1 files changed, 129 insertions, 0 deletions
diff --git a/wgengine/netstack/debug.go b/wgengine/netstack/debug.go
new file mode 100644
index 000000000..1259f63a9
--- /dev/null
+++ b/wgengine/netstack/debug.go
@@ -0,0 +1,129 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package netstack
+
+import (
+	"html/template"
+	"net/http"
+	"net/netip"
+	"slices"
+	"time"
+
+	xmaps "golang.org/x/exp/maps"
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
+
+var tcpForwarderTemplate = template.Must(template.New("").Parse(`
+<html>
+	<head>
+		<meta name="viewport" content="width=device-width, initial-scale=1">
+		<style>
+		body { font-family: monospace; font-size: 12; }
+		td { padding: 0.3em; }
+		</style>
+	</head>
+	<body>
+	<h1>TCP Forwarder</h1>
+
+	<h2>TCP Statistics</h2>
+	<table border=1>
+		<tr>
+			<th>Metric</th>
+			<th>Value</th>
+		</tr>
+		{{ range .Stats }}
+		<tr><td>{{ .Key }}</td><td>{{ .Value }}</td></tr>
+		{{ end }}
+	</table>
+
+	<h2>In-Flight Outbound Connections</h2>
+	<table border=1>
+		<tr>
+			<th>Start Time</th>
+			<th>Client IP</th>
+			<th>Remote IP</th>
+		</tr>
+		{{ range .InFlightDials }}
+		<tr>
+			<td>{{ .Start.Format "2006-01-02T15:04:05Z07:00" }} ({{ printf "%.2f" .DurationSecs }} seconds ago)</td>
+			<td>{{ .ClientIP }}</td>
+			<td>{{ .RemoteAddr }}</td>
+		</tr>
+		{{ end }}
+	</table>
+</body>
+</html>
+`))
+
+// DebugTCPForwarder writes debug information about this netstack
+// implementation's current TCP forwarder in HTML format.
+func (ns *Impl) DebugTCPForwarder(w http.ResponseWriter, r *http.Request) {
+	// Grab data while holding the mutex
+	ns.tcpDebugMu.Lock()
+	tcpDials := xmaps.Values(ns.inFlightDials)
+	ns.tcpDebugMu.Unlock()
+
+	slices.SortFunc(tcpDials, func(a, b tcpDialInfo) int {
+		return a.start.Compare(b.start)
+	})
+
+	type templateDataStats struct {
+		Key   string
+		Value uint64
+	}
+	type templateDataDial struct {
+		Start        time.Time
+		DurationSecs float64
+		ClientIP     netip.Addr
+		RemoteAddr   netip.AddrPort
+	}
+	type templateData struct {
+		Stats         []templateDataStats
+		InFlightDials []templateDataDial
+	}
+
+	var data templateData
+
+	// Statistics from gVisor
+	tcpStats := ns.ipstack.Stats().TCP
+	tcpMetrics := []struct {
+		name  string
+		field *tcpip.StatCounter
+	}{
+		{"Active Connection Openings", tcpStats.ActiveConnectionOpenings},
+		{"Passive Connection Openings", tcpStats.PassiveConnectionOpenings},
+		{"Established Connections", tcpStats.CurrentEstablished},
+		{"Connected Connections", tcpStats.CurrentConnected},
+		{"Dropped In-Flight Forwarder Connections", tcpStats.ForwardMaxInFlightDrop},
+		{"Established Resets", tcpStats.EstablishedResets},
+		{"Established Timeout", tcpStats.EstablishedTimedout},
+		{"Failed Connection Attempts", tcpStats.FailedConnectionAttempts},
+		{"Retransmits", tcpStats.Retransmits},
+		{"Timeouts", tcpStats.Timeouts},
+		{"Checksum Errors", tcpStats.ChecksumErrors},
+		{"Failed Port Reservations", tcpStats.FailedPortReservations},
+	}
+	for _, metric := range tcpMetrics {
+		data.Stats = append(data.Stats, templateDataStats{
+			Key:   metric.name,
+			Value: metric.field.Value(),
+		})
+	}
+
+	// Any in-flight DialContext calls in the TCP forwarding path.
+	now := time.Now()
+	for _, dial := range tcpDials {
+		elapsed := now.Sub(dial.start)
+		data.InFlightDials = append(data.InFlightDials, templateDataDial{
+			Start:        dial.start,
+			DurationSecs: elapsed.Seconds(),
+			ClientIP:     dial.clientRemoteIP,
+			RemoteAddr:   dial.dialAddr,
+		})
+	}
+
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	w.Header().Set("Cache-Control", "no-cache")
+	tcpForwarderTemplate.Execute(w, &data)
+}