Azure Function App HTTP Trigger Keys Automatically Changed, Causing .NET App Failures

Question

I have created an Azure HTTP Trigger Function App and consume it in my .NET application. Recently, I have encountered an issue where the function keys (passed via the x-functions-key header) are changing automatically every few days, causing my .NET app to fail with a 401 Unauthorized error.

What I have checked so far:

1. No manual regeneration of keys via the Azure Portal.
2. Restarted the Function App, but the keys still unchanged.
3. No deployments that could trigger key rotation.

Questions:

1. Why do the keys change automatically?
   • Are there default Azure policies or hidden behaviors that force key regeneration?
2. How can I prevent key rotation?
   • Is there a way to disable auto-rotation for Function App keys?
3. Best practices for avoiding downtime:
   • If keys are unavoidable, how can I retrieve them dynamically in my .NET app?

Code Example (HTTP Client):

private HttpClient Initialize()  
{  
    HttpClient httpClient = new HttpClient();  
    string headerValue = _config["HEADER_VALUE"]; // Key stored in config  
    httpClient.DefaultRequestHeaders.Add("x-functions-key", headerValue);  
    return httpClient;  
} 

public async Task SendMessage(string payload) 
{ 
	try
	 { 
		string AzureFuctionURL = ConfigurationManager.AppSettings["AzureFuctionURL"]; 
        HttpClient httpClient = Initialize(); 
        HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, AzureFuctionURL) 
		{ 
			Content = new StringContent(payload, Encoding.UTF8, "application/json") 
		};
		 
        var result = await httpClient.SendAsync(request); 
		Console.WriteLine($"Status Code: {result.StatusCode}"); 
		var responseBody = await result.Content.ReadAsStringAsync(); 
        Console.WriteLine($"Response: {responseBody}"); 
      } 
      catch (Exception ex)
      {
 		Console.WriteLine("Error sending message: " + ex.Message); 
      }
}

Error Scenario: The x-functions-key value becomes invalid after a few days, forcing me to manually update HEADER_VALUE in configuration.

Answer 1

Hello , Welcome to MS Q&A

Here's why it happens and how to prevent it:

Why Do Function App Keys Change

1. Consumption Plan Behavior
   • If you're using an Azure Consumption Plan, Function Apps can scale out and scale in dynamically. Sometimes, this results in key regenerations when instances restart or scale down. The platform may automatically remove unused or orphaned keys.
   1. App Restart or Deployment
      • If you redeploy your function app, it may regenerate keys. If you delete and recreate a function, new URLs and keys may be generated.

Key Expiry or Automatic Cleanup

• Certain keys may be deleted by Azure if they're not used for a while or if there are policy settings applied.

Solutions to Prevent Keys from Changing

Option 1: Use an App Service Plan Instead of Consumption Plan

• Switch to a Premium (EP) Plan or Dedicated (App Service Plan) where instances are always warm. This prevents function keys from changing due to scale-in/out events.

Option 2: Use a Static Key from Azure Key Vault

• Store and manage your keys securely in Key Vault instead of relying on Function App keys.
• Update your function app to fetch the key dynamically from Key Vault.

Option 3: Manually Regenerate and Store Keys

• If you want to keep the same function key, manually regenerate it via the portal or CLI. Store the key in Azure App Configuration, an environment variable, or a configuration file.

az functionapp keys set --name <FunctionAppName> --resource-group <ResourceGroupName> --key-name <CustomKeyName> --key-value <YourKeyValue>

Option 4: Use AAD Authentication Instead of Function Keys

• Instead of relying on function keys, consider AAD Authentication (MI or OAuth Tokens) for secure access. This eliminates the need to manage and persist function keys manually.

Overall...

• If you must use a Consumption Plan, store your function key in Key Vault and retrieve it dynamically.
• If you need static keys, move to a Premium or Dedicated App Service Plan.
• For security and better access management, use Azure AD authentication.

Kindly let us know if any further ques

Pls accept if it helps

Thanks

Deepanshu