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Here is a solution which accomplish that using reCaptcha:

First insert a div tag in the form and add the captcha interface using jQuery:

<script type="text/javascript" src="http://www.google.com/recaptcha/api/js/recaptcha_ajax.js"></script>
<script type="text/javascript">
 try {
  var sendBtn = document.postform18.send;
  if (sendBtn) {
   $(sendBtn).before('<div id="recaptcha_div"></div>');
 
   Recaptcha.create("6Le_KLsSAAAAAMi....", "recaptcha_div", {
    theme: "red"
   });
  }
 }
 catch (err) { }
</script>


Then create a notification subscriber that handles the post and verifies that the captcha is ok:

Imports Dynamicweb.Frontend
Imports Dynamicweb.Extensibility
Imports System.Net
Imports System.IO

<Subscribe(Notifications.Standard.Application.BeginRequest)> _
Public Class reCaptcha
 Inherits NotificationSubscriber

 Public Overrides Sub OnNotify(ByVal notification As String, ByVal args As NotificationArgs)
  'TODO: Add code here
  Dim BeginRequestArgs As Notifications.Standard.Application.BeginRequestArgs = DirectCast(args, Notifications.Standard.Application.BeginRequestArgs)
  Dim app As HttpApplication = CType(BeginRequestArgs.sender, HttpApplication)

  If app.Request.Path.ToLower = "/admin/public/formmail.aspx" AndAlso Base.ChkInteger(Base.Request("FormID")) = 18 Then
   Dim isValid As Boolean = GetCaptchaRespone()
   If Not isValid Then
    'Base.we(HttpContext.Current.Request.UrlReferrer.ToString())
    HttpContext.Current.Response.Redirect(HttpContext.Current.Request.UrlReferrer.ToString())
   End If
  End If
 End Sub

 Private Function GetCaptchaRespone() As Boolean
  Dim postData As String = "privatekey=6Le_KLsSAAAAANNhC1K-..."
  postData += "&remoteip=" & HttpContext.Current.Server.UrlEncode(HttpContext.Current.Request.UserHostAddress)
  postData += "&challenge=" & HttpContext.Current.Server.UrlEncode(Base.Request("recaptcha_challenge_field"))
  postData += "&response=" & HttpContext.Current.Server.UrlEncode(Base.Request("recaptcha_response_field"))
  Return GetResponse(postData).StartsWith("true", StringComparison.InvariantCultureIgnoreCase)
 End Function

 Private Function GetResponse(ByVal postData As String) As String

  Dim useURL As String = "http://www.google.com/recaptcha/api/verify"

  Dim objRequest As WebRequest = WebRequest.Create(useURL)
  objRequest.Timeout = 6000 'In milliseconds - in this case 6 seconds
  objRequest.Method = "POST"
  objRequest.ContentLength = postData.Length
  objRequest.ContentType = "application/x-www-form-urlencoded"

  'Create an instance of the StreamWriter class and attach the WebRequest object to it
  Dim postWriter As StreamWriter = New StreamWriter(objRequest.GetRequestStream())
  postWriter.Write(postData)
  postWriter.Close()

  'Create an instance of the WebResponse class and get the output to the rawOutput string
  Dim objResponse As WebResponse = objRequest.GetResponse()
  Dim sr As StreamReader = New StreamReader(objResponse.GetResponseStream())
  Dim rawOutput As String = sr.ReadToEnd()
  sr.Close()
  Return rawOutput
 End Function
End Class

Hi Nicolai

How would you "in best practise" implement spam protection on af DW formular, without using captcha?

Best regards
Casper
 

Fond this very nice solution, that we'll test:
http://stackoverflow.com/questions/2230453/spam-prevention-reduction-contact-form