Advanced Certificate Services Configuration with DSC

Recently I’ve been rebuilding my Hyper-V lab environment from scratch (as part of my MCSA/MCSE studying) and decided I would completely script the process using PowerShell only. My goal was to not require a single interactive session with any of the servers to setup the entire environment. This was a multi-site AD environment with several other member servers performing other duties including NPS & NAP, ADCS, WDS, WSUS, ADFS, ADRMS, IIS, DirectAccess, SQL etc. I also wanted this to include a proper Multi-tier PKI environment with both a standalone Root CA and an Enterprise Subordinate CA (as is recommended by Microsoft).

The Problem

To configure a AD CS using DSC is very straight forward using the xADCSDeployment DSC Resource. However, anyone who has installed an enterprise PKI is probably familiar with the fact that that AIA and CDP settings need to be configured for the CA’s so that any generated certificates can include these extensions. Unfortunately the xADCSDeployment DSC Resource doesn’t support setting these Certificate Services options because the underlying AD CS PowerShell cmdlets don’t allow setting these options either.

The Solution

The solution to this problem is to use the DSC Script Resource to call the CertUtil.exe application to set these values. To do this I set the SetScript resource parameter like this:

			SetScript = {
				If ($Using:Node.CADistinguishedNameSuffix) {
					& "$($ENV:SystemRoot)\system32\certutil.exe" -setreg CA\DSConfigDN "CN=Configuration,$($Using:Node.CADistinguishedNameSuffix)"
					& "$($ENV:SystemRoot)\system32\certutil.exe" -setreg CA\DSDomainDN "$($Using:Node.CADistinguishedNameSuffix)"
				}
				If ($Using:Node.CRLPublicationURLs) {
					& "$($ENV:SystemRoot)\System32\certutil.exe" -setreg CA\CRLPublicationURLs $($Using:Node.CRLPublicationURLs)
				}
				If ($Using:Node.CACertPublicationURLs) {
					& "$($ENV:SystemRoot)\System32\certutil.exe" -setreg CA\CACertPublicationURLs $($Using:Node.CACertPublicationURLs)
				}
				Restart-Service -Name CertSvc
				Add-Content -Path 'c:\windows\setup\scripts\certutil.log' -Value "Certificate Service Restarted ..."
			}

The above code expects the $Node object to contain several properties that it will use to set applicable settings in the CA server:

  • CADistinguishedNameSuffix – This is the Directory Services Distinguished Name (e.g. DC=bmdlab,DC=com). If left blank then your CDP and AIA LDAP Addresses will be incorrect.
  • CRLPublicationURLs – The CRL Publication URLs in the same format as you would normally pass to the certutil.exe application.
  • CACertPublicationURLs – The CA Cert Publication URLs (AIA Extension) in the same format as you would normally pass to the certutil.exe application.

Note: To access the $Node object in the scripts resource you’ll need to use Using keyword, otherwise the $Node object won’t be available in the external script scope.

And finally, after these values have been set the CertSvc needs to be restarted.

What about TestScript?

You might look at the above code and wonder, “won’t the CertSvc be restarted every 30 minutes no matter what?”. That is where the TestScript resource parameter comes in. We’ll use this to decide if the current values of the CA are different to what we want them to be. In this case we dive straight to the registry rather than using the CertUtil.exe application.

			TestScript = { 
				If (((Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('DSConfigDN') -ne "CN=Configuration,$($Using:Node.CADistinguishedNameSuffix)")) {
					Return $False
				}
				If (((Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('DSDomainDN') -ne "$($Using:Node.CADistinguishedNameSuffix)")) {
					Return $False
				}
				If (($Using:Node.CRLPublicationURLs) -and ((Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('CRLPublicationURLs') -ne $Using:Node.CRLPublicationURLs)) {
					Return $False
				}
				If (($Using:Node.CACertPublicationURLs) -and ((Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('CACertPublicationURLs') -ne $Using:Node.CACertPublicationURLs)) {
					Return $False
				}
				Return $True
			}

Once again I need to ensure the Using scope is used with the Node variable. If any of the node properties don’t match the registry value then false is returned which triggers SetScript. If all of the values match True is returned (meaning everything is the same) and SetScript isn’t fired.

Is That It?

Almost. Finally I needed to implement the GetScript parameter of the resource. This was the easiest part:

			GetScript = {
				Return @{
					'DSConfigDN' = (Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('DSConfigDN');
					'DSDomainDN' = (Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('DSDomainDN');
					'CRLPublicationURLs'  = (Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('CRLPublicationURLs');
					'CACertPublicationURLs'  = (Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('CACertPublicationURLs')
				}
			}

I could also adjust the TestScript to call the GetScript and then use the returned hash table to compare with the Node values instead of comparing them directly with the registry values. But I didn’t.

Here is what the final script looks like (I didn’t include everything in the DSC Configuration as there were lots of other resources for creating the machine):

		Script ADCSAdvConfig
		{
			SetScript = {
				If ($Using:Node.CADistinguishedNameSuffix) {
					& "$($ENV:SystemRoot)\system32\certutil.exe" -setreg CA\DSConfigDN "CN=Configuration,$($Using:Node.CADistinguishedNameSuffix)"
					& "$($ENV:SystemRoot)\system32\certutil.exe" -setreg CA\DSDomainDN "$($Using:Node.CADistinguishedNameSuffix)"
				}
				If ($Using:Node.CRLPublicationURLs) {
					& "$($ENV:SystemRoot)\System32\certutil.exe" -setreg CA\CRLPublicationURLs $($Using:Node.CRLPublicationURLs)
				}
				If ($Using:Node.CACertPublicationURLs) {
					& "$($ENV:SystemRoot)\System32\certutil.exe" -setreg CA\CACertPublicationURLs $($Using:Node.CACertPublicationURLs)
				}
				Restart-Service -Name CertSvc
				Add-Content -Path 'c:\windows\setup\scripts\certutil.log' -Value "Certificate Service Restarted ..."
			}
			GetScript = {
				Return @{
					'DSConfigDN' = (Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('DSConfigDN');
					'DSDomainDN' = (Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('DSDomainDN');
					'CRLPublicationURLs'  = (Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('CRLPublicationURLs');
					'CACertPublicationURLs'  = (Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('CACertPublicationURLs')
				}
			}
			TestScript = { 
				If (((Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('DSConfigDN') -ne "CN=Configuration,$($Using:Node.CADistinguishedNameSuffix)")) {
					Return $False
				}
				If (((Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('DSDomainDN') -ne "$($Using:Node.CADistinguishedNameSuffix)")) {
					Return $False
				}
				If (($Using:Node.CRLPublicationURLs) -and ((Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('CRLPublicationURLs') -ne $Using:Node.CRLPublicationURLs)) {
					Return $False
				}
				If (($Using:Node.CACertPublicationURLs) -and ((Get-ChildItem 'HKLM:\System\CurrentControlSet\Services\CertSvc\Configuration').GetValue('CACertPublicationURLs') -ne $Using:Node.CACertPublicationURLs)) {
					Return $False
				}
				Return $True
			}
			DependsOn = '[xADCSWebEnrollment]ConfigWebEnrollment'
		}

Hopefully someone will make sense of all this. It should also be useful in other similar situations where there are no relevant DSC resources.

One thought on “Advanced Certificate Services Configuration with DSC

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