Silverlight and Integrated Authentication

I’ve been meaning to write about this for a while, because it’s a reoccurring nuisance: Using integrated authentication with Silverlight. More to the point, the nuisance is the differences between Cassini (Visual Studio Web Development Server) and IIS in combination with some WCF configuration pitfalls for Silverlight enabled WCF services….

Note: Apart from driving me crazy, I’ve been stumbling over this issue quite a few times in the Silverlight forums. Thus I’m going through this in detail, explaining one or the other seemingly obvious point…

Many ASP.NET LOB applications run on the intranet with Windows integrated authentication (see also here). This way the user is instantly available from HttpContext.User, e.g. for display, and can be subjected to application security via a RoleProvider. Silverlight on the other hand runs on the client. I have written about making the user and his roles available on the client before. However, the more important part is to have this information available in the WCF services serving the data and initiating server side processing. And being WCF, they work a little different from ASP.NET, or not, or only sometimes….

 

Starting with Cassini…

Let’s assume we are developing a Silverlight application, using the defaults, i.e. Cassini, and the templates Visual Studio offers for new items. When a “Silverlight-enabled WCF service” is created, it uses the following settings:

Now there’s (already) a choice to make: Use ASP.NET compatibility? Or stay WCF only? (That question may be worth a separate post…). With ASP.NET compatibility, HttpContext.* is available within the service, including HttpContext.User. The WCF pendant of the user is OperationContext.Current.ServiceSecurityContext.PrimaryIdentity. Take the following sample implementation to see which information is available during a respective call:

The client code to test that service is simple as can be:

The XAML is boilerplate enough, but for the sake of completeness:

I choose compatibility mode, and as the client shows, HttpContext.User is available out of the box:

Great, just what an ASP.NET developer is used to. But compatibility or not, it also shows that the WCF user is not available. But! WCF is configurable, and all we have to do is set the correct configuration. In this case we have to choose Ntlm as authentication scheme:

And look what we’ve got:

Great, no problem at all. Now we have the baseline and are ready to move on to IIS.

 

Now for IIS…

Moving to IIS on the developer machine is simple. Just go to the web project settings, tab “Web”, choose “Use Local IIS Web Server”, optionally creating the respective virtual directory from here. In order to work against IIS, Visual Studio needs to run with administrative permissions.

 

Moving from Cassini to IIS however has a slew of additional pitfalls:

• The service URL
• The IIS configuration for authentication
• WCF service activation issues
• The WCF authentication scheme
• localhost vs. machine name

Usually they show up as team (which obviously doesn’t help), but let’s look at them one by one.

 

The service URL

There’s one difference between how Cassini and IIS are being addressed by the web project: Projects usually run in Cassini in the root (i.e. localhost:12345/default.aspx), while in IIS they run in a virtual directory (e.g. localhost/MyApplication/default.aspx). This may affect you whenever you are dealing with absolute and relative URLs. It will at least cause the generated service URLs to differ more than just by the port information. Of course you can recreate the service references at that point, but you don’t want to do that every time you switch between Cassini and IIS, do you?

BTW: There’s a similar issue if you are running against IIS, using localhost, and you create a service reference: This may write the machine name into the ServiceReferences.ClientConfig (depending on the proxy configuration), e.g. mymachine.networkname.com/application, rather than localhost. While these are semantically the same URLs, for Silverlight is qualifies as a cross domain call. Consequently it will look for a clientaccesspolicy.xml file which is probably not there and react with a respective security exception.

The solution with Silverlight 3 is to dynamically adjust the endpoint of the client proxy in your code to point to the service within the web, the Silverlight application was started from:

Silverlight 4 supports relative URLs out of the box:

Coming versions of the tooling will probably generate relative URLs in the first place; until then you’ll have to remember to adjust them every time you add or update a service reference.

 

The IIS configuration for authentication

This one may be obvious, but in combination with others, it may still bite you. Initially starting the application will result in the notorious NotFound exception:

Note: To be able to handle server exceptions in Silverlight you’ll have to overcome some IE plugin limitations, inhibiting access to http response 500. This can be achieved via a behavior, as described on MSDN. However this addresses exceptions thrown by the service implementation and won’t help in the infrastructure related errors I’m talking about here.

The eventlog actually contains the necessary information:

No windows authentication? Well, while Cassini automatically runs in the context of the current user, IIS needs explicitly being told that Windows Authentication is required. This is simple in IIS configuration, just enable Windows Authentication and disable Anonymous Authentication in the IIS configuration for the respective virtual directory.

 

WCF service activation issues

Running again will apparently not have changed anything at all, displaying the same error. With just a seemingly minor difference in the eventlog entry:

That’s right. The service demands to run anonymous after it just demanded to run authenticated. Call it schizophrenic.

To make a long story short, our service has two endpoints with conflicting demands: The regular service endpoint requires Windows Authentication, while the “mex” endpoint for service meta information requires Anonymous access. OK, we might re-enable anonymous access, but that wasn’t indented, so the way to work around this activation issue is to keep anonymous access disabled and to remove the “mex” endpoint from the web.config:

Curiously generating the service reference still works in Visual Studio (perhaps with login dialogs, but still)…

 

The WCF authentication scheme

We’re still not there. The next issue when running the application might be a login credentials dialog when the service is called. And no matter what you type in, it still won’t work anyway, again with the NotFound exception. Unfortunately this time without an eventlog entry.

Again, to make it short, IIS doesn’t support Ntlm as authentication scheme, we need to switch to Negotiate… .

And now it works:

Do I have to say that this configuration doesn’t run in Cassini? Right, every time you switch between IIS and Cassini you have to remember to adjust the configuration. There is another enumeration value for the authentication scheme, named IntegratedWindowsAuthentication, which would be nice – if it worked. Unfortunately those two values, Ntlm and Negotiate, are the only ones that work, under Cassini and IIS respectively.

 

localhost vs. machine name

Now it works, we get the user information as needed. For a complete picture however, we need to look at the difference between addressing the local web server via localhost or via the machine name: Calls against localhost are optimized by the operating system to bypass some of the network protocol stack and work directly against the kernel mode driver (HTTP.SYS). This affects caching as well as http sniffers like Fiddler, which both work only via the machine name.

Note: This may actually be the very reason to switch to IIS early during development, when you need Fiddler as debugger (to check the actually exchanged information). Otherwise it’s later on, when you need it as profiler (to measure network utilization). Of course you’ll want http caching enabled and working by that time.

Of course you can put the machine name in the project settings, yet this would affect all team members. Perhaps a better idea is to have the page redirect dynamically:

Another word on caching: In IIS6 caching has to be explicitly set for the .xap file, using 1 day as default cache time and allowing 1 minute at the minimum. During development this may be an issue. With IIS7 caching should be automatically set to CacheUntilChange and you may also set the cache time with a resolution in seconds.

 

Where are we?

OK, that was quite a list of pitfalls and differences between Cassini and IIS, even IIS6 and IIS7. Some of this may go away with the new IIS Express. Some will stay and remain a nuisance. Visual Studio initially guides you towards using Cassini. At one time however you’ll have to switch to IIS. And since you cannot have both at the same time, this may be an issue especially in development teams. My recommendation would be: Start with IIS right away or plan the switch as a concerted action within your team.

That’s all for now folks,
AJ.NET

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