120 lines
4.0 KiB
Go
120 lines
4.0 KiB
Go
// Copyright 2016 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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// Contains the NTP time drift detection via the SNTP protocol:
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// https://tools.ietf.org/html/rfc4330
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package discover
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import (
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"fmt"
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"net"
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"sort"
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"time"
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"github.com/ethereum/go-ethereum/log"
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)
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const (
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ntpPool = "pool.ntp.org" // ntpPool is the NTP server to query for the current time
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ntpChecks = 3 // Number of measurements to do against the NTP server
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)
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// durationSlice attaches the methods of sort.Interface to []time.Duration,
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// sorting in increasing order.
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type durationSlice []time.Duration
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func (s durationSlice) Len() int { return len(s) }
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func (s durationSlice) Less(i, j int) bool { return s[i] < s[j] }
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func (s durationSlice) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
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// checkClockDrift queries an NTP server for clock drifts and warns the user if
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// one large enough is detected.
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func checkClockDrift() {
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drift, err := sntpDrift(ntpChecks)
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if err != nil {
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return
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}
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if drift < -driftThreshold || drift > driftThreshold {
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log.Warn(fmt.Sprintf("System clock seems off by %v, which can prevent network connectivity", drift))
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log.Warn("Please enable network time synchronisation in system settings.")
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} else {
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log.Debug("NTP sanity check done", "drift", drift)
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}
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}
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// sntpDrift does a naive time resolution against an NTP server and returns the
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// measured drift. This method uses the simple version of NTP. It's not precise
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// but should be fine for these purposes.
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//
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// Note, it executes two extra measurements compared to the number of requested
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// ones to be able to discard the two extremes as outliers.
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func sntpDrift(measurements int) (time.Duration, error) {
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// Resolve the address of the NTP server
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addr, err := net.ResolveUDPAddr("udp", ntpPool+":123")
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if err != nil {
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return 0, err
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}
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// Construct the time request (empty package with only 2 fields set):
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// Bits 3-5: Protocol version, 3
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// Bits 6-8: Mode of operation, client, 3
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request := make([]byte, 48)
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request[0] = 3<<3 | 3
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// Execute each of the measurements
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drifts := []time.Duration{}
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for i := 0; i < measurements+2; i++ {
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// Dial the NTP server and send the time retrieval request
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conn, err := net.DialUDP("udp", nil, addr)
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if err != nil {
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return 0, err
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}
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defer conn.Close()
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sent := time.Now()
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if _, err = conn.Write(request); err != nil {
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return 0, err
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}
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// Retrieve the reply and calculate the elapsed time
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conn.SetDeadline(time.Now().Add(5 * time.Second))
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reply := make([]byte, 48)
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if _, err = conn.Read(reply); err != nil {
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return 0, err
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}
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elapsed := time.Since(sent)
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// Reconstruct the time from the reply data
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sec := uint64(reply[43]) | uint64(reply[42])<<8 | uint64(reply[41])<<16 | uint64(reply[40])<<24
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frac := uint64(reply[47]) | uint64(reply[46])<<8 | uint64(reply[45])<<16 | uint64(reply[44])<<24
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nanosec := sec*1e9 + (frac*1e9)>>32
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t := time.Date(1900, 1, 1, 0, 0, 0, 0, time.UTC).Add(time.Duration(nanosec)).Local()
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// Calculate the drift based on an assumed answer time of RRT/2
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drifts = append(drifts, sent.Sub(t)+elapsed/2)
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}
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// Calculate average drif (drop two extremities to avoid outliers)
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sort.Sort(durationSlice(drifts))
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drift := time.Duration(0)
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for i := 1; i < len(drifts)-1; i++ {
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drift += drifts[i]
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}
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return drift / time.Duration(measurements), nil
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}
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