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// Copyright 2020 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef NET_DNS_DNS_UDP_TRACKER_H_
#define NET_DNS_DNS_UDP_TRACKER_H_
#include <stdint.h>
#include "base/containers/circular_deque.h"
#include "base/memory/raw_ptr.h"
#include "base/time/default_tick_clock.h"
#include "base/time/time.h"
#include "net/base/net_export.h"
namespace base {
class TickClock;
} // namespace base
namespace net {
// Data tracker for DNS UDP and its usage of local ports. Intended to be owned
// by a DnsSession and thus keep track of the data session-wide. Responsible for
// related metrics and used to inform behavior based on the stored data.
//
// TODO(ericorth@chromium.org): Add methods to access the stored data or
// conclusions about it.
class NET_EXPORT_PRIVATE DnsUdpTracker {
public:
static constexpr base::TimeDelta kMaxAge = base::Minutes(10);
static constexpr size_t kMaxRecordedQueries = 256;
// How recently an ID needs to be recorded in a recent query to be considered
// "recognized".
static constexpr base::TimeDelta kMaxRecognizedIdAge = base::Seconds(15);
// Numbers of ID mismatches required to set the |low_entropy_| flag. Also
// serves as the max number of mismatches to be recorded, as no more entries
// are recorded after setting the flag.
static constexpr size_t kUnrecognizedIdMismatchThreshold = 8;
static constexpr size_t kRecognizedIdMismatchThreshold = 128;
// Number of reuses of the same port required to set the |low_entropy_| flag.
// Note: The original value of this parameter was 2, but it caused a problem
// on Windows (crbug.com/1413620). The low entropy checker in DnsUdpTracker
// was too sensitive and caused many TCP fallbacks. This happened because the
// dynamic port range for UDP on Windows is too small (only 16384 ports). This
// meant that there was a high probability (about 1%) of reusing the same port
// number three or more times out of 256 records. To avoid these unnecessary
// TCP fallbacks, the value was changed to 3. The probability of reusing the
// same port number to 4 or more times out of 256 records is 3.92566e-05. And
// if the available port count is 2048, the probability: 0.0182851. So it is
// likely to activate when getting into the low entropy.
// (See crrev.com/c/4374511 for the calculation).
static constexpr int kPortReuseThreshold = 3;
DnsUdpTracker();
~DnsUdpTracker();
DnsUdpTracker(DnsUdpTracker&&);
DnsUdpTracker& operator=(DnsUdpTracker&&);
void RecordQuery(uint16_t port, uint16_t query_id);
void RecordResponseId(uint16_t query_id, uint16_t response_id);
void RecordConnectionError(int connection_error);
// If true, the entropy from random UDP port and DNS ID has been detected to
// potentially be low, e.g. due to exhaustion of the port pool or mismatches
// on IDs.
bool low_entropy() const { return low_entropy_; }
void set_tick_clock_for_testing(base::TickClock* tick_clock) {
tick_clock_ = tick_clock;
}
private:
struct QueryData;
void PurgeOldRecords();
void SaveQuery(QueryData query);
void SaveIdMismatch(uint16_t it);
bool low_entropy_ = false;
base::circular_deque<QueryData> recent_queries_;
// Times of recent ID mismatches, separated by whether or not the ID was
// recognized from recent queries.
base::circular_deque<base::TimeTicks> recent_unrecognized_id_hits_;
base::circular_deque<base::TimeTicks> recent_recognized_id_hits_;
raw_ptr<const base::TickClock> tick_clock_ =
base::DefaultTickClock::GetInstance();
};
} // namespace net
#endif // NET_DNS_DNS_UDP_TRACKER_H_
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