diff --git a/cmd/derper/depaware.txt b/cmd/derper/depaware.txt
index 6608faaf7..9c720fa60 100644
--- a/cmd/derper/depaware.txt
+++ b/cmd/derper/depaware.txt
@@ -2,6 +2,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
 
         filippo.io/edwards25519                                      from github.com/hdevalence/ed25519consensus
         filippo.io/edwards25519/field                                from filippo.io/edwards25519
+        github.com/axiomhq/hyperloglog                               from tailscale.com/derp/derpserver
         github.com/beorn7/perks/quantile                             from github.com/prometheus/client_golang/prometheus
      💣 github.com/cespare/xxhash/v2                                 from github.com/prometheus/client_golang/prometheus
         github.com/coder/websocket                                   from tailscale.com/cmd/derper+
@@ -9,6 +10,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
         github.com/coder/websocket/internal/util                     from github.com/coder/websocket
         github.com/coder/websocket/internal/xsync                    from github.com/coder/websocket
    W 💣 github.com/dblohm7/wingoes                                   from tailscale.com/util/winutil
+        github.com/dgryski/go-metro                                  from github.com/axiomhq/hyperloglog
         github.com/fxamacker/cbor/v2                                 from tailscale.com/tka
         github.com/go-json-experiment/json                           from tailscale.com/types/opt+
         github.com/go-json-experiment/json/internal                  from github.com/go-json-experiment/json+
diff --git a/derp/derpserver/derpserver.go b/derp/derpserver/derpserver.go
index 0bbc66780..1879e0c53 100644
--- a/derp/derpserver/derpserver.go
+++ b/derp/derpserver/derpserver.go
@@ -36,6 +36,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/axiomhq/hyperloglog"
 	"go4.org/mem"
 	"golang.org/x/sync/errgroup"
 	"tailscale.com/client/local"
@@ -1643,6 +1644,12 @@ type sclient struct {
 	sawSrc map[key.NodePublic]set.Handle
 	bw     *lazyBufioWriter
 
+	// senderCardinality estimates the number of unique peers that have
+	// sent packets to this client. Owned by sendLoop, protected by
+	// senderCardinalityMu for reads from other goroutines.
+	senderCardinalityMu sync.Mutex
+	senderCardinality   *hyperloglog.Sketch
+
 	// Guarded by s.mu
 	//
 	// peerStateChange is used by mesh peers (a set of regional
@@ -1778,6 +1785,8 @@ func (c *sclient) onSendLoopDone() {
 func (c *sclient) sendLoop(ctx context.Context) error {
 	defer c.onSendLoopDone()
 
+	c.senderCardinality = hyperloglog.New()
+
 	jitter := rand.N(5 * time.Second)
 	keepAliveTick, keepAliveTickChannel := c.s.clock.NewTicker(derp.KeepAlive + jitter)
 	defer keepAliveTick.Stop()
@@ -2000,6 +2009,11 @@ func (c *sclient) sendPacket(srcKey key.NodePublic, contents []byte) (err error)
 	if withKey {
 		pktLen += key.NodePublicRawLen
 		c.noteSendFromSrc(srcKey)
+		if c.senderCardinality != nil {
+			c.senderCardinalityMu.Lock()
+			c.senderCardinality.Insert(srcKey.AppendTo(nil))
+			c.senderCardinalityMu.Unlock()
+		}
 	}
 	if err = derp.WriteFrameHeader(c.bw.bw(), derp.FrameRecvPacket, uint32(pktLen)); err != nil {
 		return err
@@ -2013,6 +2027,17 @@ func (c *sclient) sendPacket(srcKey key.NodePublic, contents []byte) (err error)
 	return err
 }
 
+// EstimatedUniqueSenders returns an estimate of the number of unique peers
+// that have sent packets to this client.
+func (c *sclient) EstimatedUniqueSenders() uint64 {
+	c.senderCardinalityMu.Lock()
+	defer c.senderCardinalityMu.Unlock()
+	if c.senderCardinality == nil {
+		return 0
+	}
+	return c.senderCardinality.Estimate()
+}
+
 // noteSendFromSrc notes that we are about to write a packet
 // from src to sclient.
 //
@@ -2295,7 +2320,8 @@ type BytesSentRecv struct {
 	Sent uint64
 	Recv uint64
 	// Key is the public key of the client which sent/received these bytes.
-	Key key.NodePublic
+	Key           key.NodePublic
+	UniqueSenders uint64 `json:",omitzero"`
 }
 
 // parseSSOutput parses the output from the specific call to ss in ServeDebugTraffic.
@@ -2349,6 +2375,11 @@ func (s *Server) ServeDebugTraffic(w http.ResponseWriter, r *http.Request) {
 			if prev.Sent < next.Sent || prev.Recv < next.Recv {
 				if pkey, ok := s.keyOfAddr[k]; ok {
 					next.Key = pkey
+					if cs, ok := s.clients[pkey]; ok {
+						if c := cs.activeClient.Load(); c != nil {
+							next.UniqueSenders = c.EstimatedUniqueSenders()
+						}
+					}
 					if err := enc.Encode(next); err != nil {
 						s.mu.Unlock()
 						return
diff --git a/derp/derpserver/derpserver_test.go b/derp/derpserver/derpserver_test.go
index 2db5f25bc..1dd86f314 100644
--- a/derp/derpserver/derpserver_test.go
+++ b/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)
+		}
+	})
+}
diff --git a/flake.nix b/flake.nix
index 505061a76..855ce555b 100644
--- a/flake.nix
+++ b/flake.nix
@@ -151,4 +151,4 @@
     });
   };
 }
-# nix-direnv cache busting line: sha256-jJSSXMyUqcJoZuqfSlBsKDQezyqS+jDkRglMMjG1K8g=
+# nix-direnv cache busting line: sha256-IkodqRYdueML7U2Hh8vRw6Et7+WII+VXuPJ3jZ2xYx8=
diff --git a/go.mod b/go.mod
index a49a9724f..bd6fe441d 100644
--- a/go.mod
+++ b/go.mod
@@ -16,6 +16,7 @@ require (
 	github.com/aws/aws-sdk-go-v2/feature/s3/manager v1.17.58
 	github.com/aws/aws-sdk-go-v2/service/s3 v1.75.3
 	github.com/aws/aws-sdk-go-v2/service/ssm v1.44.7
+	github.com/axiomhq/hyperloglog v0.0.0-20240319100328-84253e514e02
 	github.com/bradfitz/go-tool-cache v0.0.0-20251113223507-0124e698e0bd
 	github.com/bramvdbogaerde/go-scp v1.4.0
 	github.com/cilium/ebpf v0.15.0
@@ -149,6 +150,7 @@ require (
 	github.com/containerd/typeurl/v2 v2.2.3 // indirect
 	github.com/cyphar/filepath-securejoin v0.3.6 // indirect
 	github.com/deckarep/golang-set/v2 v2.8.0 // indirect
+	github.com/dgryski/go-metro v0.0.0-20180109044635-280f6062b5bc // indirect
 	github.com/docker/go-connections v0.5.0 // indirect
 	github.com/docker/go-units v0.5.0 // indirect
 	github.com/felixge/httpsnoop v1.0.4 // indirect
diff --git a/go.mod.sri b/go.mod.sri
index 66422652e..329fe9405 100644
--- a/go.mod.sri
+++ b/go.mod.sri
@@ -1 +1 @@
-sha256-jJSSXMyUqcJoZuqfSlBsKDQezyqS+jDkRglMMjG1K8g=
+sha256-IkodqRYdueML7U2Hh8vRw6Et7+WII+VXuPJ3jZ2xYx8=
diff --git a/go.sum b/go.sum
index f70fe9159..111c99ac9 100644
--- a/go.sum
+++ b/go.sum
@@ -170,6 +170,8 @@ github.com/aws/aws-sdk-go-v2/service/sts v1.33.13 h1:3LXNnmtH3TURctC23hnC0p/39Q5
 github.com/aws/aws-sdk-go-v2/service/sts v1.33.13/go.mod h1:7Yn+p66q/jt38qMoVfNvjbm3D89mGBnkwDcijgtih8w=
 github.com/aws/smithy-go v1.22.2 h1:6D9hW43xKFrRx/tXXfAlIZc4JI+yQe6snnWcQyxSyLQ=
 github.com/aws/smithy-go v1.22.2/go.mod h1:irrKGvNn1InZwb2d7fkIRNucdfwR8R+Ts3wxYa/cJHg=
+github.com/axiomhq/hyperloglog v0.0.0-20240319100328-84253e514e02 h1:bXAPYSbdYbS5VTy92NIUbeDI1qyggi+JYh5op9IFlcQ=
+github.com/axiomhq/hyperloglog v0.0.0-20240319100328-84253e514e02/go.mod h1:k08r+Yj1PRAmuayFiRK6MYuR5Ve4IuZtTfxErMIh0+c=
 github.com/beorn7/perks v0.0.0-20180321164747-3a771d992973/go.mod h1:Dwedo/Wpr24TaqPxmxbtue+5NUziq4I4S80YR8gNf3Q=
 github.com/beorn7/perks v1.0.0/go.mod h1:KWe93zE9D1o94FZ5RNwFwVgaQK1VOXiVxmqh+CedLV8=
 github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
@@ -271,6 +273,8 @@ github.com/deckarep/golang-set/v2 v2.8.0 h1:swm0rlPCmdWn9mESxKOjWk8hXSqoxOp+Zlfu
 github.com/deckarep/golang-set/v2 v2.8.0/go.mod h1:VAky9rY/yGXJOLEDv3OMci+7wtDpOF4IN+y82NBOac4=
 github.com/denis-tingaikin/go-header v0.5.0 h1:SRdnP5ZKvcO9KKRP1KJrhFR3RrlGuD+42t4429eC9k8=
 github.com/denis-tingaikin/go-header v0.5.0/go.mod h1:mMenU5bWrok6Wl2UsZjy+1okegmwQ3UgWl4V1D8gjlY=
+github.com/dgryski/go-metro v0.0.0-20180109044635-280f6062b5bc h1:8WFBn63wegobsYAX0YjD+8suexZDga5CctH4CCTx2+8=
+github.com/dgryski/go-metro v0.0.0-20180109044635-280f6062b5bc/go.mod h1:c9O8+fpSOX1DM8cPNSkX/qsBWdkD4yd2dpciOWQjpBw=
 github.com/digitalocean/go-smbios v0.0.0-20180907143718-390a4f403a8e h1:vUmf0yezR0y7jJ5pceLHthLaYf4bA5T14B6q39S4q2Q=
 github.com/digitalocean/go-smbios v0.0.0-20180907143718-390a4f403a8e/go.mod h1:YTIHhz/QFSYnu/EhlF2SpU2Uk+32abacUYA5ZPljz1A=
 github.com/distribution/reference v0.6.0 h1:0IXCQ5g4/QMHHkarYzh5l+u8T3t73zM5QvfrDyIgxBk=
diff --git a/shell.nix b/shell.nix
index d412693d9..28bdbdafb 100644
--- a/shell.nix
+++ b/shell.nix
@@ -16,4 +16,4 @@
 ) {
   src =  ./.;
 }).shellNix
-# nix-direnv cache busting line: sha256-jJSSXMyUqcJoZuqfSlBsKDQezyqS+jDkRglMMjG1K8g=
+# nix-direnv cache busting line: sha256-IkodqRYdueML7U2Hh8vRw6Et7+WII+VXuPJ3jZ2xYx8=