CAS Proxying with ASP.Net Forms Authentication

Integrating CAS Proxying with Forms Authentication in ASP.Net 2.0

To set up CAS authentication in ASP.Net is a relatively simple process if you don't implement Forms Authentication and do NOT require your application to act as a CAS proxy.
However, when you do want to make use of ASP.Net's Forms Authentication AND CAS-enable your application for proxying, the process is a little more involved.
The good news is that it still does not require much coding at all. It just requires a little knowledge about how CAS works and how you can get ASP.Net Forms Authentication to synchronize with it.

The .Net assembly (DLL) and source code needed for this article can be downloaded here. Just drop it in your project's bin folder.

For an example of how to do CAS proxying with classic ASP, see me other article CAS Proxying with Classic ASP

First drop the CASAuthentication.dll (attached) into your project's bin folder. Then, In Web.config, make the following changes:

Turn on Forms Authentication:

<!--Authentication mode configuration -->
<authentication mode="Forms">
    <forms name="casAuth" defaultUrl="Default.aspx" loginUrl="Login.aspx" />
</authentication>

Deny all unauthenticated users

<!-- Authorization configuration -->
<authorization>
  <deny users="?"/>
</authorization>

Include the CAS.Web.Security namespace in your pages section.

<!-- Pages configuration, Globally Import the CAS.Web.Security namespace so it can be used throughout your CAS application -->
<pages>
  <namespaces>
    <add namespace="CAS.Web.Security"/>
  </namespaces>
</pages>

Add the CAS host url to the appSettings section. You must name the key: CASURL

<!-- Application settings configuration -->
  <appSettings>
    <!---change CAS Url accordingly-->
    <add key="CASURL" value="https://auth.berkeley.edu/cas"/>
  </appSettings>

Add an httpModules section with the following module. It must be inside the system.web section of your web.config file.

<httpModules>
  <add name="CASAuthenticationV2" type="CAS.Web.Security.CASAuthenticationV2, CASAuthentication"/>
</httpModules >

(Optional) If you want all CASAuthentication class related errors routed to your own error page, simply add a customErrors page section to your system.web section. Mode must be set to On for the errors to be re-directed. If mode is Off or the customErrors section is not present in web.config, all errors will be written out to the current application page.
```
<customErrors mode="On" defaultRedirect="MyErrorPage.aspx">
  <error statusCode="403" redirect="NoAccess.htm"/>
  <error statusCode="404" redirect="FileNotFound.htm"/>
</ customErrors>
```
In this example, the page MyErrorPage.aspx can access the last error reported by calling the CASAuthentication.LastError property of the CASAuthentication class.

VB.Net Code Examples

The CAS proxier - (Your main default page)

Default.aspx

<% @ Page Language="VB"

%>

    Protected Sub btnRunTest_Click(ByVal sender As Object,ByVal e As System.EventArgs)
            'First, let's add some proxy arguments to send to the CAS proxy
            CASAuthentication.AddProxyArgument("arg1", "hello")
            CASAuthentication.AddProxyArgument("arg2", "world")

            If Not CASAuthentication.InvokeCASProxy(ProxyAppUrl:="{url_to_your_CAS_Proxy_Application}", _
                pgtUrl:="{secure_url_to_your_CAS_Callback_Url_Application}", _
                HttpMethodPost:= False) Then
                Response.Write(CASAuthentication.LastError)
                Return
            EndIf
    End Sub
</script>

<html>
<head>
    <title>Test calling a CAS Proxy</title>
</head>
<body>
    <p><a href="LogOut.aspx">Log out of CAS</a></p>
    <p><asp:Button ID="btnRunTest"runat="server"OnClick="btnRunTest_Click"Text="Call Test Proxy" /></p>
</body>
</html>

The CAS callback Url (This would be the same application page specified in the pgtUrl attribute in the call to InvokeCASProxy method shown above) - This assumes that your main application (the CAS proxier) and callback Url are part of the same application so that they can share application specific variables. If the callback Url is NOT part of the same application, then you must handle storing/retrieving the pgtIou/pgtId pair yourself. (e.g. store them in an external database).

IMPORTANT NOTE

Because your callback Url will be called by CAS directly, you must turn off Forms Authentication for this page. This is easily achieved by adding the following to your web.config file root configuration section:

<location path="ProxyCallback.aspx">
  <system.web>
    <authorization>
      <allow users="*"/>
    </authorization>
  </system.web>
</location>

ProxyCallback.aspx

<% @ Page Language="VB"

%>

            'We have a pgtIou/pgtId pair sent from CAS server
            'Now call the AssignPgtIDForCallingProxy method of the CASAuthentication class
            'this will store the pgtId in an application variable with it's name the value of the pgtIou
           CASAuthentication.AssignPgtIDForCallingProxy(pgtIou, pgtId)
        Else
            Response.Write("No pgtIou/pgtId pair!")
        End If

End Sub
</script>

<html>
<head>
<title>Proxy Callback Url Page</title>
</head>
<body>
</body>
</html>

The CAS Proxy (This would be the same application specified in the ProxyAppUrl attribute in the call to InvokeCASProxy method shown above)

CASProxy.aspx

<% @ Page Language="VB"

%>

        If CASAuthentication.IsAuthenticated Then
            Response.Write("Welcome " & CASAuthentication.CalNetID & ", you have been successfully authenticated with CAS!")
            Response.Write("<BR>")

Dim proxyArgs As StringBuilder = New StringBuilder
Dim proxyArgKey As String= String.Empty

'
'Since this is the proxy application, we will check it's arguments it was sent
'Normally we would know if the arguments were sent in the query string or Form post
'but for the purposes of this example, we will check both
'

            If Request.QueryString.Count > 0 Then
                'get arguments from querystring object
                For Each proxyArgKey In Request.QueryString
                    proxyArgs.AppendFormat("{0}={1} (query string)<br>", proxyArgKey, Request.QueryString.Get(proxyArgKey))
                Next
            Else
                'get arguments from form object
                For Each proxyArgKey In Request.Form
                    proxyArgs.AppendFormat("{0}={1} (form post)<br>", proxyArgKey, Request.Form.Get(proxyArgKey))
                Next
            EndIf

Response.Write(proxyArgs.ToString)
Response.Write("<BR>")

            If Not String.IsNullOrEmpty(CASAuthentication.Proxies) Then
               Response.Write(CASAuthentication.Proxies)
               Response.Write("<BR>")
          End If
        End If

End Sub
</script>

<html>
<head>
<title>Test CAS Proxied Application</title>
</head>
<body>
<a href="LogOut.aspx">Log out of CAS</a>
</body>
</html>