8 min to read.

Abstract

Virtual machines should be connected to an approved virtual network - This default Azure policy is fantastic. This policy checks if a VM is part of approved VNET; else it shows compliance message.

However it only offers VMs to be checked against single VNET name. In reality, we have flood of Azure VNETs across multiple Azure subscriptions.

So we need a policy that can check all Azure VMs against “multiple azure Virtual Networks”.

This article talks about creating a policy that allows to provision Azure VMs inside only allowed list of VNETs.

The Challenge

This policy will be built as custom. I want to do below –

Evaluate every NIC against the VNET names present in input parameter. Refer yellow and green highlight below. I need to –

1. Loop using for or for-each

2. Dynamically get values of all items present in a parameter after performing “split” on the input string.

"mode": "All",

"policyRule": {

"if": {

"allOf": [

{

"field": "type",

"equals": "Microsoft.Network/networkInterfaces"

{

"not": {

"field": "Microsoft.Network/networkInterfaces/ipconfigurations[*].subnet.id",

"contains": "[parameters('virtualNetworkIds')]"

}

]

"then": {

"effect": "[parameters('effect')]"

}

"parameters": {

"effect": {

"type": "String",

"metadata": {

"displayName": "Effect",

"description": "The effect determines what happens when the policy rule is evaluated to match"

"allowedValues": [

"Audit",

"Deny",

"Disabled"

"defaultValue": "Audit"

"virtualNetworkIds": {

"type": "String",

"metadata": {

"displayName": "Virtual network Names",

"description": "Resource name of the virtual network. Example: Add , separated multiple values."

}

The input parameter to this policy will be more than one names of VNETs. Unfortunately in Azure policy I could not find a way to iterate over input parameters by using for-each loop construct.

There is a Current function available however it can be used only when we are using Count and Where function. Also these functions are used over Field property, used as an array. These functions cant be used when input parameter is not an array.

Therefore we need a way to iterate over input parameters array and comparing every parameter value with one subnet id Field highlighted as green above.

Solution

If we see the outcome of below line - "Microsoft.Network/networkInterfaces/ipconfigurations[*].subnet.id",

It will be resolved to id format of subnet similar to below - "/subscriptions/SubId/resourceGroups/rg--net/providers/Microsoft.Network/virtualNetworks/vnet-01/subnets/default"

So we just need to use expertise string functions lie split, combine, concat etc. in a such a way that we take out only VENT name from above string. Therefore lets write code to covert to string and then split so as to get VNET name. Final code is as below –

"[split(string(field('Microsoft.Network/networkInterfaces/ipconfigurations[*].subnet.id')),'/')[8]]"

Creating Custom Policy

Login to Azure portal -> In middle top search box type “Policies” -> Select Definitions -> Create New. Then add values as shown below. The name of the policy I have given as “Azure Resources should be connected to an approved virtual networks”. Code of policy to be added under section “POLICY RULE” can be taken from github link shared below.

Also whenever we create custom policy always get it added in new Category as “My Custom Policies”. Do not add any Role Assignment. Then save to finish policy creation wizard. [click to get better view.]

Open the newly created policy and click on Assign. [click to get better view].

On the Basics tab, make “policy enforcement” option as disabled. We want to just view the report of azure resources not deployed in approved VNET. If we enforce policy means it will not allow to provision new resources in any of the VNET other than listed in parameters tab below. For testing purpose I disabled policy enforcement.

Under “Remediation” tab uncheck the option “create a managed identity”.

On Parameters tab, make sure that you add names of all VNETs against which you want to evaluate azure resources is added in below shown format only. Then Click on “Save” button for VNET names added and click on “Review and Create” to complete assignment process.

Output

Virtual Machines

I could see that Virtual Machines not listed under the VNETs I added as parameter. Refer screenshots [click to get better view] –

Same policy will also be automatically applied on Azure App Service configured with private endpoint. Refer below output –

Full and final working policy is present at this Github link –

kunalchandratre1/NicVnetPolicy:This repo has code related to Azure policy to identify Azure resources whichare not deployed in approved VNETs. (github.com)

Important –

This policy will not work for Azure Kubernetes Service, VM Scale Sets as their NIC resource provider format is different. However this policy should work as is for all types of private endpoints where NIC is create with resource provider format of Microsoft.Network/networkInterfaces/.

I have not tested with other types of private endpoint. Please test it and add your experience in comments.

Conclusion

Hope this article helped you to build custom policy and helped to achieve your compliance and governance. Let me know your views in comments section below to improve and what are your thoughts on this approach.

Happy Azure Governance!!

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