Friday, December 22, 2023

Add resource locks to individual records in a private DNS zone

 It is possible to add resource locks to individual records in private DNS zones. A scenario for this could be a critical or central A record that you don't want to be changed by mistake while still allowing for ongoing updates to the private DNS zone as part of daily operations.

If you have a centralized private DNS zone setup with Azure policy handling the DNS record creation and you also use private endpoints (PE), then there is a (perhaps small) risk that A records can be overwritten. This is the case if someone creates a new PE and associates with the same resource. The DeployIfNotExist policy will run on PE creation and replace the existing record and so that the PaaS service will resolve to a new local IP (note that if you delete the new PE again, the A record will also be deleted and the original PE will no longer have an A record and so will have to be recreated or the A record re-added).

Adding locks to individual records is described in further detail here. Note that this can currently only be done using PowerShell and can't be done via the Azure portal.

The example from MSFT looks like below:

An actual example is shown below:

# Lock a DNS record set

$lvl = "ReadOnly"

$lnm = "dontChangeMe"

$rsc = ""

$rty = "Microsoft.Network/privateDNSZones/A"

$rsg = "rg-dns-conn-001"


New-AzResourceLock -LockLevel $lvl -LockName $lnm -ResourceName $rsc -ResourceType $rty -ResourceGroupName $rsg

Note that the MSFT example uses a regular DNS zone whereas my example uses private DNS zones (marked in bold above).

See example below for when lock is applied:

Once applied, you can see (and edit and delete) the lock in the portal under the private DNS zone -> "you're private DNS zone, e.g. for blob" -> Locks, see below:

The reason that the lock type is set to ReadOnly and not CannotDelete is that the latter option will  allow the records to be overwritten which we don't want.

Azure policy: Deploy CanNotDelete locks to resource groups

 You can use Azure policy to apply resource locks to all resource groups in a defined scope. This can be useful to ensure that critical resources are not deleted by mistake. See general description of resource locks here. Note that applying locks in the environment can create unforeseen problems so it's good to proceed with a bit of caution. 

The policy in this post is based on a slightly modified version from AzAdvertizer, that can be found here. The original policy will apply locks to a list of resource group names that is specified in an array.

The modified version, which can be found here on Github, applies locks to all resource groups but excludes resource groups which are specified in the array.

Note that the only built-in roles (for the system assigned managed identity, SAMI) that can apply locks are Owner and User Access Administrator. Owner has typically too many permissions and User Access Administrator does not have the policy deployment permissions. So a custom RBAC is required.

To create a custom RBAC, go the the subscription or management group level where the policy will be applied, go to Access Control (IAM) -> Roles. From here click +Add to a new role. Add the following permissions:


The full RBAC role in JSON can be found here on Github (make sure to update the management group or subscription under assignableScopes).

When you assign the policy definition, make sure to choose the new RBAC role when creating the SAMI, see below:

Azure policy: Reducing the default evaluation time for auto A record creation for private endpoints

 I have previously written about the use of Azure policy to automatically create A records for private endpoints in centrally managed private DNS zones. You can see more here and here. And the general recommended setup from MSFT is available here.

One of the slightly annoying things about the standard Azure policies is that they use the default evaluation delay time (10 minutes) for when a DeployIfNotExist policy runs. This means that when you create a private endpoint, it takes around 10 minutes before the A record is created in the private DNS zone. This creates, in addition to the added wait time, some confusion from users as they don't know whether their deployments work or not.

This evaluation delay time can be minimized by using the optional property for DeployIfNotExist type policies called EvaluationDelay. This is described in more detail here.

There are different values that can be set for this property, but to get the best effect, I'd recommend using AfterProvisioningSuccess. This will run the policy as soon as the private endpoint has been successfully deployed.

You can see an example of a policy using this property here on Github. And below is an example as well.

Note that this property does not only apply to DNS record creation. It can be used for other DeployIfNotExist policies as well where you want the resources deployed right away.

Monday, December 11, 2023

Azure policy: Auto create DNS records using both subResource and private link resource type

 When using private endpoints at scale, the recommended setup from Microsoft is to use Azure Policy to automatically create the DNS records in the central private DNS zones when the private endpoints are created. The reason for this is that users or owners of the spokes or landing zones do not have permissions to create A records in the central private DNS zones in the Hub.

For most private DNS zones, the regular Azure policy can be used which checks for private DNS zone name and subResource id, see list here. However, there are scenarios where this is not sufficient. For example, if a region has to be specified using Recovery Services Vault, see more on that here.

Another example, and the scope of this post, is when there are overlapping subResource values such as for Synapse Analytics and Cosmos DB (which both use 'sql') or Synapse Studio and Storage accounts Web (which both use 'web'). If multiple policies are created using the same subResource, you don't know in which private DNS zone that the A record will be created and you can experience records being created first in one zone and then the other whichever policy is evaluated first.

To address this, Microsoft has created a policy that, in addition to the subResource, adds a parameter that matches on the private link resource type (also referred to as privateLinkServiceId). The policy can be found here.

The private link resource type is found in the first column in the table of private DNS zones, here. Examples of values are:

  • Microsoft.Synapse/privateLinkHubs
  • Microsoft.Synapse/workspaces
  • Microsoft.DocumentDB/databaseAccounts

For some odd reason, MSFT hardcodes the value of the private link reosurce type in the policy. I've updated the policy slightly to parameterize that value. The updated policy can be found on here on Github.

Below you can see an example of what it looks like when the policy is assigned in the portal: