mirror of https://github.com/tailscale/tailscale/
net/dns: retrample resolve.conf when another process has trampled it
When using the resolve.conf file for setting DNS, it is possible that some other services will trample the file and overwrite our set DNS server. Experiments has shown this to be a racy error depending on how quickly processes start. Make an attempt to trample back the file a limited number of times if the file is changed. Updates #16635 Signed-off-by: Claus Lensbøl <claus@tailscale.com>cmol/resolveconf_trample_trample_back
Claus Lensbøl
2 weeks ago
parent
b7658a4ad2
commit
0feb1e9a27
@ -0,0 +1,109 @@
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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//go:build linux
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package dns
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import (
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"context"
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"fmt"
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"net/netip"
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"os"
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"path/filepath"
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"testing"
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"testing/synctest"
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"github.com/illarion/gonotify/v3"
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"tailscale.com/util/dnsname"
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"tailscale.com/util/eventbus/eventbustest"
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)
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func TestDNSTrampleRecovery(t *testing.T) {
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HookWatchFile.Set(watchFile)
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synctest.Test(t, func(t *testing.T) {
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tmp := t.TempDir()
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if err := os.MkdirAll(filepath.Join(tmp, "etc"), 0700); err != nil {
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t.Fatal(err)
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}
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const resolvPath = "/etc/resolv.conf"
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fs := directFS{prefix: tmp}
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readFile := func(t *testing.T, path string) string {
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t.Helper()
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b, err := fs.ReadFile(path)
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if err != nil {
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t.Errorf("Reading DNS config: %v", err)
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}
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return string(b)
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}
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bus := eventbustest.NewBus(t)
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eventbustest.LogAllEvents(t, bus)
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m := newDirectManagerOnFS(t.Logf, nil, bus, fs)
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defer m.Close()
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if err := m.SetDNS(OSConfig{
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Nameservers: []netip.Addr{netip.MustParseAddr("8.8.8.8"), netip.MustParseAddr("8.8.4.4")},
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SearchDomains: []dnsname.FQDN{"ts.net.", "ts-dns.test."},
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MatchDomains: []dnsname.FQDN{"ignored."},
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}); err != nil {
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t.Fatal(err)
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}
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const want = `# resolv.conf(5) file generated by tailscale
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# For more info, see https://tailscale.com/s/resolvconf-overwrite
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# DO NOT EDIT THIS FILE BY HAND -- CHANGES WILL BE OVERWRITTEN
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nameserver 8.8.8.8
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nameserver 8.8.4.4
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search ts.net ts-dns.test
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`
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if got := readFile(t, resolvPath); got != want {
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t.Fatalf("resolv.conf:\n%s, want:\n%s", got, want)
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}
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tw := eventbustest.NewWatcher(t, bus)
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const trample = "Hvem er det som tramper på min bro?"
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if err := fs.WriteFile(resolvPath, []byte(trample), 0644); err != nil {
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t.Fatal(err)
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}
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synctest.Wait()
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if err := eventbustest.Expect(tw, eventbustest.Type[TrampleDNS]()); err != nil {
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t.Errorf("did not see trample event: %s", err)
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}
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})
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}
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// watchFile is generally copied from linuxtrample, but cancels the context
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// after the first call to cb() after the first trample to end the test.
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func watchFile(ctx context.Context, dir, filename string, cb func()) error {
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ctx, cancel := context.WithCancel(ctx)
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defer cancel()
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const events = gonotify.IN_ATTRIB |
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gonotify.IN_CLOSE_WRITE |
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gonotify.IN_CREATE |
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gonotify.IN_DELETE |
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gonotify.IN_MODIFY |
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gonotify.IN_MOVE
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watcher, err := gonotify.NewDirWatcher(ctx, events, dir)
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if err != nil {
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return fmt.Errorf("NewDirWatcher: %w", err)
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}
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for {
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select {
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case event := <-watcher.C:
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if event.Name == filename {
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cb()
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cancel()
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}
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case <-ctx.Done():
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return ctx.Err()
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}
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}
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}
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