github.com/aitimate-0/go-ethereum@v1.9.7/p2p/discover/ntp.go (about)

     1  // Copyright 2016 The go-ethereum Authors
     2  // This file is part of the go-ethereum library.
     3  //
     4  // The go-ethereum library is free software: you can redistribute it and/or modify
     5  // it under the terms of the GNU Lesser General Public License as published by
     6  // the Free Software Foundation, either version 3 of the License, or
     7  // (at your option) any later version.
     8  //
     9  // The go-ethereum library is distributed in the hope that it will be useful,
    10  // but WITHOUT ANY WARRANTY; without even the implied warranty of
    11  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    12  // GNU Lesser General Public License for more details.
    13  //
    14  // You should have received a copy of the GNU Lesser General Public License
    15  // along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
    16  
    17  // Contains the NTP time drift detection via the SNTP protocol:
    18  //   https://tools.ietf.org/html/rfc4330
    19  
    20  package discover
    21  
    22  import (
    23  	"fmt"
    24  	"net"
    25  	"sort"
    26  	"time"
    27  
    28  	"github.com/ethereum/go-ethereum/log"
    29  )
    30  
    31  const (
    32  	ntpPool   = "pool.ntp.org" // ntpPool is the NTP server to query for the current time
    33  	ntpChecks = 3              // Number of measurements to do against the NTP server
    34  )
    35  
    36  // durationSlice attaches the methods of sort.Interface to []time.Duration,
    37  // sorting in increasing order.
    38  type durationSlice []time.Duration
    39  
    40  func (s durationSlice) Len() int           { return len(s) }
    41  func (s durationSlice) Less(i, j int) bool { return s[i] < s[j] }
    42  func (s durationSlice) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
    43  
    44  // checkClockDrift queries an NTP server for clock drifts and warns the user if
    45  // one large enough is detected.
    46  func checkClockDrift() {
    47  	drift, err := sntpDrift(ntpChecks)
    48  	if err != nil {
    49  		return
    50  	}
    51  	if drift < -driftThreshold || drift > driftThreshold {
    52  		log.Warn(fmt.Sprintf("System clock seems off by %v, which can prevent network connectivity", drift))
    53  		log.Warn("Please enable network time synchronisation in system settings.")
    54  	} else {
    55  		log.Debug("NTP sanity check done", "drift", drift)
    56  	}
    57  }
    58  
    59  // sntpDrift does a naive time resolution against an NTP server and returns the
    60  // measured drift. This method uses the simple version of NTP. It's not precise
    61  // but should be fine for these purposes.
    62  //
    63  // Note, it executes two extra measurements compared to the number of requested
    64  // ones to be able to discard the two extremes as outliers.
    65  func sntpDrift(measurements int) (time.Duration, error) {
    66  	// Resolve the address of the NTP server
    67  	addr, err := net.ResolveUDPAddr("udp", ntpPool+":123")
    68  	if err != nil {
    69  		return 0, err
    70  	}
    71  	// Construct the time request (empty package with only 2 fields set):
    72  	//   Bits 3-5: Protocol version, 3
    73  	//   Bits 6-8: Mode of operation, client, 3
    74  	request := make([]byte, 48)
    75  	request[0] = 3<<3 | 3
    76  
    77  	// Execute each of the measurements
    78  	drifts := []time.Duration{}
    79  	for i := 0; i < measurements+2; i++ {
    80  		// Dial the NTP server and send the time retrieval request
    81  		conn, err := net.DialUDP("udp", nil, addr)
    82  		if err != nil {
    83  			return 0, err
    84  		}
    85  		defer conn.Close()
    86  
    87  		sent := time.Now()
    88  		if _, err = conn.Write(request); err != nil {
    89  			return 0, err
    90  		}
    91  		// Retrieve the reply and calculate the elapsed time
    92  		conn.SetDeadline(time.Now().Add(5 * time.Second))
    93  
    94  		reply := make([]byte, 48)
    95  		if _, err = conn.Read(reply); err != nil {
    96  			return 0, err
    97  		}
    98  		elapsed := time.Since(sent)
    99  
   100  		// Reconstruct the time from the reply data
   101  		sec := uint64(reply[43]) | uint64(reply[42])<<8 | uint64(reply[41])<<16 | uint64(reply[40])<<24
   102  		frac := uint64(reply[47]) | uint64(reply[46])<<8 | uint64(reply[45])<<16 | uint64(reply[44])<<24
   103  
   104  		nanosec := sec*1e9 + (frac*1e9)>>32
   105  
   106  		t := time.Date(1900, 1, 1, 0, 0, 0, 0, time.UTC).Add(time.Duration(nanosec)).Local()
   107  
   108  		// Calculate the drift based on an assumed answer time of RRT/2
   109  		drifts = append(drifts, sent.Sub(t)+elapsed/2)
   110  	}
   111  	// Calculate average drif (drop two extremities to avoid outliers)
   112  	sort.Sort(durationSlice(drifts))
   113  
   114  	drift := time.Duration(0)
   115  	for i := 1; i < len(drifts)-1; i++ {
   116  		drift += drifts[i]
   117  	}
   118  	return drift / time.Duration(measurements), nil
   119  }