dns_reloader.cc 3.9 KB

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  1. // Copyright (c) 2012 The Chromium Authors. All rights reserved.
  2. // Use of this source code is governed by a BSD-style license that can be
  3. // found in the LICENSE file.
  4. #include "net/dns/dns_reloader.h"
  5. #include "build/build_config.h"
  6. #if BUILDFLAG(IS_POSIX) && !BUILDFLAG(IS_APPLE) && !BUILDFLAG(IS_OPENBSD) && \
  7. !BUILDFLAG(IS_ANDROID) && !BUILDFLAG(IS_FUCHSIA)
  8. #include <resolv.h>
  9. #include "base/lazy_instance.h"
  10. #include "base/notreached.h"
  11. #include "base/synchronization/lock.h"
  12. #include "base/task/current_thread.h"
  13. #include "base/threading/thread_local.h"
  14. #include "net/base/network_change_notifier.h"
  15. namespace net {
  16. namespace {
  17. // On Linux/BSD, changes to /etc/resolv.conf can go unnoticed thus resulting
  18. // in DNS queries failing either because nameservers are unknown on startup
  19. // or because nameserver info has changed as a result of e.g. connecting to
  20. // a new network. Some distributions patch glibc to stat /etc/resolv.conf
  21. // to try to automatically detect such changes but these patches are not
  22. // universal and even patched systems such as Jaunty appear to need calls
  23. // to res_ninit to reload the nameserver information in different threads.
  24. //
  25. // To fix this, on systems with FilePathWatcher support, we use
  26. // NetworkChangeNotifier::DNSObserver to monitor /etc/resolv.conf to
  27. // enable us to respond to DNS changes and reload the resolver state.
  28. //
  29. // OpenBSD does not have thread-safe res_ninit/res_nclose so we can't do
  30. // the same trick there and most *BSD's don't yet have support for
  31. // FilePathWatcher (but perhaps the new kqueue mac code just needs to be
  32. // ported to *BSD to support that).
  33. //
  34. // Android does not have /etc/resolv.conf. The system takes care of nameserver
  35. // changes, so none of this is needed.
  36. //
  37. // TODO(crbug.com/971411): Convert to SystemDnsConfigChangeNotifier because this
  38. // really only cares about system DNS config changes, not Chrome effective
  39. // config changes.
  40. class DnsReloader : public NetworkChangeNotifier::DNSObserver {
  41. public:
  42. DnsReloader(const DnsReloader&) = delete;
  43. DnsReloader& operator=(const DnsReloader&) = delete;
  44. // NetworkChangeNotifier::DNSObserver:
  45. void OnDNSChanged() override {
  46. base::AutoLock lock(lock_);
  47. resolver_generation_++;
  48. }
  49. void MaybeReload() {
  50. ReloadState* reload_state = tls_reload_state_.Get();
  51. base::AutoLock lock(lock_);
  52. if (!reload_state) {
  53. auto new_reload_state = std::make_unique<ReloadState>();
  54. new_reload_state->resolver_generation = resolver_generation_;
  55. res_ninit(&_res);
  56. tls_reload_state_.Set(std::move(new_reload_state));
  57. } else if (reload_state->resolver_generation != resolver_generation_) {
  58. reload_state->resolver_generation = resolver_generation_;
  59. // It is safe to call res_nclose here since we know res_ninit will have
  60. // been called above.
  61. res_nclose(&_res);
  62. res_ninit(&_res);
  63. }
  64. }
  65. private:
  66. struct ReloadState {
  67. ~ReloadState() { res_nclose(&_res); }
  68. int resolver_generation;
  69. };
  70. DnsReloader() { NetworkChangeNotifier::AddDNSObserver(this); }
  71. ~DnsReloader() override {
  72. NOTREACHED(); // LeakyLazyInstance is not destructed.
  73. }
  74. base::Lock lock_; // Protects resolver_generation_.
  75. int resolver_generation_ = 0;
  76. friend struct base::LazyInstanceTraitsBase<DnsReloader>;
  77. // We use thread local storage to identify which ReloadState to interact with.
  78. base::ThreadLocalOwnedPointer<ReloadState> tls_reload_state_;
  79. };
  80. base::LazyInstance<DnsReloader>::Leaky
  81. g_dns_reloader = LAZY_INSTANCE_INITIALIZER;
  82. } // namespace
  83. void EnsureDnsReloaderInit() {
  84. g_dns_reloader.Pointer();
  85. }
  86. void DnsReloaderMaybeReload() {
  87. // This routine can be called by any of the DNS worker threads.
  88. DnsReloader* dns_reloader = g_dns_reloader.Pointer();
  89. dns_reloader->MaybeReload();
  90. }
  91. } // namespace net
  92. #endif // BUILDFLAG(IS_POSIX) && !BUILDFLAG(IS_APPLE) && !BUILDFLAG(IS_OPENBSD)
  93. // && !BUILDFLAG(IS_ANDROID)