连续十次请求后的HttpWebRequest超时

Question

我正在为特定网站编写网络抓取工具.该应用程序是一个不使用多个线程的VB.Net Windows窗体应用程序- 每个Web请求都是连续的.但是,在十次成功的页面检索之后,每个连续的请求都会超时.

我已经回顾了已经在SO上发布的类似问题,并在我的GetPage例程中实现了推荐的技术,如下所示:

Public Function GetPage(ByVal url As String) As String
    Dim result As String = String.Empty

    Dim uri As New Uri(url)
    Dim sp As ServicePoint = ServicePointManager.FindServicePoint(uri)
    sp.ConnectionLimit = 100

    Dim request As HttpWebRequest = WebRequest.Create(uri)
    request.KeepAlive = False
    request.Timeout = 15000

    Try
        Using response As HttpWebResponse = DirectCast(request.GetResponse, HttpWebResponse)
            Using dataStream As Stream = response.GetResponseStream()
                Using reader As New StreamReader(dataStream)
                    If response.StatusCode <> HttpStatusCode.OK Then
                        Throw New Exception("Got response status code: " + response.StatusCode)
                    End If
                    result = reader.ReadToEnd()
                End Using
            End Using
            response.Close()
        End Using

    Catch ex As Exception
        Dim msg As String = "Error reading page """ & url & """. " & ex.Message
        Logger.LogMessage(msg, LogOutputLevel.Diagnostics)
    End Try

    Return result

End Function

我错过了什么吗？我没有关闭或处理应该是的对象吗？看起来奇怪的是它总是在连续十次请求之后发生.

笔记:

1. 在此方法所在的类的构造函数中,我有以下内容:

   ServicePointManager.DefaultConnectionLimit = 100

2. 如果我将KeepAlive设置为true,则在五个请求之后开始超时.

3. 所有请求都针对同一域中的页面.

编辑

我在每个Web请求之间添加了两到七秒之间的延迟,这样我就不会"锤击"该站点或尝试DOS攻击.但是,问题仍然存在.

Answer 1

我认为该网站有某种 DOS 保护，当它受到大量 rapis 请求攻击时，该保护就会启动。您可能想尝试在网络请求上设置 UserAgent。