Implemented AsyncHostResolver -- asynchronous HostResolver.

BUG=60149
TEST=net_unittests --gtest_filter="AsyncHostResolver*"

Review URL: http://codereview.chromium.org/7466011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@93313 0039d316-1c4b-4281-b951-d872f2087c98

15 files changed, 1583 insertions, 212 deletions

diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index 11d8827..35070e19 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -34,6 +34,7 @@
 #include "content/browser/gpu/gpu_process_host.h"
 #include "content/browser/in_process_webkit/indexed_db_key_utility_client.h"
 #include "content/common/url_fetcher.h"
+#include "net/base/async_host_resolver.h"
 #include "net/base/cert_verifier.h"
 #include "net/base/cookie_monster.h"
 #include "net/base/dnsrr_resolver.h"
@@ -160,8 +161,24 @@ net::HostResolver* CreateGlobalHostResolver(net::NetLog* net_log) {
     }
   }
 
-  net::HostResolver* global_host_resolver =
+  net::HostResolver* global_host_resolver = NULL;
+  if (command_line.HasSwitch(switches::kDnsServer)) {
+    std::string dns_ip_string =
+      command_line.GetSwitchValueASCII(switches::kDnsServer);
+    net::IPAddressNumber dns_ip_number;
+    if (net::ParseIPLiteralToNumber(dns_ip_string, &dns_ip_number)) {
+      global_host_resolver =
+        net::CreateAsyncHostResolver(parallelism, dns_ip_number, net_log);
+    } else {
+      LOG(ERROR) << "Invalid IP address specified for --dns-server: "
+                 << dns_ip_string;
+    }
+  }
+
+  if (!global_host_resolver) {
+    global_host_resolver =
       net::CreateSystemHostResolver(parallelism, retry_attempts, net_log);
+  }
 
   // Determine if we should disable IPv6 support.
   if (!command_line.HasSwitch(switches::kEnableIPv6)) {
diff --git a/chrome/browser/net/passive_log_collector.cc b/chrome/browser/net/passive_log_collector.cc
index 6e522a7..ca321dd 100644
--- a/chrome/browser/net/passive_log_collector.cc
+++ b/chrome/browser/net/passive_log_collector.cc
@@ -78,6 +78,8 @@ PassiveLogCollector::PassiveLogCollector()
   trackers_[net::NetLog::SOURCE_EXPONENTIAL_BACKOFF_THROTTLING] =
       &exponential_backoff_throttling_tracker_;
   trackers_[net::NetLog::SOURCE_DNS_TRANSACTION] = &dns_transaction_tracker_;
+  trackers_[net::NetLog::SOURCE_ASYNC_HOST_RESOLVER_REQUEST] =
+      &async_host_resolver_request_tracker_;
   // Make sure our mapping is up-to-date.
   for (size_t i = 0; i < arraysize(trackers_); ++i)
     DCHECK(trackers_[i]) << "Unhandled SourceType: " << i;
@@ -695,3 +697,30 @@ PassiveLogCollector::DnsTransactionTracker::DoAddEntry(
   }
   return ACTION_NONE;
 }
+
+//----------------------------------------------------------------------------
+// AsyncHostResolverRequestTracker
+//----------------------------------------------------------------------------
+
+const size_t
+PassiveLogCollector::AsyncHostResolverRequestTracker::kMaxNumSources = 100;
+
+const size_t
+PassiveLogCollector::AsyncHostResolverRequestTracker::kMaxGraveyardSize = 15;
+
+PassiveLogCollector::
+    AsyncHostResolverRequestTracker::AsyncHostResolverRequestTracker()
+        : SourceTracker(kMaxNumSources, kMaxGraveyardSize, NULL) {
+}
+
+PassiveLogCollector::SourceTracker::Action
+PassiveLogCollector::AsyncHostResolverRequestTracker::DoAddEntry(
+    const ChromeNetLog::Entry& entry,
+    SourceInfo* out_info) {
+  AddEntryToSourceInfo(entry, out_info);
+  if (entry.type == net::NetLog::TYPE_ASYNC_HOST_RESOLVER_REQUEST &&
+      entry.phase == net::NetLog::PHASE_END) {
+    return ACTION_MOVE_TO_GRAVEYARD;
+  }
+  return ACTION_NONE;
+}
diff --git a/chrome/browser/net/passive_log_collector.h b/chrome/browser/net/passive_log_collector.h
index 56c60c7..28598b2 100644
--- a/chrome/browser/net/passive_log_collector.h
+++ b/chrome/browser/net/passive_log_collector.h
@@ -369,6 +369,21 @@ class PassiveLogCollector : public ChromeNetLog::ThreadSafeObserver {
     DISALLOW_COPY_AND_ASSIGN(DnsTransactionTracker);
   };
 
+  // Tracks the log entries for the last seen SOURCE_ASYNC_HOST_RESOLVER_REQUEST
+  class AsyncHostResolverRequestTracker : public SourceTracker {
+   public:
+    static const size_t kMaxNumSources;
+    static const size_t kMaxGraveyardSize;
+
+    AsyncHostResolverRequestTracker();
+
+   private:
+    virtual Action DoAddEntry(const ChromeNetLog::Entry& entry,
+                              SourceInfo* out_info);
+
+    DISALLOW_COPY_AND_ASSIGN(AsyncHostResolverRequestTracker);
+  };
+
   PassiveLogCollector();
   virtual ~PassiveLogCollector();
 
@@ -410,6 +425,7 @@ class PassiveLogCollector : public ChromeNetLog::ThreadSafeObserver {
   HttpStreamJobTracker http_stream_job_tracker_;
   ExponentialBackoffThrottlingTracker exponential_backoff_throttling_tracker_;
   DnsTransactionTracker dns_transaction_tracker_;
+  AsyncHostResolverRequestTracker async_host_resolver_request_tracker_;
 
   // This array maps each NetLog::SourceType to one of the tracker instances
   // defined above. Use of this array avoid duplicating the list of trackers
diff --git a/chrome/browser/resources/net_internals/sourceentry.js b/chrome/browser/resources/net_internals/sourceentry.js
index 3cb5fd0..1a901ca 100644
--- a/chrome/browser/resources/net_internals/sourceentry.js
+++ b/chrome/browser/resources/net_internals/sourceentry.js
@@ -321,6 +321,7 @@ SourceEntry.prototype.getDescription = function() {
           description = connectJobSourceEntry.getDescription();
       }
       break;
+    case LogSourceType.ASYNC_HOST_RESOLVER_REQUEST:
     case LogSourceType.DNS_TRANSACTION:
       description = e.params.hostname;
       break;
diff --git a/net/base/async_host_resolver.cc b/net/base/async_host_resolver.cc
new file mode 100644
index 0000000..8aa9818
--- /dev/null
+++ b/net/base/async_host_resolver.cc
@@ -0,0 +1,427 @@
+// 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/async_host_resolver.h"
+
+#include <algorithm>
+
+#include "base/bind.h"
+#include "base/logging.h"
+#include "base/rand_util.h"
+#include "base/stl_util.h"
+#include "base/values.h"
+#include "net/base/address_list.h"
+#include "net/base/dns_util.h"
+#include "net/base/net_errors.h"
+#include "net/socket/client_socket_factory.h"
+
+namespace net {
+
+namespace {
+
+// TODO(agayev): fix this when IPv6 support is added.
+uint16 QueryTypeFromAddressFamily(AddressFamily address_family) {
+  return kDNS_A;
+}
+
+int ResolveAsIp(const HostResolver::RequestInfo& info,
+                const IPAddressNumber& ip_number,
+                AddressList* addresses) {
+  if (ip_number.size() != kIPv4AddressSize)
+    return ERR_NAME_NOT_RESOLVED;
+
+  *addresses = AddressList::CreateFromIPAddressWithCname(
+      ip_number,
+      info.port(),
+      info.host_resolver_flags() & HOST_RESOLVER_CANONNAME);
+  return OK;
+}
+
+class RequestParameters : public NetLog::EventParameters {
+ public:
+  RequestParameters(const HostResolver::RequestInfo& info,
+                    const NetLog::Source& source)
+      : info_(info), source_(source) {}
+
+  virtual Value* ToValue() const {
+    DictionaryValue* dict = new DictionaryValue();
+    dict->SetString("hostname", info_.host_port_pair().ToString());
+    dict->SetInteger("address_family",
+                     static_cast<int>(info_.address_family()));
+    dict->SetBoolean("allow_cached_response", info_.allow_cached_response());
+    dict->SetBoolean("only_use_cached_response",
+                     info_.only_use_cached_response());
+    dict->SetBoolean("is_speculative", info_.is_speculative());
+    dict->SetInteger("priority", info_.priority());
+
+    if (source_.is_valid())
+      dict->Set("source_dependency", source_.ToValue());
+
+    return dict;
+  }
+
+ private:
+  const HostResolver::RequestInfo info_;
+  const NetLog::Source source_;
+};
+
+}  // namespace
+
+HostResolver* CreateAsyncHostResolver(size_t max_concurrent_resolves,
+                                      const IPAddressNumber& dns_ip,
+                                      NetLog* net_log) {
+  size_t max_transactions = max_concurrent_resolves;
+  if (max_transactions == 0)
+    max_transactions = 20;
+  size_t max_pending_requests = max_transactions * 100;
+  HostResolver* resolver = new AsyncHostResolver(
+      IPEndPoint(dns_ip, 53),
+      max_transactions,
+      max_pending_requests,
+      base::Bind(&base::RandInt),
+      NULL,
+      net_log);
+  return resolver;
+}
+
+//-----------------------------------------------------------------------------
+class AsyncHostResolver::Request {
+ public:
+  Request(const BoundNetLog& source_net_log,
+          const BoundNetLog& request_net_log,
+          int id,
+          const HostResolver::RequestInfo& info,
+          const Key& key,
+          CompletionCallback* callback,
+          AddressList* addresses)
+      : source_net_log_(source_net_log),
+        request_net_log_(request_net_log),
+        id_(id),
+        info_(info),
+        key_(key),
+        callback_(callback),
+        addresses_(addresses) {
+    DCHECK(addresses_);
+  }
+
+  int id() const { return id_; }
+  const HostResolver::RequestInfo& info() const { return info_; }
+  const Key& key() const { return key_; }
+  RequestPriority priority() const { return info_.priority(); }
+  const BoundNetLog& source_net_log() const { return source_net_log_; }
+  const BoundNetLog& request_net_log() const { return request_net_log_; }
+  const AddressList* addresses() const { return addresses_; }
+
+  void OnComplete(int result, const IPAddressList& ip_addresses) {
+    DCHECK(callback_);
+    if (result == OK)
+      *addresses_ =
+          AddressList::CreateFromIPAddressList(ip_addresses, info_.port());
+    callback_->Run(result);
+  }
+
+ private:
+  BoundNetLog source_net_log_;
+  BoundNetLog request_net_log_;
+  const int id_;
+  const HostResolver::RequestInfo info_;
+  const Key key_;
+  CompletionCallback* callback_;
+  AddressList* addresses_;
+};
+
+//-----------------------------------------------------------------------------
+AsyncHostResolver::AsyncHostResolver(const IPEndPoint& dns_server,
+                                     size_t max_transactions,
+                                     size_t max_pending_requests,
+                                     const RandIntCallback& rand_int_cb,
+                                     ClientSocketFactory* factory,
+                                     NetLog* net_log)
+    : max_transactions_(max_transactions),
+      max_pending_requests_(max_pending_requests),
+      dns_server_(dns_server),
+      rand_int_cb_(rand_int_cb),
+      factory_(factory),
+      next_request_id_(0),
+      net_log_(net_log) {
+}
+
+AsyncHostResolver::~AsyncHostResolver() {
+  for (KeyRequestListMap::iterator it = requestlist_map_.begin();
+       it != requestlist_map_.end(); ++it)
+    STLDeleteElements(&it->second);
+  STLDeleteElements(&transactions_);
+}
+
+int AsyncHostResolver::Resolve(const RequestInfo& info,
+                               AddressList* addresses,
+                               CompletionCallback* callback,
+                               RequestHandle* out_req,
+                               const BoundNetLog& source_net_log) {
+  DCHECK(addresses);
+
+  IPAddressNumber ip_number;
+  std::string dns_name;
+  int rv = ERR_UNEXPECTED;
+  if (info.hostname().empty())
+    rv = ERR_NAME_NOT_RESOLVED;
+  else if (ParseIPLiteralToNumber(info.hostname(), &ip_number))
+    rv = ResolveAsIp(info, ip_number, addresses);
+  else if (!DNSDomainFromDot(info.hostname(), &dns_name))
+    rv = ERR_NAME_NOT_RESOLVED;
+  else if (info.only_use_cached_response())  // TODO(agayev): support caching
+    rv = ERR_NAME_NOT_RESOLVED;
+
+  Request* request = CreateNewRequest(
+      info, dns_name, callback, addresses, source_net_log);
+
+  OnStart(request);
+  if (rv != ERR_UNEXPECTED) {
+    OnFinish(request, rv);
+    delete request;
+    return rv;
+  }
+
+  if (out_req)
+    *out_req = reinterpret_cast<RequestHandle>(request);
+
+  if (AttachToRequestList(request))
+    return ERR_IO_PENDING;
+  if (transactions_.size() < max_transactions_)
+    return StartNewTransactionFor(request);
+  return Enqueue(request);
+}
+
+void AsyncHostResolver::OnStart(Request* request) {
+  DCHECK(request);
+
+  request->source_net_log().BeginEvent(
+      NetLog::TYPE_ASYNC_HOST_RESOLVER,
+      make_scoped_refptr(new NetLogSourceParameter(
+          "source_dependency", request->request_net_log().source())));
+
+  request->request_net_log().BeginEvent(
+      NetLog::TYPE_ASYNC_HOST_RESOLVER_REQUEST,
+      make_scoped_refptr(new RequestParameters(
+          request->info(), request->source_net_log().source())));
+
+  FOR_EACH_OBSERVER(
+      HostResolver::Observer, observers_,
+      OnStartResolution(request->id(), request->info()));
+}
+
+void AsyncHostResolver::OnFinish(Request* request, int result) {
+  DCHECK(request);
+  bool was_resolved = result == OK;
+
+  FOR_EACH_OBSERVER(
+      HostResolver::Observer, observers_,
+      OnFinishResolutionWithStatus(
+          request->id(), was_resolved, request->info()));
+
+  request->request_net_log().EndEventWithNetErrorCode(
+      NetLog::TYPE_ASYNC_HOST_RESOLVER_REQUEST, result);
+  request->source_net_log().EndEvent(
+      NetLog::TYPE_ASYNC_HOST_RESOLVER, NULL);
+}
+
+void AsyncHostResolver::OnCancel(Request* request) {
+  DCHECK(request);
+
+  FOR_EACH_OBSERVER(
+      HostResolver::Observer, observers_,
+      OnCancelResolution(request->id(), request->info()));
+
+  request->request_net_log().AddEvent(
+      NetLog::TYPE_CANCELLED, NULL);
+  request->request_net_log().EndEvent(
+      NetLog::TYPE_ASYNC_HOST_RESOLVER_REQUEST, NULL);
+  request->source_net_log().EndEvent(
+      NetLog::TYPE_ASYNC_HOST_RESOLVER, NULL);
+}
+
+void AsyncHostResolver::CancelRequest(RequestHandle req_handle) {
+  scoped_ptr<Request> request(reinterpret_cast<Request*>(req_handle));
+  DCHECK(request.get());
+
+  KeyRequestListMap::iterator it = requestlist_map_.find(request->key());
+  if (it != requestlist_map_.end())
+    it->second.remove(request.get());
+  else
+    pending_requests_[request->priority()].remove(request.get());
+
+  OnCancel(request.get());
+}
+
+void AsyncHostResolver::AddObserver(HostResolver::Observer* observer) {
+  observers_.AddObserver(observer);
+}
+
+void AsyncHostResolver::RemoveObserver(HostResolver::Observer* observer) {
+  observers_.RemoveObserver(observer);
+}
+
+void AsyncHostResolver::SetDefaultAddressFamily(
+    AddressFamily address_family) {
+  NOTIMPLEMENTED();
+}
+
+AddressFamily AsyncHostResolver::GetDefaultAddressFamily() const {
+  return ADDRESS_FAMILY_IPV4;
+}
+
+HostResolverImpl* AsyncHostResolver::GetAsHostResolverImpl() {
+  return NULL;
+}
+
+void AsyncHostResolver::OnTransactionComplete(
+    int result,
+    const DnsTransaction* transaction,
+    const IPAddressList& ip_addresses) {
+
+  DCHECK(std::find(transactions_.begin(), transactions_.end(), transaction)
+         != transactions_.end());
+  DCHECK(requestlist_map_.find(transaction->key()) != requestlist_map_.end());
+
+  // Run callback of every request that was depending on this transaction,
+  // also notify observers.
+  RequestList& requests = requestlist_map_[transaction->key()];
+  for (RequestList::iterator it = requests.begin(); it != requests.end();
+       ++it) {
+    Request* request = *it;
+    OnFinish(request, result);
+    request->OnComplete(result, ip_addresses);
+  }
+
+  // Cleanup requests.
+  STLDeleteElements(&requests);
+  requestlist_map_.erase(transaction->key());
+
+  // Cleanup transaction and start a new one if there are pending requests.
+  delete transaction;
+  transactions_.remove(transaction);
+  ProcessPending();
+}
+
+AsyncHostResolver::Request* AsyncHostResolver::CreateNewRequest(
+    const RequestInfo& info,
+    const std::string& dns_name,
+    CompletionCallback* callback,
+    AddressList* addresses,
+    const BoundNetLog& source_net_log) {
+
+  uint16 query_type = QueryTypeFromAddressFamily(info.address_family());
+  Key key(dns_name, query_type);
+
+  BoundNetLog request_net_log = BoundNetLog::Make(net_log_,
+      NetLog::SOURCE_ASYNC_HOST_RESOLVER_REQUEST);
+
+  int id = next_request_id_++;
+  Request* request = new Request(
+      source_net_log, request_net_log, id, info, key, callback, addresses);
+  return request;
+}
+
+bool AsyncHostResolver::AttachToRequestList(Request* request) {
+  KeyRequestListMap::iterator it = requestlist_map_.find(request->key());
+  if (it == requestlist_map_.end())
+    return false;
+  it->second.push_back(request);
+  return true;
+}
+
+int AsyncHostResolver::StartNewTransactionFor(Request* request) {
+  DCHECK(requestlist_map_.find(request->key()) == requestlist_map_.end());
+  DCHECK(transactions_.size() < max_transactions_);
+
+  request->request_net_log().AddEvent(
+      NetLog::TYPE_ASYNC_HOST_RESOLVER_CREATE_DNS_TRANSACTION, NULL);
+
+  requestlist_map_[request->key()].push_back(request);
+  DnsTransaction* transaction = new DnsTransaction(
+      dns_server_,
+      request->key().first,
+      request->key().second,
+      rand_int_cb_,
+      factory_,
+      request->request_net_log(),
+      net_log_);
+  transaction->SetDelegate(this);
+  transactions_.push_back(transaction);
+  return transaction->Start();
+}
+
+int AsyncHostResolver::Enqueue(Request* request) {
+  scoped_ptr<Request> evicted_request(Insert(request));
+  if (evicted_request != NULL) {
+    int rv = ERR_HOST_RESOLVER_QUEUE_TOO_LARGE;
+    if (evicted_request.get() == request)
+      return rv;
+    evicted_request->OnComplete(rv, IPAddressList());
+  }
+  return ERR_IO_PENDING;
+}
+
+AsyncHostResolver::Request* AsyncHostResolver::Insert(Request* request) {
+  pending_requests_[request->priority()].push_back(request);
+  if (GetNumPending() > max_pending_requests_) {
+    Request* req = RemoveLowest();
+    DCHECK(req);
+    return req;
+  }
+  return NULL;
+}
+
+size_t AsyncHostResolver::GetNumPending() {
+  size_t num_pending = 0;
+  for (size_t i = 0; i < arraysize(pending_requests_); ++i)
+    num_pending += pending_requests_[i].size();
+  return num_pending;
+}
+
+AsyncHostResolver::Request* AsyncHostResolver::RemoveLowest() {
+  for (int i = static_cast<int>(arraysize(pending_requests_)) - 1;
+       i >= 0; --i) {
+    RequestList& requests = pending_requests_[i];
+    if (!requests.empty()) {
+      Request* request = requests.front();
+      requests.pop_front();
+      return request;
+    }
+  }
+  return NULL;
+}
+
+AsyncHostResolver::Request* AsyncHostResolver::RemoveHighest() {
+  for (size_t i = 0; i < arraysize(pending_requests_) - 1; ++i) {
+    RequestList& requests = pending_requests_[i];
+    if (!requests.empty()) {
+      Request* request = requests.front();
+      requests.pop_front();
+      return request;
+    }
+  }
+  return NULL;
+}
+
+void AsyncHostResolver::ProcessPending() {
+  Request* request = RemoveHighest();
+  if (!request)
+    return;
+  for (size_t i = 0; i < arraysize(pending_requests_); ++i) {
+    RequestList& requests = pending_requests_[i];
+    RequestList::iterator it = requests.begin();
+    while (it != requests.end()) {
+      if (request->key() == (*it)->key()) {
+        requestlist_map_[request->key()].push_back(*it);
+        it = requests.erase(it);
+      } else {
+        ++it;
+      }
+    }
+  }
+  StartNewTransactionFor(request);
+}
+
+}  // namespace net
diff --git a/net/base/async_host_resolver.h b/net/base/async_host_resolver.h
new file mode 100644
index 0000000..42ba997
--- /dev/null
+++ b/net/base/async_host_resolver.h
@@ -0,0 +1,156 @@
+// 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_ASYNC_HOST_RESOLVER_H_
+#define NET_BASE_ASYNC_HOST_RESOLVER_H_
+#pragma once
+
+#include <list>
+#include <map>
+#include <vector>
+
+#include "base/observer_list.h"
+#include "base/threading/non_thread_safe.h"
+#include "net/base/address_family.h"
+#include "net/base/dns_transaction.h"
+#include "net/base/host_resolver.h"
+#include "net/base/ip_endpoint.h"
+#include "net/base/net_log.h"
+#include "net/base/rand_callback.h"
+
+namespace net {
+
+class AddressesList;
+class ClientSocketFactory;
+
+class NET_API AsyncHostResolver : public HostResolver,
+                                  public DnsTransaction::Delegate,
+                                  public base::NonThreadSafe {
+ public:
+  AsyncHostResolver(const IPEndPoint& dns_server,
+                    size_t max_transactions,
+                    size_t max_pending_requests_,
+                    const RandIntCallback& rand_int,
+                    ClientSocketFactory* factory,
+                    NetLog* net_log);
+  virtual ~AsyncHostResolver();
+
+  // HostResolver interface
+  virtual int Resolve(const RequestInfo& info,
+                      AddressList* addresses,
+                      CompletionCallback* callback,
+                      RequestHandle* out_req,
+                      const BoundNetLog& source_net_log) OVERRIDE;
+  virtual void CancelRequest(RequestHandle req_handle) OVERRIDE;
+  virtual void AddObserver(HostResolver::Observer* observer) OVERRIDE;
+  virtual void RemoveObserver(HostResolver::Observer* observer) OVERRIDE;
+  virtual void SetDefaultAddressFamily(AddressFamily address_family) OVERRIDE;
+  virtual AddressFamily GetDefaultAddressFamily() const OVERRIDE;
+  virtual HostResolverImpl* GetAsHostResolverImpl() OVERRIDE;
+
+  // DnsTransaction::Delegate interface
+  virtual void OnTransactionComplete(
+      int result,
+      const DnsTransaction* transaction,
+      const IPAddressList& ip_addresses) OVERRIDE;
+
+ private:
+  FRIEND_TEST_ALL_PREFIXES(AsyncHostResolverTest, QueuedLookup);
+  FRIEND_TEST_ALL_PREFIXES(AsyncHostResolverTest, CancelPendingLookup);
+  FRIEND_TEST_ALL_PREFIXES(AsyncHostResolverTest,
+                           ResolverDestructionCancelsLookups);
+  FRIEND_TEST_ALL_PREFIXES(AsyncHostResolverTest,
+                           OverflowQueueWithLowPriorityLookup);
+  FRIEND_TEST_ALL_PREFIXES(AsyncHostResolverTest,
+                           OverflowQueueWithHighPriorityLookup);
+
+  class Request;
+
+  typedef DnsTransaction::Key Key;
+  typedef std::list<Request*> RequestList;
+  typedef std::list<const DnsTransaction*> TransactionList;
+  typedef std::map<Key, RequestList> KeyRequestListMap;
+
+  // Create a new request for the incoming Resolve() call.
+  Request* CreateNewRequest(const RequestInfo& info,
+                            const std::string& dns_name,
+                            CompletionCallback* callback,
+                            AddressList* addresses,
+                            const BoundNetLog& source_net_log);
+
+  // Called when a request has just been started.
+  void OnStart(Request* request);
+
+  // Called when a request has just completed (before its callback is run).
+  void OnFinish(Request* request, int result);
+
+  // Called when a request has been cancelled.
+  void OnCancel(Request* request);
+
+  // If there is an in-progress transaction for Request->key(), this will
+  // attach |request| to the respective list.
+  bool AttachToRequestList(Request* request);
+
+  // Will start a new transaction for |request|, will insert a new key in
+  // |requestlist_map_| and append |request| to the respective list.
+  int StartNewTransactionFor(Request* request);
+
+  // Will enqueue |request| in |pending_requests_|.
+  int Enqueue(Request* request);
+
+  // A helper used by Enqueue to insert |request| into |pending_requests_|.
+  Request* Insert(Request* request);
+
+  // Returns the number of pending requests.
+  size_t GetNumPending();
+
+  // Removes and returns a pointer to the lowest/highest priority request
+  // from |pending_requests_|.
+  Request* RemoveLowest();
+  Request* RemoveHighest();
+
+  // Once a transaction has completed, called to start a new transaction if
+  // there are pending requests.
+  void ProcessPending();
+
+  // Maximum number of concurrent transactions.
+  size_t max_transactions_;
+
+  // List of current transactions.
+  TransactionList transactions_;
+
+  // A map from Key to a list of requests waiting for the Key to resolve.
+  KeyRequestListMap requestlist_map_;
+
+  // Maximum number of pending requests.
+  size_t max_pending_requests_;
+
+  // Queues based on priority for putting pending requests.
+  RequestList pending_requests_[NUM_PRIORITIES];
+
+  // DNS server to which queries will be setn.
+  IPEndPoint dns_server_;
+
+  // Callback to be passed to DnsTransaction for generating DNS query ids.
+  RandIntCallback rand_int_cb_;
+
+  // Also passed to DnsTransaction; it's a dependency injection to aid
+  // testing, outside of unit tests, its value is always NULL.
+  ClientSocketFactory* factory_;
+
+  // The observers to notify when a request starts/ends.
+  ObserverList<HostResolver::Observer> observers_;
+
+  // Monotonically increasing ID number to assign to the next request.
+  // Observers are the only consumers of this ID number.
+  int next_request_id_;
+
+  NetLog* net_log_;
+
+  DISALLOW_COPY_AND_ASSIGN(AsyncHostResolver);
+};
+
+}  // namespace net
+
+#endif  // NET_BASE_ASYNC_HOST_RESOLVER_H_
diff --git a/net/base/async_host_resolver_unittest.cc b/net/base/async_host_resolver_unittest.cc
new file mode 100644
index 0000000..0e92981
--- /dev/null
+++ b/net/base/async_host_resolver_unittest.cc
@@ -0,0 +1,574 @@
+// 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/async_host_resolver.h"
+
+#include "base/bind.h"
+#include "base/scoped_ptr.h"
+#include "net/base/dns_test_util.h"
+#include "net/base/net_log.h"
+#include "net/base/rand_callback.h"
+#include "net/base/sys_addrinfo.h"
+#include "net/socket/socket_test_util.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace net {
+
+namespace {
+
+void VerifyAddressList(const std::vector<const char*>& ip_addresses,
+                       int port,
+                       const AddressList& addrlist) {
+  ASSERT_LT(0u, ip_addresses.size());
+  ASSERT_NE(static_cast<addrinfo*>(NULL), addrlist.head());
+
+  IPAddressNumber ip_number;
+  const struct addrinfo* ainfo = addrlist.head();
+  for (std::vector<const char*>::const_iterator i = ip_addresses.begin();
+       i != ip_addresses.end(); ++i, ainfo = ainfo->ai_next) {
+    ASSERT_NE(static_cast<addrinfo*>(NULL), ainfo);
+    EXPECT_EQ(sizeof(struct sockaddr_in), ainfo->ai_addrlen);
+
+    const struct sockaddr* sa = ainfo->ai_addr;
+    const struct sockaddr_in* sa_in = (const struct sockaddr_in*) sa;
+    EXPECT_TRUE(htons(port) == sa_in->sin_port);
+    EXPECT_STREQ(*i, NetAddressToString(sa, ainfo->ai_addrlen).c_str());
+  }
+  ASSERT_EQ(static_cast<addrinfo*>(NULL), ainfo);
+}
+
+}  // namespace
+
+static const int kPortNum = 80;
+static const size_t kMaxTransactions = 2;
+static const size_t kMaxPendingRequests = 1;
+static int transaction_ids[] = {0, 1, 2, 3};
+
+// The following fixture sets up an environment for four different lookups
+// with their data defined in dns_test_util.h.  All tests make use of these
+// predefined variables instead of each defining their own, to avoid
+// boilerplate code in every test.  Assuming every coming query is for a
+// distinct hostname, as |kMaxTransactions| is set to 2 and
+// |kMaxPendingRequests| is set to 1, first two queries start immediately
+// and the next one is sent to pending queue; as a result, the next query
+// should either fail itself or cause the pending query to fail depending
+// on its priority.
+class AsyncHostResolverTest : public testing::Test {
+ public:
+  AsyncHostResolverTest()
+      : info0_(HostPortPair(kT0HostName, kPortNum)),
+        info1_(HostPortPair(kT1HostName, kPortNum)),
+        info2_(HostPortPair(kT2HostName, kPortNum)),
+        info3_(HostPortPair(kT3HostName, kPortNum)),
+        ip_addresses0_(kT0IpAddresses,
+            kT0IpAddresses + arraysize(kT0IpAddresses)),
+        ip_addresses1_(kT1IpAddresses,
+            kT1IpAddresses + arraysize(kT1IpAddresses)),
+        ip_addresses2_(kT2IpAddresses,
+            kT2IpAddresses + arraysize(kT2IpAddresses)),
+        ip_addresses3_(kT3IpAddresses,
+            kT3IpAddresses + arraysize(kT3IpAddresses)),
+        test_prng_(std::deque<int>(
+            transaction_ids, transaction_ids + arraysize(transaction_ids))) {
+
+    rand_int_cb_ = base::Bind(&TestPrng::GetNext,
+                              base::Unretained(&test_prng_));
+    // AF_INET only for now.
+    info0_.set_address_family(ADDRESS_FAMILY_IPV4);
+    info1_.set_address_family(ADDRESS_FAMILY_IPV4);
+    info2_.set_address_family(ADDRESS_FAMILY_IPV4);
+    info3_.set_address_family(ADDRESS_FAMILY_IPV4);
+
+    // Setup socket read/writes for transaction 0.
+    writes0_.push_back(
+        MockWrite(true, reinterpret_cast<const char*>(kT0QueryDatagram),
+                  arraysize(kT0QueryDatagram)));
+    reads0_.push_back(
+         MockRead(true, reinterpret_cast<const char*>(kT0ResponseDatagram),
+                  arraysize(kT0ResponseDatagram)));
+    data0_.reset(new StaticSocketDataProvider(&reads0_[0], reads0_.size(),
+                                              &writes0_[0], writes0_.size()));
+
+    // Setup socket read/writes for transaction 1.
+    writes1_.push_back(
+        MockWrite(true, reinterpret_cast<const char*>(kT1QueryDatagram),
+                  arraysize(kT1QueryDatagram)));
+    reads1_.push_back(
+         MockRead(true, reinterpret_cast<const char*>(kT1ResponseDatagram),
+                  arraysize(kT1ResponseDatagram)));
+    data1_.reset(new StaticSocketDataProvider(&reads1_[0], reads1_.size(),
+                                              &writes1_[0], writes1_.size()));
+
+    // Setup socket read/writes for transaction 2.
+    writes2_.push_back(
+        MockWrite(true, reinterpret_cast<const char*>(kT2QueryDatagram),
+                  arraysize(kT2QueryDatagram)));
+    reads2_.push_back(
+         MockRead(true, reinterpret_cast<const char*>(kT2ResponseDatagram),
+                  arraysize(kT2ResponseDatagram)));
+    data2_.reset(new StaticSocketDataProvider(&reads2_[0], reads2_.size(),
+                                              &writes2_[0], writes2_.size()));
+
+    // Setup socket read/writes for transaction 3.
+    writes3_.push_back(
+        MockWrite(true, reinterpret_cast<const char*>(kT3QueryDatagram),
+                  arraysize(kT3QueryDatagram)));
+    reads3_.push_back(
+         MockRead(true, reinterpret_cast<const char*>(kT3ResponseDatagram),
+                  arraysize(kT3ResponseDatagram)));
+    data3_.reset(new StaticSocketDataProvider(&reads3_[0], reads3_.size(),
+                                              &writes3_[0], writes3_.size()));
+
+    factory_.AddSocketDataProvider(data0_.get());
+    factory_.AddSocketDataProvider(data1_.get());
+    factory_.AddSocketDataProvider(data2_.get());
+    factory_.AddSocketDataProvider(data3_.get());
+
+    IPEndPoint dns_server;
+    bool rv0 = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
+    DCHECK(rv0);
+
+    resolver_.reset(
+        new AsyncHostResolver(
+            dns_server, kMaxTransactions, kMaxPendingRequests, rand_int_cb_,
+            &factory_, NULL));
+  }
+
+ protected:
+  AddressList addrlist0_, addrlist1_, addrlist2_, addrlist3_;
+  HostResolver::RequestInfo info0_, info1_, info2_, info3_;
+  std::vector<MockWrite> writes0_, writes1_, writes2_, writes3_;
+  std::vector<MockRead> reads0_, reads1_, reads2_, reads3_;
+  scoped_ptr<StaticSocketDataProvider> data0_, data1_, data2_, data3_;
+  std::vector<const char*> ip_addresses0_, ip_addresses1_,
+    ip_addresses2_, ip_addresses3_;
+  MockClientSocketFactory factory_;
+  TestPrng test_prng_;
+  RandIntCallback rand_int_cb_;
+  scoped_ptr<HostResolver> resolver_;
+  TestCompletionCallback callback0_, callback1_, callback2_, callback3_;
+};
+
+TEST_F(AsyncHostResolverTest, EmptyHostLookup) {
+  info0_.set_host_port_pair(HostPortPair("", kPortNum));
+  int rv = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                              BoundNetLog());
+  EXPECT_EQ(ERR_NAME_NOT_RESOLVED, rv);
+}
+
+TEST_F(AsyncHostResolverTest, IPv4LiteralLookup) {
+  const char* kIPLiteral = "192.168.1.2";
+  info0_.set_host_port_pair(HostPortPair(kIPLiteral, kPortNum));
+  info0_.set_host_resolver_flags(HOST_RESOLVER_CANONNAME);
+  int rv = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                              BoundNetLog());
+  EXPECT_EQ(OK, rv);
+  std::vector<const char*> ip_addresses(1, kIPLiteral);
+  VerifyAddressList(ip_addresses, kPortNum, addrlist0_);
+  EXPECT_STREQ(kIPLiteral, addrlist0_.head()->ai_canonname);
+}
+
+TEST_F(AsyncHostResolverTest, IPv6LiteralLookup) {
+  info0_.set_host_port_pair(HostPortPair("2001:db8:0::42", kPortNum));
+  int rv = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                              BoundNetLog());
+  // When support for IPv6 is added, this should succeed.
+  EXPECT_EQ(ERR_NAME_NOT_RESOLVED, rv);
+}
+
+TEST_F(AsyncHostResolverTest, CachedOnlyLookup) {
+  info0_.set_only_use_cached_response(true);
+  int rv = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                              BoundNetLog());
+  // When caching is added, this should succeed.
+  EXPECT_EQ(ERR_NAME_NOT_RESOLVED, rv);
+}
+
+TEST_F(AsyncHostResolverTest, InvalidHostNameLookup) {
+  const std::string kHostName1(64, 'a');
+  info0_.set_host_port_pair(HostPortPair(kHostName1, kPortNum));
+  int rv = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                              BoundNetLog());
+  EXPECT_EQ(ERR_NAME_NOT_RESOLVED, rv);
+
+  const std::string kHostName2(4097, 'b');
+  info0_.set_host_port_pair(HostPortPair(kHostName2, kPortNum));
+  rv = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                          BoundNetLog());
+  EXPECT_EQ(ERR_NAME_NOT_RESOLVED, rv);
+}
+
+TEST_F(AsyncHostResolverTest, Lookup) {
+  int rv = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                              BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv);
+  rv = callback0_.WaitForResult();
+  EXPECT_EQ(OK, rv);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist0_);
+}
+
+TEST_F(AsyncHostResolverTest, ConcurrentLookup) {
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info1_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  int rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+
+  rv0 = callback0_.WaitForResult();
+  EXPECT_EQ(OK, rv0);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist0_);
+
+  rv1 = callback1_.WaitForResult();
+  EXPECT_EQ(OK, rv1);
+  VerifyAddressList(ip_addresses1_, kPortNum, addrlist1_);
+
+  rv2 = callback2_.WaitForResult();
+  EXPECT_EQ(OK, rv2);
+  VerifyAddressList(ip_addresses2_, kPortNum, addrlist2_);
+
+  EXPECT_EQ(3u, factory_.udp_client_sockets().size());
+}
+
+TEST_F(AsyncHostResolverTest, SameHostLookupsConsumeSingleTransaction) {
+  // We pass the info0_ to all requests.
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info0_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  int rv2 = resolver_->Resolve(info0_, &addrlist2_, &callback2_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+
+  rv0 = callback0_.WaitForResult();
+  EXPECT_EQ(OK, rv0);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist0_);
+
+  rv1 = callback1_.WaitForResult();
+  EXPECT_EQ(OK, rv1);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist1_);
+
+  rv2 = callback2_.WaitForResult();
+  EXPECT_EQ(OK, rv2);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist2_);
+
+  // Although we have three lookups, a single UDP socket was used.
+  EXPECT_EQ(1u, factory_.udp_client_sockets().size());
+}
+
+TEST_F(AsyncHostResolverTest, CancelLookup) {
+  HostResolver::RequestHandle req0 = NULL, req2 = NULL;
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, &req0,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info1_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  int rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, &req2,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+
+  resolver_->CancelRequest(req0);
+  resolver_->CancelRequest(req2);
+
+  MessageLoop::current()->RunAllPending();
+
+  EXPECT_FALSE(callback0_.have_result());
+  EXPECT_FALSE(callback2_.have_result());
+
+  rv1 = callback1_.WaitForResult();
+  EXPECT_EQ(OK, rv1);
+  VerifyAddressList(ip_addresses1_, kPortNum, addrlist1_);
+}
+
+// Tests the following scenario: start two resolutions for the same host,
+// cancel one of them, make sure that the other one completes.
+TEST_F(AsyncHostResolverTest, CancelSameHostLookup) {
+  HostResolver::RequestHandle req0 = NULL;
+
+  // Pass the info0_ to both requests.
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, &req0,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info0_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+
+  resolver_->CancelRequest(req0);
+  MessageLoop::current()->RunAllPending();
+  EXPECT_FALSE(callback0_.have_result());
+
+  rv1 = callback1_.WaitForResult();
+  EXPECT_EQ(OK, rv1);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist1_);
+
+  EXPECT_EQ(1u, factory_.udp_client_sockets().size());
+}
+
+// Test that a queued lookup completes.
+TEST_F(AsyncHostResolverTest, QueuedLookup) {
+  // kMaxTransactions is 2, thus the following requests consume all
+  // available transactions.
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info1_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+
+  // The following request will end up in queue.
+  int rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+  EXPECT_EQ(1u,
+      static_cast<AsyncHostResolver*>(resolver_.get())->GetNumPending());
+
+  // Make sure all requests complete.
+  rv0 = callback0_.WaitForResult();
+  EXPECT_EQ(OK, rv0);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist0_);
+
+  rv1 = callback1_.WaitForResult();
+  EXPECT_EQ(OK, rv1);
+  VerifyAddressList(ip_addresses1_, kPortNum, addrlist1_);
+
+  rv2 = callback2_.WaitForResult();
+  EXPECT_EQ(OK, rv2);
+  VerifyAddressList(ip_addresses2_, kPortNum, addrlist2_);
+}
+
+// Test that cancelling a queued lookup works.
+TEST_F(AsyncHostResolverTest, CancelPendingLookup) {
+  // kMaxTransactions is 2, thus the following requests consume all
+  // available transactions.
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info1_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+
+  // The following request will end up in queue.
+  HostResolver::RequestHandle req2 = NULL;
+  int rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, &req2,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+  EXPECT_EQ(1u,
+      static_cast<AsyncHostResolver*>(resolver_.get())->GetNumPending());
+
+  resolver_->CancelRequest(req2);
+
+  // Make sure first two requests complete while the cancelled one doesn't.
+  MessageLoop::current()->RunAllPending();
+  EXPECT_FALSE(callback2_.have_result());
+
+  rv0 = callback0_.WaitForResult();
+  EXPECT_EQ(OK, rv0);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist0_);
+
+  rv1 = callback1_.WaitForResult();
+  EXPECT_EQ(OK, rv1);
+  VerifyAddressList(ip_addresses1_, kPortNum, addrlist1_);
+}
+
+TEST_F(AsyncHostResolverTest, ResolverDestructionCancelsLookups) {
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info1_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  // This one is queued.
+  int rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(1u,
+      static_cast<AsyncHostResolver*>(resolver_.get())->GetNumPending());
+
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+
+  resolver_.reset();
+
+  MessageLoop::current()->RunAllPending();
+
+  EXPECT_FALSE(callback0_.have_result());
+  EXPECT_FALSE(callback1_.have_result());
+  EXPECT_FALSE(callback2_.have_result());
+}
+
+// Test that when the number of pending lookups is at max, a new lookup
+// with a priority lower than all of those in the queue fails.
+TEST_F(AsyncHostResolverTest, OverflowQueueWithLowPriorityLookup) {
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info1_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  // This one is queued and fills up the queue since its size is 1.
+  int rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(1u,
+      static_cast<AsyncHostResolver*>(resolver_.get())->GetNumPending());
+
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+
+  // This one fails.
+  info3_.set_priority(LOWEST);
+  int rv3 = resolver_->Resolve(info3_, &addrlist3_, &callback3_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_HOST_RESOLVER_QUEUE_TOO_LARGE, rv3);
+
+  MessageLoop::current()->RunAllPending();
+  EXPECT_FALSE(callback3_.have_result());
+}
+
+// Test that when the number of pending lookups is at max, a new lookup
+// with a priority higher than any of those in the queue succeeds and
+// causes the lowest priority lookup in the queue to fail.
+TEST_F(AsyncHostResolverTest, OverflowQueueWithHighPriorityLookup) {
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info1_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+
+  // Next lookup is queued.  Since this will be ejected from the queue and
+  // will not consume a socket from our factory, we are not passing it
+  // predefined members.
+  HostResolver::RequestInfo info(HostPortPair("cnn.com", 80));
+  info.set_address_family(ADDRESS_FAMILY_IPV4);
+  AddressList addrlist_fail;
+  TestCompletionCallback callback_fail;
+  int rv_fail = resolver_->Resolve(info, &addrlist_fail, &callback_fail, NULL,
+                                   BoundNetLog());
+  EXPECT_EQ(1u,
+      static_cast<AsyncHostResolver*>(resolver_.get())->GetNumPending());
+
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+  EXPECT_EQ(ERR_IO_PENDING, rv_fail);
+
+  // Lookup 2 causes the above to fail, but itself should succeed.
+  info2_.set_priority(HIGHEST);
+  int rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, NULL,
+                               BoundNetLog());
+
+  rv0 = callback0_.WaitForResult();
+  EXPECT_EQ(OK, rv0);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist0_);
+
+  rv1 = callback1_.WaitForResult();
+  EXPECT_EQ(OK, rv1);
+  VerifyAddressList(ip_addresses1_, kPortNum, addrlist1_);
+
+  rv_fail = callback_fail.WaitForResult();
+  EXPECT_EQ(ERR_HOST_RESOLVER_QUEUE_TOO_LARGE, rv_fail);
+  EXPECT_EQ(static_cast<addrinfo*>(NULL), addrlist_fail.head());
+
+  rv2 = callback2_.WaitForResult();
+  EXPECT_EQ(OK, rv2);
+  VerifyAddressList(ip_addresses2_, kPortNum, addrlist2_);
+}
+
+// Test that registering, unregistering, and notifying of observers of
+// resolution start, completion and cancellation (both due to CancelRequest
+// and resolver destruction) work.
+TEST_F(AsyncHostResolverTest, Observers) {
+  TestHostResolverObserver observer;
+  resolver_->AddObserver(&observer);
+
+  int rv0 = resolver_->Resolve(info0_, &addrlist0_, &callback0_, NULL,
+                               BoundNetLog());
+  int rv1 = resolver_->Resolve(info1_, &addrlist1_, &callback1_, NULL,
+                               BoundNetLog());
+  // We will cancel this one.
+  HostResolver::RequestHandle req2 = NULL;
+  int rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, &req2,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
+  EXPECT_EQ(ERR_IO_PENDING, rv1);
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+
+  // Cancel lookup 2.
+  resolver_->CancelRequest(req2);
+
+  // Lookup 0 and 1 should succeed.
+  rv0 = callback0_.WaitForResult();
+  EXPECT_EQ(OK, rv0);
+  VerifyAddressList(ip_addresses0_, kPortNum, addrlist0_);
+
+  rv1 = callback1_.WaitForResult();
+  EXPECT_EQ(OK, rv1);
+  VerifyAddressList(ip_addresses1_, kPortNum, addrlist1_);
+
+  // Next lookup should not have finished.
+  MessageLoop::current()->RunAllPending();
+  EXPECT_FALSE(callback2_.have_result());
+
+  // Verify observer calls.
+  EXPECT_EQ(3u, observer.start_log.size());
+  EXPECT_EQ(2u, observer.finish_log.size());
+  EXPECT_EQ(1u, observer.cancel_log.size());
+
+  // Lookup 0 started and finished.
+  EXPECT_TRUE(observer.start_log[0] ==
+              TestHostResolverObserver::StartOrCancelEntry(0, info0_));
+  EXPECT_TRUE(observer.finish_log[0] ==
+              TestHostResolverObserver::FinishEntry(0, true, info0_));
+
+  // Ditto for lookup 1.
+  EXPECT_TRUE(observer.start_log[1] ==
+              TestHostResolverObserver::StartOrCancelEntry(1, info1_));
+  EXPECT_TRUE(observer.finish_log[1] ==
+              TestHostResolverObserver::FinishEntry(1, true, info1_));
+
+  // Lookup 2 was cancelled, hence, failed to finish.
+  EXPECT_TRUE(observer.start_log[2] ==
+              TestHostResolverObserver::StartOrCancelEntry(2, info2_));
+  EXPECT_TRUE(observer.cancel_log[0] ==
+              TestHostResolverObserver::StartOrCancelEntry(2, info2_));
+
+  // Unregister observer.
+  resolver_->RemoveObserver(&observer);
+
+  // We will do lookup 2 again but will not be cancel it this time.
+  rv2 = resolver_->Resolve(info2_, &addrlist2_, &callback2_, NULL,
+                           BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv2);
+
+  // Run lookup 2 to completion.
+  rv2 = callback2_.WaitForResult();
+  EXPECT_EQ(OK, rv2);
+  VerifyAddressList(ip_addresses2_, kPortNum, addrlist2_);
+
+  // Observer log should stay the same.
+  EXPECT_EQ(3u, observer.start_log.size());
+  EXPECT_EQ(2u, observer.finish_log.size());
+  EXPECT_EQ(1u, observer.cancel_log.size());
+
+  // Re-register observer.
+  resolver_->AddObserver(&observer);
+
+  // Start lookup 3.
+  int rv3 = resolver_->Resolve(info3_, &addrlist3_, &callback3_, NULL,
+                               BoundNetLog());
+  EXPECT_EQ(ERR_IO_PENDING, rv3);
+
+  // Destroy the resolver and make sure that observer was notified of just
+  // the resolution start.
+  resolver_.reset();
+
+  EXPECT_EQ(4u, observer.start_log.size()); // Was incremented by 1.
+  EXPECT_EQ(2u, observer.finish_log.size());
+  EXPECT_EQ(1u, observer.cancel_log.size());
+
+  EXPECT_TRUE(observer.start_log[3] ==
+              TestHostResolverObserver::StartOrCancelEntry(4, info3_));
+}
+
+}  // namespace net
diff --git a/net/base/dns_test_util.cc b/net/base/dns_test_util.cc
index f448b6f..d7f6803 100644
--- a/net/base/dns_test_util.cc
+++ b/net/base/dns_test_util.cc
@@ -22,6 +22,41 @@ int TestPrng::GetNext(int min, int max) {
   return rv;
 }
 
+bool operator==(const HostResolver::RequestInfo& a,
+                const HostResolver::RequestInfo& b) {
+   return a.hostname() == b.hostname() &&
+          a.port() == b.port() &&
+          a.allow_cached_response() == b.allow_cached_response() &&
+          a.priority() == b.priority() &&
+          a.is_speculative() == b.is_speculative() &&
+          a.referrer() == b.referrer();
+}
+
+TestHostResolverObserver::TestHostResolverObserver() {
+}
+
+TestHostResolverObserver::~TestHostResolverObserver() {
+}
+
+void TestHostResolverObserver::OnStartResolution(
+    int id,
+    const HostResolver::RequestInfo& info) {
+  start_log.push_back(StartOrCancelEntry(id, info));
+}
+
+void TestHostResolverObserver::OnFinishResolutionWithStatus(
+    int id,
+    bool was_resolved,
+    const HostResolver::RequestInfo& info) {
+  finish_log.push_back(FinishEntry(id, was_resolved, info));
+}
+
+void TestHostResolverObserver::OnCancelResolution(
+    int id,
+    const HostResolver::RequestInfo& info) {
+  cancel_log.push_back(StartOrCancelEntry(id, info));
+}
+
 bool ConvertStringsToIPAddressList(
     const char* const ip_strings[], size_t size, IPAddressList* address_list) {
   DCHECK(address_list);
diff --git a/net/base/dns_test_util.h b/net/base/dns_test_util.h
index 96ce38a..04d8652 100644
--- a/net/base/dns_test_util.h
+++ b/net/base/dns_test_util.h
@@ -12,6 +12,7 @@
 #include "base/logging.h"
 #include "net/base/dns_transaction.h"
 #include "net/base/dns_util.h"
+#include "net/base/host_resolver.h"
 #include "net/base/ip_endpoint.h"
 #include "net/base/net_util.h"
 
@@ -34,6 +35,62 @@ class TestPrng {
   DISALLOW_COPY_AND_ASSIGN(TestPrng);
 };
 
+bool operator==(const HostResolver::RequestInfo& a,
+                const HostResolver::RequestInfo& b);
+
+// Observer that just makes note of how it was called. The test code can then
+// inspect to make sure it was called with the right parameters.  Used by
+// HostResolverImpl and AsyncHostResolver unit tests.
+class TestHostResolverObserver : public HostResolver::Observer {
+ public:
+  TestHostResolverObserver();
+  virtual ~TestHostResolverObserver();
+
+  // HostResolver::Observer methods:
+  virtual void OnStartResolution(int id, const HostResolver::RequestInfo& info);
+  virtual void OnFinishResolutionWithStatus(
+      int id,
+      bool was_resolved,
+      const HostResolver::RequestInfo& info);
+  virtual void OnCancelResolution(
+      int id,
+      const HostResolver::RequestInfo& info);
+
+  // Tuple (id, info).
+  struct StartOrCancelEntry {
+    StartOrCancelEntry(int id, const HostResolver::RequestInfo& info)
+        : id(id), info(info) {}
+
+    bool operator==(const StartOrCancelEntry& other) const {
+      return id == other.id && info == other.info;
+    }
+
+    int id;
+    HostResolver::RequestInfo info;
+  };
+
+  // Tuple (id, was_resolved, info).
+  struct FinishEntry {
+    FinishEntry(int id, bool was_resolved,
+                const HostResolver::RequestInfo& info)
+        : id(id), was_resolved(was_resolved), info(info) {}
+
+    bool operator==(const FinishEntry& other) const {
+      return id == other.id &&
+             was_resolved == other.was_resolved &&
+             info == other.info;
+    }
+
+    int id;
+    bool was_resolved;
+    HostResolver::RequestInfo info;
+  };
+
+  std::vector<StartOrCancelEntry> start_log;
+  std::vector<FinishEntry> finish_log;
+  std::vector<StartOrCancelEntry> cancel_log;
+};
+
 // A utility function for tests that given an array of IP literals,
 // converts it to an IPAddressList.
 bool ConvertStringsToIPAddressList(
@@ -47,29 +104,26 @@ static const char kDnsIp[] = "192.168.1.1";
 static const uint16 kDnsPort = 53;
 
 //-----------------------------------------------------------------------------
-// Query/response set for www.google.com, ID is fixed to 1.
-
-static const uint16 kT1Qtype = kDNS_A;
-
-static const char kT1DnsName[] = {
+// Query/response set for www.google.com, ID is fixed to 0.
+static const char kT0HostName[] = "www.google.com";
+static const uint16 kT0Qtype = kDNS_A;
+static const char kT0DnsName[] = {
   0x03, 'w', 'w', 'w',
   0x06, 'g', 'o', 'o', 'g', 'l', 'e',
   0x03, 'c', 'o', 'm',
   0x00
 };
-
-static const uint8 kT1QueryDatagram[] = {
+static const uint8 kT0QueryDatagram[] = {
   // query for www.google.com, type A.
-  0x00, 0x01, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00,
+  0x00, 0x00, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x03, 0x77, 0x77, 0x77,
   0x06, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x03,
   0x63, 0x6f, 0x6d, 0x00, 0x00, 0x01, 0x00, 0x01
 };
-
-static const uint8 kT1ResponseDatagram[] = {
+static const uint8 kT0ResponseDatagram[] = {
   // response contains one CNAME for www.l.google.com and the following
   // IP addresses: 74.125.226.{179,180,176,177,178}
-  0x00, 0x01, 0x81, 0x80, 0x00, 0x01, 0x00, 0x06,
+  0x00, 0x00, 0x81, 0x80, 0x00, 0x01, 0x00, 0x06,
   0x00, 0x00, 0x00, 0x00, 0x03, 0x77, 0x77, 0x77,
   0x06, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x03,
   0x63, 0x6f, 0x6d, 0x00, 0x00, 0x01, 0x00, 0x01,
@@ -87,37 +141,34 @@ static const uint8 kT1ResponseDatagram[] = {
   0x00, 0x01, 0x00, 0x00, 0x00, 0xe4, 0x00, 0x04,
   0x4a, 0x7d, 0xe2, 0xb2
 };
-
-static const char* const kT1IpAddresses[] = {
+static const char* const kT0IpAddresses[] = {
   "74.125.226.179", "74.125.226.180", "74.125.226.176",
   "74.125.226.177", "74.125.226.178"
 };
 
 //-----------------------------------------------------------------------------
-// Query/response set for codereview.chromium.org, ID is fixed to 2.
-static const uint16 kT2Qtype = kDNS_A;
-
-static const char kT2DnsName[] = {
+// Query/response set for codereview.chromium.org, ID is fixed to 1.
+static const char kT1HostName[] = "codereview.chromium.org";
+static const uint16 kT1Qtype = kDNS_A;
+static const char kT1DnsName[] = {
   0x12, 'c', 'o', 'd', 'e', 'r', 'e', 'v', 'i', 'e', 'w',
   0x10, 'c', 'h', 'r', 'o', 'm', 'i', 'u', 'm',
   0x03, 'o', 'r', 'g',
   0x00
 };
-
-static const uint8 kT2QueryDatagram[] = {
+static const uint8 kT1QueryDatagram[] = {
   // query for codereview.chromium.org, type A.
-  0x00, 0x02, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00,
+  0x00, 0x01, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x0a, 0x63, 0x6f, 0x64,
   0x65, 0x72, 0x65, 0x76, 0x69, 0x65, 0x77, 0x08,
   0x63, 0x68, 0x72, 0x6f, 0x6d, 0x69, 0x75, 0x6d,
   0x03, 0x6f, 0x72, 0x67, 0x00, 0x00, 0x01, 0x00,
   0x01
 };
-
-static const uint8 kT2ResponseDatagram[] = {
+static const uint8 kT1ResponseDatagram[] = {
   // response contains one CNAME for ghs.l.google.com and the following
   // IP address: 64.233.169.121
-  0x00, 0x02, 0x81, 0x80, 0x00, 0x01, 0x00, 0x02,
+  0x00, 0x01, 0x81, 0x80, 0x00, 0x01, 0x00, 0x02,
   0x00, 0x00, 0x00, 0x00, 0x0a, 0x63, 0x6f, 0x64,
   0x65, 0x72, 0x65, 0x76, 0x69, 0x65, 0x77, 0x08,
   0x63, 0x68, 0x72, 0x6f, 0x6d, 0x69, 0x75, 0x6d,
@@ -129,8 +180,92 @@ static const uint8 kT2ResponseDatagram[] = {
   0x35, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x01,
   0x0b, 0x00, 0x04, 0x40, 0xe9, 0xa9, 0x79
 };
+static const char* const kT1IpAddresses[] = {
+  "64.233.169.121"
+};
 
 //-----------------------------------------------------------------------------
+// Query/response set for www.ccs.neu.edu, ID is fixed to 2.
+static const char kT2HostName[] = "www.ccs.neu.edu";
+static const uint16 kT2Qtype = kDNS_A;
+static const char kT2DnsName[] = {
+  0x03, 'w', 'w', 'w',
+  0x03, 'c', 'c', 'c',
+  0x03, 'n', 'e', 'u',
+  0x03, 'e', 'd', 'u',
+  0x00
+};
+static const uint8 kT2QueryDatagram[] = {
+  // query for www.ccs.neu.edu, type A.
+  0x00, 0x02, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00,
+  0x00, 0x00, 0x00, 0x00, 0x03, 0x77, 0x77, 0x77,
+  0x03, 0x63, 0x63, 0x73, 0x03, 0x6e, 0x65, 0x75,
+  0x03, 0x65, 0x64, 0x75, 0x00, 0x00, 0x01, 0x00,
+  0x01
+};
+static const uint8 kT2ResponseDatagram[] = {
+  // response contains one CNAME for vulcan.ccs.neu.edu and the following
+  // IP address: 129.10.116.81
+  0x00, 0x02, 0x81, 0x80, 0x00, 0x01, 0x00, 0x02,
+  0x00, 0x00, 0x00, 0x00, 0x03, 0x77, 0x77, 0x77,
+  0x03, 0x63, 0x63, 0x73, 0x03, 0x6e, 0x65, 0x75,
+  0x03, 0x65, 0x64, 0x75, 0x00, 0x00, 0x01, 0x00,
+  0x01, 0xc0, 0x0c, 0x00, 0x05, 0x00, 0x01, 0x00,
+  0x00, 0x01, 0x2c, 0x00, 0x09, 0x06, 0x76, 0x75,
+  0x6c, 0x63, 0x61, 0x6e, 0xc0, 0x10, 0xc0, 0x2d,
+  0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x01, 0x2c,
+  0x00, 0x04, 0x81, 0x0a, 0x74, 0x51
+};
+static const char* const kT2IpAddresses[] = {
+  "129.10.116.81"
+};
+
+//-----------------------------------------------------------------------------
+// Query/response set for www.google.az, ID is fixed to 3.
+static const char kT3HostName[] = "www.google.az";
+static const uint16 kT3Qtype = kDNS_A;
+static const char kT3DnsName[] = {
+  0x03, 'w', 'w', 'w',
+  0x06, 'g', 'o', 'o', 'g', 'l', 'e',
+  0x02, 'a', 'z',
+  0x00
+};
+static const uint8 kT3QueryDatagram[] = {
+  // query for www.google.az, type A.
+  0x00, 0x03, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00,
+  0x00, 0x00, 0x00, 0x00, 0x03, 0x77, 0x77, 0x77,
+  0x06, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x02,
+  0x61, 0x7a, 0x00, 0x00, 0x01, 0x00, 0x01
+};
+static const uint8 kT3ResponseDatagram[] = {
+  // response contains www.google.com as CNAME for www.google.az and
+  // www.l.google.com as CNAME for www.google.com and the following
+  // IP addresses: 74.125.226.{178,179,180,176,177}
+  0x00, 0x03, 0x81, 0x80, 0x00, 0x01, 0x00, 0x07,
+  0x00, 0x00, 0x00, 0x00, 0x03, 0x77, 0x77, 0x77,
+  0x06, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x02,
+  0x61, 0x7a, 0x00, 0x00, 0x01, 0x00, 0x01, 0xc0,
+  0x0c, 0x00, 0x05, 0x00, 0x01, 0x00, 0x01, 0x50,
+  0x99, 0x00, 0x10, 0x03, 0x77, 0x77, 0x77, 0x06,
+  0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x03, 0x63,
+  0x6f, 0x6d, 0x00, 0xc0, 0x2b, 0x00, 0x05, 0x00,
+  0x01, 0x00, 0x01, 0x50, 0x99, 0x00, 0x08, 0x03,
+  0x77, 0x77, 0x77, 0x01, 0x6c, 0xc0, 0x2f, 0xc0,
+  0x47, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00,
+  0x15, 0x00, 0x04, 0x4a, 0x7d, 0xe2, 0xb2, 0xc0,
+  0x47, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00,
+  0x15, 0x00, 0x04, 0x4a, 0x7d, 0xe2, 0xb3, 0xc0,
+  0x47, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00,
+  0x15, 0x00, 0x04, 0x4a, 0x7d, 0xe2, 0xb4, 0xc0,
+  0x47, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00,
+  0x15, 0x00, 0x04, 0x4a, 0x7d, 0xe2, 0xb0, 0xc0,
+  0x47, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00,
+  0x15, 0x00, 0x04, 0x4a, 0x7d, 0xe2, 0xb1
+};
+static const char* const kT3IpAddresses[] = {
+  "74.125.226.178", "74.125.226.179", "74.125.226.180",
+  "74.125.226.176", "74.125.226.177"
+};
 
 }  // namespace net
 
diff --git a/net/base/dns_transaction_unittest.cc b/net/base/dns_transaction_unittest.cc
index 6c532f2..2e557e9 100644
--- a/net/base/dns_transaction_unittest.cc
+++ b/net/base/dns_transaction_unittest.cc
@@ -53,48 +53,48 @@ class TestDelegate : public DnsTransaction::Delegate {
 
 
 TEST(DnsTransactionTest, NormalQueryResponseTest) {
-  MockWrite writes1[] = {
-    MockWrite(true, reinterpret_cast<const char*>(kT1QueryDatagram),
-              arraysize(kT1QueryDatagram))
+  MockWrite writes0[] = {
+    MockWrite(true, reinterpret_cast<const char*>(kT0QueryDatagram),
+              arraysize(kT0QueryDatagram))
   };
 
-  MockRead reads1[] = {
-    MockRead(true, reinterpret_cast<const char*>(kT1ResponseDatagram),
-             arraysize(kT1ResponseDatagram))
+  MockRead reads0[] = {
+    MockRead(true, reinterpret_cast<const char*>(kT0ResponseDatagram),
+             arraysize(kT0ResponseDatagram))
   };
 
-  StaticSocketDataProvider data(reads1, arraysize(reads1),
-                                writes1, arraysize(writes1));
+  StaticSocketDataProvider data(reads0, arraysize(reads0),
+                                writes0, arraysize(writes0));
   MockClientSocketFactory factory;
   factory.AddSocketDataProvider(&data);
 
-  TestPrng test_prng(std::deque<int>(1, 1));
+  TestPrng test_prng(std::deque<int>(1, 0));
   RandIntCallback rand_int_cb =
       base::Bind(&TestPrng::GetNext, base::Unretained(&test_prng));
-  std::string t1_dns_name(kT1DnsName, arraysize(kT1DnsName));
+  std::string t0_dns_name(kT0DnsName, arraysize(kT0DnsName));
 
   IPEndPoint dns_server;
-  bool rv0 = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
-  ASSERT_TRUE(rv0);
+  bool rv = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
+  ASSERT_TRUE(rv);
 
-  DnsTransaction t(dns_server, t1_dns_name, kT1Qtype, rand_int_cb, &factory,
+  DnsTransaction t(dns_server, t0_dns_name, kT1Qtype, rand_int_cb, &factory,
                    BoundNetLog(), NULL);
 
   TestDelegate delegate;
   t.SetDelegate(&delegate);
 
   IPAddressList expected_ip_addresses;
-  rv0 = ConvertStringsToIPAddressList(kT1IpAddresses,
-                                      arraysize(kT1IpAddresses),
-                                      &expected_ip_addresses);
-  ASSERT_TRUE(rv0);
+  rv = ConvertStringsToIPAddressList(kT0IpAddresses,
+                                     arraysize(kT0IpAddresses),
+                                     &expected_ip_addresses);
+  ASSERT_TRUE(rv);
 
-  int rv = t.Start();
-  EXPECT_EQ(ERR_IO_PENDING, rv);
+  int rv0 = t.Start();
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
 
   MessageLoop::current()->Run();
 
-  EXPECT_TRUE(DnsTransaction::Key(t1_dns_name, kT1Qtype) == t.key());
+  EXPECT_TRUE(DnsTransaction::Key(t0_dns_name, kT0Qtype) == t.key());
   EXPECT_EQ(OK, delegate.result());
   EXPECT_EQ(&t, delegate.transaction());
   EXPECT_TRUE(expected_ip_addresses == delegate.ip_addresses());
@@ -104,42 +104,42 @@ TEST(DnsTransactionTest, NormalQueryResponseTest) {
 }
 
 TEST(DnsTransactionTest, MismatchedQueryResponseTest) {
-  MockWrite writes1[] = {
-    MockWrite(true, reinterpret_cast<const char*>(kT1QueryDatagram),
-              arraysize(kT1QueryDatagram))
+  MockWrite writes0[] = {
+    MockWrite(true, reinterpret_cast<const char*>(kT0QueryDatagram),
+              arraysize(kT0QueryDatagram))
   };
 
-  MockRead reads2[] = {
-    MockRead(true, reinterpret_cast<const char*>(kT2ResponseDatagram),
-             arraysize(kT2ResponseDatagram))
+  MockRead reads1[] = {
+    MockRead(true, reinterpret_cast<const char*>(kT1ResponseDatagram),
+             arraysize(kT1ResponseDatagram))
   };
 
-  StaticSocketDataProvider data(reads2, arraysize(reads2),
-                                writes1, arraysize(writes1));
+  StaticSocketDataProvider data(reads1, arraysize(reads1),
+                                writes0, arraysize(writes0));
   MockClientSocketFactory factory;
   factory.AddSocketDataProvider(&data);
 
-  TestPrng test_prng(std::deque<int>(1, 1));
+  TestPrng test_prng(std::deque<int>(1, 0));
   RandIntCallback rand_int_cb =
       base::Bind(&TestPrng::GetNext, base::Unretained(&test_prng));
-  std::string t1_dns_name(kT1DnsName, arraysize(kT1DnsName));
+  std::string t0_dns_name(kT0DnsName, arraysize(kT0DnsName));
 
   IPEndPoint dns_server;
-  bool rv0 = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
-  ASSERT_TRUE(rv0);
+  bool rv = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
+  ASSERT_TRUE(rv);
 
-  DnsTransaction t(dns_server, t1_dns_name, kT1Qtype, rand_int_cb, &factory,
+  DnsTransaction t(dns_server, t0_dns_name, kT1Qtype, rand_int_cb, &factory,
                    BoundNetLog(), NULL);
 
   TestDelegate delegate;
   t.SetDelegate(&delegate);
 
-  int rv = t.Start();
-  EXPECT_EQ(ERR_IO_PENDING, rv);
+  int rv0 = t.Start();
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
 
   MessageLoop::current()->Run();
 
-  EXPECT_TRUE(DnsTransaction::Key(t1_dns_name, kT1Qtype) == t.key());
+  EXPECT_TRUE(DnsTransaction::Key(t0_dns_name, kT0Qtype) == t.key());
   EXPECT_EQ(ERR_DNS_MALFORMED_RESPONSE, delegate.result());
   EXPECT_EQ(0u, delegate.ip_addresses().size());
   EXPECT_EQ(&t, delegate.transaction());
@@ -150,35 +150,35 @@ TEST(DnsTransactionTest, MismatchedQueryResponseTest) {
 // Test that after the first timeout we do a fresh connection and if we get
 // a response on the new connection, we return it.
 TEST(DnsTransactionTest, FirstTimeoutTest) {
-  MockWrite writes1[] = {
-    MockWrite(true, reinterpret_cast<const char*>(kT1QueryDatagram),
-              arraysize(kT1QueryDatagram))
+  MockWrite writes0[] = {
+    MockWrite(true, reinterpret_cast<const char*>(kT0QueryDatagram),
+              arraysize(kT0QueryDatagram))
   };
 
-  MockRead reads1[] = {
-    MockRead(true, reinterpret_cast<const char*>(kT1ResponseDatagram),
-             arraysize(kT1ResponseDatagram))
+  MockRead reads0[] = {
+    MockRead(true, reinterpret_cast<const char*>(kT0ResponseDatagram),
+             arraysize(kT0ResponseDatagram))
   };
 
+  scoped_refptr<DelayedSocketData> socket0_data(
+      new DelayedSocketData(2, NULL, 0, writes0, arraysize(writes0)));
   scoped_refptr<DelayedSocketData> socket1_data(
-      new DelayedSocketData(2, NULL, 0, writes1, arraysize(writes1)));
-  scoped_refptr<DelayedSocketData> socket2_data(
-      new DelayedSocketData(0, reads1, arraysize(reads1),
-                            writes1, arraysize(writes1)));
+      new DelayedSocketData(0, reads0, arraysize(reads0),
+                            writes0, arraysize(writes0)));
   MockClientSocketFactory factory;
+  factory.AddSocketDataProvider(socket0_data.get());
   factory.AddSocketDataProvider(socket1_data.get());
-  factory.AddSocketDataProvider(socket2_data.get());
 
-  TestPrng test_prng(std::deque<int>(2, 1));
+  TestPrng test_prng(std::deque<int>(2, 0));
   RandIntCallback rand_int_cb =
       base::Bind(&TestPrng::GetNext, base::Unretained(&test_prng));
-  std::string t1_dns_name(kT1DnsName, arraysize(kT1DnsName));
+  std::string t0_dns_name(kT0DnsName, arraysize(kT0DnsName));
 
   IPEndPoint dns_server;
-  bool rv0 = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
-  ASSERT_TRUE(rv0);
+  bool rv = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
+  ASSERT_TRUE(rv);
 
-  DnsTransaction t(dns_server, t1_dns_name, kT1Qtype, rand_int_cb, &factory,
+  DnsTransaction t(dns_server, t0_dns_name, kT1Qtype, rand_int_cb, &factory,
                    BoundNetLog(), NULL);
 
   TestDelegate delegate;
@@ -189,25 +189,26 @@ TEST(DnsTransactionTest, FirstTimeoutTest) {
                                    kTimeoutsMs + arraysize(kTimeoutsMs)));
 
   IPAddressList expected_ip_addresses;
-  rv0 = ConvertStringsToIPAddressList(kT1IpAddresses,
-                                      arraysize(kT1IpAddresses),
-                                      &expected_ip_addresses);
-  ASSERT_TRUE(rv0);
+  rv = ConvertStringsToIPAddressList(kT0IpAddresses,
+                                     arraysize(kT0IpAddresses),
+                                     &expected_ip_addresses);
+  ASSERT_TRUE(rv);
+
+  int rv0 = t.Start();
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
 
-  int rv = t.Start();
-  EXPECT_EQ(ERR_IO_PENDING, rv);
 
   MessageLoop::current()->Run();
 
-  EXPECT_TRUE(DnsTransaction::Key(t1_dns_name, kT1Qtype) == t.key());
+  EXPECT_TRUE(DnsTransaction::Key(t0_dns_name, kT0Qtype) == t.key());
   EXPECT_EQ(OK, delegate.result());
   EXPECT_EQ(&t, delegate.transaction());
   EXPECT_TRUE(expected_ip_addresses == delegate.ip_addresses());
 
+  EXPECT_TRUE(socket0_data->at_read_eof());
+  EXPECT_TRUE(socket0_data->at_write_eof());
   EXPECT_TRUE(socket1_data->at_read_eof());
   EXPECT_TRUE(socket1_data->at_write_eof());
-  EXPECT_TRUE(socket2_data->at_read_eof());
-  EXPECT_TRUE(socket2_data->at_write_eof());
   EXPECT_EQ(2u, factory.udp_client_sockets().size());
 }
 
@@ -215,38 +216,38 @@ TEST(DnsTransactionTest, FirstTimeoutTest) {
 // the second timeout we do another fresh connection, and if we get a
 // response on the second connection, we return it.
 TEST(DnsTransactionTest, SecondTimeoutTest) {
-  MockWrite writes1[] = {
-    MockWrite(true, reinterpret_cast<const char*>(kT1QueryDatagram),
-              arraysize(kT1QueryDatagram))
+  MockWrite writes0[] = {
+    MockWrite(true, reinterpret_cast<const char*>(kT0QueryDatagram),
+              arraysize(kT0QueryDatagram))
   };
 
-  MockRead reads1[] = {
-    MockRead(true, reinterpret_cast<const char*>(kT1ResponseDatagram),
-             arraysize(kT1ResponseDatagram))
+  MockRead reads0[] = {
+    MockRead(true, reinterpret_cast<const char*>(kT0ResponseDatagram),
+             arraysize(kT0ResponseDatagram))
   };
 
+  scoped_refptr<DelayedSocketData> socket0_data(
+      new DelayedSocketData(2, NULL, 0, writes0, arraysize(writes0)));
   scoped_refptr<DelayedSocketData> socket1_data(
-      new DelayedSocketData(2, NULL, 0, writes1, arraysize(writes1)));
+      new DelayedSocketData(2, NULL, 0, writes0, arraysize(writes0)));
   scoped_refptr<DelayedSocketData> socket2_data(
-      new DelayedSocketData(2, NULL, 0, writes1, arraysize(writes1)));
-  scoped_refptr<DelayedSocketData> socket3_data(
-      new DelayedSocketData(0, reads1, arraysize(reads1),
-                            writes1, arraysize(writes1)));
+      new DelayedSocketData(0, reads0, arraysize(reads0),
+                            writes0, arraysize(writes0)));
   MockClientSocketFactory factory;
+  factory.AddSocketDataProvider(socket0_data.get());
   factory.AddSocketDataProvider(socket1_data.get());
   factory.AddSocketDataProvider(socket2_data.get());
-  factory.AddSocketDataProvider(socket3_data.get());
 
-  TestPrng test_prng(std::deque<int>(3, 1));
+  TestPrng test_prng(std::deque<int>(3, 0));
   RandIntCallback rand_int_cb =
       base::Bind(&TestPrng::GetNext, base::Unretained(&test_prng));
-  std::string t1_dns_name(kT1DnsName, arraysize(kT1DnsName));
+  std::string t0_dns_name(kT0DnsName, arraysize(kT0DnsName));
 
   IPEndPoint dns_server;
-  bool rv0 = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
-  ASSERT_TRUE(rv0);
+  bool rv = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
+  ASSERT_TRUE(rv);
 
-  DnsTransaction t(dns_server, t1_dns_name, kT1Qtype, rand_int_cb, &factory,
+  DnsTransaction t(dns_server, t0_dns_name, kT1Qtype, rand_int_cb, &factory,
                    BoundNetLog(), NULL);
 
   TestDelegate delegate;
@@ -257,27 +258,27 @@ TEST(DnsTransactionTest, SecondTimeoutTest) {
                                    kTimeoutsMs + arraysize(kTimeoutsMs)));
 
   IPAddressList expected_ip_addresses;
-  rv0 = ConvertStringsToIPAddressList(kT1IpAddresses,
-                                      arraysize(kT1IpAddresses),
-                                      &expected_ip_addresses);
-  ASSERT_TRUE(rv0);
+  rv = ConvertStringsToIPAddressList(kT0IpAddresses,
+                                     arraysize(kT0IpAddresses),
+                                     &expected_ip_addresses);
+  ASSERT_TRUE(rv);
 
-  int rv = t.Start();
-  EXPECT_EQ(ERR_IO_PENDING, rv);
+  int rv0 = t.Start();
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
 
   MessageLoop::current()->Run();
 
-  EXPECT_TRUE(DnsTransaction::Key(t1_dns_name, kT1Qtype) == t.key());
+  EXPECT_TRUE(DnsTransaction::Key(t0_dns_name, kT1Qtype) == t.key());
   EXPECT_EQ(OK, delegate.result());
   EXPECT_EQ(&t, delegate.transaction());
   EXPECT_TRUE(expected_ip_addresses == delegate.ip_addresses());
 
+  EXPECT_TRUE(socket0_data->at_read_eof());
+  EXPECT_TRUE(socket0_data->at_write_eof());
   EXPECT_TRUE(socket1_data->at_read_eof());
   EXPECT_TRUE(socket1_data->at_write_eof());
   EXPECT_TRUE(socket2_data->at_read_eof());
   EXPECT_TRUE(socket2_data->at_write_eof());
-  EXPECT_TRUE(socket3_data->at_read_eof());
-  EXPECT_TRUE(socket3_data->at_write_eof());
   EXPECT_EQ(3u, factory.udp_client_sockets().size());
 }
 
@@ -285,32 +286,32 @@ TEST(DnsTransactionTest, SecondTimeoutTest) {
 // the second timeout we do another fresh connection and after the third
 // timeout we give up and return a timeout error.
 TEST(DnsTransactionTest, ThirdTimeoutTest) {
-  MockWrite writes1[] = {
-    MockWrite(true, reinterpret_cast<const char*>(kT1QueryDatagram),
-              arraysize(kT1QueryDatagram))
+  MockWrite writes0[] = {
+    MockWrite(true, reinterpret_cast<const char*>(kT0QueryDatagram),
+              arraysize(kT0QueryDatagram))
   };
 
+  scoped_refptr<DelayedSocketData> socket0_data(
+      new DelayedSocketData(2, NULL, 0, writes0, arraysize(writes0)));
   scoped_refptr<DelayedSocketData> socket1_data(
-      new DelayedSocketData(2, NULL, 0, writes1, arraysize(writes1)));
+      new DelayedSocketData(2, NULL, 0, writes0, arraysize(writes0)));
   scoped_refptr<DelayedSocketData> socket2_data(
-      new DelayedSocketData(2, NULL, 0, writes1, arraysize(writes1)));
-  scoped_refptr<DelayedSocketData> socket3_data(
-      new DelayedSocketData(2, NULL, 0, writes1, arraysize(writes1)));
+      new DelayedSocketData(2, NULL, 0, writes0, arraysize(writes0)));
   MockClientSocketFactory factory;
+  factory.AddSocketDataProvider(socket0_data.get());
   factory.AddSocketDataProvider(socket1_data.get());
   factory.AddSocketDataProvider(socket2_data.get());
-  factory.AddSocketDataProvider(socket3_data.get());
 
-  TestPrng test_prng(std::deque<int>(3, 1));
+  TestPrng test_prng(std::deque<int>(3, 0));
   RandIntCallback rand_int_cb =
       base::Bind(&TestPrng::GetNext, base::Unretained(&test_prng));
-  std::string t1_dns_name(kT1DnsName, arraysize(kT1DnsName));
+  std::string t0_dns_name(kT0DnsName, arraysize(kT0DnsName));
 
   IPEndPoint dns_server;
-  bool rv0 = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
-  ASSERT_TRUE(rv0);
+  bool rv = CreateDnsAddress(kDnsIp, kDnsPort, &dns_server);
+  ASSERT_TRUE(rv);
 
-  DnsTransaction t(dns_server, t1_dns_name, kT1Qtype, rand_int_cb, &factory,
+  DnsTransaction t(dns_server, t0_dns_name, kT1Qtype, rand_int_cb, &factory,
                    BoundNetLog(), NULL);
 
   TestDelegate delegate;
@@ -320,21 +321,21 @@ TEST(DnsTransactionTest, ThirdTimeoutTest) {
       std::vector<base::TimeDelta>(kTimeoutsMs,
                                    kTimeoutsMs + arraysize(kTimeoutsMs)));
 
-  int rv = t.Start();
-  EXPECT_EQ(ERR_IO_PENDING, rv);
+  int rv0 = t.Start();
+  EXPECT_EQ(ERR_IO_PENDING, rv0);
 
   MessageLoop::current()->Run();
 
-  EXPECT_TRUE(DnsTransaction::Key(t1_dns_name, kT1Qtype) == t.key());
+  EXPECT_TRUE(DnsTransaction::Key(t0_dns_name, kT0Qtype) == t.key());
   EXPECT_EQ(ERR_DNS_TIMED_OUT, delegate.result());
   EXPECT_EQ(&t, delegate.transaction());
 
+  EXPECT_TRUE(socket0_data->at_read_eof());
+  EXPECT_TRUE(socket0_data->at_write_eof());
   EXPECT_TRUE(socket1_data->at_read_eof());
   EXPECT_TRUE(socket1_data->at_write_eof());
   EXPECT_TRUE(socket2_data->at_read_eof());
   EXPECT_TRUE(socket2_data->at_write_eof());
-  EXPECT_TRUE(socket3_data->at_read_eof());
-  EXPECT_TRUE(socket3_data->at_write_eof());
   EXPECT_EQ(3u, factory.udp_client_sockets().size());
 }
 
diff --git a/net/base/host_resolver.h b/net/base/host_resolver.h
index d46fcc3..77977c1 100644
--- a/net/base/host_resolver.h
+++ b/net/base/host_resolver.h
@@ -14,6 +14,7 @@
 #include "net/base/completion_callback.h"
 #include "net/base/host_port_pair.h"
 #include "net/base/net_api.h"
+#include "net/base/net_util.h"
 #include "net/base/request_priority.h"
 
 namespace net {
@@ -209,6 +210,11 @@ NET_API HostResolver* CreateSystemHostResolver(size_t max_concurrent_resolves,
                                                size_t max_retry_attempts,
                                                NetLog* net_log);
 
+// Creates a HostResolver implementation that sends actual DNS queries to
+// the specified DNS server and parses response and returns results.
+NET_API HostResolver* CreateAsyncHostResolver(size_t max_concurrent_resolves,
+                                              const IPAddressNumber& dns_ip,
+                                              NetLog* net_log);
 }  // namespace net
 
 #endif  // NET_BASE_HOST_RESOLVER_H_
diff --git a/net/base/host_resolver_impl_unittest.cc b/net/base/host_resolver_impl_unittest.cc
index 52b7f89..a833599 100644
--- a/net/base/host_resolver_impl_unittest.cc
+++ b/net/base/host_resolver_impl_unittest.cc
@@ -16,6 +16,7 @@
 #include "base/time.h"
 #include "net/base/address_list.h"
 #include "net/base/completion_callback.h"
+#include "net/base/dns_test_util.h"
 #include "net/base/mock_host_resolver.h"
 #include "net/base/net_errors.h"
 #include "net/base/net_log_unittest.h"
@@ -979,73 +980,6 @@ TEST_F(HostResolverImplTest, BypassCache) {
   MessageLoop::current()->Run();
 }
 
-bool operator==(const HostResolver::RequestInfo& a,
-                const HostResolver::RequestInfo& b) {
-   return a.hostname() == b.hostname() &&
-          a.port() == b.port() &&
-          a.allow_cached_response() == b.allow_cached_response() &&
-          a.priority() == b.priority() &&
-          a.is_speculative() == b.is_speculative() &&
-          a.referrer() == b.referrer();
-}
-
-// Observer that just makes note of how it was called. The test code can then
-// inspect to make sure it was called with the right parameters.
-class CapturingObserver : public HostResolver::Observer {
- public:
-  // DnsResolutionObserver methods:
-  virtual void OnStartResolution(int id,
-                                 const HostResolver::RequestInfo& info) {
-    start_log.push_back(StartOrCancelEntry(id, info));
-  }
-
-  virtual void OnFinishResolutionWithStatus(
-      int id,
-      bool was_resolved,
-      const HostResolver::RequestInfo& info) {
-    finish_log.push_back(FinishEntry(id, was_resolved, info));
-  }
-
-  virtual void OnCancelResolution(int id,
-                                  const HostResolver::RequestInfo& info) {
-    cancel_log.push_back(StartOrCancelEntry(id, info));
-  }
-
-  // Tuple (id, info).
-  struct StartOrCancelEntry {
-    StartOrCancelEntry(int id, const HostResolver::RequestInfo& info)
-        : id(id), info(info) {}
-
-    bool operator==(const StartOrCancelEntry& other) const {
-      return id == other.id && info == other.info;
-    }
-
-    int id;
-    HostResolver::RequestInfo info;
-  };
-
-  // Tuple (id, was_resolved, info).
-  struct FinishEntry {
-    FinishEntry(int id, bool was_resolved,
-                const HostResolver::RequestInfo& info)
-        : id(id), was_resolved(was_resolved), info(info) {}
-
-    bool operator==(const FinishEntry& other) const {
-      return id == other.id &&
-             was_resolved == other.was_resolved &&
-             info == other.info;
-    }
-
-    int id;
-    bool was_resolved;
-    HostResolver::RequestInfo info;
-  };
-
-  std::vector<StartOrCancelEntry> start_log;
-  std::vector<FinishEntry> finish_log;
-  std::vector<StartOrCancelEntry> cancel_log;
-};
-
 // Test that registering, unregistering, and notifying of observers works.
 // Does not test the cancellation notification since all resolves are
 // synchronous.
@@ -1053,7 +987,7 @@ TEST_F(HostResolverImplTest, Observers) {
   scoped_ptr<HostResolver> host_resolver(
       CreateHostResolverImpl(NULL));
 
-  CapturingObserver observer;
+  TestHostResolverObserver observer;
 
   host_resolver->AddObserver(&observer);
 
@@ -1078,9 +1012,9 @@ TEST_F(HostResolverImplTest, Observers) {
   EXPECT_EQ(1U, observer.finish_log.size());
   EXPECT_EQ(0U, observer.cancel_log.size());
   EXPECT_TRUE(observer.start_log[0] ==
-              CapturingObserver::StartOrCancelEntry(0, info1));
+              TestHostResolverObserver::StartOrCancelEntry(0, info1));
   EXPECT_TRUE(observer.finish_log[0] ==
-              CapturingObserver::FinishEntry(0, true, info1));
+              TestHostResolverObserver::FinishEntry(0, true, info1));
 
   // Resolve "host1" again -- this time it  will be served from cache, but it
   // should still notify of completion.
@@ -1092,9 +1026,9 @@ TEST_F(HostResolverImplTest, Observers) {
   EXPECT_EQ(2U, observer.finish_log.size());
   EXPECT_EQ(0U, observer.cancel_log.size());
   EXPECT_TRUE(observer.start_log[1] ==
-              CapturingObserver::StartOrCancelEntry(1, info1));
+              TestHostResolverObserver::StartOrCancelEntry(1, info1));
   EXPECT_TRUE(observer.finish_log[1] ==
-              CapturingObserver::FinishEntry(1, true, info1));
+              TestHostResolverObserver::FinishEntry(1, true, info1));
 
   // Resolve "host2", setting referrer to "http://foobar.com"
   HostResolver::RequestInfo info2(HostPortPair("host2", 70));
@@ -1106,9 +1040,9 @@ TEST_F(HostResolverImplTest, Observers) {
   EXPECT_EQ(3U, observer.finish_log.size());
   EXPECT_EQ(0U, observer.cancel_log.size());
   EXPECT_TRUE(observer.start_log[2] ==
-              CapturingObserver::StartOrCancelEntry(2, info2));
+              TestHostResolverObserver::StartOrCancelEntry(2, info2));
   EXPECT_TRUE(observer.finish_log[2] ==
-              CapturingObserver::FinishEntry(2, true, info2));
+              TestHostResolverObserver::FinishEntry(2, true, info2));
 
   // Unregister the observer.
   host_resolver->RemoveObserver(&observer);
@@ -1128,7 +1062,7 @@ TEST_F(HostResolverImplTest, Observers) {
 //  (1) Delete the HostResolver while job is outstanding.
 //  (2) Call HostResolver::CancelRequest() while a request is outstanding.
 TEST_F(HostResolverImplTest, CancellationObserver) {
-  CapturingObserver observer;
+  TestHostResolverObserver observer;
   {
     // Create a host resolver and attach an observer.
     scoped_ptr<HostResolver> host_resolver(
@@ -1155,7 +1089,7 @@ TEST_F(HostResolverImplTest, CancellationObserver) {
     EXPECT_EQ(0U, observer.cancel_log.size());
 
     EXPECT_TRUE(observer.start_log[0] ==
-                CapturingObserver::StartOrCancelEntry(0, info1));
+                TestHostResolverObserver::StartOrCancelEntry(0, info1));
 
     // Cancel the request.
     host_resolver->CancelRequest(req);
@@ -1165,7 +1099,7 @@ TEST_F(HostResolverImplTest, CancellationObserver) {
     EXPECT_EQ(1U, observer.cancel_log.size());
 
     EXPECT_TRUE(observer.cancel_log[0] ==
-                CapturingObserver::StartOrCancelEntry(0, info1));
+                TestHostResolverObserver::StartOrCancelEntry(0, info1));
 
     // Start an async request for (host2:60)
     HostResolver::RequestInfo info2(HostPortPair("host2", 60));
@@ -1179,7 +1113,7 @@ TEST_F(HostResolverImplTest, CancellationObserver) {
     EXPECT_EQ(1U, observer.cancel_log.size());
 
     EXPECT_TRUE(observer.start_log[1] ==
-                CapturingObserver::StartOrCancelEntry(1, info2));
+                TestHostResolverObserver::StartOrCancelEntry(1, info2));
 
     // Upon exiting this scope, HostResolver is destroyed, so all requests are
     // implicitly cancelled.
@@ -1194,7 +1128,7 @@ TEST_F(HostResolverImplTest, CancellationObserver) {
 
   HostResolver::RequestInfo info(HostPortPair("host2", 60));
   EXPECT_TRUE(observer.cancel_log[1] ==
-              CapturingObserver::StartOrCancelEntry(1, info));
+              TestHostResolverObserver::StartOrCancelEntry(1, info));
 }
 
 // Test that IP address changes flush the cache.
@@ -1364,7 +1298,7 @@ TEST_F(HostResolverImplTest, HigherPriorityRequestsStartedFirst) {
       new HostResolverImpl(resolver_proc, CreateDefaultCache(), kMaxJobs,
                            kRetryAttempts, NULL));
 
-  CapturingObserver observer;
+  TestHostResolverObserver observer;
   host_resolver->AddObserver(&observer);
 
   // Note that at this point the CapturingHostResolverProc is blocked, so any
diff --git a/net/base/net_log_event_type_list.h b/net/base/net_log_event_type_list.h
index f0c2333..8e57c96 100644
--- a/net/base/net_log_event_type_list.h
+++ b/net/base/net_log_event_type_list.h
@@ -1077,8 +1077,8 @@ EVENT_TYPE(THROTTLING_GOT_CUSTOM_RETRY_AFTER)
 //
 // {
 //   "net_error": <The net error code for the failure>,
-//   "ip_address_list": <The result of the resolution process -- list of IP
-//                       addresses>,
+//   "ip_address_list": <The result of the resolution process,
+//                       an IPAddressList>
 // }
 EVENT_TYPE(DNS_TRANSACTION)
 
@@ -1093,3 +1093,39 @@ EVENT_TYPE(DNS_TRANSACTION)
 //                           attempt>,
 //   }
 EVENT_TYPE(DNS_TRANSACTION_ATTEMPT_STARTED)
+
+// ------------------------------------------------------------------------
+// AsyncHostResolver
+// ------------------------------------------------------------------------
+
+// The start/end of waiting on a host resolve (DNS) request.
+// The BEGIN phase contains the following parameters:
+//
+//   {
+//     "source_dependency": <Source id of the request being waited on>,
+//   }
+EVENT_TYPE(ASYNC_HOST_RESOLVER)
+
+// The start/end of a host resolve (DNS) request.
+//
+// The BEGIN phase contains the following parameters:
+//
+//   {
+//     "hostname": <Hostname associated with the request>,
+//     "address_family": <Address family of the request>,
+//     "allow_cached_response": <Whether to allow cached response>,
+//     "only_use_cached_response": <Use cached results only>,
+//     "is_speculative": <Whether the lookup is speculative>,
+//     "priority": <Priority of the request>,
+//     "source_dependency": <Source id, if any, of what created the request>,
+//   }
+//
+// The END phase will contain this parameter:
+//   {
+//     "net_error": <The net error code integer>,
+//   }
+EVENT_TYPE(ASYNC_HOST_RESOLVER_REQUEST)
+
+// This event is created when a new DnsTransaction is about to be created
+// for a request.
+EVENT_TYPE(ASYNC_HOST_RESOLVER_CREATE_DNS_TRANSACTION)
diff --git a/net/base/net_log_source_type_list.h b/net/base/net_log_source_type_list.h
index 2fcd534..de80d14 100644
--- a/net/base/net_log_source_type_list.h
+++ b/net/base/net_log_source_type_list.h
@@ -21,5 +21,6 @@ SOURCE_TYPE(MEMORY_CACHE_ENTRY, 10)
 SOURCE_TYPE(HTTP_STREAM_JOB, 11)
 SOURCE_TYPE(EXPONENTIAL_BACKOFF_THROTTLING, 12)
 SOURCE_TYPE(DNS_TRANSACTION, 13)
+SOURCE_TYPE(ASYNC_HOST_RESOLVER_REQUEST, 14)
 
-SOURCE_TYPE(COUNT, 14)  // Always keep this as the last entry.
+SOURCE_TYPE(COUNT, 15)  // Always keep this as the last entry.
diff --git a/net/net.gyp b/net/net.gyp
index f83cc76..12cfe91 100644
--- a/net/net.gyp
+++ b/net/net.gyp
@@ -31,6 +31,8 @@
         'base/address_list_net_log_param.h',
         'base/asn1_util.cc',
         'base/asn1_util.h',
+        'base/async_host_resolver.cc',
+        'base/async_host_resolver.h',
         'base/auth.cc',
         'base/auth.h',
         'base/backoff_entry.cc',
@@ -872,6 +874,7 @@
       ],
       'sources': [
         'base/address_list_unittest.cc',
+        'base/async_host_resolver_unittest.cc',
         'base/backoff_entry_unittest.cc',
         'base/cert_database_nss_unittest.cc',
         'base/cert_verifier_unittest.cc',