wgengine/magicsock: export packet drop metric for outbound errors

This required sharing the dropped packet metric between two packages
(tstun and magicsock), so I've moved its definition to util/usermetric.

Updates tailscale/corp#22075

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

net/tstun/wrap.go

@@ -213,24 +213,14 @@ type Wrapper struct {
 }
 
 type metrics struct {
-	inboundDroppedPacketsTotal  *tsmetrics.MultiLabelMap[dropPacketLabel]
-	outboundDroppedPacketsTotal *tsmetrics.MultiLabelMap[dropPacketLabel]
+	inboundDroppedPacketsTotal  *tsmetrics.MultiLabelMap[usermetric.DropLabels]
+	outboundDroppedPacketsTotal *tsmetrics.MultiLabelMap[usermetric.DropLabels]
 }
 
 func registerMetrics(reg *usermetric.Registry) *metrics {
 	return &metrics{
-		inboundDroppedPacketsTotal: usermetric.NewMultiLabelMapWithRegistry[dropPacketLabel](
-			reg,
-			"tailscaled_inbound_dropped_packets_total",
-			"counter",
-			"Counts the number of dropped packets received by the node from other peers",
-		),
-		outboundDroppedPacketsTotal: usermetric.NewMultiLabelMapWithRegistry[dropPacketLabel](
-			reg,
-			"tailscaled_outbound_dropped_packets_total",
-			"counter",
-			"Counts the number of packets dropped while being sent to other peers",
-		),
+		inboundDroppedPacketsTotal:  reg.DroppedPacketsInbound(),
+		outboundDroppedPacketsTotal: reg.DroppedPacketsOutbound(),
 	}
 }
 
@@ -886,8 +876,8 @@ func (t *Wrapper) filterPacketOutboundToWireGuard(p *packet.Parsed, pc *peerConf
 
 	if filt.RunOut(p, t.filterFlags) != filter.Accept {
 		metricPacketOutDropFilter.Add(1)
-		t.metrics.outboundDroppedPacketsTotal.Add(dropPacketLabel{
-			Reason: DropReasonACL,
+		t.metrics.outboundDroppedPacketsTotal.Add(usermetric.DropLabels{
+			Reason: usermetric.ReasonACL,
 		}, 1)
 		return filter.Drop, gro
 	}
@@ -1158,8 +1148,8 @@ func (t *Wrapper) filterPacketInboundFromWireGuard(p *packet.Parsed, captHook ca
 
 	if outcome != filter.Accept {
 		metricPacketInDropFilter.Add(1)
-		t.metrics.inboundDroppedPacketsTotal.Add(dropPacketLabel{
-			Reason: DropReasonACL,
+		t.metrics.inboundDroppedPacketsTotal.Add(usermetric.DropLabels{
+			Reason: usermetric.ReasonACL,
 		}, 1)
 
 		// Tell them, via TSMP, we're dropping them due to the ACL.
@@ -1239,8 +1229,8 @@ func (t *Wrapper) Write(buffs [][]byte, offset int) (int, error) {
 		t.noteActivity()
 		_, err := t.tdevWrite(buffs, offset)
 		if err != nil {
-			t.metrics.inboundDroppedPacketsTotal.Add(dropPacketLabel{
-				Reason: DropReasonError,
+			t.metrics.inboundDroppedPacketsTotal.Add(usermetric.DropLabels{
+				Reason: usermetric.ReasonError,
 			}, int64(len(buffs)))
 		}
 		return len(buffs), err
@@ -1482,20 +1472,6 @@ var (
 	metricPacketOutDropSelfDisco = clientmetric.NewCounter("tstun_out_to_wg_drop_self_disco")
 )
 
-type DropReason string
-
-const (
-	DropReasonACL   DropReason = "acl"
-	DropReasonError DropReason = "error"
-)
-
-type dropPacketLabel struct {
-	// Reason indicates what we have done with the packet, and has the following values:
-	// - acl (rejected packets because of ACL)
-	// - error (rejected packets because of an error)
-	Reason DropReason
-}
-
 func (t *Wrapper) InstallCaptureHook(cb capture.Callback) {
 	t.captureHook.Store(cb)
 }

net/tstun/wrap_test.go

@@ -441,13 +441,13 @@ func TestFilter(t *testing.T) {
 	}
 
 	var metricInboundDroppedPacketsACL, metricInboundDroppedPacketsErr, metricOutboundDroppedPacketsACL int64
-	if m, ok := tun.metrics.inboundDroppedPacketsTotal.Get(dropPacketLabel{Reason: DropReasonACL}).(*expvar.Int); ok {
+	if m, ok := tun.metrics.inboundDroppedPacketsTotal.Get(usermetric.DropLabels{Reason: usermetric.ReasonACL}).(*expvar.Int); ok {
 		metricInboundDroppedPacketsACL = m.Value()
 	}
-	if m, ok := tun.metrics.inboundDroppedPacketsTotal.Get(dropPacketLabel{Reason: DropReasonError}).(*expvar.Int); ok {
+	if m, ok := tun.metrics.inboundDroppedPacketsTotal.Get(usermetric.DropLabels{Reason: usermetric.ReasonError}).(*expvar.Int); ok {
 		metricInboundDroppedPacketsErr = m.Value()
 	}
-	if m, ok := tun.metrics.outboundDroppedPacketsTotal.Get(dropPacketLabel{Reason: DropReasonACL}).(*expvar.Int); ok {
+	if m, ok := tun.metrics.outboundDroppedPacketsTotal.Get(usermetric.DropLabels{Reason: usermetric.ReasonACL}).(*expvar.Int); ok {
 		metricOutboundDroppedPacketsACL = m.Value()
 	}
 

util/usermetric/metrics.go

@@ -0,0 +1,69 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// This file contains user-facing metrics that are used by multiple packages.
+// Use it to define more common metrics. Any changes to the registry and
+// metric types should be in usermetric.go.
+
+package usermetric
+
+import (
+	"sync"
+
+	"tailscale.com/metrics"
+)
+
+// Metrics contains user-facing metrics that are used by multiple packages.
+type Metrics struct {
+	initOnce sync.Once
+
+	droppedPacketsInbound  *metrics.MultiLabelMap[DropLabels]
+	droppedPacketsOutbound *metrics.MultiLabelMap[DropLabels]
+}
+
+// DropReason is the reason why a packet was dropped.
+type DropReason string
+
+const (
+	// ReasonACL means that the packet was not permitted by ACL.
+	ReasonACL DropReason = "acl"
+
+	// ReasonError means that the packet was dropped because of an error.
+	ReasonError DropReason = "error"
+)
+
+// DropLabels contains common label(s) for dropped packet counters.
+type DropLabels struct {
+	Reason DropReason
+}
+
+// initOnce initializes the common metrics.
+func (r *Registry) initOnce() {
+	r.m.initOnce.Do(func() {
+		r.m.droppedPacketsInbound = NewMultiLabelMapWithRegistry[DropLabels](
+			r,
+			"tailscaled_inbound_dropped_packets_total",
+			"counter",
+			"Counts the number of dropped packets received by the node from other peers",
+		)
+		r.m.droppedPacketsOutbound = NewMultiLabelMapWithRegistry[DropLabels](
+			r,
+			"tailscaled_outbound_dropped_packets_total",
+			"counter",
+			"Counts the number of packets dropped while being sent to other peers",
+		)
+	})
+}
+
+// DroppedPacketsOutbound returns the outbound dropped packet metric, creating it
+// if necessary.
+func (r *Registry) DroppedPacketsOutbound() *metrics.MultiLabelMap[DropLabels] {
+	r.initOnce()
+	return r.m.droppedPacketsOutbound
+}
+
+// DroppedPacketsInbound returns the inbound dropped packet metric.
+func (r *Registry) DroppedPacketsInbound() *metrics.MultiLabelMap[DropLabels] {
+	r.initOnce()
+	return r.m.droppedPacketsInbound
+}

util/usermetric/usermetric.go

@@ -19,6 +19,9 @@ import (
 // Registry tracks user-facing metrics of various Tailscale subsystems.
 type Registry struct {
 	vars expvar.Map
+
+	// m contains common metrics owned by the registry.
+	m Metrics
 }
 
 // NewMultiLabelMapWithRegistry creates and register a new

wgengine/magicsock/derp.go

@@ -674,6 +674,9 @@ func (c *Conn) runDerpWriter(ctx context.Context, dc *derphttp.Client, ch <-chan
 			if err != nil {
 				c.logf("magicsock: derp.Send(%v): %v", wr.addr, err)
 				metricSendDERPError.Add(1)
+				if !wr.isDisco {
+					c.metrics.outboundPacketsDroppedErrors.Add(1)
+				}
 			} else if !wr.isDisco {
 				c.metrics.outboundPacketsDERPTotal.Add(1)
 				c.metrics.outboundBytesDERPTotal.Add(int64(len(wr.b)))

wgengine/magicsock/magicsock.go

@@ -127,6 +127,10 @@ type metrics struct {
 	outboundBytesIPv4Total expvar.Int
 	outboundBytesIPv6Total expvar.Int
 	outboundBytesDERPTotal expvar.Int
+
+	// outboundPacketsDroppedErrors is the total number of outbound packets
+	// dropped due to errors.
+	outboundPacketsDroppedErrors expvar.Int
 }
 
 // A Conn routes UDP packets and actively manages a list of its endpoints.
@@ -605,6 +609,8 @@ func registerMetrics(reg *usermetric.Registry) *metrics {
 		"counter",
 		"Counts the number of bytes sent to other peers",
 	)
+	outboundPacketsDroppedErrors := reg.DroppedPacketsOutbound()
+
 	m := new(metrics)
 
 	// Map clientmetrics to the usermetric counters.
@@ -631,6 +637,8 @@ func registerMetrics(reg *usermetric.Registry) *metrics {
 	outboundBytesTotal.Set(pathDirectV6, &m.outboundBytesIPv6Total)
 	outboundBytesTotal.Set(pathDERP, &m.outboundBytesDERPTotal)
 
+	outboundPacketsDroppedErrors.Set(usermetric.DropLabels{Reason: usermetric.ReasonError}, &m.outboundPacketsDroppedErrors)
+
 	return m
 }
 
@@ -1202,8 +1210,13 @@ func (c *Conn) networkDown() bool { return !c.networkUp.Load() }
 // Send implements conn.Bind.
 //
 // See https://pkg.go.dev/golang.zx2c4.com/wireguard/conn#Bind.Send
-func (c *Conn) Send(buffs [][]byte, ep conn.Endpoint) error {
+func (c *Conn) Send(buffs [][]byte, ep conn.Endpoint) (err error) {
 	n := int64(len(buffs))
+	defer func() {
+		if err != nil {
+			c.metrics.outboundPacketsDroppedErrors.Add(n)
+		}
+	}()
 	metricSendData.Add(n)
 	if c.networkDown() {
 		metricSendDataNetworkDown.Add(n)

wgengine/magicsock/magicsock_test.go

@@ -63,6 +63,7 @@ import (
 	"tailscale.com/types/nettype"
 	"tailscale.com/types/ptr"
 	"tailscale.com/util/cibuild"
+	"tailscale.com/util/must"
 	"tailscale.com/util/racebuild"
 	"tailscale.com/util/set"
 	"tailscale.com/util/usermetric"
@@ -3083,3 +3084,27 @@ func TestMaybeRebindOnError(t *testing.T) {
 		}
 	})
 }
+
+func TestNetworkDownSendErrors(t *testing.T) {
+	netMon := must.Get(netmon.New(t.Logf))
+	defer netMon.Close()
+
+	reg := new(usermetric.Registry)
+	conn := must.Get(NewConn(Options{
+		DisablePortMapper: true,
+		Logf:              t.Logf,
+		NetMon:            netMon,
+		Metrics:           reg,
+	}))
+	defer conn.Close()
+
+	conn.SetNetworkUp(false)
+	if err := conn.Send([][]byte{{00}}, &lazyEndpoint{}); err == nil {
+		t.Error("expected error, got nil")
+	}
+	resp := httptest.NewRecorder()
+	reg.Handler(resp, new(http.Request))
+	if !strings.Contains(resp.Body.String(), `tailscaled_outbound_dropped_packets_total{reason="error"} 1`) {
+		t.Errorf("expected NetworkDown to increment packet dropped metric; got %q", resp.Body.String())
+	}
+}