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:

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