How Can I Capture HTTP Response Headers in a Chrome Extension?

Capturing HTTP Response Headers in a Chrome Extension

In pursuit of monitoring HTTP responses within a Chrome extension, developers often face a lack of APIs tailored to this specific task. This article aims to provide a comprehensive solution by utilizing a script injection mechanism combined with custom event listeners.

Injecting a Script

The key to capturing responses is to inject a script into the website, which involves modifying both the manifest.json and the injected script.

• manifest.json: Configure the "content_scripts" section to specify which websites to inject the script into and the resources to include.
• inject.js: Inject the actual capture script, "injected.js," into the DOM.

Capturing HTTP Responses

The injected script named "injected.js" hijacks the native XMLHttpRequest object to monitor all network requests and responses. When a response is received, the script retrieves both the response headers and, if applicable, the response body.

Implementing the Capture Script

Within "injected.js":

1. Patch the "open" function to record the request details (method, URL, start time) and request headers.
2. Patch the "setRequestHeader" function to capture additional request headers.
3. Patch the "send" function to add an event listener that grabs the response data upon completion.

Obtaining the Response

When the request is completed, the event listener added in step 3 triggers and logs the response details, including:

• URL
• Request headers (JSON-formatted)
• Response headers
• Response body (JSON-formatted, if possible)

Conclusion

By utilizing script injection and event listeners, developers can easily capture and examine the response headers of HTTP requests made by a website. This enables valuable analysis and monitoring capabilities within Chrome extensions.

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