Troubleshoot Azure Front Door with RefString

A guide to understand and use RefStrings for diagnosing and resolving issues with Azure Front Door.

Prerequisites

• You must have an Azure Front Door profile. To create a profile, see Creating an Azure Front Door profile.

What is a RefString?

A RefString is a short string appended by Azure Front Door to the HTTP response headers of each request. It provides details on how the request was processed, including the point of presence (POP) and backend status.

RefStrings can help you troubleshoot and resolve issues with Azure Front Door, such as cache misses, routing errors, backend failures, and latency problems. You can identify the root cause and take appropriate actions to fix it by analyzing the RefStrings of the requests.

Note

If you encounter an error page from Microsoft services, it will already include a RefString for the request that generated the error page. In such cases, you can skip directly to the diagnostic step.

How to gather a RefString

To gather a RefString, you need to capture the HTTP response headers of the requests and look for the header named X-Azure-Ref. This header contains the RefString, encoded in Base64. You can use different methods to capture the HTTP response headers, depending on your preference and situation. Here are a few examples of how to obtain a RefString from various browsers and applications:

Microsoft Edge Browser

1. Open the browser's developer tools by pressing F12 or Ctrl+Shift+I.

2. Go to the Network tab.

3. Refresh the page or perform the action that triggers the request.

4. Locate the specific request in the list and find the X-Azure-Ref header in the response headers section.

5. Copy the value of the X-Azure-Ref header to use with the RefString troubleshooting tool in the Azure portal.

For more information, see Inspect network activity - Microsoft Edge Developer documentation.

Example of how to obtain a RefString from Microsoft Edge Browser:

Google Chrome

For Google Chrome browsers, see Inspect network activity - Google Chrome Developer documentation.

cURL

To obtain headers with cURL, use the -I or —include option to include the HTTP response headers in the output. Look for the X-Azure-Ref header in the output, and copy the value of the header.

Fiddler

1. Launch Fiddler and start capturing HTTP traffic. Refresh the page or perform the action that generates the request.

2. Choose the request from the list and navigate to the Inspectors tab.

3. Switch to the Raw view, locate the X-Azure-Ref header in the response headers, copy its value, and decode it using a Base64 decoder.

To learn more about viewing and capturing network traffic with Fiddler, see Web Debugging - Capture Network Traffic.

How to use a RefString with some of our troubleshooting tools

Azure Front Door uses a RefString to manage 4xx and 5xx errors. The following are the steps to use the diagnostic tool with a RefString for tracking and diagnosing connectivity issues:

1. Navigate to your Azure Front Door Profile.

2. Select the Diagnose and solve problems menu.

   

3. Scroll down and select Connectivity under the Common problems section.

   

4. In the box What issue are you having? select Select a problem subtype and choose 4xx and 5xx errors on the drop down-menu, then select the Next.

   

5. Enter your RefString in the box within the 4xx and 5xx errors section. You input the Restring given to you from your request under the Tracking Reference – RefString field.

   

6. Finally, select Run Diagnostics to identify the cause of the issue, which explains the failure if it's a known problem.

   An example of a result displaying an issue:

   

   Note

   The diagnostic capabilities may require up to 15 minutes to deliver results. We ask for your patience that you allow the process to finish before taking further action.

Alternative option

If you choose not to use the diagnostic tool, you can include a RefString when submitting a support ticket. Additionally, you can enable the Access Logs feature to receive updates on RefString data directly in the Azure portal. For more information on tracking references and access log parameters, see Monitor metrics and logs in Azure Front Door.

This article highlights specific fields in access logs that help identify various types of errors:

• Cache misses: RefString indicate whether a request was served from the cache and provide reasons if it wasn't.

  Example: NOCACHE means the request wasn't eligible for caching, MISS means no valid cache entry existed, and STALE means the cache entry was expired.

• Routing errors: RefString can reveal if a request was routed correctly to the backend and the reason.

  Example: FALLBACK means rerouted due to primary backend issues, and OVERRIDE means sent to an alternative backend against routing rules.

• Backend failures: RefString indicate if delivery to the backend succeeded and explain any issues.

  Example: TIMEOUT means the response took too long, CONNFAIL means connection failed, and ERROR indicates an error response from the backend.

• Latency problems: RefString detail Azure Front Door's processing time and stage durations.

  Example: DURATION shows total handling time, RTT shows round-trip time, and TTFB shows the time taken to receive the first byte from the backend.

Next steps

• To learn more about navigating common issues, see Front Door Troubleshooting Issues.
• For answers to common questions, see Azure Front Door FAQ.