Why are graphics not appearing on some websites?

Overview

When using Web Filter to block categories, and also using the Pass Sites list to pass some websites, these passed websites may load without graphics. That is because these sites are loading graphics from another domain/site which is blocked by your Category settings. This graphics site will often, but not always, categorize in the same category as the site which it contains graphics for.

Referrers

When linking to offsite content, some sites will use a special tag called a referrer. This essentially alerts your web browser that the additional, offsite content is trustworthy and can be loaded even though it's hosted on a different website/URL.

In Web Filter > Advanced, you can see an option for "Pass if referrer matches any Pass Site". This option may reduce issues with referred traffic, but it is not always effective for a couple of reasons:

  • While referred traffic was common in the earlier days of the internet, many sites are no longer using this method at all.
  • The referrer 'tag' is contained in the packet headers. Much of the web is now encrypted HTTPS traffic, which cannot be read by Web Filter unless you are using SSL Inspector to inspect that site.

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Example

A good example of this is Pinterest. Pinterest.com is categorized under Social Networking and you can pass via Pass Sites, even when blocking the Social Networking category. However, pinterest.com loads images from pinimg.com, which is categorized under Content Delivery Networks. Since it is accessed without a referrer, content from this site might be blocked by the category block for Content Delivery Networks. The only resolution to this problem is to create a Pass Site entry for *pinimg.com*.
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You can see this traffic in Reports at Reports > Web Filter > All Web Events. Look for the session to the site you loaded without graphics and look for other sites associated with the session that are blocked by a category block. Pass this blocked site to have graphics load properly.

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