// 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/dns_transaction.h"

#include <deque>
#include <memory>
#include <string>
#include <utility>
#include <vector>

#include "base/big_endian.h"
#include "base/bind.h"
#include "base/location.h"
#include "base/macros.h"
#include "base/memory/ptr_util.h"
#include "base/memory/ref_counted.h"
#include "base/memory/weak_ptr.h"
#include "base/metrics/histogram_macros.h"
#include "base/profiler/scoped_tracker.h"
#include "base/rand_util.h"
#include "base/single_thread_task_runner.h"
#include "base/stl_util.h"
#include "base/strings/string_piece.h"
#include "base/threading/non_thread_safe.h"
#include "base/threading/thread_task_runner_handle.h"
#include "base/timer/timer.h"
#include "base/values.h"
#include "net/base/completion_callback.h"
#include "net/base/io_buffer.h"
#include "net/base/ip_address.h"
#include "net/base/ip_endpoint.h"
#include "net/base/net_errors.h"
#include "net/dns/dns_protocol.h"
#include "net/dns/dns_query.h"
#include "net/dns/dns_response.h"
#include "net/dns/dns_session.h"
#include "net/dns/dns_util.h"
#include "net/log/net_log.h"
#include "net/log/net_log_capture_mode.h"
#include "net/log/net_log_event_type.h"
#include "net/log/net_log_source.h"
#include "net/log/net_log_with_source.h"
#include "net/socket/stream_socket.h"
#include "net/udp/datagram_client_socket.h"

namespace net {

namespace {

// Provide a common macro to simplify code and readability. We must use a
// macro as the underlying HISTOGRAM macro creates static variables.
#define DNS_HISTOGRAM(name, time) UMA_HISTOGRAM_CUSTOM_TIMES(name, time, \
    base::TimeDelta::FromMilliseconds(1), base::TimeDelta::FromHours(1), 100)

// Count labels in the fully-qualified name in DNS format.
int CountLabels(const std::string& name) {
  size_t count = 0;
  for (size_t i = 0; i < name.size() && name[i]; i += name[i] + 1)
    ++count;
  return count;
}

bool IsIPLiteral(const std::string& hostname) {
  IPAddress ip;
  return ip.AssignFromIPLiteral(hostname);
}

std::unique_ptr<base::Value> NetLogStartCallback(
    const std::string* hostname,
    uint16_t qtype,
    NetLogCaptureMode /* capture_mode */) {
  std::unique_ptr<base::DictionaryValue> dict(new base::DictionaryValue());
  dict->SetString("hostname", *hostname);
  dict->SetInteger("query_type", qtype);
  return std::move(dict);
}

// ----------------------------------------------------------------------------

// A single asynchronous DNS exchange, which consists of sending out a
// DNS query, waiting for a response, and returning the response that it
// matches. Logging is done in the socket and in the outer DnsTransaction.
class DnsAttempt {
 public:
  explicit DnsAttempt(unsigned server_index)
      : result_(ERR_FAILED), server_index_(server_index) {}

  virtual ~DnsAttempt() {}
  // Starts the attempt. Returns ERR_IO_PENDING if cannot complete synchronously
  // and calls |callback| upon completion.
  virtual int Start(const CompletionCallback& callback) = 0;

  // Returns the query of this attempt.
  virtual const DnsQuery* GetQuery() const = 0;

  // Returns the response or NULL if has not received a matching response from
  // the server.
  virtual const DnsResponse* GetResponse() const = 0;

  // Returns the net log bound to the source of the socket.
  virtual const NetLogWithSource& GetSocketNetLog() const = 0;

  // Returns the index of the destination server within DnsConfig::nameservers.
  unsigned server_index() const { return server_index_; }

  // Returns a Value representing the received response, along with a reference
  // to the NetLog source source of the UDP socket used.  The request must have
  // completed before this is called.
  std::unique_ptr<base::Value> NetLogResponseCallback(
      NetLogCaptureMode capture_mode) const {
    DCHECK(GetResponse()->IsValid());

    std::unique_ptr<base::DictionaryValue> dict(new base::DictionaryValue());
    dict->SetInteger("rcode", GetResponse()->rcode());
    dict->SetInteger("answer_count", GetResponse()->answer_count());
    GetSocketNetLog().source().AddToEventParameters(dict.get());
    return std::move(dict);
  }

  void set_result(int result) {
    result_ = result;
  }

  // True if current attempt is pending (waiting for server response).
  bool is_pending() const {
    return result_ == ERR_IO_PENDING;
  }

  // True if attempt is completed (received server response).
  bool is_completed() const {
    return (result_ == OK) || (result_ == ERR_NAME_NOT_RESOLVED) ||
        (result_ == ERR_DNS_SERVER_REQUIRES_TCP);
  }

 private:
  // Result of last operation.
  int result_;

  const unsigned server_index_;
};

class DnsUDPAttempt : public DnsAttempt {
 public:
  DnsUDPAttempt(unsigned server_index,
                std::unique_ptr<DnsSession::SocketLease> socket_lease,
                std::unique_ptr<DnsQuery> query)
      : DnsAttempt(server_index),
        next_state_(STATE_NONE),
        received_malformed_response_(false),
        socket_lease_(std::move(socket_lease)),
        query_(std::move(query)) {}

  // DnsAttempt:
  int Start(const CompletionCallback& callback) override {
    DCHECK_EQ(STATE_NONE, next_state_);
    callback_ = callback;
    start_time_ = base::TimeTicks::Now();
    next_state_ = STATE_SEND_QUERY;
    return DoLoop(OK);
  }

  const DnsQuery* GetQuery() const override { return query_.get(); }

  const DnsResponse* GetResponse() const override {
    const DnsResponse* resp = response_.get();
    return (resp != NULL && resp->IsValid()) ? resp : NULL;
  }

  const NetLogWithSource& GetSocketNetLog() const override {
    return socket_lease_->socket()->NetLog();
  }

 private:
  enum State {
    STATE_SEND_QUERY,
    STATE_SEND_QUERY_COMPLETE,
    STATE_READ_RESPONSE,
    STATE_READ_RESPONSE_COMPLETE,
    STATE_NONE,
  };

  DatagramClientSocket* socket() {
    return socket_lease_->socket();
  }

  int DoLoop(int result) {
    CHECK_NE(STATE_NONE, next_state_);
    int rv = result;
    do {
      State state = next_state_;
      next_state_ = STATE_NONE;
      switch (state) {
        case STATE_SEND_QUERY:
          rv = DoSendQuery();
          break;
        case STATE_SEND_QUERY_COMPLETE:
          rv = DoSendQueryComplete(rv);
          break;
        case STATE_READ_RESPONSE:
          rv = DoReadResponse();
          break;
        case STATE_READ_RESPONSE_COMPLETE:
          rv = DoReadResponseComplete(rv);
          break;
        default:
          NOTREACHED();
          break;
      }
    } while (rv != ERR_IO_PENDING && next_state_ != STATE_NONE);

    set_result(rv);
    // If we received a malformed response, and are now waiting for another one,
    // indicate to the transaction that the server might be misbehaving.
    if (rv == ERR_IO_PENDING && received_malformed_response_)
      return ERR_DNS_MALFORMED_RESPONSE;
    if (rv == OK) {
      DCHECK_EQ(STATE_NONE, next_state_);
      DNS_HISTOGRAM("AsyncDNS.UDPAttemptSuccess",
                    base::TimeTicks::Now() - start_time_);
    } else if (rv != ERR_IO_PENDING) {
      DNS_HISTOGRAM("AsyncDNS.UDPAttemptFail",
                    base::TimeTicks::Now() - start_time_);
    }
    return rv;
  }

  int DoSendQuery() {
    next_state_ = STATE_SEND_QUERY_COMPLETE;
    return socket()->Write(query_->io_buffer(),
                           query_->io_buffer()->size(),
                           base::Bind(&DnsUDPAttempt::OnIOComplete,
                                      base::Unretained(this)));
  }

  int DoSendQueryComplete(int rv) {
    DCHECK_NE(ERR_IO_PENDING, rv);
    if (rv < 0)
      return rv;

    // Writing to UDP should not result in a partial datagram.
    if (rv != query_->io_buffer()->size())
      return ERR_MSG_TOO_BIG;

    next_state_ = STATE_READ_RESPONSE;
    return OK;
  }

  int DoReadResponse() {
    next_state_ = STATE_READ_RESPONSE_COMPLETE;
    response_.reset(new DnsResponse());
    return socket()->Read(response_->io_buffer(),
                          response_->io_buffer()->size(),
                          base::Bind(&DnsUDPAttempt::OnIOComplete,
                                     base::Unretained(this)));
  }

  int DoReadResponseComplete(int rv) {
    DCHECK_NE(ERR_IO_PENDING, rv);
    if (rv < 0)
      return rv;

    DCHECK(rv);
    if (!response_->InitParse(rv, *query_)) {
      // Other implementations simply ignore mismatched responses. Since each
      // DnsUDPAttempt binds to a different port, we might find that responses
      // to previously timed out queries lead to failures in the future.
      // Our solution is to make another attempt, in case the query truly
      // failed, but keep this attempt alive, in case it was a false alarm.
      received_malformed_response_ = true;
      next_state_ = STATE_READ_RESPONSE;
      return OK;
    }
    if (response_->flags() & dns_protocol::kFlagTC)
      return ERR_DNS_SERVER_REQUIRES_TCP;
    // TODO(szym): Extract TTL for NXDOMAIN results. http://crbug.com/115051
    if (response_->rcode() == dns_protocol::kRcodeNXDOMAIN)
      return ERR_NAME_NOT_RESOLVED;
    if (response_->rcode() != dns_protocol::kRcodeNOERROR)
      return ERR_DNS_SERVER_FAILED;

    return OK;
  }

  void OnIOComplete(int rv) {
    rv = DoLoop(rv);
    if (rv != ERR_IO_PENDING)
      callback_.Run(rv);
  }

  State next_state_;
  bool received_malformed_response_;
  base::TimeTicks start_time_;

  std::unique_ptr<DnsSession::SocketLease> socket_lease_;
  std::unique_ptr<DnsQuery> query_;

  std::unique_ptr<DnsResponse> response_;

  CompletionCallback callback_;

  DISALLOW_COPY_AND_ASSIGN(DnsUDPAttempt);
};

class DnsTCPAttempt : public DnsAttempt {
 public:
  DnsTCPAttempt(unsigned server_index,
                std::unique_ptr<StreamSocket> socket,
                std::unique_ptr<DnsQuery> query)
      : DnsAttempt(server_index),
        next_state_(STATE_NONE),
        socket_(std::move(socket)),
        query_(std::move(query)),
        length_buffer_(new IOBufferWithSize(sizeof(uint16_t))),
        response_length_(0) {}

  // DnsAttempt:
  int Start(const CompletionCallback& callback) override {
    DCHECK_EQ(STATE_NONE, next_state_);
    callback_ = callback;
    start_time_ = base::TimeTicks::Now();
    next_state_ = STATE_CONNECT_COMPLETE;
    int rv = socket_->Connect(base::Bind(&DnsTCPAttempt::OnIOComplete,
                                         base::Unretained(this)));
    if (rv == ERR_IO_PENDING) {
      set_result(rv);
      return rv;
    }
    return DoLoop(rv);
  }

  const DnsQuery* GetQuery() const override { return query_.get(); }

  const DnsResponse* GetResponse() const override {
    const DnsResponse* resp = response_.get();
    return (resp != NULL && resp->IsValid()) ? resp : NULL;
  }

  const NetLogWithSource& GetSocketNetLog() const override {
    return socket_->NetLog();
  }

 private:
  enum State {
    STATE_CONNECT_COMPLETE,
    STATE_SEND_LENGTH,
    STATE_SEND_QUERY,
    STATE_READ_LENGTH,
    STATE_READ_LENGTH_COMPLETE,
    STATE_READ_RESPONSE,
    STATE_READ_RESPONSE_COMPLETE,
    STATE_NONE,
  };

  int DoLoop(int result) {
    CHECK_NE(STATE_NONE, next_state_);
    int rv = result;
    do {
      State state = next_state_;
      next_state_ = STATE_NONE;
      switch (state) {
        case STATE_CONNECT_COMPLETE:
          rv = DoConnectComplete(rv);
          break;
        case STATE_SEND_LENGTH:
          rv = DoSendLength(rv);
          break;
        case STATE_SEND_QUERY:
          rv = DoSendQuery(rv);
          break;
        case STATE_READ_LENGTH:
          rv = DoReadLength(rv);
          break;
        case STATE_READ_LENGTH_COMPLETE:
          rv = DoReadLengthComplete(rv);
          break;
        case STATE_READ_RESPONSE:
          rv = DoReadResponse(rv);
          break;
        case STATE_READ_RESPONSE_COMPLETE:
          rv = DoReadResponseComplete(rv);
          break;
        default:
          NOTREACHED();
          break;
      }
    } while (rv != ERR_IO_PENDING && next_state_ != STATE_NONE);

    set_result(rv);
    if (rv == OK) {
      DCHECK_EQ(STATE_NONE, next_state_);
      DNS_HISTOGRAM("AsyncDNS.TCPAttemptSuccess",
                    base::TimeTicks::Now() - start_time_);
    } else if (rv != ERR_IO_PENDING) {
      DNS_HISTOGRAM("AsyncDNS.TCPAttemptFail",
                    base::TimeTicks::Now() - start_time_);
    }
    return rv;
  }

  int DoConnectComplete(int rv) {
    // TODO(rvargas): Remove ScopedTracker below once crbug.com/462784 is fixed.
    tracked_objects::ScopedTracker tracking_profile(
        FROM_HERE_WITH_EXPLICIT_FUNCTION(
            "462784 DnsTCPAttempt::DoConnectComplete"));

    DCHECK_NE(ERR_IO_PENDING, rv);
    if (rv < 0)
      return rv;

    uint16_t query_size = static_cast<uint16_t>(query_->io_buffer()->size());
    if (static_cast<int>(query_size) != query_->io_buffer()->size())
      return ERR_FAILED;
    base::WriteBigEndian<uint16_t>(length_buffer_->data(), query_size);
    buffer_ =
        new DrainableIOBuffer(length_buffer_.get(), length_buffer_->size());
    next_state_ = STATE_SEND_LENGTH;
    return OK;
  }

  int DoSendLength(int rv) {
    DCHECK_NE(ERR_IO_PENDING, rv);
    if (rv < 0)
      return rv;

    buffer_->DidConsume(rv);
    if (buffer_->BytesRemaining() > 0) {
      next_state_ = STATE_SEND_LENGTH;
      return socket_->Write(
          buffer_.get(),
          buffer_->BytesRemaining(),
          base::Bind(&DnsTCPAttempt::OnIOComplete, base::Unretained(this)));
    }
    buffer_ = new DrainableIOBuffer(query_->io_buffer(),
                                    query_->io_buffer()->size());
    next_state_ = STATE_SEND_QUERY;
    return OK;
  }

  int DoSendQuery(int rv) {
    DCHECK_NE(ERR_IO_PENDING, rv);
    if (rv < 0)
      return rv;

    buffer_->DidConsume(rv);
    if (buffer_->BytesRemaining() > 0) {
      next_state_ = STATE_SEND_QUERY;
      return socket_->Write(
          buffer_.get(),
          buffer_->BytesRemaining(),
          base::Bind(&DnsTCPAttempt::OnIOComplete, base::Unretained(this)));
    }
    buffer_ =
        new DrainableIOBuffer(length_buffer_.get(), length_buffer_->size());
    next_state_ = STATE_READ_LENGTH;
    return OK;
  }

  int DoReadLength(int rv) {
    DCHECK_EQ(OK, rv);

    next_state_ = STATE_READ_LENGTH_COMPLETE;
    return ReadIntoBuffer();
  }

  int DoReadLengthComplete(int rv) {
    DCHECK_NE(ERR_IO_PENDING, rv);
    if (rv < 0)
      return rv;
    if (rv == 0)
      return ERR_CONNECTION_CLOSED;

    buffer_->DidConsume(rv);
    if (buffer_->BytesRemaining() > 0) {
      next_state_ = STATE_READ_LENGTH;
      return OK;
    }

    base::ReadBigEndian<uint16_t>(length_buffer_->data(), &response_length_);
    // Check if advertised response is too short. (Optimization only.)
    if (response_length_ < query_->io_buffer()->size())
      return ERR_DNS_MALFORMED_RESPONSE;
    // Allocate more space so that DnsResponse::InitParse sanity check passes.
    response_.reset(new DnsResponse(response_length_ + 1));
    buffer_ = new DrainableIOBuffer(response_->io_buffer(), response_length_);
    next_state_ = STATE_READ_RESPONSE;
    return OK;
  }

  int DoReadResponse(int rv) {
    DCHECK_EQ(OK, rv);

    next_state_ = STATE_READ_RESPONSE_COMPLETE;
    return ReadIntoBuffer();
  }

  int DoReadResponseComplete(int rv) {
    DCHECK_NE(ERR_IO_PENDING, rv);
    if (rv < 0)
      return rv;
    if (rv == 0)
      return ERR_CONNECTION_CLOSED;

    buffer_->DidConsume(rv);
    if (buffer_->BytesRemaining() > 0) {
      next_state_ = STATE_READ_RESPONSE;
      return OK;
    }

    if (!response_->InitParse(buffer_->BytesConsumed(), *query_))
      return ERR_DNS_MALFORMED_RESPONSE;
    if (response_->flags() & dns_protocol::kFlagTC)
      return ERR_UNEXPECTED;
    // TODO(szym): Frankly, none of these are expected.
    if (response_->rcode() == dns_protocol::kRcodeNXDOMAIN)
      return ERR_NAME_NOT_RESOLVED;
    if (response_->rcode() != dns_protocol::kRcodeNOERROR)
      return ERR_DNS_SERVER_FAILED;

    return OK;
  }

  void OnIOComplete(int rv) {
    rv = DoLoop(rv);
    if (rv != ERR_IO_PENDING)
      callback_.Run(rv);
  }

  int ReadIntoBuffer() {
    return socket_->Read(
        buffer_.get(),
        buffer_->BytesRemaining(),
        base::Bind(&DnsTCPAttempt::OnIOComplete, base::Unretained(this)));
  }

  State next_state_;
  base::TimeTicks start_time_;

  std::unique_ptr<StreamSocket> socket_;
  std::unique_ptr<DnsQuery> query_;
  scoped_refptr<IOBufferWithSize> length_buffer_;
  scoped_refptr<DrainableIOBuffer> buffer_;

  uint16_t response_length_;
  std::unique_ptr<DnsResponse> response_;

  CompletionCallback callback_;

  DISALLOW_COPY_AND_ASSIGN(DnsTCPAttempt);
};

// ----------------------------------------------------------------------------

// Implements DnsTransaction. Configuration is supplied by DnsSession.
// The suffix list is built according to the DnsConfig from the session.
// The timeout for each DnsUDPAttempt is given by DnsSession::NextTimeout.
// The first server to attempt on each query is given by
// DnsSession::NextFirstServerIndex, and the order is round-robin afterwards.
// Each server is attempted DnsConfig::attempts times.
class DnsTransactionImpl : public DnsTransaction,
                           public base::NonThreadSafe,
                           public base::SupportsWeakPtr<DnsTransactionImpl> {
 public:
  DnsTransactionImpl(DnsSession* session,
                     const std::string& hostname,
                     uint16_t qtype,
                     const DnsTransactionFactory::CallbackType& callback,
                     const NetLogWithSource& net_log)
      : session_(session),
        hostname_(hostname),
        qtype_(qtype),
        callback_(callback),
        net_log_(net_log),
        qnames_initial_size_(0),
        attempts_count_(0),
        had_tcp_attempt_(false),
        first_server_index_(0) {
    DCHECK(session_.get());
    DCHECK(!hostname_.empty());
    DCHECK(!callback_.is_null());
    DCHECK(!IsIPLiteral(hostname_));
  }

  ~DnsTransactionImpl() override {
    if (!callback_.is_null()) {
      net_log_.EndEventWithNetErrorCode(NetLogEventType::DNS_TRANSACTION,
                                        ERR_ABORTED);
    }  // otherwise logged in DoCallback or Start
  }

  const std::string& GetHostname() const override {
    DCHECK(CalledOnValidThread());
    return hostname_;
  }

  uint16_t GetType() const override {
    DCHECK(CalledOnValidThread());
    return qtype_;
  }

  void Start() override {
    DCHECK(!callback_.is_null());
    DCHECK(attempts_.empty());
    net_log_.BeginEvent(NetLogEventType::DNS_TRANSACTION,
                        base::Bind(&NetLogStartCallback, &hostname_, qtype_));
    AttemptResult result(PrepareSearch(), NULL);
    if (result.rv == OK) {
      qnames_initial_size_ = qnames_.size();
      if (qtype_ == dns_protocol::kTypeA)
        UMA_HISTOGRAM_COUNTS("AsyncDNS.SuffixSearchStart", qnames_.size());
      result = ProcessAttemptResult(StartQuery());
    }

    // Must always return result asynchronously, to avoid reentrancy.
    if (result.rv != ERR_IO_PENDING) {
      base::ThreadTaskRunnerHandle::Get()->PostTask(
          FROM_HERE,
          base::Bind(&DnsTransactionImpl::DoCallback, AsWeakPtr(), result));
    }
  }

 private:
  // Wrapper for the result of a DnsUDPAttempt.
  struct AttemptResult {
    AttemptResult(int rv, const DnsAttempt* attempt)
        : rv(rv), attempt(attempt) {}

    int rv;
    const DnsAttempt* attempt;
  };

  // Prepares |qnames_| according to the DnsConfig.
  int PrepareSearch() {
    const DnsConfig& config = session_->config();

    std::string labeled_hostname;
    if (!DNSDomainFromDot(hostname_, &labeled_hostname))
      return ERR_INVALID_ARGUMENT;

    if (hostname_.back() == '.') {
      // It's a fully-qualified name, no suffix search.
      qnames_.push_back(labeled_hostname);
      return OK;
    }

    int ndots = CountLabels(labeled_hostname) - 1;

    if (ndots > 0 && !config.append_to_multi_label_name) {
      qnames_.push_back(labeled_hostname);
      return OK;
    }

    // Set true when |labeled_hostname| is put on the list.
    bool had_hostname = false;

    if (ndots >= config.ndots) {
      qnames_.push_back(labeled_hostname);
      had_hostname = true;
    }

    std::string qname;
    for (size_t i = 0; i < config.search.size(); ++i) {
      // Ignore invalid (too long) combinations.
      if (!DNSDomainFromDot(hostname_ + "." + config.search[i], &qname))
        continue;
      if (qname.size() == labeled_hostname.size()) {
        if (had_hostname)
          continue;
        had_hostname = true;
      }
      qnames_.push_back(qname);
    }

    if (ndots > 0 && !had_hostname)
      qnames_.push_back(labeled_hostname);

    return qnames_.empty() ? ERR_DNS_SEARCH_EMPTY : OK;
  }

  void DoCallback(AttemptResult result) {
    DCHECK(!callback_.is_null());
    DCHECK_NE(ERR_IO_PENDING, result.rv);
    const DnsResponse* response = result.attempt ?
        result.attempt->GetResponse() : NULL;
    CHECK(result.rv != OK || response != NULL);

    timer_.Stop();
    RecordLostPacketsIfAny();
    if (result.rv == OK)
      UMA_HISTOGRAM_COUNTS("AsyncDNS.AttemptCountSuccess", attempts_count_);
    else
      UMA_HISTOGRAM_COUNTS("AsyncDNS.AttemptCountFail", attempts_count_);

    if (response && qtype_ == dns_protocol::kTypeA) {
      UMA_HISTOGRAM_COUNTS("AsyncDNS.SuffixSearchRemain", qnames_.size());
      UMA_HISTOGRAM_COUNTS("AsyncDNS.SuffixSearchDone",
                           qnames_initial_size_ - qnames_.size());
    }

    DnsTransactionFactory::CallbackType callback = callback_;
    callback_.Reset();

    net_log_.EndEventWithNetErrorCode(NetLogEventType::DNS_TRANSACTION,
                                      result.rv);
    callback.Run(this, result.rv, response);
  }

  // Makes another attempt at the current name, |qnames_.front()|, using the
  // next nameserver.
  AttemptResult MakeAttempt() {
    unsigned attempt_number = attempts_.size();

    uint16_t id = session_->NextQueryId();
    std::unique_ptr<DnsQuery> query;
    if (attempts_.empty()) {
      query.reset(new DnsQuery(id, qnames_.front(), qtype_));
    } else {
      query = attempts_[0]->GetQuery()->CloneWithNewId(id);
    }

    const DnsConfig& config = session_->config();

    unsigned server_index =
        (first_server_index_ + attempt_number) % config.nameservers.size();
    // Skip over known failed servers.
    server_index = session_->NextGoodServerIndex(server_index);

    std::unique_ptr<DnsSession::SocketLease> lease =
        session_->AllocateSocket(server_index, net_log_.source());

    bool got_socket = !!lease.get();

    DnsUDPAttempt* attempt =
        new DnsUDPAttempt(server_index, std::move(lease), std::move(query));

    attempts_.push_back(base::WrapUnique(attempt));
    ++attempts_count_;

    if (!got_socket)
      return AttemptResult(ERR_CONNECTION_REFUSED, NULL);

    net_log_.AddEvent(
        NetLogEventType::DNS_TRANSACTION_ATTEMPT,
        attempt->GetSocketNetLog().source().ToEventParametersCallback());

    int rv = attempt->Start(
        base::Bind(&DnsTransactionImpl::OnUdpAttemptComplete,
                   base::Unretained(this), attempt_number,
                   base::TimeTicks::Now()));
    if (rv == ERR_IO_PENDING) {
      base::TimeDelta timeout = session_->NextTimeout(server_index,
                                                      attempt_number);
      timer_.Start(FROM_HERE, timeout, this, &DnsTransactionImpl::OnTimeout);
    }
    return AttemptResult(rv, attempt);
  }

  AttemptResult MakeTCPAttempt(const DnsAttempt* previous_attempt) {
    DCHECK(previous_attempt);
    DCHECK(!had_tcp_attempt_);

    unsigned server_index = previous_attempt->server_index();

    std::unique_ptr<StreamSocket> socket(
        session_->CreateTCPSocket(server_index, net_log_.source()));

    // TODO(szym): Reuse the same id to help the server?
    uint16_t id = session_->NextQueryId();
    std::unique_ptr<DnsQuery> query =
        previous_attempt->GetQuery()->CloneWithNewId(id);

    RecordLostPacketsIfAny();
    // Cancel all other attempts, no point waiting on them.
    attempts_.clear();

    unsigned attempt_number = attempts_.size();

    DnsTCPAttempt* attempt =
        new DnsTCPAttempt(server_index, std::move(socket), std::move(query));

    attempts_.push_back(base::WrapUnique(attempt));
    ++attempts_count_;
    had_tcp_attempt_ = true;

    net_log_.AddEvent(
        NetLogEventType::DNS_TRANSACTION_TCP_ATTEMPT,
        attempt->GetSocketNetLog().source().ToEventParametersCallback());

    int rv = attempt->Start(base::Bind(&DnsTransactionImpl::OnAttemptComplete,
                                       base::Unretained(this),
                                       attempt_number));
    if (rv == ERR_IO_PENDING) {
      // Custom timeout for TCP attempt.
      base::TimeDelta timeout = timer_.GetCurrentDelay() * 2;
      timer_.Start(FROM_HERE, timeout, this, &DnsTransactionImpl::OnTimeout);
    }
    return AttemptResult(rv, attempt);
  }

  // Begins query for the current name. Makes the first attempt.
  AttemptResult StartQuery() {
    std::string dotted_qname = DNSDomainToString(qnames_.front());
    net_log_.BeginEvent(NetLogEventType::DNS_TRANSACTION_QUERY,
                        NetLog::StringCallback("qname", &dotted_qname));

    first_server_index_ = session_->NextFirstServerIndex();
    RecordLostPacketsIfAny();
    attempts_.clear();
    had_tcp_attempt_ = false;
    return MakeAttempt();
  }

  void OnUdpAttemptComplete(unsigned attempt_number,
                            base::TimeTicks start,
                            int rv) {
    DCHECK_LT(attempt_number, attempts_.size());
    const DnsAttempt* attempt = attempts_[attempt_number].get();
    if (attempt->GetResponse()) {
      session_->RecordRTT(attempt->server_index(),
                          base::TimeTicks::Now() - start);
    }
    OnAttemptComplete(attempt_number, rv);
  }

  void OnAttemptComplete(unsigned attempt_number, int rv) {
    if (callback_.is_null())
      return;
    DCHECK_LT(attempt_number, attempts_.size());
    const DnsAttempt* attempt = attempts_[attempt_number].get();
    AttemptResult result = ProcessAttemptResult(AttemptResult(rv, attempt));
    if (result.rv != ERR_IO_PENDING)
      DoCallback(result);
  }

  // Record packet loss for any incomplete attempts.
  void RecordLostPacketsIfAny() {
    // Loop through attempts until we find first that is completed
    size_t first_completed = 0;
    for (first_completed = 0; first_completed < attempts_.size();
        ++first_completed) {
      if (attempts_[first_completed]->is_completed())
        break;
    }
    // If there were no completed attempts, then we must be offline, so don't
    // record any attempts as lost packets.
    if (first_completed == attempts_.size())
      return;

    size_t num_servers = session_->config().nameservers.size();
    std::vector<int> server_attempts(num_servers);
    for (size_t i = 0; i < first_completed; ++i) {
      unsigned server_index = attempts_[i]->server_index();
      int server_attempt = server_attempts[server_index]++;
      // Don't record lost packet unless attempt is in pending state.
      if (!attempts_[i]->is_pending())
        continue;
      session_->RecordLostPacket(server_index, server_attempt);
    }
  }

  void LogResponse(const DnsAttempt* attempt) {
    if (attempt && attempt->GetResponse()) {
      net_log_.AddEvent(NetLogEventType::DNS_TRANSACTION_RESPONSE,
                        base::Bind(&DnsAttempt::NetLogResponseCallback,
                                   base::Unretained(attempt)));
    }
  }

  bool MoreAttemptsAllowed() const {
    if (had_tcp_attempt_)
      return false;
    const DnsConfig& config = session_->config();
    return attempts_.size() < config.attempts * config.nameservers.size();
  }

  // Resolves the result of a DnsAttempt until a terminal result is reached
  // or it will complete asynchronously (ERR_IO_PENDING).
  AttemptResult ProcessAttemptResult(AttemptResult result) {
    while (result.rv != ERR_IO_PENDING) {
      LogResponse(result.attempt);

      switch (result.rv) {
        case OK:
          session_->RecordServerSuccess(result.attempt->server_index());
          net_log_.EndEventWithNetErrorCode(
              NetLogEventType::DNS_TRANSACTION_QUERY, result.rv);
          DCHECK(result.attempt);
          DCHECK(result.attempt->GetResponse());
          return result;
        case ERR_NAME_NOT_RESOLVED:
          session_->RecordServerSuccess(result.attempt->server_index());
          net_log_.EndEventWithNetErrorCode(
              NetLogEventType::DNS_TRANSACTION_QUERY, result.rv);
          // Try next suffix.
          qnames_.pop_front();
          if (qnames_.empty()) {
            return AttemptResult(ERR_NAME_NOT_RESOLVED, NULL);
          } else {
            result = StartQuery();
          }
          break;
        case ERR_CONNECTION_REFUSED:
        case ERR_DNS_TIMED_OUT:
          if (result.attempt)
            session_->RecordServerFailure(result.attempt->server_index());
          if (MoreAttemptsAllowed()) {
            result = MakeAttempt();
          } else {
            return result;
          }
          break;
        case ERR_DNS_SERVER_REQUIRES_TCP:
          result = MakeTCPAttempt(result.attempt);
          break;
        default:
          // Server failure.
          DCHECK(result.attempt);
          if (result.attempt != attempts_.back().get()) {
            // This attempt already timed out. Ignore it.
            session_->RecordServerFailure(result.attempt->server_index());
            return AttemptResult(ERR_IO_PENDING, NULL);
          }
          if (MoreAttemptsAllowed()) {
            result = MakeAttempt();
          } else if (result.rv == ERR_DNS_MALFORMED_RESPONSE &&
                     !had_tcp_attempt_) {
            // For UDP only, ignore the response and wait until the last attempt
            // times out.
            return AttemptResult(ERR_IO_PENDING, NULL);
          } else {
            return AttemptResult(result.rv, NULL);
          }
          break;
      }
    }
    return result;
  }

  void OnTimeout() {
    if (callback_.is_null())
      return;
    DCHECK(!attempts_.empty());
    AttemptResult result = ProcessAttemptResult(
        AttemptResult(ERR_DNS_TIMED_OUT, attempts_.back().get()));
    if (result.rv != ERR_IO_PENDING)
      DoCallback(result);
  }

  scoped_refptr<DnsSession> session_;
  std::string hostname_;
  uint16_t qtype_;
  // Cleared in DoCallback.
  DnsTransactionFactory::CallbackType callback_;

  NetLogWithSource net_log_;

  // Search list of fully-qualified DNS names to query next (in DNS format).
  std::deque<std::string> qnames_;
  size_t qnames_initial_size_;

  // List of attempts for the current name.
  std::vector<std::unique_ptr<DnsAttempt>> attempts_;
  // Count of attempts, not reset when |attempts_| vector is cleared.
  int  attempts_count_;
  bool had_tcp_attempt_;

  // Index of the first server to try on each search query.
  int first_server_index_;

  base::OneShotTimer timer_;

  DISALLOW_COPY_AND_ASSIGN(DnsTransactionImpl);
};

// ----------------------------------------------------------------------------

// Implementation of DnsTransactionFactory that returns instances of
// DnsTransactionImpl.
class DnsTransactionFactoryImpl : public DnsTransactionFactory {
 public:
  explicit DnsTransactionFactoryImpl(DnsSession* session) {
    session_ = session;
  }

  std::unique_ptr<DnsTransaction> CreateTransaction(
      const std::string& hostname,
      uint16_t qtype,
      const CallbackType& callback,
      const NetLogWithSource& net_log) override {
    return std::unique_ptr<DnsTransaction>(new DnsTransactionImpl(
        session_.get(), hostname, qtype, callback, net_log));
  }

 private:
  scoped_refptr<DnsSession> session_;
};

}  // namespace

// static
std::unique_ptr<DnsTransactionFactory> DnsTransactionFactory::CreateFactory(
    DnsSession* session) {
  return std::unique_ptr<DnsTransactionFactory>(
      new DnsTransactionFactoryImpl(session));
}

}  // namespace net