A beginner-friendly, real-world walkthrough of diagnosing and fixing networking issues in a Linux Azure Function App running on Elastic Premium.
When an Azure Function App doesn't even show its runtime version, it is very easy to start looking at the application code.
But what if you haven't deployed any code yet?
That was exactly the situation I faced while setting up a new Linux Azure Function App on the Elastic Premium EP1 plan in Microsoft Azure. The Azure Portal could not properly load the runtime information, and ZIP deployment was failing before giving us any useful application-level logs.
There was no Python code to debug.
That was the first important clue.
The problem was not in the application. It was in the networking and storage connectivity required by the Function App itself.
The final solution involved connecting the Function App to the correct Virtual Network (VNet) and then creating a separate Private Endpoint and Private DNS configuration for another storage account used by the application.
This article explains what happened, why it happened, and how to troubleshoot similar problems step by step.
The Architecture We Were Working With
The environment was not a completely public Azure setup.
The Function App was running on Linux + Azure Functions Elastic Premium (EP1), and the environment already had a shared VNet containing private resources.
The storage account associated with the Function App was also protected by network restrictions and was reachable through private networking.
Few existing Function App was already working in this environment.
The new Function App, however, was not connected to the same VNet.
That small difference was enough to prevent the new Function App from reaching the storage it needed.
Azure Functions depends on storage for important runtime and application operations. For Premium plans, Azure Files is also commonly used for function app content and deployment-related functionality. Microsoft specifically notes that the storage account used by a Function App must be accessible to the app, and that Premium plans can use Azure Files for function code and configuration.
So the important question became:
If the storage account is private, how is the Function App supposed to reach it?
The answer is through the VNet.
The First Problem: "Runtime Unreachable"
Immediately after creating the Function App, we noticed a few unusual things.
The Azure Portal couldn't properly display the Functions runtime version. The host appeared unreachable, and ZIP deployment was failing without giving us the kind of application logs we would normally expect.
At first, this can be confusing.
You might think:
Is function_app.py wrong?
Is requirements.txt missing something?
Is the Python version incorrect?
Did the ZIP package have the wrong structure?
But none of those explanations made sense because the application hadn't even been deployed yet.
That changes the troubleshooting approach. If the Function App cannot initialize its host before your code is running, you should first investigate the infrastructure it depends on.
One of the most important dependencies is storage.
Azure Functions uses the storage account configured for the Function App for core operations. On Elastic Premium, Azure Files and queues can also be used to store application content and configuration. Microsoft documents AzureWebJobsStorage as the default storage connection and WEBSITE_CONTENTAZUREFILECONNECTIONSTRING as the storage connection used for Azure Files content on Elastic Premium and Consumption plans.
In our environment, this storage account was protected by network restrictions and had a private endpoint. Existing Function Apps could access it because those Function Apps had already been integrated with the correct VNet.
The new Function App wasn't.
So we had a simple mismatch:
Private storage → VNet access required → new Function App wasn't connected to the VNet.
VNet Integration: What Does It Actually Do?
This is one of the concepts that can be confusing when you are new to Azure networking.
VNet integration is primarily an outbound networking feature.
It gives the Function App a path into a VNet so that the app can reach resources inside that network, including resources exposed through private endpoints. It does not mean that your Function App suddenly becomes privately accessible from the VNet.
These are two different concepts:
VNet Integration controls how the Function App makes outbound connections.
Private Endpoint gives an Azure resource a private IP address inside a VNet.
Microsoft's documentation specifically describes regional VNet integration as an outbound capability that allows Azure Functions to access resources in the VNet, including private endpoints.
That distinction becomes extremely important when troubleshooting.
The First Fix: Connect the Function App to the VNet
The easiest way to understand the required configuration was to look at the existing Function App that was already working. We checked its networking configuration and identified the VNet and the subnet used for VNet integration.
Then we configured the new Function App using the same network architecture.
From the Azure Portal, go to:
Function App → Networking → Virtual Network Integration
Then select Add virtual network integration.
Choose the VNet used by the existing application and select the dedicated integration subnet, if in your case no such VNet exist, then create a VNet first.
For an Elastic Premium Linux Function App, the integration subnet must be delegated to:
Microsoft.Web/serverFarms
Microsoft currently documents /28 as the minimum subnet size for Elastic Premium VNet integration and recommends a larger subnet for Linux Premium workloads, because each running instance consumes an IP address and scaling can temporarily require additional addresses. You can also keep the default size when you setup the subnet.
One important rule is:
Do not use your private-endpoint subnet as the Function App integration subnet.
These subnets have different purposes.
The integration subnet is used by the Function App for VNet integration.
The private-endpoint subnet contains the private IP addresses assigned to private endpoints.
Microsoft's own Function App networking samples use separate subnets for these two purposes.
Once the integration was completed, the Function App was restarted.
After giving Azure a few minutes to complete the networking changes, the Function runtime became accessible again.
That was a strong indication that we had identified the first problem correctly.
But Then We Found Another Problem
Getting the Function App runtime working was only the first part. The application also needed to communicate with another storage account. This was not the storage account used by the Function App itself.
It was a separate business-content storage account used by the application to store processed documents.
*Example scenerio- *
You have a task that triggers the function app on a specific time to place a file from an API to your blob storage account. Now this storage account is not the same that the function app uses.
We had a simple health-check function that tested connectivity to different dependencies.
The Key Vault check was successful.
The Blob Storage check was failing.
The error looked similar to:
Failed to resolve 'contentarchive.blob.core.windows.net'
No address associated with hostname
This error was extremely useful. Why?
Because it wasn't an authentication error.
It wasn't saying:
AuthorizationPermissionMismatch
It wasn't saying:
403 Forbidden
It was saying that the hostname could not be resolved.
That pointed us toward DNS and private endpoint configuration.
Why VNet Integration Didn't Automatically Fix the Second Storage Account
This is another important concept. Connecting your Function App to a VNet does not automatically give the Function App private connectivity to every Azure resource.
The destination resource must also be configured correctly.
In our case, the second storage account did not yet have the required private endpoint.
So we effectively had this:
Function App
|
| VNet Integration
↓
VNet
|
X
Private path to content storage didn't exist
The solution was to create the private networking path for that specific storage account.
Private Endpoint: What Does It Do?
A Private Endpoint creates a private network interface with a private IP address inside your VNet and connects that IP to a specific Azure service.
For example, if you create a Blob private endpoint for a storage account, the Blob service becomes reachable through a private IP.
But there is an important detail here:
Private endpoints are tied to a specific service sub-resource.
For Azure Storage, Blob, File, Queue, Table, and other services have separate endpoints and corresponding DNS zones.
For example:
Blob → privatelink.blob.core.windows.net
File → privatelink.file.core.windows.net
Queue → privatelink.queue.core.windows.net
Table → privatelink.table.core.windows.net
Microsoft's current private endpoint documentation confirms these service-specific DNS zones.
So creating a Blob private endpoint doesn't automatically create private connectivity for Azure Files or Queue.
This distinction matters particularly for Function Apps because the Function runtime and the application itself may use different storage services.
The Second Fix: Create a Private Endpoint for the Content Storage
We opened the business-content storage account and went to:
Networking → Private Endpoint Connections
Then we created a new private endpoint.
For the target sub-resource, we selected:
Blob
We selected the same VNet used by the Function App, but importantly, we selected the private-endpoint subnet, not the Function App's integration subnet.
During the configuration, we also enabled integration with a Private DNS Zone.
For Azure Blob Storage, the standard private DNS zone is:
privatelink.blob.core.windows.net
Microsoft recommends this naming convention for Blob private endpoints.
Once the private endpoint was provisioned and the connection was approved, the next step was verifying DNS.
Why DNS Was So Important
Creating a private endpoint gives the service a private IP. But your application doesn't normally connect to the storage account by typing its private IP.
It still uses the normal hostname:
contentarchive.blob.core.windows.net
The job of Private DNS is to make that hostname resolve to the correct private endpoint.
Conceptually, we want:
contentarchive.blob.core.windows.net
↓
Private DNS resolution
↓
Private IP
↓
Private Endpoint
↓
Storage Account
Without the correct DNS configuration, the application might continue trying to resolve the hostname through the normal public DNS path—or fail to resolve it in the private networking context.
Microsoft specifically states that applications calling private endpoints must have DNS resolution configured so the service hostname resolves to the private endpoint.
Checking the Private DNS Zone
After creating the private endpoint, we checked:
Private DNS Zones → privatelink.blob.core.windows.net
Inside the zone, we checked the record sets.
There should be an A record for the storage account that points to the private endpoint's IP address.
For example:
contentarchive
↓
10.x.x.x
The exact IP will obviously be different in every environment.
We then compared that address with the private IP assigned to the private endpoint.
The addresses should match.
We also checked the Virtual Network Links section.
The VNet used by the Function App must be linked to the Private DNS Zone so that workloads inside that VNet can resolve the private name.
Microsoft's DNS documentation describes the use of Private DNS Zones and VNet links for private endpoint name resolution.
One more useful detail is that when a private endpoint is created with the recommended DNS zone configuration, Azure can automatically manage the corresponding private DNS record through the private endpoint's DNS zone group.
Restart and Test Again
Once the networking and DNS configuration was complete, we restarted the Function App.
Then we ran the health-check function again.
The result changed from a DNS failure to a successful Blob connection.
A simplified version of the health-check response looked like this:
{
"checks": {
"key_vault": {
"ok": true
},
"blob": {
"ok": true
}
},
"overall_ok": true
}
This is where health checks become extremely useful.
Instead of simply knowing that "the Function isn't working," we can test individual dependencies.
For example:
Function App
|
+---- Key Vault → OK
|
+---- Blob Storage → DNS failure
After fixing DNS:
Function App
|
+---- Key Vault → OK
|
+---- Blob Storage → OK
That tells us much more about where the problem actually exists.
An Important Troubleshooting Pattern
There is a useful pattern to remember when troubleshooting private networking.
If you see:
Failed to resolve hostname
No address associated with hostname
DNS resolution failed
start by investigating DNS and private endpoint configuration.
If DNS starts working but you then receive something like:
403 Forbidden
AuthorizationPermissionMismatch
Network access denied
that is actually progress.
It means the application can now find the destination, but something else is blocking the request.
At that point, investigate:
- Storage firewall and network rules
- Private endpoint approval
- Identity or access permissions
- Storage RBAC
- Connection configuration
- The specific Storage service being accessed
This is much more effective than changing application code randomly.
One Storage Account Does Not Mean One Private Endpoint
This is probably the most important lesson from the second issue.
Suppose your architecture looks like this:
Function App
|
↓
VNet
|
+---- Storage Account A
|
+---- Storage Account B
|
+---- Key Vault
You cannot assume that because Storage Account A has a private endpoint, Storage Account B is automatically private.
Each Azure resource needs its own private connectivity configuration.
And even within one Storage Account, different services can have different private endpoint requirements.
For example:
Storage Account
|
+---- Blob → Blob private endpoint
|
+---- File → File private endpoint
|
+---- Queue → Queue private endpoint
The exact configuration depends on which services your Function App actually uses.
This is particularly important for Azure Functions because the Function App's own storage requirements may include Azure Files in Elastic Premium, while your application may separately access Blob Storage for business data.
What About ZIP Deployment?
Once the networking problem was fixed, deployment could be investigated separately.
For a Python Function App, the ZIP package needs to contain the required project files at the root of the package.
For example:
function_app.py
host.json
requirements.txt
The important point is that host.json should be at the root of the extracted deployment package.
For Linux Function Apps using remote build, Microsoft currently documents these application settings:
SCM_DO_BUILD_DURING_DEPLOYMENT=true
ENABLE_ORYX_BUILD=true
Remote build allows Azure to build dependencies in the target Linux environment rather than relying on packages built on your local machine.
This is a deployment concern, though—not something to troubleshoot first when the Function host itself cannot start because it cannot reach its required storage.
That distinction saved us a lot of unnecessary debugging.
The Troubleshooting Flow I Would Follow Next Time
If I encounter a new Function App that is unreachable before any application code has been deployed, I would follow this order.
First, check the Function App itself
Confirm that the Function App is running, the hosting plan is correct, and the runtime configuration is present.
If the runtime itself cannot load, don't immediately start debugging function_app.py.
Next, check the storage dependency
Look at the Function App's storage configuration, including AzureWebJobsStorage and, for Elastic Premium, the Azure Files content configuration where applicable.
Ask:
Can this Function App actually reach the storage account it depends on?
If the storage account is publicly accessible, the problem may be elsewhere.
If it is restricted to a VNet or private endpoint, networking becomes a primary suspect.
Then, check VNet integration
Verify that the Function App is integrated with the correct VNet.
Also verify that the integration subnet is separate from the private-endpoint subnet and has the correct delegation.
For Elastic Premium Linux, Microsoft documents Microsoft.Web/serverFarms as the required delegation.
Then, check the private endpoint
For every private Azure service the application needs to access, verify that the appropriate private endpoint exists and is approved.
Don't assume that another resource's private endpoint covers it.
Then, check DNS
Make sure the correct Private DNS Zone exists and is linked to the VNet.
For Blob Storage, that means:
privatelink.blob.core.windows.net
Then verify that the storage account has the expected A record pointing to the private endpoint IP.
Finally, test the application
Once the infrastructure is confirmed, run a small health check that tests dependencies individually.
That gives you a much clearer picture than testing the entire application at once.
Quick Checklist
When troubleshooting an Azure Function App that cannot reach a private storage account, I would check these in order:
- Function App storage: Is the storage account used by the Function App accessible from the Function App's network path?
- VNet integration: Is the Function App connected to the correct VNet?
- Integration subnet: Is the correct dedicated subnet being used and delegated to Microsoft.Web/serverFarms for Elastic Premium?
- Private endpoint: Does the destination storage account have the required private endpoint for the service being accessed?
- Subnet separation: Is the private endpoint using its own subnet rather than the Function App's integration subnet?
- Private DNS: Does the appropriate Private DNS Zone exist and is it linked to the VNet?
- DNS record: Does the storage account hostname resolve to the private endpoint's IP?
- Access permissions: Once networking works, does the Function App have the required identity/RBAC or other authentication permissions?
- Deployment: Only after the platform and network are healthy, investigate ZIP packaging, dependencies, and remote build configuration.
References
The following Microsoft Learn resources were used to verify the Azure networking and deployment concepts discussed in this article:
Azure Functions networking options
Storage considerations for Azure Functions
Azure Private Endpoint DNS configuration
Use private endpoints to integrate Azure Functions with a virtual network
Deployment technologies in Azure Functions
Run your Functions from a Package File in Azure
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