net/dns/resolver: add debug HTML handler to see what DNS traffic was forwarded

Change-Id: I6b790e92dcc608515ac8b178f2271adc9fd98f78 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2 years ago · 0aa4c6f147
parent ae319b4636
commit 0aa4c6f147
4 changed files with 114 additions and 0 deletions
--- a/health/health.go
+++ b/health/health.go
@ -9,6 +9,7 @@ package health
 import (
 	"errors"
 	"fmt"
+	"net/http"
 	"os"
 	"runtime"
 	"sort"
@ -28,6 +29,8 @@ var (
 	watchers = map[*watchHandle]func(Subsystem, error){} // opt func to run if error state changes
 	timer    *time.Timer

+	debugHandler = map[string]http.Handler{}
+
 	inMapPoll               bool
 	inMapPollSince          time.Time
 	lastMapPollEndedAt      time.Time
@ -116,6 +119,18 @@ func SetNetworkCategoryHealth(err error) { set(SysNetworkCategory, err) }

 func NetworkCategoryHealth() error { return get(SysNetworkCategory) }

+func RegisterDebugHandler(typ string, h http.Handler) {
+	mu.Lock()
+	defer mu.Unlock()
+	debugHandler[typ] = h
+}
+
+func DebugHandler(typ string) http.Handler {
+	mu.Lock()
+	defer mu.Unlock()
+	return debugHandler[typ]
+}
+
 func get(key Subsystem) error {
 	mu.Lock()
 	defer mu.Unlock()
--- a/ipn/ipnlocal/peerapi.go
+++ b/ipn/ipnlocal/peerapi.go
@ -32,6 +32,7 @@ import (
 	"golang.org/x/net/dns/dnsmessage"
 	"inet.af/netaddr"
 	"tailscale.com/client/tailscale/apitype"
+	"tailscale.com/health"
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn"
 	"tailscale.com/logtail/backoff"
@ -556,6 +557,9 @@ func (h *peerAPIHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	case "/v0/magicsock":
 		h.handleServeMagicsock(w, r)
 		return
+	case "/v0/dnsfwd":
+		h.handleServeDNSFwd(w, r)
+		return
 	}
 	who := h.peerUser.DisplayName
 	fmt.Fprintf(w, `<html>
@ -808,6 +812,19 @@ func (h *peerAPIHandler) handleServeMetrics(w http.ResponseWriter, r *http.Reque
 	clientmetric.WritePrometheusExpositionFormat(w)
 }

+func (h *peerAPIHandler) handleServeDNSFwd(w http.ResponseWriter, r *http.Request) {
+	if !h.isSelf {
+		http.Error(w, "not owner", http.StatusForbidden)
+		return
+	}
+	dh := health.DebugHandler("dnsfwd")
+	if dh == nil {
+		http.Error(w, "not wired up", 500)
+		return
+	}
+	dh.ServeHTTP(w, r)
+}
+
 func (h *peerAPIHandler) replyToDNSQueries() bool {
 	if h.isSelf {
 		// If the peer is owned by the same user, just allow it
--- a/net/dns/resolver/debug.go
+++ b/net/dns/resolver/debug.go
@ -0,0 +1,78 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package resolver
+
+import (
+	"fmt"
+	"html"
+	"net/http"
+	"strconv"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"tailscale.com/health"
+)
+
+func init() {
+	health.RegisterDebugHandler("dnsfwd", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		n, _ := strconv.Atoi(r.FormValue("n"))
+		if n <= 0 {
+			n = 100
+		} else if n > 10000 {
+			n = 10000
+		}
+		fl, ok := fwdLogAtomic.Load().(*fwdLog)
+		if !ok || n != len(fl.ent) {
+			fl = &fwdLog{ent: make([]fwdLogEntry, n)}
+			fwdLogAtomic.Store(fl)
+		}
+		fl.ServeHTTP(w, r)
+	}))
+}
+
+var fwdLogAtomic atomic.Value // of *fwdLog
+
+type fwdLog struct {
+	mu  sync.Mutex
+	pos int // ent[pos] is next entry
+	ent []fwdLogEntry
+}
+
+type fwdLogEntry struct {
+	Domain string
+	Time   time.Time
+}
+
+func (fl *fwdLog) addName(name string) {
+	if fl == nil {
+		return
+	}
+	fl.mu.Lock()
+	defer fl.mu.Unlock()
+	if len(fl.ent) == 0 {
+		return
+	}
+	fl.ent[fl.pos] = fwdLogEntry{Domain: name, Time: time.Now()}
+	fl.pos++
+	if fl.pos == len(fl.ent) {
+		fl.pos = 0
+	}
+}
+
+func (fl *fwdLog) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	fl.mu.Lock()
+	defer fl.mu.Unlock()
+
+	fmt.Fprintf(w, "<html><h1>DNS forwards</h1>")
+	now := time.Now()
+	for i := 0; i < len(fl.ent); i++ {
+		ent := fl.ent[(i+fl.pos)%len(fl.ent)]
+		if ent.Domain == "" {
+			continue
+		}
+		fmt.Fprintf(w, "%v ago: %v<br>\n", now.Sub(ent.Time).Round(time.Second), html.EscapeString(ent.Domain))
+	}
+}
--- a/net/dns/resolver/forwarder.go
+++ b/net/dns/resolver/forwarder.go
@ -611,6 +611,10 @@ func (f *forwarder) forwardWithDestChan(ctx context.Context, query packet, respo
 		}
 	}

+	if fl, ok := fwdLogAtomic.Load().(*fwdLog); ok {
+		fl.addName(string(domain))
+	}
+
 	clampEDNSSize(query.bs, maxResponseBytes)

 	if len(resolvers) == 0 {