[net/dns] Resolve AF_UNSPEC on dual-stacked systems. Sort addresses according to RFC3484.

Original review: http://codereview.chromium.org/10442098/

BUG=113993
TEST=./net_unittests --gtest_filter=AddressSorter*:HostResolverImplDnsTest.DnsTaskUnspec

Review URL: https://chromiumcodereview.appspot.com/10855179

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151750 0039d316-1c4b-4281-b951-d872f2087c98

13 files changed, 1301 insertions, 67 deletions

diff --git a/net/dns/address_sorter.h b/net/dns/address_sorter.h
new file mode 100644
index 0000000..6ac9430
--- /dev/null
+++ b/net/dns/address_sorter.h
@@ -0,0 +1,46 @@
+// Copyright (c) 2012 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_DNS_ADDRESS_SORTER_H_
+#define NET_DNS_ADDRESS_SORTER_H_
+
+#include "base/basictypes.h"
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+#include "net/base/net_export.h"
+
+namespace net {
+
+class AddressList;
+
+// Sorts AddressList according to RFC3484, by likelihood of successful
+// connection. Depending on the platform, the sort could be performed
+// asynchronously by the OS, or synchronously by local implementation.
+// AddressSorter does not necessarily preserve port numbers on the sorted list.
+class NET_EXPORT AddressSorter {
+ public:
+  typedef base::Callback<void(bool success,
+                              const AddressList& list)> CallbackType;
+
+  virtual ~AddressSorter() {}
+
+  // Sorts |list|, which must include at least one IPv6 address.
+  // Calls |callback| upon completion. Could complete synchronously. Could
+  // complete after this AddressSorter is destroyed.
+  virtual void Sort(const AddressList& list,
+                    const CallbackType& callback) const = 0;
+
+  // Creates platform-dependent AddressSorter.
+  static scoped_ptr<AddressSorter> CreateAddressSorter();
+
+ protected:
+  AddressSorter() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(AddressSorter);
+};
+
+}  // namespace net
+
+#endif  // NET_DNS_ADDRESS_SORTER_H_
diff --git a/net/dns/address_sorter_posix.cc b/net/dns/address_sorter_posix.cc
new file mode 100644
index 0000000..807cbf8
--- /dev/null
+++ b/net/dns/address_sorter_posix.cc
@@ -0,0 +1,428 @@
+// Copyright (c) 2012 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/dns/address_sorter_posix.h"
+
+#include <netinet/in.h>
+
+#if defined(OS_MACOSX) || defined(OS_BSD)
+#include <sys/socket.h>  // Must be included before ifaddrs.h.
+#include <ifaddrs.h>
+#include <net/if.h>
+#include <netinet/in_var.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#endif
+
+#include <algorithm>
+
+#include "base/eintr_wrapper.h"
+#include "base/logging.h"
+#include "base/memory/scoped_vector.h"
+#include "net/socket/client_socket_factory.h"
+#include "net/udp/datagram_client_socket.h"
+
+#if defined(OS_LINUX)
+#include "net/base/address_tracker_linux.h"
+#endif
+
+namespace net {
+
+namespace {
+
+// Address sorting is performed according to RFC3484 with revisions.
+// http://tools.ietf.org/html/draft-ietf-6man-rfc3484bis-06
+// Precedence and label are separate to support override through /etc/gai.conf.
+
+// Returns true if |p1| should precede |p2| in the table.
+// Sorts table by decreasing prefix size to allow longest prefix matching.
+bool ComparePolicy(const AddressSorterPosix::PolicyEntry& p1,
+                   const AddressSorterPosix::PolicyEntry& p2) {
+  return p1.prefix_length > p2.prefix_length;
+}
+
+// Creates sorted PolicyTable from |table| with |size| entries.
+AddressSorterPosix::PolicyTable LoadPolicy(
+    AddressSorterPosix::PolicyEntry* table,
+    size_t size) {
+  AddressSorterPosix::PolicyTable result(table, table + size);
+  std::sort(result.begin(), result.end(), ComparePolicy);
+  return result;
+}
+
+// Search |table| for matching prefix of |address|. |table| must be sorted by
+// descending prefix (prefix of another prefix must be later in table).
+unsigned GetPolicyValue(const AddressSorterPosix::PolicyTable& table,
+                        const IPAddressNumber& address) {
+  if (address.size() == kIPv4AddressSize)
+    return GetPolicyValue(table, ConvertIPv4NumberToIPv6Number(address));
+  for (unsigned i = 0; i < table.size(); ++i) {
+    const AddressSorterPosix::PolicyEntry& entry = table[i];
+    IPAddressNumber prefix(entry.prefix, entry.prefix + kIPv6AddressSize);
+    if (IPNumberMatchesPrefix(address, prefix, entry.prefix_length))
+      return entry.value;
+  }
+  NOTREACHED();
+  // The last entry is the least restrictive, so assume it's default.
+  return table.back().value;
+}
+
+bool IsIPv6Multicast(const IPAddressNumber& address) {
+  DCHECK_EQ(kIPv6AddressSize, address.size());
+  return address[0] == 0xFF;
+}
+
+AddressSorterPosix::AddressScope GetIPv6MulticastScope(
+    const IPAddressNumber& address) {
+  DCHECK_EQ(kIPv6AddressSize, address.size());
+  return static_cast<AddressSorterPosix::AddressScope>(address[1] & 0x0F);
+}
+
+bool IsIPv6Loopback(const IPAddressNumber& address) {
+  DCHECK_EQ(kIPv6AddressSize, address.size());
+  // IN6_IS_ADDR_LOOPBACK
+  unsigned char kLoopback[kIPv6AddressSize] = {
+    0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 0, 1,
+  };
+  return address == IPAddressNumber(kLoopback, kLoopback + kIPv6AddressSize);
+}
+
+bool IsIPv6LinkLocal(const IPAddressNumber& address) {
+  DCHECK_EQ(kIPv6AddressSize, address.size());
+  // IN6_IS_ADDR_LINKLOCAL
+  return (address[0] == 0xFE) && ((address[1] & 0xC0) == 0x80);
+}
+
+bool IsIPv6SiteLocal(const IPAddressNumber& address) {
+  DCHECK_EQ(kIPv6AddressSize, address.size());
+  // IN6_IS_ADDR_SITELOCAL
+  return (address[0] == 0xFE) && ((address[1] & 0xC0) == 0xC0);
+}
+
+AddressSorterPosix::AddressScope GetScope(
+    const AddressSorterPosix::PolicyTable& ipv4_scope_table,
+    const IPAddressNumber& address) {
+  if (address.size() == kIPv6AddressSize) {
+    if (IsIPv6Multicast(address)) {
+      return GetIPv6MulticastScope(address);
+    } else if (IsIPv6Loopback(address) || IsIPv6LinkLocal(address)) {
+      return AddressSorterPosix::SCOPE_LINKLOCAL;
+    } else if (IsIPv6SiteLocal(address)) {
+      return AddressSorterPosix::SCOPE_SITELOCAL;
+    } else {
+      return AddressSorterPosix::SCOPE_GLOBAL;
+    }
+  } else if (address.size() == kIPv4AddressSize) {
+    return static_cast<AddressSorterPosix::AddressScope>(
+        GetPolicyValue(ipv4_scope_table, address));
+  } else {
+    NOTREACHED();
+    return AddressSorterPosix::SCOPE_NODELOCAL;
+  }
+}
+
+// Default policy table. RFC 3484, Section 2.1.
+AddressSorterPosix::PolicyEntry kDefaultPrecedenceTable[] = {
+  // ::1/128 -- loopback
+  { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 }, 128, 50 },
+  // ::/0 -- any
+  { { }, 0, 40 },
+  // ::ffff:0:0/96 -- IPv4 mapped
+  { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF }, 96, 35 },
+  // 2002::/16 -- 6to4
+  { { 0x20, 0x02, }, 16, 30 },
+  // 2001::/32 -- Teredo
+  { { 0x20, 0x01, 0, 0 }, 32, 5 },
+  // fc00::/7 -- unique local address
+  { { 0xFC }, 7, 3 },
+  // ::/96 -- IPv4 compatible
+  { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }, 96, 1 },
+  // fec0::/10 -- site-local expanded scope
+  { { 0xFE, 0xC0 }, 10, 1 },
+  // 3ffe::/16 -- 6bone
+  { { 0x3F, 0xFE }, 16, 1 },
+};
+
+AddressSorterPosix::PolicyEntry kDefaultLabelTable[] = {
+  // ::1/128 -- loopback
+  { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 }, 128, 0 },
+  // ::/0 -- any
+  { { }, 0, 1 },
+  // ::ffff:0:0/96 -- IPv4 mapped
+  { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF }, 96, 4 },
+  // 2002::/16 -- 6to4
+  { { 0x20, 0x02, }, 16, 2 },
+  // 2001::/32 -- Teredo
+  { { 0x20, 0x01, 0, 0 }, 32, 5 },
+  // fc00::/7 -- unique local address
+  { { 0xFC }, 7, 13 },
+  // ::/96 -- IPv4 compatible
+  { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }, 96, 3 },
+  // fec0::/10 -- site-local expanded scope
+  { { 0xFE, 0xC0 }, 10, 11 },
+  // 3ffe::/16 -- 6bone
+  { { 0x3F, 0xFE }, 16, 12 },
+};
+
+// Default mapping of IPv4 addresses to scope.
+AddressSorterPosix::PolicyEntry kDefaultIPv4ScopeTable[] = {
+  { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF, 0x7F }, 104,
+      AddressSorterPosix::SCOPE_LINKLOCAL },
+  { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF, 0xA9, 0xFE }, 112,
+      AddressSorterPosix::SCOPE_LINKLOCAL },
+  { { }, 0, AddressSorterPosix::SCOPE_GLOBAL },
+};
+
+// Returns number of matching initial bits between the addresses |a1| and |a2|.
+unsigned CommonPrefixLength(const IPAddressNumber& a1,
+                            const IPAddressNumber& a2) {
+  DCHECK_EQ(a1.size(), a2.size());
+  for (size_t i = 0; i < a1.size(); ++i) {
+    unsigned diff = a1[i] ^ a2[i];
+    if (!diff)
+      continue;
+    for (unsigned j = 0; j < CHAR_BIT; ++j) {
+      if (diff & (1 << (CHAR_BIT - 1)))
+        return i * CHAR_BIT + j;
+      diff <<= 1;
+    }
+    NOTREACHED();
+  }
+  return a1.size() * CHAR_BIT;
+}
+
+// Computes the number of leading 1-bits in |mask|.
+unsigned MaskPrefixLength(const IPAddressNumber& mask) {
+  IPAddressNumber all_ones(mask.size(), 0xFF);
+  return CommonPrefixLength(mask, all_ones);
+}
+
+struct DestinationInfo {
+  IPAddressNumber address;
+  AddressSorterPosix::AddressScope scope;
+  unsigned precedence;
+  unsigned label;
+  const AddressSorterPosix::SourceAddressInfo* src;
+  unsigned common_prefix_length;
+};
+
+// Returns true iff |dst_a| should precede |dst_b| in the address list.
+// RFC 3484, section 6.
+bool CompareDestinations(const DestinationInfo* dst_a,
+                         const DestinationInfo* dst_b) {
+  // Rule 1: Avoid unusable destinations.
+  // Unusable destinations are already filtered out.
+  DCHECK(dst_a->src);
+  DCHECK(dst_b->src);
+
+  // Rule 2: Prefer matching scope.
+  bool scope_match1 = (dst_a->src->scope == dst_a->scope);
+  bool scope_match2 = (dst_b->src->scope == dst_b->scope);
+  if (scope_match1 != scope_match2)
+    return scope_match1;
+
+  // Rule 3: Avoid deprecated addresses.
+  if (dst_a->src->deprecated != dst_b->src->deprecated)
+    return !dst_a->src->deprecated;
+
+  // Rule 4: Prefer home addresses.
+  if (dst_a->src->home != dst_b->src->home)
+    return dst_a->src->home;
+
+  // Rule 5: Prefer matching label.
+  bool label_match1 = (dst_a->src->label == dst_a->label);
+  bool label_match2 = (dst_b->src->label == dst_b->label);
+  if (label_match1 != label_match2)
+    return label_match1;
+
+  // Rule 6: Prefer higher precedence.
+  if (dst_a->precedence != dst_b->precedence)
+    return dst_a->precedence > dst_b->precedence;
+
+  // Rule 7: Prefer native transport.
+  if (dst_a->src->native != dst_b->src->native)
+    return dst_a->src->native;
+
+  // Rule 8: Prefer smaller scope.
+  if (dst_a->scope != dst_b->scope)
+    return dst_a->scope < dst_b->scope;
+
+  // Rule 9: Use longest matching prefix. Only for matching address families.
+  if (dst_a->address.size() == dst_b->address.size()) {
+    if (dst_a->common_prefix_length != dst_b->common_prefix_length)
+      return dst_a->common_prefix_length > dst_b->common_prefix_length;
+  }
+
+  // Rule 10: Leave the order unchanged.
+  // stable_sort takes care of that.
+  return false;
+}
+
+}  // namespace
+
+AddressSorterPosix::AddressSorterPosix(ClientSocketFactory* socket_factory)
+    : socket_factory_(socket_factory),
+      precedence_table_(LoadPolicy(kDefaultPrecedenceTable,
+                                   arraysize(kDefaultPrecedenceTable))),
+      label_table_(LoadPolicy(kDefaultLabelTable,
+                              arraysize(kDefaultLabelTable))),
+      ipv4_scope_table_(LoadPolicy(kDefaultIPv4ScopeTable,
+                              arraysize(kDefaultIPv4ScopeTable))) {
+  NetworkChangeNotifier::AddIPAddressObserver(this);
+  OnIPAddressChanged();
+}
+
+AddressSorterPosix::~AddressSorterPosix() {
+  NetworkChangeNotifier::RemoveIPAddressObserver(this);
+}
+
+void AddressSorterPosix::Sort(const AddressList& list,
+                              const CallbackType& callback) const {
+  DCHECK(CalledOnValidThread());
+  ScopedVector<DestinationInfo> sort_list;
+
+  for (size_t i = 0; i < list.size(); ++i) {
+    scoped_ptr<DestinationInfo> info(new DestinationInfo());
+    info->address = list[i].address();
+    info->scope = GetScope(ipv4_scope_table_, info->address);
+    info->precedence = GetPolicyValue(precedence_table_, info->address);
+    info->label = GetPolicyValue(label_table_, info->address);
+
+    // Each socket can only be bound once.
+    scoped_ptr<DatagramClientSocket> socket(
+        socket_factory_->CreateDatagramClientSocket(
+            DatagramSocket::DEFAULT_BIND,
+            RandIntCallback(),
+            NULL /* NetLog */,
+            NetLog::Source()));
+
+    // Even though no packets are sent, cannot use port 0 in Connect.
+    IPEndPoint dest(info->address, 80 /* port */);
+    int rv = socket->Connect(dest);
+    if (rv != OK) {
+      LOG(WARNING) << "Could not connect to " << dest.ToStringWithoutPort()
+                   << " reason " << rv;
+      continue;
+    }
+    // Filter out unusable destinations.
+    IPEndPoint src;
+    rv = socket->GetLocalAddress(&src);
+    if (rv != OK) {
+      LOG(WARNING) << "Could not get local address for "
+                   << src.ToStringWithoutPort() << " reason " << rv;
+      continue;
+    }
+
+    SourceAddressInfo& src_info = source_map_[src.address()];
+    if (src_info.scope == SCOPE_UNDEFINED) {
+      // If |source_info_| is out of date, |src| might be missing, but we still
+      // want to sort, even though the HostCache will be cleared soon.
+      FillPolicy(src.address(), &src_info);
+    }
+    info->src = &src_info;
+
+    if (info->address.size() == src.address().size()) {
+      info->common_prefix_length = std::min(
+          CommonPrefixLength(info->address, src.address()),
+          info->src->prefix_length);
+    }
+    sort_list.push_back(info.release());
+  }
+
+  std::stable_sort(sort_list.begin(), sort_list.end(), CompareDestinations);
+
+  AddressList result;
+  for (size_t i = 0; i < sort_list.size(); ++i)
+    result.push_back(IPEndPoint(sort_list[i]->address, 0 /* port */));
+
+  callback.Run(true, result);
+}
+
+void AddressSorterPosix::OnIPAddressChanged() {
+  DCHECK(CalledOnValidThread());
+  source_map_.clear();
+#if defined(OS_LINUX)
+  const internal::AddressTrackerLinux* tracker =
+      NetworkChangeNotifier::GetAddressTracker();
+  if (!tracker)
+    return;
+  typedef internal::AddressTrackerLinux::AddressMap AddressMap;
+  AddressMap map = tracker->GetAddressMap();
+  for (AddressMap::const_iterator it = map.begin(); it != map.end(); ++it) {
+    const IPAddressNumber& address = it->first;
+    const struct ifaddrmsg& msg = it->second;
+    SourceAddressInfo& info = source_map_[address];
+    info.native = false;  // TODO(szym): obtain this via netlink.
+    info.deprecated = msg.ifa_flags & IFA_F_DEPRECATED;
+    info.home = msg.ifa_flags & IFA_F_HOMEADDRESS;
+    info.prefix_length = msg.ifa_prefixlen;
+    FillPolicy(address, &info);
+  }
+#elif defined(OS_MACOSX) || defined(OS_BSD)
+  // It's not clear we will receive notification when deprecated flag changes.
+  // Socket for ioctl.
+  int ioctl_socket = socket(AF_INET6, SOCK_DGRAM, 0);
+  if (ioctl_socket < 0)
+    return;
+
+  struct ifaddrs* addrs;
+  int rv = getifaddrs(&addrs);
+  if (rv < 0) {
+    LOG(WARNING) << "getifaddrs failed " << rv;
+    close(ioctl_socket);
+    return;
+  }
+
+  for (struct ifaddrs* ifa = addrs; ifa != NULL; ifa = ifa->ifa_next) {
+    IPEndPoint src;
+    if (!src.FromSockAddr(ifa->ifa_addr, ifa->ifa_addr->sa_len)) {
+      LOG(WARNING) << "FromSockAddr failed";
+      continue;
+    }
+    SourceAddressInfo& info = source_map_[src.address()];
+    // Note: no known way to fill in |native| and |home|.
+    info.native = info.home = info.deprecated = false;
+    if (ifa->ifa_addr->sa_family == AF_INET6) {
+      struct in6_ifreq ifr = {};
+      strncpy(ifr.ifr_name, ifa->ifa_name, sizeof(ifr.ifr_name) - 1);
+      DCHECK_LE(ifa->ifa_addr->sa_len, sizeof(ifr.ifr_ifru.ifru_addr));
+      memcpy(&ifr.ifr_ifru.ifru_addr, ifa->ifa_addr, ifa->ifa_addr->sa_len);
+      int rv = ioctl(ioctl_socket, SIOCGIFAFLAG_IN6, &ifr);
+      if (rv > 0) {
+        info.deprecated = ifr.ifr_ifru.ifru_flags & IN6_IFF_DEPRECATED;
+      } else {
+        LOG(WARNING) << "SIOCGIFAFLAG_IN6 failed " << rv;
+      }
+    }
+    if (ifa->ifa_netmask) {
+      IPEndPoint netmask;
+      if (netmask.FromSockAddr(ifa->ifa_netmask, ifa->ifa_addr->sa_len)) {
+        info.prefix_length = MaskPrefixLength(netmask.address());
+      } else {
+        LOG(WARNING) << "FromSockAddr failed on netmask";
+      }
+    }
+    FillPolicy(src.address(), &info);
+  }
+  freeifaddrs(addrs);
+  close(ioctl_socket);
+#endif
+}
+
+void AddressSorterPosix::FillPolicy(const IPAddressNumber& address,
+                                    SourceAddressInfo* info) const {
+  DCHECK(CalledOnValidThread());
+  info->scope = GetScope(ipv4_scope_table_, address);
+  info->label = GetPolicyValue(label_table_, address);
+}
+
+// static
+scoped_ptr<AddressSorter> AddressSorter::CreateAddressSorter() {
+  return scoped_ptr<AddressSorter>(
+      new AddressSorterPosix(ClientSocketFactory::GetDefaultFactory()));
+}
+
+}  // namespace net
+
diff --git a/net/dns/address_sorter_posix.h b/net/dns/address_sorter_posix.h
new file mode 100644
index 0000000..1c88ad2
--- /dev/null
+++ b/net/dns/address_sorter_posix.h
@@ -0,0 +1,94 @@
+// Copyright (c) 2012 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_DNS_ADDRESS_SORTER_POSIX_H_
+#define NET_DNS_ADDRESS_SORTER_POSIX_H_
+
+#include <map>
+#include <vector>
+
+#include "base/threading/non_thread_safe.h"
+#include "net/base/address_list.h"
+#include "net/base/net_errors.h"
+#include "net/base/net_export.h"
+#include "net/base/net_util.h"
+#include "net/base/network_change_notifier.h"
+#include "net/dns/address_sorter.h"
+
+namespace net {
+
+class ClientSocketFactory;
+
+// This implementation uses explicit policy to perform the sorting. It is not
+// thread-safe and always completes synchronously.
+class NET_EXPORT_PRIVATE AddressSorterPosix
+    : public AddressSorter,
+      public base::NonThreadSafe,
+      public NetworkChangeNotifier::IPAddressObserver {
+ public:
+  // Generic policy entry.
+  struct PolicyEntry {
+    // IPv4 addresses must be mapped to IPv6.
+    unsigned char prefix[kIPv6AddressSize];
+    unsigned prefix_length;
+    unsigned value;
+  };
+
+  typedef std::vector<PolicyEntry> PolicyTable;
+
+  enum AddressScope {
+    SCOPE_UNDEFINED = 0,
+    SCOPE_NODELOCAL = 1,
+    SCOPE_LINKLOCAL = 2,
+    SCOPE_SITELOCAL = 5,
+    SCOPE_ORGLOCAL = 8,
+    SCOPE_GLOBAL = 14,
+  };
+
+  struct SourceAddressInfo {
+    // Values read from policy tables.
+    AddressScope scope;
+    unsigned label;
+
+    // Values from the OS, matter only if more than one source address is used.
+    unsigned prefix_length;
+    bool deprecated;  // vs. preferred RFC4862
+    bool home;        // vs. care-of RFC6275
+    bool native;
+  };
+
+  typedef std::map<IPAddressNumber, SourceAddressInfo> SourceAddressMap;
+
+  explicit AddressSorterPosix(ClientSocketFactory* socket_factory);
+  virtual ~AddressSorterPosix();
+
+  // AddressSorter:
+  virtual void Sort(const AddressList& list,
+                    const CallbackType& callback) const OVERRIDE;
+
+ private:
+  friend class AddressSorterPosixTest;
+
+  // NetworkChangeNotifier::IPAddressObserver:
+  virtual void OnIPAddressChanged() OVERRIDE;
+
+  // Fills |info| with values for |address| from policy tables.
+  void FillPolicy(const IPAddressNumber& address,
+                  SourceAddressInfo* info) const;
+
+  // Mutable to allow using default values for source addresses which were not
+  // found in most recent OnIPAddressChanged.
+  mutable SourceAddressMap source_map_;
+
+  ClientSocketFactory* socket_factory_;
+  PolicyTable precedence_table_;
+  PolicyTable label_table_;
+  PolicyTable ipv4_scope_table_;
+
+  DISALLOW_COPY_AND_ASSIGN(AddressSorterPosix);
+};
+
+}  // namespace net
+
+#endif  // NET_DNS_ADDRESS_SORTER_POSIX_H_
diff --git a/net/dns/address_sorter_posix_unittest.cc b/net/dns/address_sorter_posix_unittest.cc
new file mode 100644
index 0000000..96cbfc6
--- /dev/null
+++ b/net/dns/address_sorter_posix_unittest.cc
@@ -0,0 +1,325 @@
+// Copyright (c) 2012 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/dns/address_sorter_posix.h"
+
+#include "base/bind.h"
+#include "base/logging.h"
+#include "net/base/net_errors.h"
+#include "net/base/net_util.h"
+#include "net/base/test_completion_callback.h"
+#include "net/socket/client_socket_factory.h"
+#include "net/udp/datagram_client_socket.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace net {
+namespace {
+
+// Used to map destination address to source address.
+typedef std::map<IPAddressNumber, IPAddressNumber> AddressMapping;
+
+IPAddressNumber ParseIP(const std::string& str) {
+  IPAddressNumber addr;
+  CHECK(ParseIPLiteralToNumber(str, &addr));
+  return addr;
+}
+
+// A mock socket which binds to source address according to AddressMapping.
+class TestUDPClientSocket : public DatagramClientSocket {
+ public:
+  explicit TestUDPClientSocket(const AddressMapping* mapping)
+      : mapping_(mapping), connected_(false)  {}
+
+  virtual ~TestUDPClientSocket() {}
+
+  virtual int Read(IOBuffer*, int, const CompletionCallback&) OVERRIDE {
+    NOTIMPLEMENTED();
+    return OK;
+  }
+  virtual int Write(IOBuffer*, int, const CompletionCallback&) OVERRIDE {
+    NOTIMPLEMENTED();
+    return OK;
+  }
+  virtual bool SetReceiveBufferSize(int32) OVERRIDE {
+    return true;
+  }
+  virtual bool SetSendBufferSize(int32) OVERRIDE {
+    return true;
+  }
+
+  virtual void Close() OVERRIDE {}
+  virtual int GetPeerAddress(IPEndPoint* address) const OVERRIDE {
+    NOTIMPLEMENTED();
+    return OK;
+  }
+  virtual int GetLocalAddress(IPEndPoint* address) const OVERRIDE {
+    if (!connected_)
+      return ERR_UNEXPECTED;
+    *address = local_endpoint_;
+    return OK;
+  }
+
+  virtual int Connect(const IPEndPoint& remote) OVERRIDE {
+    if (connected_)
+      return ERR_UNEXPECTED;
+    AddressMapping::const_iterator it = mapping_->find(remote.address());
+    if (it == mapping_->end())
+      return ERR_FAILED;
+    connected_ = true;
+    local_endpoint_ = IPEndPoint(it->second, 39874 /* arbitrary port */);
+    return OK;
+  }
+
+  virtual const BoundNetLog& NetLog() const OVERRIDE {
+    return net_log_;
+  }
+
+ private:
+  BoundNetLog net_log_;
+  const AddressMapping* mapping_;
+  bool connected_;
+  IPEndPoint local_endpoint_;
+
+  DISALLOW_COPY_AND_ASSIGN(TestUDPClientSocket);
+};
+
+// Creates TestUDPClientSockets and maintains an AddressMapping.
+class TestSocketFactory : public ClientSocketFactory {
+ public:
+  TestSocketFactory() {}
+  virtual ~TestSocketFactory() {}
+
+  virtual DatagramClientSocket* CreateDatagramClientSocket(
+      DatagramSocket::BindType,
+      const RandIntCallback&,
+      NetLog*,
+      const NetLog::Source&) OVERRIDE {
+    return new TestUDPClientSocket(&mapping_);
+  }
+  virtual StreamSocket* CreateTransportClientSocket(
+      const AddressList&,
+      NetLog*,
+      const NetLog::Source&) OVERRIDE {
+    NOTIMPLEMENTED();
+    return NULL;
+  }
+  virtual SSLClientSocket* CreateSSLClientSocket(
+      ClientSocketHandle*,
+      const HostPortPair&,
+      const SSLConfig&,
+      const SSLClientSocketContext&) OVERRIDE {
+    NOTIMPLEMENTED();
+    return NULL;
+  }
+  virtual void ClearSSLSessionCache() OVERRIDE {
+    NOTIMPLEMENTED();
+  }
+
+  void AddMapping(const IPAddressNumber& dst, const IPAddressNumber& src) {
+    mapping_[dst] = src;
+  }
+
+ private:
+  AddressMapping mapping_;
+
+  DISALLOW_COPY_AND_ASSIGN(TestSocketFactory);
+};
+
+void OnSortComplete(AddressList* result_buf,
+                    const CompletionCallback& callback,
+                    bool success,
+                    const AddressList& result) {
+  EXPECT_TRUE(success);
+  if (success)
+    *result_buf = result;
+  callback.Run(OK);
+}
+
+}  // namespace
+
+class AddressSorterPosixTest : public testing::Test {
+ protected:
+  AddressSorterPosixTest() : sorter_(&socket_factory_) {}
+
+  void AddMapping(const std::string& dst, const std::string& src) {
+    socket_factory_.AddMapping(ParseIP(dst), ParseIP(src));
+  }
+
+  AddressSorterPosix::SourceAddressInfo* GetSourceInfo(
+      const std::string& addr) {
+    IPAddressNumber address = ParseIP(addr);
+    AddressSorterPosix::SourceAddressInfo* info = &sorter_.source_map_[address];
+    if (info->scope == AddressSorterPosix::SCOPE_UNDEFINED)
+      sorter_.FillPolicy(address, info);
+    return info;
+  }
+
+  // Verify that NULL-terminated |addresses| matches (-1)-terminated |order|
+  // after sorting.
+  void Verify(const char* addresses[], const int order[]) {
+    AddressList list;
+    for (const char** addr = addresses; *addr != NULL; ++addr)
+      list.push_back(IPEndPoint(ParseIP(*addr), 80));
+    for (size_t i = 0; order[i] >= 0; ++i)
+      CHECK_LT(order[i], static_cast<int>(list.size()));
+
+    AddressList result;
+    TestCompletionCallback callback;
+    sorter_.Sort(list, base::Bind(&OnSortComplete, &result,
+                                  callback.callback()));
+    callback.WaitForResult();
+
+    for (size_t i = 0; (i < result.size()) || (order[i] >= 0); ++i) {
+      IPEndPoint expected = order[i] >= 0 ? list[order[i]] : IPEndPoint();
+      IPEndPoint actual = i < result.size() ? result[i] : IPEndPoint();
+      EXPECT_TRUE(expected.address() == actual.address()) <<
+          "Address out of order at position " << i << "\n" <<
+          "  Actual: " << actual.ToStringWithoutPort() << "\n" <<
+          "Expected: " << expected.ToStringWithoutPort();
+    }
+  }
+
+  TestSocketFactory socket_factory_;
+  AddressSorterPosix sorter_;
+};
+
+// Rule 1: Avoid unusable destinations.
+TEST_F(AddressSorterPosixTest, Rule1) {
+  AddMapping("10.0.0.231", "10.0.0.1");
+  const char* addresses[] = { "::1", "10.0.0.231", "127.0.0.1", NULL };
+  const int order[] = { 1, -1 };
+  Verify(addresses, order);
+}
+
+// Rule 2: Prefer matching scope.
+TEST_F(AddressSorterPosixTest, Rule2) {
+  AddMapping("3002::1", "4000::10");      // matching global
+  AddMapping("ff32::1", "fe81::10");      // matching link-local
+  AddMapping("fec1::1", "fec1::10");      // matching node-local
+  AddMapping("3002::2", "::1");           // global vs. link-local
+  AddMapping("fec1::2", "fe81::10");      // site-local vs. link-local
+  AddMapping("8.0.0.1", "169.254.0.10");  // global vs. link-local
+  // In all three cases, matching scope is preferred.
+  const int order[] = { 1, 0, -1 };
+  const char* addresses1[] = { "3002::2", "3002::1", NULL };
+  Verify(addresses1, order);
+  const char* addresses2[] = { "fec1::2", "ff32::1", NULL };
+  Verify(addresses2, order);
+  const char* addresses3[] = { "8.0.0.1", "fec1::1", NULL };
+  Verify(addresses3, order);
+}
+
+// Rule 3: Avoid deprecated addresses.
+TEST_F(AddressSorterPosixTest, Rule3) {
+  // Matching scope.
+  AddMapping("3002::1", "4000::10");
+  GetSourceInfo("4000::10")->deprecated = true;
+  AddMapping("3002::2", "4000::20");
+  const char* addresses[] = { "3002::1", "3002::2", NULL };
+  const int order[] = { 1, 0, -1 };
+  Verify(addresses, order);
+}
+
+// Rule 4: Prefer home addresses.
+TEST_F(AddressSorterPosixTest, Rule4) {
+  AddMapping("3002::1", "4000::10");
+  AddMapping("3002::2", "4000::20");
+  GetSourceInfo("4000::20")->home = true;
+  const char* addresses[] = { "3002::1", "3002::2", NULL };
+  const int order[] = { 1, 0, -1 };
+  Verify(addresses, order);
+}
+
+// Rule 5: Prefer matching label.
+TEST_F(AddressSorterPosixTest, Rule5) {
+  AddMapping("::1", "::1");                       // matching loopback
+  AddMapping("::ffff:1234:1", "::ffff:1234:10");  // matching IPv4-mapped
+  AddMapping("2001::1", "::ffff:1234:10");        // Teredo vs. IPv4-mapped
+  AddMapping("2002::1", "2001::10");              // 6to4 vs. Teredo
+  const int order[] = { 1, 0, -1 };
+  {
+    const char* addresses[] = { "2001::1", "::1", NULL };
+    Verify(addresses, order);
+  }
+  {
+    const char* addresses[] = { "2002::1", "::ffff:1234:1", NULL };
+    Verify(addresses, order);
+  }
+}
+
+// Rule 6: Prefer higher precedence.
+TEST_F(AddressSorterPosixTest, Rule6) {
+  AddMapping("::1", "::1");                       // loopback
+  AddMapping("ff32::1", "fe81::10");              // multicast
+  AddMapping("::ffff:1234:1", "::ffff:1234:10");  // IPv4-mapped
+  AddMapping("2001::1", "2001::10");              // Teredo
+  const char* addresses[] = { "2001::1", "::ffff:1234:1", "ff32::1", "::1",
+                              NULL };
+  const int order[] = { 3, 2, 1, 0, -1 };
+  Verify(addresses, order);
+}
+
+// Rule 7: Prefer native transport.
+TEST_F(AddressSorterPosixTest, Rule7) {
+  AddMapping("3002::1", "4000::10");
+  AddMapping("3002::2", "4000::20");
+  GetSourceInfo("4000::20")->native = true;
+  const char* addresses[] = { "3002::1", "3002::2", NULL };
+  const int order[] = { 1, 0, -1 };
+  Verify(addresses, order);
+}
+
+// Rule 8: Prefer smaller scope.
+TEST_F(AddressSorterPosixTest, Rule8) {
+  // Matching scope. Should precede the others by Rule 2.
+  AddMapping("fe81::1", "fe81::10");  // link-local
+  AddMapping("3000::1", "4000::10");  // global
+  // Mismatched scope.
+  AddMapping("ff32::1", "4000::10");  // link-local
+  AddMapping("ff35::1", "4000::10");  // site-local
+  AddMapping("ff38::1", "4000::10");  // org-local
+  const char* addresses[] = { "ff38::1", "3000::1", "ff35::1", "ff32::1",
+                              "fe81::1", NULL };
+  const int order[] = { 4, 1, 3, 2, 0, -1 };
+  Verify(addresses, order);
+}
+
+// Rule 9: Use longest matching prefix.
+TEST_F(AddressSorterPosixTest, Rule9) {
+  AddMapping("3000::1", "3000:ffff::10");  // 16 bit match
+  GetSourceInfo("3000:ffff::10")->prefix_length = 16;
+  AddMapping("4000::1", "4000::10");       // 123 bit match, limited to 15
+  GetSourceInfo("4000::10")->prefix_length = 15;
+  AddMapping("4002::1", "4000::10");       // 14 bit match
+  AddMapping("4080::1", "4000::10");       // 8 bit match
+  const char* addresses[] = { "4080::1", "4002::1", "4000::1", "3000::1",
+                              NULL };
+  const int order[] = { 3, 2, 1, 0, -1 };
+  Verify(addresses, order);
+}
+
+// Rule 10: Leave the order unchanged.
+TEST_F(AddressSorterPosixTest, Rule10) {
+  AddMapping("4000::1", "4000::10");
+  AddMapping("4000::2", "4000::10");
+  AddMapping("4000::3", "4000::10");
+  const char* addresses[] = { "4000::1", "4000::2", "4000::3", NULL };
+  const int order[] = { 0, 1, 2, -1 };
+  Verify(addresses, order);
+}
+
+TEST_F(AddressSorterPosixTest, MultipleRules) {
+  AddMapping("::1", "::1");           // loopback
+  AddMapping("ff32::1", "fe81::10");  // link-local multicast
+  AddMapping("ff3e::1", "4000::10");  // global multicast
+  AddMapping("4000::1", "4000::10");  // global unicast
+  AddMapping("ff32::2", "fe81::20");  // deprecated link-local multicast
+  GetSourceInfo("fe81::20")->deprecated = true;
+  const char* addresses[] = { "ff3e::1", "ff32::2", "4000::1", "ff32::1", "::1",
+                              "8.0.0.1", NULL };
+  const int order[] = { 4, 3, 0, 2, 1, -1 };
+  Verify(addresses, order);
+}
+
+}  // namespace net
diff --git a/net/dns/address_sorter_unittest.cc b/net/dns/address_sorter_unittest.cc
new file mode 100644
index 0000000..0c2be88
--- /dev/null
+++ b/net/dns/address_sorter_unittest.cc
@@ -0,0 +1,66 @@
+// Copyright (c) 2012 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/dns/address_sorter.h"
+
+#if defined(OS_WIN)
+#include <winsock2.h>
+#endif
+
+#include "base/bind.h"
+#include "base/logging.h"
+#include "net/base/address_list.h"
+#include "net/base/net_util.h"
+#include "net/base/test_completion_callback.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+#if defined(OS_WIN)
+#include "net/base/winsock_init.h"
+#endif
+
+namespace net {
+namespace {
+
+IPEndPoint MakeEndPoint(const std::string& str) {
+  IPAddressNumber addr;
+  CHECK(ParseIPLiteralToNumber(str, &addr));
+  return IPEndPoint(addr, 0);
+}
+
+void OnSortComplete(AddressList* result_buf,
+                    const CompletionCallback& callback,
+                    bool success,
+                    const AddressList& result) {
+  if (success)
+    *result_buf = result;
+  callback.Run(success ? OK : ERR_FAILED);
+}
+
+TEST(AddressSorterTest, Sort) {
+  int expected_result = OK;
+#if defined(OS_WIN)
+  EnsureWinsockInit();
+  SOCKET sock = socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);
+  if (sock == INVALID_SOCKET) {
+    expected_result = ERR_FAILED;
+  } else {
+    closesocket(sock);
+  }
+#endif
+  scoped_ptr<AddressSorter> sorter(AddressSorter::CreateAddressSorter());
+  AddressList list;
+  list.push_back(MakeEndPoint("10.0.0.1"));
+  list.push_back(MakeEndPoint("8.8.8.8"));
+  list.push_back(MakeEndPoint("::1"));
+  list.push_back(MakeEndPoint("2001:4860:4860::8888"));
+
+  AddressList result;
+  TestCompletionCallback callback;
+  sorter->Sort(list, base::Bind(&OnSortComplete, &result,
+                                callback.callback()));
+  EXPECT_EQ(expected_result, callback.WaitForResult());
+}
+
+}  // namespace
+}  // namespace net
diff --git a/net/dns/address_sorter_win.cc b/net/dns/address_sorter_win.cc
new file mode 100644
index 0000000..0f1c504
--- /dev/null
+++ b/net/dns/address_sorter_win.cc
@@ -0,0 +1,198 @@
+// Copyright (c) 2012 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/dns/address_sorter.h"
+
+#include <winsock2.h>
+
+#include <algorithm>
+
+#include "base/bind.h"
+#include "base/location.h"
+#include "base/logging.h"
+#include "base/threading/worker_pool.h"
+#include "base/win/windows_version.h"
+#include "net/base/address_list.h"
+#include "net/base/ip_endpoint.h"
+#include "net/base/winsock_init.h"
+
+namespace net {
+
+namespace {
+
+class AddressSorterWin : public AddressSorter {
+ public:
+  AddressSorterWin() {
+    EnsureWinsockInit();
+  }
+
+  virtual ~AddressSorterWin() {}
+
+  // AddressSorter:
+  virtual void Sort(const AddressList& list,
+                    const CallbackType& callback) const OVERRIDE {
+    DCHECK(!list.empty());
+    scoped_refptr<Job> job = new Job(list, callback);
+  }
+
+ private:
+  // Executes the SIO_ADDRESS_LIST_SORT ioctl on the WorkerPool, and
+  // performs the necessary conversions to/from AddressList.
+  class Job : public base::RefCountedThreadSafe<Job> {
+   public:
+    Job(const AddressList& list, const CallbackType& callback)
+        : callback_(callback),
+          buffer_size_(sizeof(SOCKET_ADDRESS_LIST) +
+                       list.size() * (sizeof(SOCKET_ADDRESS) +
+                                      sizeof(SOCKADDR_STORAGE))),
+          input_buffer_(reinterpret_cast<SOCKET_ADDRESS_LIST*>(
+              malloc(buffer_size_))),
+          output_buffer_(reinterpret_cast<SOCKET_ADDRESS_LIST*>(
+              malloc(buffer_size_))),
+          success_(false) {
+      input_buffer_->iAddressCount = list.size();
+      SOCKADDR_STORAGE* storage = reinterpret_cast<SOCKADDR_STORAGE*>(
+          input_buffer_->Address + input_buffer_->iAddressCount);
+
+      for (size_t i = 0; i < list.size(); ++i) {
+        IPEndPoint ipe = list[i];
+        // Addresses must be sockaddr_in6.
+        if (ipe.GetFamily() == AF_INET) {
+          ipe = IPEndPoint(ConvertIPv4NumberToIPv6Number(ipe.address()),
+                           ipe.port());
+        }
+
+        struct sockaddr* addr = reinterpret_cast<struct sockaddr*>(storage + i);
+        socklen_t addr_len = sizeof(SOCKADDR_STORAGE);
+        bool result = ipe.ToSockAddr(addr, &addr_len);
+        DCHECK(result);
+        input_buffer_->Address[i].lpSockaddr = addr;
+        input_buffer_->Address[i].iSockaddrLength = addr_len;
+      }
+
+      if (!base::WorkerPool::PostTaskAndReply(
+          FROM_HERE,
+          base::Bind(&Job::Run, this),
+          base::Bind(&Job::OnComplete, this),
+          false /* task is slow */)) {
+        LOG(ERROR) << "WorkerPool::PostTaskAndReply failed";
+        OnComplete();
+      }
+    }
+
+   private:
+    friend class base::RefCountedThreadSafe<Job>;
+    ~Job() {}
+
+    // Executed on the WorkerPool.
+    void Run() {
+      SOCKET sock = socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);
+      if (sock == INVALID_SOCKET)
+        return;
+      DWORD result_size = 0;
+      int result = WSAIoctl(sock, SIO_ADDRESS_LIST_SORT, input_buffer_.get(),
+                            buffer_size_, output_buffer_.get(), buffer_size_,
+                            &result_size, NULL, NULL);
+      if (result == SOCKET_ERROR) {
+        LOG(ERROR) << "SIO_ADDRESS_LIST_SORT failed " << WSAGetLastError();
+      } else {
+        success_ = true;
+      }
+      closesocket(sock);
+    }
+
+    // Executed on the calling thread.
+    void OnComplete() {
+      AddressList list;
+      if (success_) {
+        list.reserve(output_buffer_->iAddressCount);
+        for (int i = 0; i < output_buffer_->iAddressCount; ++i) {
+          IPEndPoint ipe;
+          ipe.FromSockAddr(output_buffer_->Address[i].lpSockaddr,
+                           output_buffer_->Address[i].iSockaddrLength);
+          // Unmap V4MAPPED IPv6 addresses so that Happy Eyeballs works.
+          if (IsIPv4Mapped(ipe.address())) {
+            ipe = IPEndPoint(ConvertIPv4MappedToIPv4(ipe.address()),
+                                                     ipe.port());
+          }
+          list.push_back(ipe);
+        }
+      }
+      callback_.Run(success_, list);
+    }
+
+    const CallbackType callback_;
+    const size_t buffer_size_;
+    scoped_ptr_malloc<SOCKET_ADDRESS_LIST> input_buffer_;
+    scoped_ptr_malloc<SOCKET_ADDRESS_LIST> output_buffer_;
+    bool success_;
+
+    DISALLOW_COPY_AND_ASSIGN(Job);
+  };
+
+  DISALLOW_COPY_AND_ASSIGN(AddressSorterWin);
+};
+
+// Merges |list_ipv4| and |list_ipv6| before passing it to |callback|, but
+// only if |success| is true.
+void MergeResults(const AddressSorter::CallbackType& callback,
+                  const AddressList& list_ipv4,
+                  bool success,
+                  const AddressList& list_ipv6) {
+  if (!success) {
+    callback.Run(false, AddressList());
+    return;
+  }
+  AddressList list;
+  list.insert(list.end(), list_ipv6.begin(), list_ipv6.end());
+  list.insert(list.end(), list_ipv4.begin(), list_ipv4.end());
+  callback.Run(true, list);
+}
+
+// Wrapper for AddressSorterWin which does not sort IPv4 or IPv4-mapped
+// addresses but always puts them at the end of the list. Needed because the
+// SIO_ADDRESS_LIST_SORT does not support IPv4 addresses on Windows XP.
+class AddressSorterWinXP : public AddressSorter {
+ public:
+  AddressSorterWinXP() {}
+  virtual ~AddressSorterWinXP() {}
+
+  // AddressSorter:
+  virtual void Sort(const AddressList& list,
+                    const CallbackType& callback) const OVERRIDE {
+    AddressList list_ipv4;
+    AddressList list_ipv6;
+    for (size_t i = 0; i < list.size(); ++i) {
+      const IPEndPoint& ipe = list[i];
+      if (ipe.GetFamily() == AF_INET) {
+        list_ipv4.push_back(ipe);
+      } else {
+        list_ipv6.push_back(ipe);
+      }
+    }
+    if (!list_ipv6.empty()) {
+      sorter_.Sort(list_ipv6, base::Bind(&MergeResults, callback, list_ipv4));
+    } else {
+      NOTREACHED() << "Should not be called with IPv4-only addresses.";
+      callback.Run(true, list);
+    }
+  }
+
+ private:
+  AddressSorterWin sorter_;
+
+  DISALLOW_COPY_AND_ASSIGN(AddressSorterWinXP);
+};
+
+}  // namespace
+
+// static
+scoped_ptr<AddressSorter> AddressSorter::CreateAddressSorter() {
+  if (base::win::GetVersion() < base::win::VERSION_VISTA)
+    return scoped_ptr<AddressSorter>(new AddressSorterWinXP());
+  return scoped_ptr<AddressSorter>(new AddressSorterWin());
+}
+
+}  // namespace net
+
diff --git a/net/dns/dns_client.cc b/net/dns/dns_client.cc
index 5381452..859fc27 100644
--- a/net/dns/dns_client.cc
+++ b/net/dns/dns_client.cc
@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "base/rand_util.h"
 #include "net/base/net_log.h"
+#include "net/dns/address_sorter.h"
 #include "net/dns/dns_config_service.h"
 #include "net/dns/dns_session.h"
 #include "net/dns/dns_transaction.h"
@@ -18,7 +19,9 @@ namespace {
 
 class DnsClientImpl : public DnsClient {
  public:
-  explicit DnsClientImpl(NetLog* net_log) : net_log_(net_log) {}
+  explicit DnsClientImpl(NetLog* net_log)
+      : address_sorter_(AddressSorter::CreateAddressSorter()),
+        net_log_(net_log) {}
 
   virtual void SetConfig(const DnsConfig& config) OVERRIDE {
     factory_.reset();
@@ -40,9 +43,14 @@ class DnsClientImpl : public DnsClient {
     return session_.get() ? factory_.get() : NULL;
   }
 
+  virtual AddressSorter* GetAddressSorter() OVERRIDE {
+    return address_sorter_.get();
+  }
+
  private:
   scoped_refptr<DnsSession> session_;
   scoped_ptr<DnsTransactionFactory> factory_;
+  scoped_ptr<AddressSorter> address_sorter_;
 
   NetLog* net_log_;
 };
diff --git a/net/dns/dns_client.h b/net/dns/dns_client.h
index 13aa0bf8..650c7d0 100644
--- a/net/dns/dns_client.h
+++ b/net/dns/dns_client.h
@@ -10,12 +10,14 @@
 
 namespace net {
 
+class AddressSorter;
 struct DnsConfig;
 class DnsTransactionFactory;
 class NetLog;
 
-// Convenience wrapper allows easy injection of DnsTransaction into
-// HostResolverImpl.
+// Convenience wrapper which allows easy injection of DnsTransaction into
+// HostResolverImpl. Pointers returned by the Get* methods are only guaranteed
+// to remain valid until next time SetConfig is called.
 class NET_EXPORT DnsClient {
  public:
   virtual ~DnsClient() {}
@@ -29,6 +31,9 @@ class NET_EXPORT DnsClient {
   // Returns NULL if the current config is not valid.
   virtual DnsTransactionFactory* GetTransactionFactory() = 0;
 
+  // Returns NULL if the current config is not valid.
+  virtual AddressSorter* GetAddressSorter() = 0;
+
   // Creates default client.
   static scoped_ptr<DnsClient> CreateClient(NetLog* net_log);
 };
diff --git a/net/dns/dns_response.cc b/net/dns/dns_response.cc
index 4ad6465..4ab7296 100644
--- a/net/dns/dns_response.cc
+++ b/net/dns/dns_response.cc
@@ -280,15 +280,13 @@ DnsResponse::Result DnsResponse::ParseToAddressList(
   }
 
   // TODO(szym): Extract TTL for NODATA results. http://crbug.com/115051
-  if (ip_addresses.empty())
-    return DNS_NO_ADDRESSES;
 
   // getcanonname in eglibc returns the first owner name of an A or AAAA RR.
   // If the response passed all the checks so far, then |expected_name| is it.
   *addr_list = AddressList::CreateFromIPAddressList(ip_addresses,
                                                     expected_name);
   *ttl = base::TimeDelta::FromSeconds(std::min(cname_ttl_sec, addr_ttl_sec));
-  return DNS_SUCCESS;
+  return DNS_PARSE_OK;
 }
 
 }  // namespace net
diff --git a/net/dns/dns_response.h b/net/dns/dns_response.h
index 5ade458..fe80c30 100644
--- a/net/dns/dns_response.h
+++ b/net/dns/dns_response.h
@@ -81,7 +81,7 @@ class NET_EXPORT_PRIVATE DnsResponse {
  public:
   // Possible results from ParseToAddressList.
   enum Result {
-    DNS_SUCCESS = 0,
+    DNS_PARSE_OK = 0,
     DNS_MALFORMED_RESPONSE,    // DnsRecordParser failed before the end of
                                // packet.
     DNS_MALFORMED_CNAME,       // Could not parse CNAME out of RRDATA.
@@ -90,7 +90,7 @@ class NET_EXPORT_PRIVATE DnsResponse {
     DNS_SIZE_MISMATCH,         // Got an address but size does not match.
     DNS_CNAME_AFTER_ADDRESS,   // Found CNAME after an address record.
     DNS_ADDRESS_TTL_MISMATCH,  // TTL of all address records are not identical.
-    DNS_NO_ADDRESSES,          // No address records found.
+    DNS_NO_ADDRESSES,          // OBSOLETE. No longer used.
     // Only add new values here.
     DNS_PARSE_RESULT_MAX,      // Bounding value for histograms.
   };
diff --git a/net/dns/dns_response_unittest.cc b/net/dns/dns_response_unittest.cc
index bb053bc..6ed6f06 100644
--- a/net/dns/dns_response_unittest.cc
+++ b/net/dns/dns_response_unittest.cc
@@ -299,7 +299,7 @@ TEST(DnsResponseTest, ParseToAddressList) {
     DnsResponse response(t.response_data, t.response_size, t.query_size);
     AddressList addr_list;
     base::TimeDelta ttl;
-    EXPECT_EQ(DnsResponse::DNS_SUCCESS,
+    EXPECT_EQ(DnsResponse::DNS_PARSE_OK,
               response.ParseToAddressList(&addr_list, &ttl));
     std::vector<const char*> expected_addresses(
         t.expected_addresses,
@@ -429,8 +429,9 @@ TEST(DnsResponseTest, ParseToAddressListFail) {
       DnsResponse::DNS_CNAME_AFTER_ADDRESS },
     { kResponseTTLMismatch, arraysize(kResponseTTLMismatch),
       DnsResponse::DNS_ADDRESS_TTL_MISMATCH },
+    // Not actually a failure, just an empty result.
     { kResponseNoAddresses, arraysize(kResponseNoAddresses),
-      DnsResponse::DNS_NO_ADDRESSES },
+      DnsResponse::DNS_PARSE_OK },
   };
 
   const size_t kQuerySize = 12 + 7;
diff --git a/net/dns/dns_test_util.cc b/net/dns/dns_test_util.cc
index 051f595..73d208c 100644
--- a/net/dns/dns_test_util.cc
+++ b/net/dns/dns_test_util.cc
@@ -14,6 +14,7 @@
 #include "net/base/dns_util.h"
 #include "net/base/io_buffer.h"
 #include "net/base/net_errors.h"
+#include "net/dns/address_sorter.h"
 #include "net/dns/dns_client.h"
 #include "net/dns/dns_config_service.h"
 #include "net/dns/dns_protocol.h"
@@ -25,19 +26,29 @@
 namespace net {
 namespace {
 
-// A DnsTransaction which responds with loopback to all queries starting with
-// "ok", fails synchronously on all queries starting with "er", and NXDOMAIN to
-// all others.
+// A DnsTransaction which uses MockDnsClientRuleList to determine the response.
 class MockTransaction : public DnsTransaction,
                         public base::SupportsWeakPtr<MockTransaction> {
  public:
-  MockTransaction(const std::string& hostname,
+  MockTransaction(const MockDnsClientRuleList& rules,
+                  const std::string& hostname,
                   uint16 qtype,
                   const DnsTransactionFactory::CallbackType& callback)
-      : hostname_(hostname),
+      : result_(MockDnsClientRule::FAIL_SYNC),
+        hostname_(hostname),
         qtype_(qtype),
         callback_(callback),
         started_(false) {
+    // Find the relevant rule which matches |qtype| and prefix of |hostname|.
+    for (size_t i = 0; i < rules.size(); ++i) {
+      const std::string& prefix = rules[i].prefix;
+      if ((rules[i].qtype == qtype) &&
+          (hostname.size() >= prefix.size()) &&
+          (hostname.compare(0, prefix.size(), prefix) == 0)) {
+        result_ = rules[i].result;
+        break;
+      }
+    }
   }
 
   virtual const std::string& GetHostname() const OVERRIDE {
@@ -51,7 +62,7 @@ class MockTransaction : public DnsTransaction,
   virtual int Start() OVERRIDE {
     EXPECT_FALSE(started_);
     started_ = true;
-    if (hostname_.substr(0, 2) == "er")
+    if (MockDnsClientRule::FAIL_SYNC == result_)
       return ERR_NAME_NOT_RESOLVED;
     // Using WeakPtr to cleanly cancel when transaction is destroyed.
     MessageLoop::current()->PostTask(
@@ -62,54 +73,66 @@ class MockTransaction : public DnsTransaction,
 
  private:
   void Finish() {
-    if (hostname_.substr(0, 2) == "ok") {
-      std::string qname;
-      DNSDomainFromDot(hostname_, &qname);
-      DnsQuery query(0, qname, qtype_);
-
-      DnsResponse response;
-      char* buffer = response.io_buffer()->data();
-      int nbytes = query.io_buffer()->size();
-      memcpy(buffer, query.io_buffer()->data(), nbytes);
-
-      const uint16 kPointerToQueryName =
-          static_cast<uint16>(0xc000 | sizeof(net::dns_protocol::Header));
-
-      const uint32 kTTL = 86400;  // One day.
-
-      // Size of RDATA which is a IPv4 or IPv6 address.
-      size_t rdata_size = qtype_ == net::dns_protocol::kTypeA ?
-                          net::kIPv4AddressSize : net::kIPv6AddressSize;
-
-      // 12 is the sum of sizes of the compressed name reference, TYPE,
-      // CLASS, TTL and RDLENGTH.
-      size_t answer_size = 12 + rdata_size;
-
-      // Write answer with loopback IP address.
-      reinterpret_cast<dns_protocol::Header*>(buffer)->ancount =
-          base::HostToNet16(1);
-      BigEndianWriter writer(buffer + nbytes, answer_size);
-      writer.WriteU16(kPointerToQueryName);
-      writer.WriteU16(qtype_);
-      writer.WriteU16(net::dns_protocol::kClassIN);
-      writer.WriteU32(kTTL);
-      writer.WriteU16(rdata_size);
-      if (qtype_ == net::dns_protocol::kTypeA) {
-        char kIPv4Loopback[] = { 0x7f, 0, 0, 1 };
-        writer.WriteBytes(kIPv4Loopback, sizeof(kIPv4Loopback));
-      } else {
-        char kIPv6Loopback[] = { 0, 0, 0, 0, 0, 0, 0, 0,
-                                 0, 0, 0, 0, 0, 0, 0, 1 };
-        writer.WriteBytes(kIPv6Loopback, sizeof(kIPv6Loopback));
-      }
-
-      EXPECT_TRUE(response.InitParse(nbytes + answer_size, query));
-      callback_.Run(this, OK, &response);
-    } else {
-      callback_.Run(this, ERR_NAME_NOT_RESOLVED, NULL);
+    switch (result_) {
+      case MockDnsClientRule::EMPTY:
+      case MockDnsClientRule::OK: {
+        std::string qname;
+        DNSDomainFromDot(hostname_, &qname);
+        DnsQuery query(0, qname, qtype_);
+
+        DnsResponse response;
+        char* buffer = response.io_buffer()->data();
+        int nbytes = query.io_buffer()->size();
+        memcpy(buffer, query.io_buffer()->data(), nbytes);
+        dns_protocol::Header* header =
+            reinterpret_cast<dns_protocol::Header*>(buffer);
+        header->flags |= dns_protocol::kFlagResponse;
+
+        if (MockDnsClientRule::OK == result_) {
+          const uint16 kPointerToQueryName =
+              static_cast<uint16>(0xc000 | sizeof(*header));
+
+          const uint32 kTTL = 86400;  // One day.
+
+          // Size of RDATA which is a IPv4 or IPv6 address.
+          size_t rdata_size = qtype_ == net::dns_protocol::kTypeA ?
+                              net::kIPv4AddressSize : net::kIPv6AddressSize;
+
+          // 12 is the sum of sizes of the compressed name reference, TYPE,
+          // CLASS, TTL and RDLENGTH.
+          size_t answer_size = 12 + rdata_size;
+
+          // Write answer with loopback IP address.
+          header->ancount = base::HostToNet16(1);
+          BigEndianWriter writer(buffer + nbytes, answer_size);
+          writer.WriteU16(kPointerToQueryName);
+          writer.WriteU16(qtype_);
+          writer.WriteU16(net::dns_protocol::kClassIN);
+          writer.WriteU32(kTTL);
+          writer.WriteU16(rdata_size);
+          if (qtype_ == net::dns_protocol::kTypeA) {
+            char kIPv4Loopback[] = { 0x7f, 0, 0, 1 };
+            writer.WriteBytes(kIPv4Loopback, sizeof(kIPv4Loopback));
+          } else {
+            char kIPv6Loopback[] = { 0, 0, 0, 0, 0, 0, 0, 0,
+                                     0, 0, 0, 0, 0, 0, 0, 1 };
+            writer.WriteBytes(kIPv6Loopback, sizeof(kIPv6Loopback));
+          }
+          nbytes += answer_size;
+        }
+        EXPECT_TRUE(response.InitParse(nbytes, query));
+        callback_.Run(this, OK, &response);
+      } break;
+      case MockDnsClientRule::FAIL_ASYNC:
+        callback_.Run(this, ERR_NAME_NOT_RESOLVED, NULL);
+        break;
+      default:
+        NOTREACHED();
+        break;
     }
   }
 
+  MockDnsClientRule::Result result_;
   const std::string hostname_;
   const uint16 qtype_;
   DnsTransactionFactory::CallbackType callback_;
@@ -120,7 +143,8 @@ class MockTransaction : public DnsTransaction,
 // A DnsTransactionFactory which creates MockTransaction.
 class MockTransactionFactory : public DnsTransactionFactory {
  public:
-  MockTransactionFactory() {}
+  explicit MockTransactionFactory(const MockDnsClientRuleList& rules)
+      : rules_(rules) {}
   virtual ~MockTransactionFactory() {}
 
   virtual scoped_ptr<DnsTransaction> CreateTransaction(
@@ -129,14 +153,29 @@ class MockTransactionFactory : public DnsTransactionFactory {
       const DnsTransactionFactory::CallbackType& callback,
       const BoundNetLog&) OVERRIDE {
     return scoped_ptr<DnsTransaction>(
-        new MockTransaction(hostname, qtype, callback));
+        new MockTransaction(rules_, hostname, qtype, callback));
+  }
+
+ private:
+  MockDnsClientRuleList rules_;
+};
+
+class MockAddressSorter : public AddressSorter {
+ public:
+  virtual ~MockAddressSorter() {}
+  virtual void Sort(const AddressList& list,
+                    const CallbackType& callback) const OVERRIDE {
+    // Do nothing.
+    callback.Run(true, list);
   }
 };
 
 // MockDnsClient provides MockTransactionFactory.
 class MockDnsClient : public DnsClient {
  public:
-  explicit MockDnsClient(const DnsConfig& config) : config_(config) {}
+  MockDnsClient(const DnsConfig& config,
+                const MockDnsClientRuleList& rules)
+      : config_(config), factory_(rules) {}
   virtual ~MockDnsClient() {}
 
   virtual void SetConfig(const DnsConfig& config) OVERRIDE {
@@ -151,16 +190,22 @@ class MockDnsClient : public DnsClient {
     return config_.IsValid() ? &factory_ : NULL;
   }
 
+  virtual AddressSorter* GetAddressSorter() OVERRIDE {
+    return &address_sorter_;
+  }
+
  private:
   DnsConfig config_;
   MockTransactionFactory factory_;
+  MockAddressSorter address_sorter_;
 };
 
 }  // namespace
 
 // static
-scoped_ptr<DnsClient> CreateMockDnsClient(const DnsConfig& config) {
-  return scoped_ptr<DnsClient>(new MockDnsClient(config));
+scoped_ptr<DnsClient> CreateMockDnsClient(const DnsConfig& config,
+                                          const MockDnsClientRuleList& rules) {
+  return scoped_ptr<DnsClient>(new MockDnsClient(config, rules));
 }
 
 MockDnsConfigService::~MockDnsConfigService() {
diff --git a/net/dns/dns_test_util.h b/net/dns/dns_test_util.h
index 53a0620..2d03d0e 100644
--- a/net/dns/dns_test_util.h
+++ b/net/dns/dns_test_util.h
@@ -5,6 +5,9 @@
 #ifndef NET_DNS_DNS_TEST_UTIL_H_
 #define NET_DNS_DNS_TEST_UTIL_H_
 
+#include <string>
+#include <vector>
+
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "net/dns/dns_config_service.h"
@@ -166,8 +169,25 @@ static const int kT3TTL = 0x00000015;
 static const unsigned kT3RecordCount = arraysize(kT3IpAddresses) + 2;
 
 class DnsClient;
+
+struct MockDnsClientRule {
+  enum Result {
+    FAIL_SYNC,   // Fail synchronously with ERR_NAME_NOT_RESOLVED.
+    FAIL_ASYNC,  // Fail asynchronously with ERR_NAME_NOT_RESOLVED.
+    EMPTY,       // Return an empty response.
+    OK,          // Return a response with loopback address.
+  };
+
+  std::string prefix;
+  uint16 qtype;
+  Result result;
+};
+
+typedef std::vector<MockDnsClientRule> MockDnsClientRuleList;
+
 // Creates mock DnsClient for testing HostResolverImpl.
-scoped_ptr<DnsClient> CreateMockDnsClient(const DnsConfig& config);
+scoped_ptr<DnsClient> CreateMockDnsClient(const DnsConfig& config,
+                                          const MockDnsClientRuleList& rules);
 
 class MockDnsConfigService : public DnsConfigService {
  public: