derp/derpserver: add a unique sender cardinality estimate

Adds an observation point that may identify potentially abusive traffic
patterns at outlier values.

Updates tailscale/corp#24681

Signed-off-by: James Tucker <james@tailscale.com>

derp/derpserver/derpserver_test.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"crypto/x509"
 	"encoding/asn1"
+	"encoding/binary"
 	"expvar"
 	"fmt"
 	"log"
@@ -20,6 +21,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/axiomhq/hyperloglog"
 	qt "github.com/frankban/quicktest"
 	"go4.org/mem"
 	"golang.org/x/time/rate"
@@ -755,6 +757,35 @@ func TestParseSSOutput(t *testing.T) {
 	}
 }
 
+func TestServeDebugTrafficUniqueSenders(t *testing.T) {
+	s := New(key.NewNode(), t.Logf)
+	defer s.Close()
+
+	clientKey := key.NewNode().Public()
+	c := &sclient{
+		key:               clientKey,
+		s:                 s,
+		logf:              logger.Discard,
+		senderCardinality: hyperloglog.New(),
+	}
+
+	for i := 0; i < 5; i++ {
+		c.senderCardinality.Insert(key.NewNode().Public().AppendTo(nil))
+	}
+
+	s.mu.Lock()
+	cs := &clientSet{}
+	cs.activeClient.Store(c)
+	s.clients[clientKey] = cs
+	s.mu.Unlock()
+
+	estimate := c.EstimatedUniqueSenders()
+	t.Logf("Estimated unique senders: %d", estimate)
+	if estimate < 4 || estimate > 6 {
+		t.Errorf("EstimatedUniqueSenders() = %d, want ~5 (4-6 range)", estimate)
+	}
+}
+
 func TestGetPerClientSendQueueDepth(t *testing.T) {
 	c := qt.New(t)
 	envKey := "TS_DEBUG_DERP_PER_CLIENT_SEND_QUEUE_DEPTH"
@@ -780,3 +811,167 @@ func TestGetPerClientSendQueueDepth(t *testing.T) {
 		})
 	}
 }
+
+func TestSenderCardinality(t *testing.T) {
+	s := New(key.NewNode(), t.Logf)
+	defer s.Close()
+
+	c := &sclient{
+		key:  key.NewNode().Public(),
+		s:    s,
+		logf: logger.WithPrefix(t.Logf, "test client: "),
+	}
+
+	if got := c.EstimatedUniqueSenders(); got != 0 {
+		t.Errorf("EstimatedUniqueSenders() before init = %d, want 0", got)
+	}
+
+	c.senderCardinality = hyperloglog.New()
+
+	if got := c.EstimatedUniqueSenders(); got != 0 {
+		t.Errorf("EstimatedUniqueSenders() with no senders = %d, want 0", got)
+	}
+
+	senders := make([]key.NodePublic, 10)
+	for i := range senders {
+		senders[i] = key.NewNode().Public()
+		c.senderCardinality.Insert(senders[i].AppendTo(nil))
+	}
+
+	estimate := c.EstimatedUniqueSenders()
+	t.Logf("Estimated unique senders after 10 inserts: %d", estimate)
+
+	if estimate < 8 || estimate > 12 {
+		t.Errorf("EstimatedUniqueSenders() = %d, want ~10 (8-12 range)", estimate)
+	}
+
+	for i := 0; i < 5; i++ {
+		c.senderCardinality.Insert(senders[i].AppendTo(nil))
+	}
+
+	estimate2 := c.EstimatedUniqueSenders()
+	t.Logf("Estimated unique senders after duplicates: %d", estimate2)
+
+	if estimate2 < 8 || estimate2 > 12 {
+		t.Errorf("EstimatedUniqueSenders() after duplicates = %d, want ~10 (8-12 range)", estimate2)
+	}
+}
+
+func TestSenderCardinality100(t *testing.T) {
+	s := New(key.NewNode(), t.Logf)
+	defer s.Close()
+
+	c := &sclient{
+		key:               key.NewNode().Public(),
+		s:                 s,
+		logf:              logger.WithPrefix(t.Logf, "test client: "),
+		senderCardinality: hyperloglog.New(),
+	}
+
+	numSenders := 100
+	for i := 0; i < numSenders; i++ {
+		c.senderCardinality.Insert(key.NewNode().Public().AppendTo(nil))
+	}
+
+	estimate := c.EstimatedUniqueSenders()
+	t.Logf("Estimated unique senders for 100 actual senders: %d", estimate)
+
+	if estimate < 85 || estimate > 115 {
+		t.Errorf("EstimatedUniqueSenders() = %d, want ~100 (85-115 range)", estimate)
+	}
+}
+
+func TestSenderCardinalityTracking(t *testing.T) {
+	s := New(key.NewNode(), t.Logf)
+	defer s.Close()
+
+	c := &sclient{
+		key:               key.NewNode().Public(),
+		s:                 s,
+		logf:              logger.WithPrefix(t.Logf, "test client: "),
+		senderCardinality: hyperloglog.New(),
+	}
+
+	zeroKey := key.NodePublic{}
+	if zeroKey != (key.NodePublic{}) {
+		c.senderCardinality.Insert(zeroKey.AppendTo(nil))
+	}
+
+	if estimate := c.EstimatedUniqueSenders(); estimate != 0 {
+		t.Errorf("EstimatedUniqueSenders() after zero key = %d, want 0", estimate)
+	}
+
+	sender1 := key.NewNode().Public()
+	sender2 := key.NewNode().Public()
+
+	if sender1 != (key.NodePublic{}) {
+		c.senderCardinality.Insert(sender1.AppendTo(nil))
+	}
+	if sender2 != (key.NodePublic{}) {
+		c.senderCardinality.Insert(sender2.AppendTo(nil))
+	}
+
+	estimate := c.EstimatedUniqueSenders()
+	t.Logf("Estimated unique senders after 2 senders: %d", estimate)
+
+	if estimate < 1 || estimate > 3 {
+		t.Errorf("EstimatedUniqueSenders() = %d, want ~2 (1-3 range)", estimate)
+	}
+}
+
+func BenchmarkHyperLogLogInsert(b *testing.B) {
+	hll := hyperloglog.New()
+	sender := key.NewNode().Public()
+	senderBytes := sender.AppendTo(nil)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		hll.Insert(senderBytes)
+	}
+}
+
+func BenchmarkHyperLogLogInsertUnique(b *testing.B) {
+	hll := hyperloglog.New()
+
+	b.ResetTimer()
+
+	buf := make([]byte, 32)
+	for i := 0; i < b.N; i++ {
+		binary.LittleEndian.PutUint64(buf, uint64(i))
+		hll.Insert(buf)
+	}
+}
+
+func BenchmarkHyperLogLogEstimate(b *testing.B) {
+	hll := hyperloglog.New()
+
+	for i := 0; i < 100; i++ {
+		hll.Insert(key.NewNode().Public().AppendTo(nil))
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = hll.Estimate()
+	}
+}
+
+func BenchmarkSenderCardinalityOverhead(b *testing.B) {
+	hll := hyperloglog.New()
+	sender := key.NewNode().Public()
+
+	b.Run("WithTracking", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			if hll != nil {
+				hll.Insert(sender.AppendTo(nil))
+			}
+		}
+	})
+
+	b.Run("WithoutTracking", func(b *testing.B) {
+		b.ReportAllocs()
+		for i := 0; i < b.N; i++ {
+			_ = sender.AppendTo(nil)
+		}
+	})
+}