Oct 292009

Using the NetTcpBinding on a WCF service is secure by default. Unless you override the default settings, you will enjoy Transport Security using Windows authentication and the EncrpytAndSign protection level. When you create a new WCF service library, Visual Studio creates a config file with the following identity block:

   24           <identity>

   25             <dns value=”localhost“/>

   26           </identity>


If you wipe this config file clean like me to write a much cleaner and shorter config file, this identity block is the first thing to go. Sadly, most people also add a binding configuration with <security mode=”None”/>. I have done this too in an Intranet environment. The samples and book examples out there don’t show how to write an actual production environment service that cares for different machines in the same domain. While the default settings work when testing on your local machine, they don?t work in a simple Intranet environment.

Most of the difficulty I experienced when starting to work with WCF was getting security to work with the TCP binding. Everything worked so easily during development, but everything broke down once deployed to the development server. It didn’t help that the only errors I saw were timeout exceptions. If I had known about the Service Trace Viewer, I could have easily determine the cause and Googled (Bing wasn’t around then) for a solution. Instead, I chose the easier (and much less secure) way out; rely on my firewall and turn security off.

As mentioned before, the NetTcpBinding is secure by default with transport security using Windows authentication. The problem most experience when moving the service to a different machine is caused by NT authentication failing. If you use svcutil to generate your client config file and your host doesn’t have the identity block mentioned above, svcutil will not add a key piece of information to the client config file. The missing element is, you guessed it, the identity block. Without it, you will likely get an exception and see a stack trace similar to this:

[System.ServiceModel.Security.SecurityNegotiationException: A call to SSPI failed, see inner exception.]

[System.Security.Authentication.AuthenticationException: A call to SSPI failed, see inner exception.]

[System.ComponentModel.Win32Exception: The target principal name is incorrect.]

If you add tracing to your client, you will see that without specifying an identity block WCF will make the call with a DNS identity set to the name of the host. Notice the blue arrows.


You can see that the EndpointReference does not have an <Identity> block. Without that identity block, WCF cannot create a valid ServicePrincipalName. You can find this in Reflector, following this path:

  • System.ServiceModel.Channels.WindowsStreamSecurityUpgradeProvider+WindowsStreamSecurityUpgradeInitiator.OnInitiateUpgrade() – This is where the SecurityNegociationException is being thrown.
  • System.ServiceModel.Channels.WindowsStreamSecurityUpgradeProvider+WindowsStreamSecurityUpgradeInitiator.InitiateUpgradePrepare() – This method populates an EndpointIdentity and ServicePrincipalName to be used immediately after for NT authentication.


When the identity is not specified, it falls back to trying to create an SPN from the host address. I have seen this work on a machine that has two DNS names, using the DNS name that does not match the NETBIOS or AD name for the machine. I’m not exactly sure why that works.

Having any of the following identity blocks in your client config file will cause WCF to take the first path that successfully creates an SPN needed to perform NT authentication in the AuthenticateAsClient method called from OnInitiateUpgrade():

  • <dns value=”serviceHostName”/>
  • <dns/>
  • <servicePrincipalName value=”domainhostServiceUserAccount”/>
  • <servicePrincipalName/>

Having these <Identity> settings in your client config file adds the appropriate <Identity> settings in the <EndpointReference> used when opening the channel.


Security seems more mysterious when going rogue and writing your own config files. If you go rogue, make sure you use the appropriate <Identity> blocks. With this mystery solved, <security mode=?None?/> is a thing of the past. Now we can keep our services secure in an Intranet environment.

Oct 142009

Web services are just the tip of the iceberg in WCFI was privileged to attend the IDesign WCF Master Class last week. It only comes to the USA one time each year, and is presented by the one and only Juval Lowy. The class is held at the training center on the Microsoft Silicon Valley campus in Mountain View, CA. Five very intense days of WCF covering all aspects of WCF from essentials like the ABCs to the most intricate details about advanced topics like concurrency, security, transactions, and the service bus.

What we’ve been told sold about WCF from Microsoft is truly just the tip of the iceberg. Juval presents countless examples that prove WCF is not just about web services. WCF is the evolution of .NET, providing world-class features that no class should ever be without.

Demos, samples, and labs are presented using .NET 3.5 and 4.0 with an emphasis on the new features and functionality in 4.0. Discovery and announcements are the most underrated and unknown new features of WCF 4.0. After seeing Juval’s demos on discovery and announcement, I can’t imagine creating services without them.

More than all of the WCF content, the class gives you a lot to think about regarding architecture, the framework, and engineering principles. Juval’s mastery of .NET is evident in his ServiceModelEx library that extends almost all aspects of WCF and the service bus. His “one line of code” motto makes it possible for all of us to configure our WCF services with ease. The ServiceModelEx library is a good example for all developers to know and understand how to “do .NET” the right way. It exemplifies the best of what .NET and WCF have to offer.

Check out the IDesign website to get the WCF Resource CD (containing many of the examples and demos from the class). Also note the next class dates and sign up for the IDesign newsletter.