Networking

using System;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Text;

class TcpChat
{
    static void Main(string[] args)
    {
        if(args.Length == 0)
        {
            Console.WriteLine("Basic TCP chat");
            Console.WriteLine();
            Console.WriteLine("Usage:");
            Console.WriteLine("tcpchat server <port>");
            Console.WriteLine("tcpchat client <url> <port>");
            return;
        }

        try
        {
            Run(args);
        }
        catch(IOException)
        {
            Console.WriteLine("--- Connection lost");
        }
        catch(SocketException ex)
        {
            Console.WriteLine("--- Can't connect: " + ex.Message);
        }
    }
    
    static void Run(string[] args)
    {
        TcpClient client;
        NetworkStream stream;
        byte[] buffer = new byte[256];
        var encoding = Encoding.ASCII;

        if(args[0].StartsWith("s", StringComparison.InvariantCultureIgnoreCase))
        {
            var port = int.Parse(args[1]);
            var listener = new TcpListener(IPAddress.Any, port);
            listener.Start();
            Console.WriteLine("--- Waiting for a connection...");
            client = listener.AcceptTcpClient();
        }
        else
        {
            var hostName = args[1];
            var port = int.Parse(args[2]);
            client = new TcpClient();
            client.Connect(hostName, port);
        }

        stream = client.GetStream();
        Console.WriteLine("--- Connected. Start typing! (exit with Ctrl-C)");

        while(true)
        {
            if(Console.KeyAvailable)
            {
                var lineToSend = Console.ReadLine();
                var bytesToSend = encoding.GetBytes(lineToSend + "\r\n");
                stream.Write(bytesToSend, 0, bytesToSend.Length);
                stream.Flush();
            }

            if (stream.DataAvailable)
            {
                var receivedBytesCount = stream.Read(buffer, 0, buffer.Length);
                var receivedString = encoding.GetString(buffer, 0, receivedBytesCount);
                Console.Write(receivedString);
            }
        }
    }
}

using System;
using System.Globalization;
using System.Linq;
using System.Net;
using System.Net.Sockets;

class SntpClient
{
    const int SntpPort = 123;
    static DateTime BaseDate = new DateTime(1900, 1, 1);

    static void Main(string[] args)
    {
        if(args.Length == 0) {
            Console.WriteLine("Simple SNTP client");
            Console.WriteLine();
            Console.WriteLine("Usage: sntpclient <sntp server url> [<local timezone>]");
            Console.WriteLine();
            Console.WriteLine("<local timezone>: a number between -12 and 12 as hours from UTC");
            Console.WriteLine("(append .5 for an extra half an hour)");
            return;
        }

        double localTimeZoneInHours = 0;
        if(args.Length > 1)
            localTimeZoneInHours = double.Parse(args[1], CultureInfo.InvariantCulture);

        var udpClient = new UdpClient();
        udpClient.Client.ReceiveTimeout = 5000;

        var sntpRequest = new byte[48];
        sntpRequest[0] = 0x23; //LI=0 (no warning), VN=4, Mode=3 (client)

        udpClient.Send(
            dgram: sntpRequest,
            bytes: sntpRequest.Length,
            hostname: args[0],
            port: SntpPort);

        byte[] sntpResponse;
        try
        {
            IPEndPoint remoteEndpoint = null;
            sntpResponse = udpClient.Receive(ref remoteEndpoint);
        }
        catch(SocketException)
        {
            Console.WriteLine("*** No response received from the server");
            return;
        }

        uint numberOfSeconds;
        if(BitConverter.IsLittleEndian)
            numberOfSeconds = BitConverter.ToUInt32(
                sntpResponse.Skip(40).Take(4).Reverse().ToArray()
                ,0);
        else
            numberOfSeconds = BitConverter.ToUInt32(sntpResponse, 40);
        
        var date = BaseDate.AddSeconds(numberOfSeconds).AddHours(localTimeZoneInHours);

        Console.WriteLine(
            $"Current date in server: {date:yyyy-MM-dd HH:mm:ss} UTC{localTimeZoneInHours:+0.#;-0.#;.}");
    }
}