通过NTP协议进行时间同步

 1  class Program
 2     {
 3         static void Main(string[] args)
 4         {
 5             GetNetworkTime();
 6         }
 7
 8         public static DateTime GetNetworkTime()
 9         {
10             //default Windows time server
11             const string ntpServer = "clock.via.net";
12
13             // NTP message size - 16 bytes of the digest (RFC 2030)
14             var ntpData = new byte[48];
15
16             //Setting the Leap Indicator, Version Number and Mode values
17             ntpData[0] = 0x1B; //LI = 0 (no warning), VN = 3 (IPv4 only), Mode = 3 (Client Mode)
18
19             var addresses = Dns.GetHostEntry(ntpServer).AddressList;
20
21             //The UDP port number assigned to NTP is 123
22             var ipEndPoint = new IPEndPoint(addresses[0], 123);
23             //NTP uses UDP
24             var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
25
26             socket.Connect(ipEndPoint);
27
28             //Stops code hang if NTP is blocked
29             socket.ReceiveTimeout = 3000;
30
31             socket.Send(ntpData);
32             socket.Receive(ntpData);
33             socket.Close();
34
35             //Offset to get to the "Transmit Timestamp" field (time at which the reply
36             //departed the server for the client, in 64-bit timestamp format."
37             const byte serverReplyTime = 40;
38
39             //Get the seconds part
40             ulong intPart = BitConverter.ToUInt32(ntpData, serverReplyTime);
41
42             //Get the seconds fraction
43             ulong fractPart = BitConverter.ToUInt32(ntpData, serverReplyTime + 4);
44
45             //Convert From big-endian to little-endian
46             intPart = SwapEndianness(intPart);
47             fractPart = SwapEndianness(fractPart);
48
49             var milliseconds = (intPart * 1000) + ((fractPart * 1000) / 0x100000000L);
50
51             //**UTC** time
52             var networkDateTime = (new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)).AddMilliseconds((long)milliseconds);
53
54             return networkDateTime.ToLocalTime();
55         }
56
57         // stackoverflow.com/a/3294698/162671
58         static uint SwapEndianness(ulong x)
59         {
60             return (uint)(((x & 0x000000ff) << 24) +
61                            ((x & 0x0000ff00) << 8) +
62                            ((x & 0x00ff0000) >> 8) +
63                            ((x & 0xff000000) >> 24));
64         }
65     }

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