In PowerShell 5.0 Microsoft introduced a new property PSDSCRunAsCredential. This property allows you to run your DSC resource under specific credentials. If you do not specify anything the DSC agent or LocalConfigurationManager (LCM) performs any operation in the LocalSystem context. In most cases this is good enough and provides enough permission to fulfill its task. But there are scenarios where you need e.g. the script resource to configure certain tasks on the server which require domain credentials. This could be e.g. creating a Group Managed Service Account (gMSA) in your domain and adding it to your server. If you would try to run this script resource using LocalSystem you are not able to authenticate against Active Directory and the script fails. Because to create and install a gMSA you need to authenticate against Active Directory. If go a step further and let’s assume you want to use the same DSC configuration in two different domains Domain A and Domain B. This means you would need to run the script resource under two different domain credentials, otherwise you would not be able to run the script. This is when the PSDSCRunAsCredential property comes into play.

Here a simple example how you could use ONE configuration to run in TWO different environments. The following TestScript consumes two credentials $MastaCredential and $OtherCredential. If the computer which runs the DSC configuration is joined to the masta.ad domain the script resource should be executed under $MastaCredential, if it is another domain it should use $OtherCredential. To visualize which user is executing the script I used Write-Verbose $env:USERNAME which will expose the current user.

$MastaCredential = Get-Credential
$OtherCredential = Get-Credential
$ConfigData = @{
	AllNodes = @(
	@{
	NodeName  = "localhost"
	PSDscAllowPlainTextPassword = $true
	MastaUser = $MastaCredential
	OtherUser = $OtherCredential
	}
		)
	}

Configuration TestScript {
    Node $AllNodes.NodeName
        {
            Script ChangeUser
            {
	            SetScript = 
	            {         
		            Write-Verbose $env:USERNAME
	            }	
											
	            TestScript = 
	            {
                    $false
	            }
	            GetScript = 
	            {           
		            Write-Verbose $env:USERNAME		
	            }
PsDscRunAsCredential =  if ((Get-WmiObject Win32_ComputerSystem).Domain -ieq 'masta.ad' ) {$Node.MastaUser} else {$Node.OtherUser}
            }	
      }
}
TestScript -ConfigurationData $ConfigData -OutputPath c:\Temp      
Start-DscConfiguration -Wait -Force -Verbose c:\Temp

If I run the configuration on a computer in the masta.ad domain it will run under MastaUser…

image

If I simulate a different domain (change the if statement so it does not match the domain name) a different user is used in my case OtherUser…

image

Having just a simple if statement in combination with the PSDSCRunAsCredential property in place, let’s you use the script resource (or any other resource) in different environments where domain credentials are required.

I think it is a very cool approach!

Leave a Comment