CreateHttpClientWithReusePort.cs

// The workaround is utilizing ConnectCallback, which is a .NET 5+ feature.
// For more info see:
// https://devblogs.microsoft.com/dotnet/net-5-new-networking-improvements/#socketshttphandler-extension-points
public HttpClient CreateHttpClientWithReusePort()
{
    SocketsHttpHandler handler = new SocketsHttpHandler
    {
        ConnectCallback = ReusePortConnectAsync
    };
    return new HttpClient(handler);

    static async ValueTask<Stream> ReusePortConnectAsync(SocketsHttpConnectionContext context, CancellationToken cancellationToken)
    {
        // Default socket creation logic
        Socket socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
        socket.NoDelay = true;
        
        // Enable SO_REUSE_UNICASTPORT:
        socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseUnicastPort, 1);

        try
        {
            await socket.ConnectAsync(context.DnsEndPoint, cancellationToken).ConfigureAwait(false);
            return new NetworkStream(socket, ownsSocket: true);
        }
        catch
        {
            socket.Dispose();
            throw;
        }
    }
}