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Real World HttpModule Examples

Back when I first discovered HttpHandlers I remember being ecstatic that the control that I thought I lost when moving from PHP to ASP.NET was finally returned, but when I first discovered HttpModules I remember almost passing out at the level of power and control you get over the ASP.NET request pipeline.  Since then, I've used HttpHandlers, HttpHandlerFactories, and HttpModules in many projects, but I'm noticing that while many people have heard of them, many have no idea what you would ever use them for.  So, I would like to give a few examples.

The first example is really simple.  On my blog, I didn't ant anyone to alias my web site or access it in any other way than by going to www.netfxharmonics.com.  Since HttpModules allow you to plug into the ASP.NET request pipeline, I was able to quick write an HttpModule to do exactly what I wanted:

public class FixDomainHttpModule : IHttpModule
{
    public void Dispose( ) {
    }

    public void Init(HttpApplication context) {
        context.BeginRequest += delegate(Object sender, EventArgs ea) {
            HttpApplication ha = sender as HttpApplication;
            String absoluteUrl = ha.Context.Request.Url.ToString( ).ToLower( );
            if (ha != null) {
                if (MinimaConfiguration.ForceSpecifiedDomain) {
                    if (!absoluteUrl.StartsWith(MinimaConfiguration.Domain.ToLower( ))) {
                        context.Response.Redirect(MinimaConfiguration.Domain);
                    }
                }
            }
        };
    }
}

...with this web.config:

<httpModules>
  <add name="FixDomainHttpModule" type="Minima.Web.HttpExtensions.FixDomainHttpModule" />
</httpModules>

By doing this I don't need to put a check in each page or in my MasterPage.  Since HttpModules are for the entire application, even URLs accessing my images are forced to be done by www.netfxharmonics.com.

Another example is a simple authentication system for a system I was working on a while back.  The application allowed anyone logged into active directory to access it's resources, but only certain people logged into active directory would be authorized to the use application (i.e. anyone could access images and CSS, but only a few people could use the system).  Knowing that the .NET framework is the model for all .NET development, I looked at the machine's web.config to see how ASP.NET implemented its windows and form authentication.  As it turns out, it does so by HttpModules.  So, I figured that the best way to solve this problem was by creating an HttpModule, not by throwing a hack into each of my WebForms or my MasterPages.  Furthermore, since ASP.NET uses the web.config for its configuration, including authentication configuration, I wanted to allow configuration of my authentication module to be via the web.config.  The general way I wanted to configure my HttpModule would be by a custom configuration section like this:

<Jampad>
    <Security RegistrationPage="~/Pages/Register.aspx" />
</Jampad>

The code for the HttpModule was extremely simple and required only a few minutes to throw together.  If the page being accessed is a WebForm and is not the RegistrationPage set in web.config, then the system's Person table is checked to see if the user logged into the machine has an account in the application.  If not, then there is a redirect to the RegistrationPage.  Simple.  Imagine how insane that would have been if you wanted to test for security on each page.

public class JampadSecurityModule : IHttpModule
{
    public void Dispose( ) {
    }

    public void Init(HttpApplication context) {
        context.BeginRequest += delegate(Object sender, EventArgs ea) {
            HttpApplication ha = sender as HttpApplication;

            if (ha != null) {
                CheckSecurity(context);
            }
        };
    }

    private void CheckSecurity(HttpApplication context) {
        SecurityConfigSection cs = (SecurityConfigSection)ConfigurationManager.GetSection("Jampad/Security");

        if (String.IsNullOrEmpty(cs.Security.RegistrationPage)) {
            throw new SecurityException("Security RegistrationPage is required.");
        }

        if (cs == null) {
            return;
        }

        if (!context.Request.Url.AbsoluteUri.Contains(cs.Security.RegistrationPage) &&
            context.Request.Url.AbsoluteUri.EndsWith(".aspx")
            ) {
            PersonCollection pc = new PersonCollection( );
            pc.GetMulti(new PredicateExpression(PersonFields.NTLogin==ActiveDirectoryFacade.NTUserName));

            if(pc.Count < 1){
                context.Response.Redirect(cs.Security.RegistrationPage);
            }
        }
    }
}

Again, plugging this into the web.config makes everything automatically happen:

<httpModules>
    <add name="JampadSecurityModule " type="Jampad.Security.JampadSecurityModule" />
</httpModules>

Recently I had to reuse my implementation in an environment that would not allow me to use LLBLGen Pro, so I had to rewrite the above 2 lines of LLBLGen Pro code into a series of Strongly-Typed DataSet tricks.  This implementation also had a LoginPage and an AccessDeniedPage in the custom configuration section, but other than that it was the same idea.  You could actually take the idea further by checking if the person is currently authenticated and if they aren't do a check on the Person table.  If they have access to the application, then PersonLastLoginTime column to the current time.  You could do many things with this implementation that would be rather crazy to do in a WebForm or MasterPage.

Another example of an HttpModule would be my custom access control system I built into my blog.  I'm not going to paste the code here as it's just the same idea as the other examples, but I will explain the concept.  Basically I created a series of tables in my blog's SQL Server database that held information on access control.  In the Access table I put columns such as AccessDirection (allow, deny), AccessType (IP Address, UserAgent, HTTP Referral), AccessText, AccessMessage, and AccessRedirect.  My HTTPModule would filter every ASP.NET request through this table to figure out what to do with it.  For example, I could block an IP address by creating a table record for 'deny', 'IP Address', '10.2.1.9', 'Your access has been denied.', NULL.  Immediately upon inserting the row, that IP address is blocked.  I could also block certain UserAgents (actually this was the original point of this HttpModule--bots these days have little respect for the robots.txt file).  I could also block requests that were from a different web site.  This would allow me to stop people from leaching images off my web site for use on their own.  With a simple HttpModule I was able to do all this in about an hour.  By the way, one record I was tempted to create was the following: 'deny', 'UserAgent', 'MSIE', NULL, 'http://www.getfirefox.com/'.  But I didn't :)

Now when would you use an HttpModule versus an HttpHandler?  Well, just think about what the difference is.  An HttpHandler handles a specific address pattern for a specific set of HTTP verbs, while every request in an application goes through an HttpModule.  So, if you wanted to have an image creator at /ImageCreator.imgx, then you need to register .imgx to IIS and then register your image creation HttpHandler in your web.config to handle that address (in case you forgot, web browsers care about the Content-Type, not the file extension.  In this example, your HttpHandler would set the Content-Type as 'image/png' or whatever your image type is.  That's how a web browser will know what to do with a file.  It has nothing to do with the file extensions; that's just for IIS.)  On the other hand, if you wanted to block all traffic from a specific web site, then you would create an HttpModule, becayse HttpModules handle all traffic on an application.  So, if you just remember this fundamental difference in purpose between the two, then shouldn't have my problems in the future.