vnetDNSServersAppend.json

{
  "properties": {
    "displayName": "Custom DNS management - set Virtual Network DNS servers",
    "policyType": "Custom",
    "mode": "All",
    "description": "Custom DNS servers must be set to the AD DC IPs.",
    "parameters": {
      "customDNSServers": {
        "type": "Array",
        "metadata": {
          "displayName": "Custom DNS servers",
          "description": "The list of required custom DNS servers."
        }
      }
    },
    "policyRule": {
      "if": {
        "allOf": [
          {
            "field": "type",
            "equals": "Microsoft.Network/virtualNetworks"
          },
          {
            "anyOf": [
              {
                "count": {
                  "field": "Microsoft.Network/virtualNetworks/dhcpOptions.dnsServers[*]"
                },
                "notEquals": "[length(parameters('customDNSServers'))]"
              },
              {
                "count": {
                  "field": "Microsoft.Network/virtualNetworks/dhcpOptions.dnsServers[*]",
                  "where": {
                    "field": "Microsoft.Network/virtualNetworks/dhcpOptions.dnsServers[*]",
                    "in": "[parameters('customDNSServers')]"
                  }
                },
                "notEquals": "[length(field('Microsoft.Network/virtualNetworks/dhcpOptions.dnsServers[*]'))]"
              }
            ]
          }
        ]
      },
      "then": {
        "effect": "append",
        "details": [
          {
            "field": "Microsoft.Network/virtualNetworks/dhcpOptions.dnsServers",
            "value": "[parameters('customDNSServers')]"
          }
        ]
      }
    }
  }
}