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How to Capture Traffic in Browser

Sometimes support might ask you to capture the traffic that the proxy sent to the browser. This short article explains how to do that using built-in tools of Microsoft Edge, Google Chrome or Mozilla Firefox.

Close All Browser Instances

To keep the irrelevant information to the minimum, it is first recommended to close all browser windows and ideally all running applications on your workstation.

Start Browser in Incognito Mode

Now open a new incognito (in-private) page as shown on the following screenshot. This ensures the browser does not cache any resources when rendering sites giving actual picture of proxy requests/responses to the support team.

Empty Private Browsing Page

Open Browser Tools

Click F12 to open built in browser network debugging tools and select the Network tab on the top window, then make sure the Persist Logs or Preserve Logs checkbox is set. Also make sure the Disable Cache checkbox is also set.

Preserve Log

Reproduce the Problem

Navigate to the problematic site and try to reproduce the problem that you are having with the proxy. Note how the internal HTTP requests and responses are rendered in the Nework tab. Errors in JavaScript will be shown on the Console tab.

Reproducing the Problem

Export HAR File

Now click the Export HAR File button as shown on the following screenshot. Archive this file and send it to for inspection. Note currently HAR file generated by Mozilla Firefox has much more debugging information than Google Chrome or Microsoft Edge - so ideally please use Mozilla Firefox to generate the HAR file.

Exporting HAR File

Blocked Resources

Usually the problems with the site rendering happen because proxy blocked some requests and responses from the browser. Such items can be identified by the 403 Blocked responses. The headers and or contents of the blocked response will usually explain why it was blocked.

Request Blocked

Incident ID            13028 on 2023-May-08 11:39:31
Scanned By             categories in 0 milliseconds
Detection Results      Blocked
Detection Info user_tracking
Assigned Categories    user_tracking
Requested URI

Changing web filtering policies or bypassing web filtering completely for that site may fix the problems.