Uwaa/HTTP/HttpServer.cs

using System.Net;
using System.Net.Security;
using System.Net.Sockets;
using System.Security.Cryptography.X509Certificates;
using Uwaa.HTTP.Routing;
using Uwaa.HTTP.Websockets;

namespace Uwaa.HTTP;

/// <summary>
/// A hypertext transfer protocol server which can listen on a port and deliver content over HTTP, optionally over SSL (HTTPS).
/// </summary>
public sealed class HttpServer
{
    /// <summary>
    /// The router to be used to find and serve content to clients.
    /// </summary>
    public readonly RouterBase Router;

    /// <summary>
    /// If non-null, all connections will be secured via SSL using this certificate.
    /// </summary>
    public readonly X509Certificate? Certificate;

    /// <summary>
    /// Maximum number of connections per IP before new connections are rejected.
    /// </summary>
    /// <remarks>
    /// Prevents Slow Loris attacks.
    /// </remarks>
    public int MaxConnections = 20;

    /// <summary>
    /// The port the server will listen on.
    /// </summary>
    public int Port => ((IPEndPoint)listener.LocalEndpoint).Port;

    /// <summary>
    /// Called when a client establishes a connection with the server.
    /// </summary>
    public event Action<TcpClient>? OnConnectionBegin;

    /// <summary>
    /// Called when a connection has terminated.
    /// </summary>
    public event Action<IPEndPoint>? OnConnectionEnd;

    /// <summary>
    /// Called when a request has been served a response.
    /// </summary>
    public event Action<HttpRequest, HttpResponse>? OnResponse;

    /// <summary>
    /// The maximum time the socket may be inactive before it is presumed dead and closed.
    /// </summary>
    public TimeSpan Timeout = TimeSpan.FromSeconds(20);

    readonly Dictionary<IPAddress, int> IPCounts = new Dictionary<IPAddress, int>();

    readonly SemaphoreSlim IPCountsLock = new SemaphoreSlim(1, 1);

    readonly TcpListener listener;

    public HttpServer(int port, X509Certificate? certificate, RouterBase router)
    {
        if (OperatingSystem.IsWindows() && certificate is X509Certificate2 cert)
        {
            //Hack because SslStream is stupid on windows
            certificate = new X509Certificate2(cert.Export(X509ContentType.Pfx));
        }

        listener = new TcpListener(IPAddress.Any, port);
        Certificate = certificate;
        Router = router;
    }

    /// <summary>
    /// Begins listening for new connections.
    /// </summary>
    public async void Start()
    {
        listener.Start();

        while (true)
            HandleClient(await listener.AcceptTcpClientAsync());
    }

    async void HandleClient(TcpClient client)
    {
        OnConnectionBegin?.Invoke(client);

        if (client.Client.RemoteEndPoint is not IPEndPoint endpoint)
            return;

        //Count IPs to prevent Slow Loris attacks
        try
        {
            await IPCountsLock.WaitAsync();
            if (IPCounts.TryGetValue(endpoint.Address, out int count))
            {
                if (count >= MaxConnections)
                {
                    //Too many connections
                    client.Close();
                    return;
                }
                
                IPCounts[endpoint.Address] = count + 1;
            }
            else
            {
                IPCounts[endpoint.Address] = 1;
            }
        }
        finally
        {
            IPCountsLock.Release();
        }

        try
        {
            //Setup client
            Stream stream = client.GetStream();
            client.LingerState = new LingerOption(true, 5);

            if (Certificate != null)
            {
                //Pass through SSL stream
                SslStream ssl = new SslStream(stream);
                await ssl.AuthenticateAsServerAsync(Certificate).WaitAsync(Timeout);
                stream = ssl;
            }

            //HTTP request-response loop
            while (client.Connected)
            {
                HttpStream httpStream = new HttpStream(stream);
                try
                {
                    HttpClientInfo clientInfo = new HttpClientInfo(client, endpoint);
                    HttpRequest req = await httpStream.ReadRequest().WaitAsync(Timeout);

                    //Parse path
                    ArraySegment<string> pathSpl = req.Path.Split('/', StringSplitOptions.RemoveEmptyEntries);
                    if (pathSpl.Count == 0)
                    {
                        pathSpl = new string[] { string.Empty };
                    }
                    else
                    {
                        for (int i = 0; i < pathSpl.Count; i++)
                            pathSpl[i] = WebUtility.UrlDecode(pathSpl[i]);
                    }

                    //Execute
                    HttpResponse? response = (await Router.GetResponse(req, clientInfo, pathSpl)) ?? HttpResponse.NotFound("Router produced no response");

                    OnResponse?.Invoke(req, response);
                    await response.WriteTo(httpStream).WaitAsync(Timeout);

                    if (response is SwitchingProtocols swp)
                    {
                        //Create and run websocket
                        WebsocketRemote ws = new WebsocketRemote(req, clientInfo, httpStream, swp.Fields.WebSocketProtocol);
                        CloseStatus closeStatus = await swp.Callback(ws);
                        ws.Close(closeStatus);
                        break; //Close
                    }
                    else
                    {
                        if (response.StatusCode is not >= 200 or not < 300 || (req.Fields.Connection == ConnectionType.Close))
                            break; //Close
                    }
                }
                catch (HttpException e)
                {
                    try
                    {
                        await new HttpResponse(400, e.Message).WriteTo(httpStream).WaitAsync(Timeout);
                    }
                    catch { }
                    break;
                }
                catch (TimeoutException)
                {
                    //Timeout
                    break;
                }
                catch (IOException)
                {
                    //Disconnect
                    throw;
                }
                catch (SocketException)
                {
                    //Disconnect
                    throw;
                }
                catch (Exception)
                {
                    await new HttpResponse(500, "Internal server error").WriteTo(httpStream).WaitAsync(Timeout);
                    throw;
                }
            }
        }
        catch (Exception)
        {
            //Swallow exceptions to prevent the server from crashing.
            //When debugging, use a debugger to break on exceptions.
        }
        finally
        {
            client.Close();
            try
            {
                await IPCountsLock.WaitAsync();
                if (IPCounts[endpoint.Address] <= 1)
                {
                    IPCounts.Remove(endpoint.Address);
                }
                else
                {
                    IPCounts[endpoint.Address]--;
                }
            }
            finally
            {
                IPCountsLock.Release();
            }

            OnConnectionEnd?.Invoke(endpoint);
        }
    }
}