Best practices - Find out why a web page displays no images

Use rule tracing to find out why a requested web page appears on a client system, but with text only and without displaying any images.

Imagine a sample issue, where a user requests access to the CNN channel homepage from a browser on a client of Web Gateway. The page appears, but displays only text.

You can use rule tracing to see whether a CNN server that provides the images on the homepage might have been blocked and why this happened.

Task

  1. On the user interface of Web Gateway, select Troubleshooting.
    If you are using several Web Gateway appliances in a Central Management configuration, make sure you are logged on to the appliance that the client in question is connected to.
  2. On the troubleshooting tree, select Rule Tracing Central.
  3. Create a trace.
    1. In the input field on the top left, type the IP address of the client system that had a request blocked, then click Go on the toggle button next to the input field.

      Requests for web access sent from the client are now traced and entries for trace files are displayed in the output field on the lower left.

      The Go on the toggle button turns into a cross to let you stop the process when no more tracing is needed.

    2. On the client system, refresh the browser or click or enter ccn.com again to reproduce the issue.

      Trace file entries appear in the output field on Web Gateway.

      Note: Depending on the amount of data that is being transferred, it can take a while until the trace file entries appear.
    3. When you have reproduced the issue and the trace file entries have appeared, click the toggle button again to stop the tracing.
  4. Review the trace file entries.

    For every request that has been traced, a time stamp and the requested URL are shown.

    At the beginning of an entry, a symbol for the most impacting action that was executed when processing the request is also shown. The most impacting of all actions is the Block action.

    When reviewing the trace file entries, you will see several entries with the blocking symbol and a URL beginning with cdn.turner.com/ccn. These are probably trace files for requests to access the CCN server that provides the images.
  5. Select a trace file entry with cdn.turner.com/ccn.
    Information on this trace appears in the rules and details panes on the right.
  6. Review the rules pane.

    The pane shows the rules that were processed for the request that was traced. The view stops at the last rule that applied before rule processing stopped. The rule is highlighted.

    This way you can see that Block URLs whose category is in Category Block List is the last rule that applied.

  7. Review the details pane.

    On the two tabs of the details pane, more tracing information is shown.

    On the Top Properties tab, you will see, among other information, that the URL.Categories property had the value Business when the rule mentioned above was processed.

Results

This completes your rule tracing activities for this issue. Images from the CNN server were not displayed because the URLs that were submitted for accessing this server have fallen into the Business category and this category is on a blocklist.

If you want to see the images displayed, you need to reconfigure the web security policy for your network and put, for example, cdn.turner.com/ccn/* on a URL whitelist.