all: fork tailscale Linux dns manager package

With modification to fit our use case.

internal/dns/direct_linux.go

@@ -0,0 +1,62 @@
+// 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.
+
+package dns
+
+import (
+	"context"
+
+	"github.com/illarion/gonotify"
+)
+
+func (m *directManager) runFileWatcher() {
+	in, err := gonotify.NewInotify()
+	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 |
+		gonotify.IN_CREATE |
+		gonotify.IN_DELETE |
+		gonotify.IN_MODIFY |
+		gonotify.IN_MOVE
+
+	if err := in.AddWatch("/etc/", events); err != nil {
+		m.logf("dns: inotify addwatch: %v", err)
+		return
+	}
+	for {
+		events, err := in.Read()
+		if ctx.Err() != nil {
+			return
+		}
+		if err != nil {
+			m.logf("dns: inotify read: %v", err)
+			return
+		}
+		var match bool
+		for _, ev := range events {
+			if ev.Name == resolvConf {
+				match = true
+				break
+			}
+		}
+		if !match {
+			continue
+		}
+		m.checkForFileTrample()
+	}
+}
+
+func (m *directManager) closeInotifyOnDone(ctx context.Context, in *gonotify.Inotify) {
+	<-ctx.Done()
+	_ = in.Close()
+}