Why can I not access some fields of a publicIpAddress in template output?

Question

In my template output, I'd like to return the allocated public IP address. I tried the following in the "outputs" section: (in an output of 'object' type)

"ipobj":   "[reference(variables('publicIPAddressName'),'2020-07-01','Full')]"

This works and returns the whole publicIpAddress object:

  "ipobj": {
    "apiVersion": "2020-07-01",
    "location": "eastus",
    "sku": {
      "name": "Basic",
      "tier": "Regional"
    },
    "properties": {
      "provisioningState": "Succeeded",
      "resourceGuid": "...",
      "ipAddress": "...",
      "publicIPAddressVersion": "IPv4",
      "publicIPAllocationMethod": "Dynamic",
      "idleTimeoutInMinutes": 4,
      "dnsSettings": {
        "domainNameLabel": "simplelinuxvm-zktwk4fzmy5p4",
        "fqdn": "simplelinuxvm-zktwk4fzmy5p4.eastus.cloudapp.azure.com"
      },
      "ipTags": [],
      "ipConfiguration": {
        "id": "/subscriptions/.../resourceGroups/.../providers/Microsoft.Network/networkInterfaces/simpleLinuxVMNetInt/ipConfigurations/ipconfig1"
      }
    },
    "subscriptionId": "...",
    "resourceGroupName": "...",
    "scope": "",
    "resourceId": "Microsoft.Network/publicIpAddresses/simpleLinuxVMPublicIP",
    "referenceApiVersion": "2020-07-01",
    "condition": true,
    "isConditionTrue": true,
    "isTemplateResource": false,
    "isAction": false,
    "provisioningOperation": "Read"
  }

This is in line with the API documentation of the publicIpAddress object. And I can retrieve some of the properties as well, using the reference without the 'Full' parameter, which, according to the Azure doc, should return the properties of the object. These work:

"[reference(variables('publicIPAddressName')).dnsSettings.fqdn]"
"[reference(variables('publicIPAddressName')).publicIPAddressVersion]"

But some other properties are not accessible:

"[reference(variables('publicIPAddressName')).ipAddress]"
"[reference(variables('publicIPAddressName'),'2020-07-01','Full').properties.ipAddress]"

According to the error message, (in case of the middle example):

The template output '...' is not valid: The language expression property 'ipAddress' doesn't exist, available properties are 'provisioningState, resourceGuid, publicIPAddressVersion, publicIPAllocationMethod, idleTimeoutInMinutes, dnsSettings, ipTags'

So it seems like Azure doesn't let me access the properties which are however there in the full output. Is there any explanation/intention behind that, or a workaround at least ?

Note: the same happens if I define a "string" output (i.e. not as part of an object) as

"ip": {
    "type": "string",
    "value": "[reference(variables('publicIPAddressName')).ipAddress]" 
}

or

"ip": {
    "type": "string",
    "value": "[reference(variables('publicIPAddressName'),'2020-07-01','Full').properties.ipAddress]"
},

Answer 1

When you output the properties of publicIpAddress object, you should pay attention to define the correct type that matches the output value.

"outputs": {
    "publicIpipAddress": {
        "type": "string",
        "value": "[reference(parameters('publicIPAddresses_vmc_backupPublicIP_name'),'2020-07-01','Full').properties.ipAddress]"
    },
    "idleTimeoutInMinutes": {
        "type": "int",
        "value": "[reference(parameters('publicIPAddresses_vmc_backupPublicIP_name'),'2020-07-01','Full').properties.idleTimeoutInMinutes]"
    }
}

Answer 2

I faced similar issue recently, and below are some of the references where the reason for this scenario is mentioned.

1. According to this:

   i)

   The 'ipaddress' property only exists if the 'publicIPAllocationMethod' is set to 'Static'. ( A Static Public IP Address). If you did this on an IP Address that was not static, it would return an error, but work for those that were static.

   ii)

   It will only have the 'ipAddress' Property when attached to a service or VM which is running. If the VM is 'Stopped', it will not have the ipAddress property.

2. And, looks like there is an open issue in github in this link, related to this. Refer to this, in case it gets resolved in the future.

3. Check this link as well, where it is mentioned that:

   This is a known limitation in the platform where a dynamic public IP address doesn't resolve itself until the VM is up and running. There are two options to workaround:

   i) Create the Public IP Address in static mode. That will ensure that Public IP address is immediately allocated. However, note that you might incur additional charges.

   ii) Change the dependency from Public IP address to the Virtual Machine that the IP address is attached to. This will ensure that the public IP address is always available.