Can .NET intercept and change css files?

Question

UPDATE 1:

I have now setup IIS6 so .NET can handle calls to .css files. What do I have to do now to get it to change css files based on the referal

Answer 1

I do something somewhat similar for files served up by our content management system. If a http handler is turned on, it inspects the filename and path to see if the user has access to the resource. If the user does, it streams the file, otherwise it returns a 401 not authorized.

I don't see why you couldn't use a handler to jump into the pipeline for the css file, check the host name, and stream out the other css file instead (if applicable). This is straightforward in IIS7 with an integrated pipeline (you didn't specify), but is also possible in IIS6 if you let a css extension be processed by .net.

Let me know if you're interested in this approach and I'll track down some code.

Edit - Here's some code

This is not exactly what you're looking for, but you may be able to get some ideas.

NOTE: This is in IIS7 with an integrated pipeline, so in IIS6 you'll have to make a change so that .css files are handled by the .net process.

Public Class FileManagerFileAuthorization
    Implements IHttpHandler

    Public ReadOnly Property IsReusable As Boolean Implements System.Web.IHttpHandler.IsReusable
        Get
            Return True
        End Get
    End Property

    Public Sub ProcessRequest(ByVal context As System.Web.HttpContext) Implements System.Web.IHttpHandler.ProcessRequest

        Dim req As HttpRequest = context.Request
        Dim absolutePath As String = req.Path
        Dim fileName As String = Path.GetFileName(absolutePath)
        Dim physicalPathAndFileName As String = HttpContext.Current.Server.MapPath(absolutePath)

        If File.Exists(physicalPathAndFileName) Then

            ' roles that the user is part of. If the user is not authenticated they are part of the public role only
            Dim memberRoles As String()
            If req.IsAuthenticated = False Then
                memberRoles = New String() {ConfigurationManager.AppSettings("PublicRole")}
            Else
                Dim r As New Roles()
                memberRoles = r.GetRolesForUser("")
            End If

            ' check permissions: transmit file or deliver 401
            Dim folderVirtualPath As String = Path.GetDirectoryName(absolutePath).Replace("\"c, "/"c)
            Dim permissions As FileManager.FolderPermissions = FileManager.GetFolderPermissions(folderVirtualPath, memberRoles)
            If permissions And FileManager.FolderPermissions.View Then
                context.Response.ContentType = FileManager.GetContentType(fileName)
                context.Response.AddHeader("Content-Length", New FileInfo(physicalPathAndFileName).Length.ToString())
                context.Response.TransmitFile(physicalPathAndFileName)
            Else
                ' unauthorized
                context.Response.StatusCode = 401
            End If

        Else
            ' file not found
            context.Response.StatusCode = 404
        End If

        context.Response.End()

    End Sub

End Class

And the web.config - and again - this is IIS7 so you'll be using the <httpHandlers/> section under system.web section. I'm looking for any file inside the Userfiles directory, but I think you could point right to a file with this.

<system.webServer>
  <handlers>
    <add name="FileManagerFileAuthorization" path="Userfiles*" verb="GET" type="FileManagerFileAuthorization" resourceType="Unspecified" preCondition="integratedMode" />
  </handlers>
</system.webServer>

Note:

To allow .net to handle non-.net files in IIS, you must allow the .net process to handle the processing of these files. To do this, open IIS manager, navigate to the website, and click properties. Go to the 'home directory' tab and click 'configuration.' Add a wildcard mapping, and choose the .net dll. If you're unsure, copy the link from .ascx below.

Since it's IIS6, you won't be able to use the system.webServer section above, you'll need to add http handlers the old way. This link explains it: http://msdn.microsoft.com/en-us/library/46c5ddfy.aspx

Answer 2

As I stated in my comment re: one of the answers, I think the http_referer header will point to the content page that links/imports the requested css file, not the search results page that linked to the content page. So even if search results are on http://intranetv2, Request.ServerVariables["HTTP_REFERER'] during the css file request will be http://intranet and you'll continue to get the old css file.

Seems like you'll have to figure out a way to either serve up the 10000 content pages from the new http://intranetv2 domain, or come up with a way to set a flag during the content aspx page request (maybe in global.asax Application_BeginRequest) that can be read and acted upon during the css file request (by an HttpHandler, like others have suggested).

Not sure what the appropriate signaling mechanism would be though. It needs to work per-user, not per-application as well as be available during and persist across multiple disparate file requests, and the choices are constrained by the use of .NET 1.1.

Answer 3

Yes, you should be able to use an HttpModule on your "old" application to intercept the calls to the old CSS. Based on the version of IIS you have on your server, you may need to do some configuration to make sure .NET is handling calls to .css files, otherwise your HttpModule would not be called. See this question for reference.

Once .NET is handling calls to CSS, you can dynamically switch the css dynamically in case the request is for the "old" css file.

Here's an old article (since you are on .NET 1.1) that should point you in the right direction for the implementation and the configuration of IIS: "URL Rewriting in ASP.NET". Basically what you are doing is very similar, since you are "rewriting" a specific URL (the one to your old CSS file) to point to different content.

Answer 4

Based on the domain name, you can dynamically control the part of the page from the code behind (.vb or .cs files or even a class). This will give you control to replace the css file based on the domian name. Capture the domain name in the code and then replace the css file/link, infact the whole part in the code behind. you can do this in C# or in VB.

Answer 5

1. In IIS, Setup the HttpHandler to receive all the file types you want (says you have done this)
2. user Server.MapPath() on HttpRequest.Url.AbsolutePath to get the physical path
3. Modify the path according to the domain
4. Write the file to the response stream.

Here is a handler (simplified) that I use routinely to server alternate files for different domains:

using System;
using System.IO;
using System.Web;
public class MultiDomainFileHandler : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        string filePath = GetDomainSpecificFilePath(context.Request.Url.Host,
            context.Server.MapPath(context.Request.Url.AbsolutePath));

        if (File.Exists(filePath))
        {
            switch (Path.GetExtension(filePath).ToLower())
            {
                case ".css": context.Response.ContentType = "text/css"; break;
                case ".jpg":
                case ".jpeg": context.Response.ContentType = "image/jpeg"; break;
                //other types you want to handle
                default: context.Request.ContentType = "application/octet-stream"; break;
            }
            context.Response.WriteFile(filePath); //Write the file to response
        }
        else context.Response.StatusCode = 404;
    }

    private string GetDomainSpecificFilePath(string domain, string originalPath)
    {
        string prefix = "";
        switch (domain.ToLower())
        {
            case "intranetv2": prefix = FILE_PREFIX_INTRANETV2; break;
            case "www.example.com": prefix = FILE_PREFIX_EXAMPLE_DOT_COM; break;
            //other domains you want to handle
        }
        string dir = Path.GetDirectoryName(originalPath);
        string fileName = prefix + Path.GetFileName(originalPath);
        return Path.Combine(dir, fileName);
    }

    const string FILE_PREFIX_INTRANETV2 = "v2.", FILE_PREFIX_EXAMPLE_DOT_COM = "ex.com.";
    public bool IsReusable { get { return false; } }
}

Now, you simple need to have alternate files in the same directories. E.g:

/Images/logo.jpg

/Images/v2.logo.jpg

/Styles/mystyle.css

/Styles/v2.mystyle.css

I hope this helps :)

Answer 6

I wouldn't recommend using httpModule as it will be called upon for each and every request which might deteriorate performance. Whereas, you can use httpHandlers to handle only specific paths. Therefore, my vote would be to use httpHandlers.

But there is a glitch. By default, IIS 6 does not pass requests for non ASP.Net extension (read extensions other than .aspx, .ashx, .axd and all that) to ASP.Net by default.

Therefore, you need to add ISAPI module for CSS extension to pass the request to aspnet_isapi.dll (you can find the complete path from .aspx extension handler).

This link might help in setting up ISAPI module.

Once ASP.Net has started handling .CSS extension, write a httpHandler with your logic and add the following line under httphandlers section in web.config file

Say your httpHandler is CSSHttpHandler then the code would be something like this.

<add verb="HEAD,GET" path="*.css" type="CSSHttpHandler">

Hope this helps.