github.com/Bytom/bytom@v1.1.2-0.20210127130405-ae40204c0b09/p2p/discover/dht/ntp.go (about)

     1  // Contains the NTP time drift detection via the SNTP protocol:
     2  //   https://tools.ietf.org/html/rfc4330
     3  
     4  package dht
     5  
     6  import (
     7  	"fmt"
     8  	"net"
     9  	"sort"
    10  	"strings"
    11  	"time"
    12  
    13  	log "github.com/sirupsen/logrus"
    14  )
    15  
    16  const (
    17  	ntpPool   = "pool.ntp.org" // ntpPool is the NTP server to query for the current time
    18  	ntpChecks = 3              // Number of measurements to do against the NTP server
    19  )
    20  
    21  // durationSlice attaches the methods of sort.Interface to []time.Duration,
    22  // sorting in increasing order.
    23  type durationSlice []time.Duration
    24  
    25  func (s durationSlice) Len() int           { return len(s) }
    26  func (s durationSlice) Less(i, j int) bool { return s[i] < s[j] }
    27  func (s durationSlice) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
    28  
    29  // checkClockDrift queries an NTP server for clock drifts and warns the user if
    30  // one large enough is detected.
    31  func checkClockDrift() {
    32  	drift, err := sntpDrift(ntpChecks)
    33  	if err != nil {
    34  		return
    35  	}
    36  	if drift < -driftThreshold || drift > driftThreshold {
    37  		warning := fmt.Sprintf("System clock seems off by %v, which can prevent network connectivity", drift)
    38  		howtofix := fmt.Sprintf("Please enable network time synchronisation in system settings")
    39  		separator := strings.Repeat("-", len(warning))
    40  
    41  		log.WithFields(log.Fields{"module": logModule}).Warn(separator)
    42  		log.WithFields(log.Fields{"module": logModule}).Warn(warning)
    43  		log.WithFields(log.Fields{"module": logModule}).Warn(howtofix)
    44  		log.WithFields(log.Fields{"module": logModule}).Warn(separator)
    45  	} else {
    46  		log.WithFields(log.Fields{"module": logModule, "drift": drift}).Debug(fmt.Sprintf("Sanity NTP check reported all ok"))
    47  	}
    48  }
    49  
    50  // sntpDrift does a naive time resolution against an NTP server and returns the
    51  // measured drift. This method uses the simple version of NTP. It's not precise
    52  // but should be fine for these purposes.
    53  //
    54  // Note, it executes two extra measurements compared to the number of requested
    55  // ones to be able to discard the two extremes as outliers.
    56  func sntpDrift(measurements int) (time.Duration, error) {
    57  	// Resolve the address of the NTP server
    58  	addr, err := net.ResolveUDPAddr("udp", ntpPool+":123")
    59  	if err != nil {
    60  		return 0, err
    61  	}
    62  	// Construct the time request (empty package with only 2 fields set):
    63  	//   Bits 3-5: Protocol version, 3
    64  	//   Bits 6-8: Mode of operation, client, 3
    65  	request := make([]byte, 48)
    66  	request[0] = 3<<3 | 3
    67  
    68  	// Execute each of the measurements
    69  	drifts := []time.Duration{}
    70  	for i := 0; i < measurements+2; i++ {
    71  		// Dial the NTP server and send the time retrieval request
    72  		conn, err := net.DialUDP("udp", nil, addr)
    73  		if err != nil {
    74  			return 0, err
    75  		}
    76  		defer conn.Close()
    77  
    78  		sent := time.Now()
    79  		if _, err = conn.Write(request); err != nil {
    80  			return 0, err
    81  		}
    82  		// Retrieve the reply and calculate the elapsed time
    83  		conn.SetDeadline(time.Now().Add(5 * time.Second))
    84  
    85  		reply := make([]byte, 48)
    86  		if _, err = conn.Read(reply); err != nil {
    87  			return 0, err
    88  		}
    89  		elapsed := time.Since(sent)
    90  
    91  		// Reconstruct the time from the reply data
    92  		sec := uint64(reply[43]) | uint64(reply[42])<<8 | uint64(reply[41])<<16 | uint64(reply[40])<<24
    93  		frac := uint64(reply[47]) | uint64(reply[46])<<8 | uint64(reply[45])<<16 | uint64(reply[44])<<24
    94  
    95  		nanosec := sec*1e9 + (frac*1e9)>>32
    96  
    97  		t := time.Date(1900, 1, 1, 0, 0, 0, 0, time.UTC).Add(time.Duration(nanosec)).Local()
    98  
    99  		// Calculate the drift based on an assumed answer time of RRT/2
   100  		drifts = append(drifts, sent.Sub(t)+elapsed/2)
   101  	}
   102  	// Calculate average drif (drop two extremities to avoid outliers)
   103  	sort.Sort(durationSlice(drifts))
   104  
   105  	drift := time.Duration(0)
   106  	for i := 1; i < len(drifts)-1; i++ {
   107  		drift += drifts[i]
   108  	}
   109  	return drift / time.Duration(measurements), nil
   110  }