all: update tailscale.com to v1.74.0

internal/dns/direct_linux.go

@@ -1,26 +1,26 @@
-// Copyright (c) 2022 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.
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
 
 package dns
 
 import (
+	"bytes"
 	"context"
 
-	"github.com/illarion/gonotify"
+	"github.com/illarion/gonotify/v2"
+	"tailscale.com/health"
 )
 
 func (m *directManager) runFileWatcher() {
-	in, err := gonotify.NewInotify()
+	ctx, cancel := context.WithCancel(m.ctx)
+	defer cancel()
+	in, err := gonotify.NewInotify(ctx)
 	if err != nil {
 		// Oh well, we tried. This is all best effort for now, to
 		// surface warnings to users.
 		m.logf("dns: inotify new: %v", err)
 		return
 	}
-	ctx, cancel := context.WithCancel(m.ctx)
-	defer cancel()
-	go m.closeInotifyOnDone(ctx, in)
 
 	const events = gonotify.IN_ATTRIB |
 		gonotify.IN_CLOSE_WRITE |
@@ -56,7 +56,53 @@ func (m *directManager) runFileWatcher() {
 	}
 }
 
-func (m *directManager) closeInotifyOnDone(ctx context.Context, in *gonotify.Inotify) {
-	<-ctx.Done()
-	_ = in.Close()
+var resolvTrampleWarnable = health.Register(&health.Warnable{
+	Code:     "ctrld-resolv-conf-overwritten",
+	Severity: health.SeverityMedium,
+	Title:    "Linux DNS configuration issue",
+	Text:     health.StaticMessage("Linux DNS config not ideal. /etc/resolv.conf overwritten. See https://tailscale.com/s/dns-fight"),
+})
+
+// checkForFileTrample checks whether /etc/resolv.conf has been trampled
+// by another program on the system. (e.g. a DHCP client)
+func (m *directManager) checkForFileTrample() {
+	m.mu.Lock()
+	want := m.wantResolvConf
+	lastWarn := m.lastWarnContents
+	m.mu.Unlock()
+
+	if want == nil {
+		return
+	}
+
+	cur, err := m.fs.ReadFile(resolvConf)
+	if err != nil {
+		m.logf("trample: read error: %v", err)
+		return
+	}
+	if bytes.Equal(cur, want) {
+		m.health.SetHealthy(resolvTrampleWarnable)
+		if lastWarn != nil {
+			m.mu.Lock()
+			m.lastWarnContents = nil
+			m.mu.Unlock()
+			m.logf("trample: resolv.conf again matches expected content")
+		}
+		return
+	}
+	if bytes.Equal(cur, lastWarn) {
+		// We already logged about this, so not worth doing it again.
+		return
+	}
+
+	m.mu.Lock()
+	m.lastWarnContents = cur
+	m.mu.Unlock()
+
+	show := cur
+	if len(show) > 1024 {
+		show = show[:1024]
+	}
+	m.logf("trample: resolv.conf changed from what we expected. did some other program interfere? current contents: %q", show)
+	m.health.SetUnhealthy(resolvTrampleWarnable, nil)
 }