ASP.NET detect hotlink image views

Question

Is there a way to detect hotlink image views in ASP.NET / IIS 7 ? I don't want to block the viewers, I just need to increment the image views counter for each static image when someone clicks on my images in google image search.

Answer 1

This is pretty easy. You can simply check the Referrer request header, and log the request if it doesn't match your local domain(s). Something like this should work:

using System;
using System.Linq;
using System.Web;

namespace ImageLogger
{
    public class ImageLoggerModule : IHttpModule
    {
        public void Init(HttpApplication context)
        {
            context.LogRequest += new EventHandler(context_LogRequest);
        }

        void context_LogRequest(object sender, EventArgs e)
        {
            var context = HttpContext.Current;

            // perhaps you have a better way to check if the file needs logging,
            // e.g.: it is a file in a certain folder
            switch (context.Request.Url.AbsolutePath.Split('.').Last().ToLower())
            {
                case "png":
                case "jpg":
                case "gif":
                case "bmp":
                    if (context.Request.UrlReferrer != null)
                    {
                        if (!"localhost".Equals(
                            context.Request.UrlReferrer.Host, 
                            StringComparison.CurrentCultureIgnoreCase)
                            )
                        {
                            // request is not from local domain --> log request
                        }
                    }
                    break;
            }
        }

        public void Dispose()
        {
        }
    }
}

In the web.config you link this module in the modules section:

<system.webServer>
    <modules>
        <add name="ImageLogger" type="ImageLogger.ImageLoggerModule"/>

This only works in Integrated Mode in IIS - in classic mode ASP.NET never gets any events for static files.

Now that I think about it; you could scrap the current logging altogether (in the page, I guess?), and just use this module, and get rid of the referrer logic. That way you only have one place where you do your logging.