Understanding "Provisional Headers" Warning in Chrome's Network Inspector

Introduction

When debugging web applications, developers often use browser tools like Google Chrome’s Developer Tools to inspect network requests. One common message you might encounter is “Caution: provisional headers are shown.” This tutorial will explain what this warning means and why it appears.

Understanding Provisional Headers

The Network Inspector in Chrome provides detailed information about each request made by the web page, including headers sent to and received from the server. Occasionally, when inspecting a resource, you might see a message: “Caution: provisional headers are shown.” This occurs because some requests do not complete normally, often due to being blocked or intercepted.

Why Do Provisional Headers Appear?

Provisional headers appear in several scenarios:

1. Blocked Requests:

   • If an extension (e.g., AdBlock) blocks a request, Chrome cannot obtain the real server response. The provisional headers displayed are placeholders indicating that the actual communication with the server didn’t occur.

2. Cached Resources:

   • When loading resources from cache, the request might not be sent to the network again, resulting in only placeholder headers being shown since no new server interaction happens.

3. Unsent Requests:

   • Sometimes, requests like CORS preflight (OPTIONS) are made but fail or timeout before a response is received. This can lead to provisional headers being displayed.

4. SSL Port Issues:

   • In certain configurations where SSL requests are directed to non-standard ports (e.g., port 8081 instead of the typical port 443), Chrome might not handle these correctly, leading to blocked requests and provisional headers.

Investigating Blocked Requests

To identify what’s blocking a request, you can use Chrome’s net-internals tool:

For Latest Versions of Chrome:

• Type chrome://net-export/ in the address bar.
• Start recording network activity and save the log file locally.
• Open the problematic page, then revisit net-internals to analyze recorded logs.

For Older Versions of Chrome:

• Navigate to chrome://net-internals.
• Open the problematic page and check for events related to your resource using the URL as a reference in the event list.

Addressing Common Issues

1. Extensions: Disable extensions temporarily or use incognito mode (which disables most extensions) to see if they are causing the issue.

2. CORS Preflight Requests:

   • Ensure that backend services handle OPTIONS requests properly. A simple response can prevent timeouts and allow subsequent requests to proceed without encountering provisional headers.

3. SSL Configuration:

   • Consider using standard ports for SSL connections or proxying traffic through a conventional SSL port if you’re running custom configurations on non-standard ports.

4. Chrome Flags:

   • Advanced users might experiment with Chrome flags (chrome://flags/), such as disabling site isolation features, to see if they resolve specific issues, though these changes can affect browser stability and security.

Conclusion

The “Caution: provisional headers are shown” message is a useful indicator that something about the request’s execution wasn’t typical. By understanding the scenarios that lead to this message and using available tools to investigate, developers can identify and fix underlying problems in their web applications efficiently.

Remember, handling blocked requests or misconfigurations properly not only resolves the immediate issue but also contributes to a more robust and reliable application experience for end users.