocksTotal.WithLabelValues("example.com", "require_header").Inc()
+}
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -14,6 +14,7 @@ import (
 	"git.wntrmute.dev/kyle/mc-proxy/internal/config"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/firewall"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/l7"
+	"git.wntrmute.dev/kyle/mc-proxy/internal/metrics"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/proxy"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/proxyproto"
 	"git.wntrmute.dev/kyle/mc-proxy/internal/sni"
@@ -204,6 +205,17 @@ func (s *Server) Version() string {
 	return s.version
 }

+// listenerAddrForRoute finds the listener address that owns the given hostname.
+func (s *Server) listenerAddrForRoute(hostname string) string {
+	key := strings.ToLower(hostname)
+	for _, ls := range s.listeners {
+		if _, ok := ls.lookupRoute(key); ok {
+			return ls.Addr
+		}
+	}
+	return "unknown"
+}
+
 // TotalConnections returns the total number of active connections.
 func (s *Server) TotalConnections() int64 {
 	var total int64
@@ -289,6 +301,7 @@ func (s *Server) serve(ctx context.Context, ln net.Listener, ls *ListenerState)

 		s.wg.Add(1)
 		ls.ActiveConnections.Add(1)
+		metrics.ConnectionsActive.WithLabelValues(ls.Addr).Inc()
 		go s.handleConn(ctx, conn, ls)
 	}
 }
@@ -307,6 +320,7 @@ func (s *Server) forceCloseAll() {
 func (s *Server) handleConn(ctx context.Context, conn net.Conn, ls *ListenerState) {
 	defer s.wg.Done()
 	defer ls.ActiveConnections.Add(-1)
+	defer metrics.ConnectionsActive.WithLabelValues(ls.Addr).Dec()
 	defer conn.Close()

 	ls.connMu.Lock()
@@ -340,8 +354,9 @@ func (s *Server) handleConn(ctx context.Context, conn net.Conn, ls *ListenerStat
 		}
 	}

-	if s.fw.Blocked(addr) {
-		s.logger.Debug("blocked by firewall", "addr", addr)
+	if blocked, reason := s.fw.BlockedWithReason(addr); blocked {
+		metrics.FirewallBlockedTotal.WithLabelValues(reason).Inc()
+		s.logger.Debug("blocked by firewall", "addr", addr, "reason", reason)
 		return
 	}

@@ -368,7 +383,11 @@ func (s *Server) handleConn(ctx context.Context, conn net.Conn, ls *ListenerStat

 // handleL4 handles an L4 (passthrough) connection.
 func (s *Server) handleL4(ctx context.Context, conn net.Conn, addr netip.Addr, clientAddrPort netip.AddrPort, hostname string, route RouteInfo, peeked []byte) {
+	metrics.ConnectionsTotal.WithLabelValues(s.listenerAddrForRoute(hostname), "l4").Inc()
+
+	dialStart := time.Now()
 	backendConn, err := net.DialTimeout("tcp", route.Backend, s.cfg.Proxy.ConnectTimeout.Duration)
+	metrics.BackendDialDuration.WithLabelValues(route.Backend).Observe(time.Since(dialStart).Seconds())
 	if err != nil {
 		s.logger.Error("backend dial failed", "hostname", hostname, "backend", route.Backend, "error", err)
 		return
@@ -391,6 +410,9 @@ func (s *Server) handleL4(ctx context.Context, conn net.Conn, addr netip.Addr, c
 		s.logger.Debug("relay ended", "hostname", hostname, "error", err)
 	}

+	metrics.TransferredBytesTotal.WithLabelValues("client_to_backend", hostname).Add(float64(result.ClientBytes))
+	metrics.TransferredBytesTotal.WithLabelValues("backend_to_client", hostname).Add(float64(result.BackendBytes))
+
 	s.logger.Info("connection closed",
 		"addr", addr,
 		"hostname", hostname,
@@ -401,6 +423,8 @@ func (s *Server) handleL4(ctx context.Context, conn net.Conn, addr netip.Addr, c

 // handleL7 handles an L7 (TLS-terminating) connection.
 func (s *Server) handleL7(ctx context.Context, conn net.Conn, addr netip.Addr, clientAddrPort netip.AddrPort, hostname string, route RouteInfo, peeked []byte) {
+	metrics.ConnectionsTotal.WithLabelValues(s.listenerAddrForRoute(hostname), "l7").Inc()
+
 	s.logger.Debug("L7 proxying", "addr", addr, "hostname", hostname, "backend", route.Backend)

 	var policies []l7.PolicyRule
@@ -409,6 +433,7 @@ func (s *Server) handleL7(ctx context.Context, conn net.Conn, addr netip.Addr, c
 	}

 	rc := l7.RouteConfig{
+		Hostname:          hostname,
 		Backend:           route.Backend,
 		TLSCert:           route.TLSCert,
 		TLSKey:            route.TLSKey,