Automating Windows Series – Chef and DSC

Posted: December 4, 2015 in Uncategorized

This is part of a series of posts on automating Windows and deploying systems in the real world of enterprise operations. I’ve focused the series on the those nuances and problems that I’ve hit trying to deploy the various software stacks found in a typical Windows centric enterprise.

At some point we will hit the need to use DSC resources in Chef recipes. The reality is the Chef cookbooks for Windows only go so far and most Microsoft products outside of Windows itself have no coverage at all. All the DSC resources available are now open source just like Chef cookbooks and are expanding all the time:

I suspect we will see more and more DSC resources come out of Microsoft’s product teams as well.

Chef DSC Resources

There are a couple of key requirements for computers to leverage DSC in Chef. Windows Managmement Framework 5 (Feb Preview or better) is needed for DSC to be available and Chef Client 12.5 or better is required to provide the two Chef resources that allow DSC resources to be consumed in cookbooks.

UPDATE: Chef Client 12.6 has just been released which removes the limitations below (see the changelog for full details, strangely the inclusion of the timeout attribute isn’t in the changelog but was included in the master branch). There is a requirement for a recent version of WMF 5 to be installed as Microsft relaxed the LCM requirements.

There are two Chef resources for using DSC with chef; dsc_script and dsc_resource. These are effectively mutually exclusive as they require the DSC Local Configuration Manager’s RefreshMode to be configured to Push (dsc_script) or Disabled (dsc_resource).

Currently the main limitation of dsc_resource is that this resource has a hard timeout of 10 minutes, thus its not useful for DSC resources like xSQLServerSetup that can run for some time. I’ve logged this issue with Chef in GitHub. It is also a closer analogue of other Chef resources than dsc_script is which makes writing declarations easy if you have prior Chef experience.

dsc_script is more flexible in terms of being able to use more of the DSC functionality, like being able to pass configuration data or use powershell code within the DSC definition. Downside is that it converts the declaration into a MOF file so securing credentials requires that you use the DSC method for encrypting strings. This involves having a certificate that supports encryption loaded on the computer and defining that certificate thumbprint in your Chef recipe. This leads to a bit of double handling where you should use Chef Vault to secure the credentials in the recipe but then have to effectively unencrypt and re-encrypt the secured string for DSC to embed into a MOF file.

Personally I prefer the dsc_resource approach as it’s clean and simple to use in a Chef recipe and I don’t have to deal with the re-encryption of secure strings (and all the troubleshooting pain it brings).

Getting Modules Installed

There are a few ways you can achieve this, you could bundle up the modules as zip files and distribute them within the a Chef cookbook or you can use the new package manager in WMF5..

The easy way – WMF5 Package Manager

By using the Chef resource “powershell_script” you can invoke the install-package cmdlet and use the get-package cmdlet as a guard. Note the use of the -force switch which will force the install even if the package repository is trusted. This isn’t good practice and a better way to do it is to  pre-define the trusted repositories and not use the -force switch.

powershell_script "install dsc module" do
  code 'install-package -name "xResourceModule" -force
  not_if 'if(get-package -name "xResourceModule"){$true}else{$false}

end

Some good references for Powershell’s package manager are below:

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