Revert 99677 (didn't help) - Revert 99666 (sync tests started failing on mac10.6:

http://build.chromium.org/p/chromium/builders/Mac10.6%20Sync/builds/9198

- Linux: Monitor resolv.conf for changes and use that to reload resolver.

BUG=67734
TEST=manual testing by poking at resolv.conf

Review URL: http://codereview.chromium.org/6903061

TBR=craig.schlenter@chromium.org
Review URL: http://codereview.chromium.org/7833030

TBR=thakis@chromium.org
Review URL: http://codereview.chromium.org/7737034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@99693 0039d316-1c4b-4281-b951-d872f2087c98

11 files changed, 284 insertions, 152 deletions

diff --git a/net/base/dns_reload_timer.cc b/net/base/dns_reload_timer.cc
deleted file mode 100644
index 89bda59..0000000
--- a/net/base/dns_reload_timer.cc
+++ /dev/null
@@ -1,100 +0,0 @@
-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "net/base/dns_reload_timer.h"
-
-#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
-#include "base/lazy_instance.h"
-#include "base/threading/thread_local_storage.h"
-#include "base/time.h"
-
-namespace {
-
-// On Linux/BSD, changes to /etc/resolv.conf can go unnoticed thus resulting
-// in DNS queries failing either because nameservers are unknown on startup
-// or because nameserver info has changed as a result of e.g. connecting to
-// a new network. Some distributions patch glibc to stat /etc/resolv.conf
-// to try to automatically detect such changes but these patches are not
-// universal and even patched systems such as Jaunty appear to need calls
-// to res_ninit to reload the nameserver information in different threads.
-//
-// We adopt the Mozilla solution here which is to call res_ninit when
-// lookups fail and to rate limit the reloading to once per second per
-// thread.
-//
-// OpenBSD does not have thread-safe res_ninit/res_nclose so we can't do
-// the same trick there.
-
-// Keep a timer per calling thread to rate limit the calling of res_ninit.
-class DnsReloadTimer {
- public:
-  // Check if the timer for the calling thread has expired. When no
-  // timer exists for the calling thread, create one.
-  bool Expired() {
-    const base::TimeDelta kRetryTime = base::TimeDelta::FromSeconds(1);
-    base::TimeTicks now = base::TimeTicks::Now();
-    base::TimeTicks* timer_ptr =
-      static_cast<base::TimeTicks*>(tls_index_.Get());
-
-    if (!timer_ptr) {
-      timer_ptr = new base::TimeTicks();
-      *timer_ptr = base::TimeTicks::Now();
-      tls_index_.Set(timer_ptr);
-      // Return true to reload dns info on the first call for each thread.
-      return true;
-    } else if (now - *timer_ptr > kRetryTime) {
-      *timer_ptr = now;
-      return true;
-    } else {
-      return false;
-    }
-  }
-
-  // Free the allocated timer.
-  static void SlotReturnFunction(void* data) {
-    base::TimeTicks* tls_data = static_cast<base::TimeTicks*>(data);
-    delete tls_data;
-  }
-
- private:
-  friend struct base::DefaultLazyInstanceTraits<DnsReloadTimer>;
-
-  DnsReloadTimer() {
-    // During testing the DnsReloadTimer Singleton may be created and destroyed
-    // multiple times. Initialize the ThreadLocalStorage slot only once.
-    if (!tls_index_.initialized())
-      tls_index_.Initialize(SlotReturnFunction);
-  }
-
-  ~DnsReloadTimer() {
-  }
-
-  // We use thread local storage to identify which base::TimeTicks to
-  // interact with.
-  static base::ThreadLocalStorage::Slot tls_index_ ;
-
-  DISALLOW_COPY_AND_ASSIGN(DnsReloadTimer);
-};
-
-// A TLS slot to the TimeTicks for the current thread.
-// static
-base::ThreadLocalStorage::Slot DnsReloadTimer::tls_index_(
-    base::LINKER_INITIALIZED);
-
-base::LazyInstance<DnsReloadTimer,
-                   base::LeakyLazyInstanceTraits<DnsReloadTimer> >
-    g_dns_reload_timer(base::LINKER_INITIALIZED);
-
-}  // namespace
-
-namespace net {
-
-bool DnsReloadTimerHasExpired() {
-  DnsReloadTimer* dns_timer = g_dns_reload_timer.Pointer();
-  return dns_timer->Expired();
-}
-
-}  // namespace net
-
-#endif  // defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
diff --git a/net/base/dns_reload_timer.h b/net/base/dns_reload_timer.h
deleted file mode 100644
index fdd90a7..0000000
--- a/net/base/dns_reload_timer.h
+++ /dev/null
@@ -1,22 +0,0 @@
-// Copyright (c) 2009 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef NET_BASE_DNS_RELOAD_TIMER_H_
-#define NET_BASE_DNS_RELOAD_TIMER_H_
-#pragma once
-
-#include "build/build_config.h"
-
-#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
-namespace net {
-
-// DnsReloadTimerExpired tests the thread local DNS reload timer and, if it has
-// expired, returns true and resets the timer. See comments in
-// host_resolver_proc.cc for details.
-bool DnsReloadTimerHasExpired();
-
-}  // namespace net
-#endif
-
-#endif  // NET_BASE_DNS_RELOAD_TIMER_H_
diff --git a/net/base/dns_reloader.cc b/net/base/dns_reloader.cc
new file mode 100644
index 0000000..d45c145
--- /dev/null
+++ b/net/base/dns_reloader.cc
@@ -0,0 +1,122 @@
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "net/base/dns_reloader.h"
+
+#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
+
+#include <resolv.h>
+
+#include "base/basictypes.h"
+#include "base/lazy_instance.h"
+#include "base/logging.h"
+#include "base/message_loop.h"
+#include "base/synchronization/lock.h"
+#include "base/threading/thread_local_storage.h"
+#include "net/base/network_change_notifier.h"
+
+namespace {
+
+// On Linux/BSD, changes to /etc/resolv.conf can go unnoticed thus resulting
+// in DNS queries failing either because nameservers are unknown on startup
+// or because nameserver info has changed as a result of e.g. connecting to
+// a new network. Some distributions patch glibc to stat /etc/resolv.conf
+// to try to automatically detect such changes but these patches are not
+// universal and even patched systems such as Jaunty appear to need calls
+// to res_ninit to reload the nameserver information in different threads.
+//
+// To fix this, on systems with FilePathWatcher support, we use
+// NetworkChangeNotifier::DNSObserver to monitor /etc/resolv.conf to
+// enable us to respond to DNS changes and reload the resolver state.
+//
+// OpenBSD does not have thread-safe res_ninit/res_nclose so we can't do
+// the same trick there and most *BSD's don't yet have support for
+// FilePathWatcher (but perhaps the new kqueue mac code just needs to be
+// ported to *BSD to support that).
+
+class DnsReloader : public net::NetworkChangeNotifier::DNSObserver {
+ public:
+  struct ReloadState {
+    int resolver_generation;
+  };
+
+  // NetworkChangeNotifier::OnDNSChanged methods:
+  virtual void OnDNSChanged() OVERRIDE {
+    DCHECK_EQ(MessageLoop::current()->type(), MessageLoop::TYPE_IO);
+    base::AutoLock l(lock_);
+    resolver_generation_++;
+  }
+
+  void MaybeReload() {
+    ReloadState* reload_state = static_cast<ReloadState*>(tls_index_.Get());
+    base::AutoLock l(lock_);
+
+    if (!reload_state) {
+      reload_state = new ReloadState();
+      reload_state->resolver_generation = resolver_generation_;
+      res_ninit(&_res);
+      tls_index_.Set(reload_state);
+    } else if (reload_state->resolver_generation != resolver_generation_) {
+      reload_state->resolver_generation = resolver_generation_;
+      // It is safe to call res_nclose here since we know res_ninit will have
+      // been called above.
+      res_nclose(&_res);
+      res_ninit(&_res);
+    }
+  }
+
+  // Free the allocated state.
+  static void SlotReturnFunction(void* data) {
+    ReloadState* reload_state = static_cast<ReloadState*>(data);
+    if (reload_state)
+      res_nclose(&_res);
+    delete reload_state;
+  }
+
+ private:
+  DnsReloader() : resolver_generation_(0) {
+    tls_index_.Initialize(SlotReturnFunction);
+    net::NetworkChangeNotifier::AddDNSObserver(this);
+  }
+
+  ~DnsReloader() {
+    NOTREACHED();  // LeakyLazyInstance is not destructed.
+  }
+
+  base::Lock lock_;  // Protects resolver_generation_.
+  int resolver_generation_;
+  friend struct base::DefaultLazyInstanceTraits<DnsReloader>;
+
+  // We use thread local storage to identify which ReloadState to interact with.
+  static base::ThreadLocalStorage::Slot tls_index_ ;
+
+  DISALLOW_COPY_AND_ASSIGN(DnsReloader);
+};
+
+// A TLS slot to the ReloadState for the current thread.
+// static
+base::ThreadLocalStorage::Slot DnsReloader::tls_index_(
+    base::LINKER_INITIALIZED);
+
+base::LazyInstance<DnsReloader,
+                   base::LeakyLazyInstanceTraits<DnsReloader> >
+    g_dns_reloader(base::LINKER_INITIALIZED);
+
+}  // namespace
+
+namespace net {
+
+void EnsureDnsReloaderInit() {
+  DnsReloader* t ALLOW_UNUSED = g_dns_reloader.Pointer();
+}
+
+void DnsReloaderMaybeReload() {
+  // This routine can be called by any of the DNS worker threads.
+  DnsReloader* dns_reloader = g_dns_reloader.Pointer();
+  dns_reloader->MaybeReload();
+}
+
+}  // namespace net
+
+#endif  // defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
diff --git a/net/base/dns_reloader.h b/net/base/dns_reloader.h
new file mode 100644
index 0000000..899fbf3
--- /dev/null
+++ b/net/base/dns_reloader.h
@@ -0,0 +1,24 @@
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef NET_BASE_DNS_RELOADER_H_
+#define NET_BASE_DNS_RELOADER_H_
+#pragma once
+
+#include "build/build_config.h"
+
+#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
+namespace net {
+
+// Call on the network thread before calling DnsReloaderMaybeReload() anywhere.
+void EnsureDnsReloaderInit();
+
+// Call on the worker thread before doing a DNS lookup to reload the
+// resolver for that thread by doing res_ninit() if required.
+void DnsReloaderMaybeReload();
+
+}  // namespace net
+#endif  // defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
+
+#endif  // NET_BASE_DNS_RELOADER_H_
diff --git a/net/base/dnsrr_resolver.cc b/net/base/dnsrr_resolver.cc
index 2cfb0cf..e0d43d9 100644
--- a/net/base/dnsrr_resolver.cc
+++ b/net/base/dnsrr_resolver.cc
@@ -20,7 +20,7 @@
 #include "base/synchronization/lock.h"
 #include "base/task.h"
 #include "base/threading/worker_pool.h"
-#include "net/base/dns_reload_timer.h"
+#include "net/base/dns_reloader.h"
 #include "net/base/dns_util.h"
 #include "net/base/net_errors.h"
 
@@ -184,26 +184,18 @@ class RRResolverWorker {
     }
 
     bool r = true;
+#if defined(OS_MACOSX) || defined(OS_OPENBSD)
     if ((_res.options & RES_INIT) == 0) {
       if (res_ninit(&_res) != 0)
         r = false;
     }
+#else
+    DnsReloaderMaybeReload();
+#endif
 
     if (r) {
       unsigned long saved_options = _res.options;
       r = Do();
-
-#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
-      if (!r && DnsReloadTimerHasExpired()) {
-        // When there's no network connection, _res may not be initialized by
-        // getaddrinfo. Therefore, we call res_nclose only when there are ns
-        // entries.
-        if (_res.nscount > 0)
-          res_nclose(&_res);
-        if (res_ninit(&_res) == 0)
-          r = Do();
-      }
-#endif
       _res.options = saved_options;
     }
 
diff --git a/net/base/host_resolver_impl.cc b/net/base/host_resolver_impl.cc
index 840d263..7d6e3e2 100644
--- a/net/base/host_resolver_impl.cc
+++ b/net/base/host_resolver_impl.cc
@@ -30,6 +30,7 @@
 #include "base/values.h"
 #include "net/base/address_list.h"
 #include "net/base/address_list_net_log_param.h"
+#include "net/base/dns_reloader.h"
 #include "net/base/host_port_pair.h"
 #include "net/base/host_resolver_proc.h"
 #include "net/base/net_errors.h"
@@ -1090,6 +1091,10 @@ HostResolverImpl::HostResolverImpl(
     additional_resolver_flags_ |= HOST_RESOLVER_LOOPBACK_ONLY;
 #endif
   NetworkChangeNotifier::AddIPAddressObserver(this);
+#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
+  EnsureDnsReloaderInit();
+  NetworkChangeNotifier::AddDNSObserver(this);
+#endif
 }
 
 HostResolverImpl::~HostResolverImpl() {
@@ -1104,6 +1109,9 @@ HostResolverImpl::~HostResolverImpl() {
     cur_completing_job_->Cancel();
 
   NetworkChangeNotifier::RemoveIPAddressObserver(this);
+#if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD)
+  NetworkChangeNotifier::RemoveDNSObserver(this);
+#endif
 
   // Delete the job pools.
   for (size_t i = 0u; i < arraysize(job_pools_); ++i)
@@ -1644,4 +1652,17 @@ void HostResolverImpl::OnIPAddressChanged() {
   // |this| may be deleted inside AbortAllInProgressJobs().
 }
 
+void HostResolverImpl::OnDNSChanged() {
+  // If the DNS server has changed, existing cached info could be wrong so we
+  // have to drop our internal cache :( Note that OS level DNS caches, such
+  // as NSCD's cache should be dropped automatically by the OS when
+  // resolv.conf changes so we don't need to do anything to clear that cache.
+  if (cache_.get())
+    cache_->clear();
+  // Existing jobs will have been sent to the original server so they need to
+  // be aborted. TODO(Craig): Should these jobs be restarted?
+  AbortAllInProgressJobs();
+  // |this| may be deleted inside AbortAllInProgressJobs().
+}
+
 }  // namespace net
diff --git a/net/base/host_resolver_impl.h b/net/base/host_resolver_impl.h
index 30772df..4c4581e 100644
--- a/net/base/host_resolver_impl.h
+++ b/net/base/host_resolver_impl.h
@@ -62,7 +62,8 @@ namespace net {
 class NET_EXPORT HostResolverImpl
     : public HostResolver,
       NON_EXPORTED_BASE(public base::NonThreadSafe),
-      public NetworkChangeNotifier::IPAddressObserver {
+      public NetworkChangeNotifier::IPAddressObserver,
+      public NetworkChangeNotifier::DNSObserver {
  public:
   // The index into |job_pools_| for the various job pools. Pools with a higher
   // index have lower priority.
@@ -304,6 +305,9 @@ class NET_EXPORT HostResolverImpl
     retry_factor_ = retry_factor;
   }
 
+  // NetworkChangeNotifier::OnDNSChanged methods:
+  virtual void OnDNSChanged();
+
   // Cache of host resolution results.
   scoped_ptr<HostCache> cache_;
 
diff --git a/net/base/host_resolver_proc.cc b/net/base/host_resolver_proc.cc
index 755f119..331675a 100644
--- a/net/base/host_resolver_proc.cc
+++ b/net/base/host_resolver_proc.cc
@@ -8,7 +8,7 @@
 
 #include "base/logging.h"
 #include "net/base/address_list.h"
-#include "net/base/dns_reload_timer.h"
+#include "net/base/dns_reloader.h"
 #include "net/base/net_errors.h"
 #include "net/base/sys_addrinfo.h"
 
@@ -183,22 +183,12 @@ int SystemHostResolverProc(const std::string& host,
   // Restrict result set to only this socket type to avoid duplicates.
   hints.ai_socktype = SOCK_STREAM;
 
-  int err = getaddrinfo(host.c_str(), NULL, &hints, &ai);
-  bool should_retry = false;
 #if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD) && \
     !defined(OS_ANDROID)
-  // If we fail, re-initialise the resolver just in case there have been any
-  // changes to /etc/resolv.conf and retry. See http://crbug.com/11380 for info.
-  if (err && DnsReloadTimerHasExpired()) {
-    // When there's no network connection, _res may not be initialized by
-    // getaddrinfo. Therefore, we call res_nclose only when there are ns
-    // entries.
-    if (_res.nscount > 0)
-      res_nclose(&_res);
-    if (!res_ninit(&_res))
-      should_retry = true;
-  }
+  DnsReloaderMaybeReload();
 #endif
+  int err = getaddrinfo(host.c_str(), NULL, &hints, &ai);
+  bool should_retry = false;
   // If the lookup was restricted (either by address family, or address
   // detection), and the results where all localhost of a single family,
   // maybe we should retry.  There were several bugs related to these
diff --git a/net/base/network_change_notifier.cc b/net/base/network_change_notifier.cc
index 2eb59ad..29b983e 100644
--- a/net/base/network_change_notifier.cc
+++ b/net/base/network_change_notifier.cc
@@ -90,6 +90,13 @@ void NetworkChangeNotifier::AddOnlineStateObserver(
   }
 }
 
+void NetworkChangeNotifier::AddDNSObserver(DNSObserver* observer) {
+  if (g_network_change_notifier) {
+    g_network_change_notifier->resolver_state_observer_list_->AddObserver(
+        observer);
+  }
+}
+
 void NetworkChangeNotifier::RemoveIPAddressObserver(
     IPAddressObserver* observer) {
   if (g_network_change_notifier) {
@@ -106,13 +113,23 @@ void NetworkChangeNotifier::RemoveOnlineStateObserver(
   }
 }
 
+void NetworkChangeNotifier::RemoveDNSObserver(DNSObserver* observer) {
+  if (g_network_change_notifier) {
+    g_network_change_notifier->resolver_state_observer_list_->RemoveObserver(
+        observer);
+  }
+}
+
 NetworkChangeNotifier::NetworkChangeNotifier()
     : ip_address_observer_list_(
         new ObserverListThreadSafe<IPAddressObserver>(
             ObserverListBase<IPAddressObserver>::NOTIFY_EXISTING_ONLY)),
       online_state_observer_list_(
         new ObserverListThreadSafe<OnlineStateObserver>(
-            ObserverListBase<OnlineStateObserver>::NOTIFY_EXISTING_ONLY)) {
+            ObserverListBase<OnlineStateObserver>::NOTIFY_EXISTING_ONLY)),
+      resolver_state_observer_list_(
+        new ObserverListThreadSafe<DNSObserver>(
+            ObserverListBase<DNSObserver>::NOTIFY_EXISTING_ONLY)) {
   DCHECK(!g_network_change_notifier);
   g_network_change_notifier = this;
 }
@@ -124,6 +141,13 @@ void NetworkChangeNotifier::NotifyObserversOfIPAddressChange() {
   }
 }
 
+void NetworkChangeNotifier::NotifyObserversOfDNSChange() {
+  if (g_network_change_notifier) {
+    g_network_change_notifier->resolver_state_observer_list_->Notify(
+        &DNSObserver::OnDNSChanged);
+  }
+}
+
 void NetworkChangeNotifier::NotifyObserversOfOnlineStateChange() {
   if (g_network_change_notifier) {
     g_network_change_notifier->online_state_observer_list_->Notify(
diff --git a/net/base/network_change_notifier.h b/net/base/network_change_notifier.h
index dad71c3..39b9e05 100644
--- a/net/base/network_change_notifier.h
+++ b/net/base/network_change_notifier.h
@@ -50,6 +50,22 @@ class NET_EXPORT NetworkChangeNotifier {
     DISALLOW_COPY_AND_ASSIGN(OnlineStateObserver);
   };
 
+  class NET_EXPORT DNSObserver {
+   public:
+    virtual ~DNSObserver() {}
+
+    // Will be called when the DNS resolver of the system may have changed.
+    // This is only used on Linux currently and watches /etc/resolv.conf
+    // and /etc/hosts
+    virtual void OnDNSChanged() = 0;
+
+   protected:
+    DNSObserver() {}
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(DNSObserver);
+  };
+
   virtual ~NetworkChangeNotifier();
 
   // See the description of NetworkChangeNotifier::IsOffline().
@@ -89,6 +105,7 @@ class NET_EXPORT NetworkChangeNotifier {
   // thread), in which case it will simply do nothing.
   static void AddIPAddressObserver(IPAddressObserver* observer);
   static void AddOnlineStateObserver(OnlineStateObserver* observer);
+  static void AddDNSObserver(DNSObserver* observer);
 
   // Unregisters |observer| from receiving notifications.  This must be called
   // on the same thread on which AddObserver() was called.  Like AddObserver(),
@@ -99,6 +116,7 @@ class NET_EXPORT NetworkChangeNotifier {
   // there's no reason to use the API in this risky way, so don't do it.
   static void RemoveIPAddressObserver(IPAddressObserver* observer);
   static void RemoveOnlineStateObserver(OnlineStateObserver* observer);
+  static void RemoveDNSObserver(DNSObserver* observer);
 
   // Allow unit tests to trigger notifications.
   static void NotifyObserversOfIPAddressChangeForTests() {
@@ -113,12 +131,15 @@ class NET_EXPORT NetworkChangeNotifier {
   // tests.
   static void NotifyObserversOfIPAddressChange();
   static void NotifyObserversOfOnlineStateChange();
+  static void NotifyObserversOfDNSChange();
 
  private:
   const scoped_refptr<ObserverListThreadSafe<IPAddressObserver> >
       ip_address_observer_list_;
   const scoped_refptr<ObserverListThreadSafe<OnlineStateObserver> >
       online_state_observer_list_;
+  const scoped_refptr<ObserverListThreadSafe<DNSObserver> >
+      resolver_state_observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(NetworkChangeNotifier);
 };
diff --git a/net/base/network_change_notifier_linux.cc b/net/base/network_change_notifier_linux.cc
index 3937de0..45ee139 100644
--- a/net/base/network_change_notifier_linux.cc
+++ b/net/base/network_change_notifier_linux.cc
@@ -7,19 +7,48 @@
 #include <errno.h>
 #include <sys/socket.h>
 
+#include "base/bind.h"
+#include "base/callback_old.h"
 #include "base/compiler_specific.h"
 #include "base/eintr_wrapper.h"
+#include "base/file_util.h"
+#include "base/files/file_path_watcher.h"
 #include "base/task.h"
 #include "base/threading/thread.h"
 #include "net/base/net_errors.h"
 #include "net/base/network_change_notifier_netlink_linux.h"
 
+using ::base::files::FilePathWatcher;
+
 namespace net {
 
 namespace {
 
 const int kInvalidSocket = -1;
 
+class DNSWatchDelegate : public FilePathWatcher::Delegate {
+ public:
+  explicit DNSWatchDelegate(Callback0::Type* callback)
+      : callback_(callback) {}
+  virtual ~DNSWatchDelegate() {}
+  // FilePathWatcher::Delegate interface
+  virtual void OnFilePathChanged(const FilePath& path) OVERRIDE;
+  virtual void OnFilePathError(const FilePath& path) OVERRIDE;
+ private:
+  scoped_ptr<Callback0::Type> callback_;
+  DISALLOW_COPY_AND_ASSIGN(DNSWatchDelegate);
+};
+
+void DNSWatchDelegate::OnFilePathChanged(const FilePath& path) {
+  // Calls NetworkChangeNotifier::NotifyObserversOfDNSChange().
+  if (callback_.get())
+    callback_->Run();
+}
+
+void DNSWatchDelegate::OnFilePathError(const FilePath& path) {
+  LOG(ERROR) << "DNSWatchDelegate::OnFilePathError for " << path.value();
+}
+
 }  // namespace
 
 class NetworkChangeNotifierLinux::Thread
@@ -42,6 +71,10 @@ class NetworkChangeNotifierLinux::Thread
     NetworkChangeNotifier::NotifyObserversOfIPAddressChange();
   }
 
+  void NotifyObserversOfDNSChange() {
+    NetworkChangeNotifier::NotifyObserversOfDNSChange();
+  }
+
   // Starts listening for netlink messages.  Also handles the messages if there
   // are any available on the netlink socket.
   void ListenForNotifications();
@@ -58,17 +91,36 @@ class NetworkChangeNotifierLinux::Thread
   // Technically only needed for ChromeOS, but it's ugly to #ifdef out.
   ScopedRunnableMethodFactory<Thread> method_factory_;
 
+  // Used to watch for changes to /etc/resolv.conf and /etc/hosts.
+  scoped_ptr<base::files::FilePathWatcher> resolv_file_watcher_;
+  scoped_ptr<base::files::FilePathWatcher> hosts_file_watcher_;
+  scoped_refptr<DNSWatchDelegate> file_watcher_delegate_;
+
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 NetworkChangeNotifierLinux::Thread::Thread()
     : base::Thread("NetworkChangeNotifier"),
       netlink_fd_(kInvalidSocket),
-      ALLOW_THIS_IN_INITIALIZER_LIST(method_factory_(this)) {}
+      ALLOW_THIS_IN_INITIALIZER_LIST(method_factory_(this)) {
+}
 
 NetworkChangeNotifierLinux::Thread::~Thread() {}
 
 void NetworkChangeNotifierLinux::Thread::Init() {
+  resolv_file_watcher_.reset(new FilePathWatcher);
+  hosts_file_watcher_.reset(new FilePathWatcher);
+  file_watcher_delegate_ = new DNSWatchDelegate(NewCallback(this,
+      &NetworkChangeNotifierLinux::Thread::NotifyObserversOfDNSChange));
+  if (!resolv_file_watcher_->Watch(
+          FilePath(FILE_PATH_LITERAL("/etc/resolv.conf")),
+          file_watcher_delegate_.get())) {
+    LOG(ERROR) << "Failed to setup watch for /etc/resolv.conf";
+  }
+  if (!hosts_file_watcher_->Watch(FilePath(FILE_PATH_LITERAL("/etc/hosts")),
+          file_watcher_delegate_.get())) {
+    LOG(ERROR) << "Failed to setup watch for /etc/hosts";
+  }
   netlink_fd_ = InitializeNetlinkSocket();
   if (netlink_fd_ < 0) {
     netlink_fd_ = kInvalidSocket;
@@ -84,6 +136,10 @@ void NetworkChangeNotifierLinux::Thread::CleanUp() {
     netlink_fd_ = kInvalidSocket;
     netlink_watcher_.StopWatchingFileDescriptor();
   }
+  // Kill watchers early to make sure they won't try to call
+  // into us via the delegate during destruction.
+  resolv_file_watcher_.reset();
+  hosts_file_watcher_.reset();
 }
 
 void NetworkChangeNotifierLinux::Thread::OnFileCanReadWithoutBlocking(int fd) {