Tuesday, October 10, 2023

Add custom rule to new NSGs via Azure Policy

 For governance, or operational, reasons there may be a need to ensure that certain rules are applied to all NSGs that are created within a certain scope.

This can be achieved using Azure Policy with a deployIfNotExist function.

Such a policy has already been created and is ready to use from AzAdvertizer.net, see link here:

I ran a quick test to verify the functionality and it works as expected. At the time of creation of the NSG, the policy kicks in an applies the rule right away.

The policy will let you specify one rule. So for multiple rules additional assignments can be created.

The policy looks for a suffix (the last part of the name) in the NSG name and only applies the rule if there's a match. You can re-arrange the check and have it look for a prefix instead, I have uploaded an example here on Github (can be copied in as a new definition via Azure Portal -> Policy -> Definitions).

If you want to apply the rule to all NSGs, then simply remove this check, see marked part below:

The policy is parameterized, so when you create an assignment it will request you to add all relevant parameters. See example below:

Note that some of the parameters such as destinationPortRange are arrays. They should be added in the format ["3389"] (for port 3389..).

Below is a screenshot of the inbound rule added post NSG deployment:

Since this is a deployIfNotExist policy, this means the Assignment requires a system assigned managed identity (or a user assigned managed identity) with Network Contributor permissions which will be automatically created when you create the assignment if you have enough permissions.

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