Using Lets Encrypt certificates with Azure Stack

Anyone who has deployed an Azure Stack integrated system will know that one of the crucial items to get right from the outset are the PKI certificates that will be used for external services, such as the portal, blob storage, ARM API’s. For production environments, Microsoft recommend having separate certificates for each of the endpoints, some of which require wildcard SSL certs. These tend to be more expensive if purchasing from a third party.

Of course, you could use an Enterprise CA based within your environment, but I’ve seen too many issues where intermediate CA’s are used to sign the SSL certificates, due to the public key not existing in the Microsoft or various Linux distro’s Trusted CA publishers store.

Side note: When A VM is provisioned on Azure Stack, only the root CA is imported by the WA agent. The intermediate CA public cert needs to be manually imported.

With all that in mind, I was lucky enough to work with a client who had the idea to cut costs, but to use a CA that is trusted universally; that being Lets Encrypt. As the strapline states on their website, ‘Lets Encrypt is a free, automated and open Certificate Authority’ .

Free? I like how much that costs, there must be a catch? Well, yes and no. The certs have a 90 day lifespan, so you’ll have to ensure you rotate the certificates before they expire, but this can be automated, so not a big deal. The documentation for expiration alerts say that you’ll be alerted when they’re within 30 days, but I’ve found you get warnings within 90 days, so be aware that you don’t just ignore them and they do eventually expire - make a note in your calendar!

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So what do we need for this to implement this solution?

Here are the pre-reqs:

• An Azure subscription

• An Azure DNS Zone for your domain

• A Service Principal within your Azure AD tenant

• Azure PowerShell modules. If using the AZ modules, the Enable-AzureRmAlias should be set. N.B. This does not work with the Azure Stack PowerShell modules, as the AzureRM.Dns modules currently included do not support the creation of CAA records - we need this capability!

• Azure Stack Readiness checker PowerShell Module.

• Posh-ACME PowerShell module https://github.com/rmbolger/Posh-ACME

• Oh, and my Azure Stack Lets Encrypt PoSh module :)

For anyone wondering how to setup Azure DNS zones for use with Azure Stack, please see my blog post here.

Assuming you’ve installed / imported / configured / downloaded all of the pre-reqs, extract the contents of the zip file downloaded for the Azure Stack Lets Encrypt Module and then:

1. Run the new-DNSTxtRole.ps1 script that is included within the Azure Stack Lets Encrypt zip file. It will prompt you for your credentials to connect to your Azure Subscription and then create a new called ‘DNS Zone Contributor’ within your subscription that you can use to assign least privileges to the Service Principal on your Resource Group that contains your DNS Zone. It restricts rights to creating TXT records so that Lets Encrypt can use them to verify that you are in fact the owner of the domain.

Here’s what the script contains:

# Use this to create a role to assign to service principal used by Posh-Acme

$profile = Connect-AzureRMAccount

$roleDef = Get-AzureRmRoleDefinition -Name "DNS Zone Contributor"
$roleDef.Id = $null
$roleDef.Name = "DNS TXT Contributor"
$roleDef.Description = "Manage DNS TXT records only."
$roleDef.Actions.RemoveRange(0,$roleDef.Actions.Count)
$roleDef.Actions.Add("Microsoft.Network/dnsZones/TXT/*")
$roleDef.Actions.Add("Microsoft.Network/dnsZones/read")
$roleDef.Actions.Add("Microsoft.Authorization/*/read")
$roleDef.Actions.Add("Microsoft.Insights/alertRules/*")
$roleDef.Actions.Add("Microsoft.ResourceHealth/availabilityStatuses/read")
$roleDef.Actions.Add("Microsoft.Resources/deployments/read")
$roleDef.Actions.Add("Microsoft.Resources/subscriptions/resourceGroups/read")
$roleDef.AssignableScopes.Clear()
$roleDef.AssignableScopes.Add("/subscriptions/$($profile.Context.Subscription.Id)")

$role = New-AzureRmRoleDefinition $roleDef
$role

2. Add your Service Principal to your DNS Zone as a ‘DNS Zone Contributor’. I named my Service Principal LetsEncryptAzureStack in this example.

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From the portal, find your DNS Zone resource, and select Access Control (IAM). From the blade that opens, select +Add.

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1. Select the DNS Zone Contributor as for the Role

2. In the Select field, type in the name of your Service Principal. Select it, then press Save
   

3 Now we need to create some CAA records within the domain. This is required by Lets Encrypt in order to create the certificates. I created a function within the module to handle this. From an elevated PowerShell session navigate to the folder you extracted the module to and run:

Import-Module .\AzSLetsEncrypt.psm1

Once imported, ensure you’ve connected to your Azure Subscription within PowerShell and run the following:

#Connect to Azure
Connect-AzureRmAccount
New-AzsDnsCaaRecords -ResourceGroup <ResourceGroup> -Region <Azure Stack Region Name> -FQDN <FQDN for the DNZ Zone> -PaaS

Enter the Resource Group that hosts your DNS Zone, The name of your Azure Stack region and the FQDN for the domain. Optionally, if you specify the PaaS switch, it will create the CAA records for the PaaS endpoints too. If everything is in place, you should hopefully see something that looks like this in your Azure DNS Zone: This step only has to be run once, unless you delete the CAA records.

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4 Now we’ve got all of that in place, we can create our certificates! Assuming you’ve still got the module imported, you can use the New-AzsPkiLECertificates function to create your certs. Rather than explain all the options, I’ve created a wrapper script :

$DebugPreference = 'continue'

$Region = '<Azure Stack Region Name>'
$FQDN = '<FQDN>'
$DNSResourceGroup = '<DNS Zone Resource Group>'

$Params = @{
 RegionName = $Region
 FQDN = $FQDN
 ServicePrincipal = '<Service Principal GUID>'
 ServicePrincipalSecret = '<Service Principal Secret>'
 pfxPass = 'P@ssword!'
 SubscriptionId = '<Subscription ID>'
 TenantId = '<Tenant ID>'
 CertPath = 'c:\azsCerts'
}

$scriptpath = $PSScriptRoot
cd $scriptpath
import-module .\AzSLetsEncrypt.psm1


#New-AzsDnsCaaRecords -ResourceGroup $DNSResourceGroup -Region $Region -FQDN $FQDN -PaaS
New-AzsPkiLECertificates @Params -Force -paas

The function will create the core certificates and if you select the PaaS switch, the optional App service and SQL RP certs. It will take a while to run as, as there is an element of waiting for the TXT records that are created for validation to replicate.

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Once all the certificates have been created, they are validated to ensure they are compatible for use with Azure Stack using the Microsoft.AzureStack.ReadinessChecker module.:

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Once the script has completed, you should have a folder structure that looks like this:

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I’ve used these certificates in two installations now and they’ve worked with no problems.

As the Posh-Acme module is used to generate the certs, the data is stored here:

%userprofile%\AppData\Local\Posh-ACME

 If you need to recreate the certs before the allowed renewal period within the Posh-ACME module, or have some issues, you can delete the sub folders in this location and re-run the script.

The folder structure that’s generated should match the requirements for the Azure Stack install routine / certificate rotation. Check out the documentation here on how to rotate the certificates if they’re coming towards their expiration date.

I hope this is of some use and allows you to save some money!