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Software and strings used in blocking the referring URL

There are many "personal privacy" or "personal firewall" products which use a client-side proxy to modify what a user's browser sends and receives.

The most common use of the proxy is to strip cookies, Javascript, and banner ads before the browser receives them. A less common function is to strip or modify the browser's Cookie, Referer, or User-Agent header which is sent which each request. When the Referer header is stripped or modified, tracking programs like AXS are not able to "make sense" of the visit. Instead of showing the URL that was visited, AXS may show nothing, may show a forged web address, or may show the standard string that the client-side proxy uses to replace the Referer.

AXS was created in 1997 before such client-side proxies were common; it doesn't handle the missing Referer field very well.

Common replacement strings are:

referrer blocked by AdSubtract

Referrer is being replaced by AdSubstract Pro, a program whose main purpose is to strip banner ads. See adsubtract.com.

Field blocked by Outpost (http://www.agnitum.com)

Referrer is being replaced by Outpost, a personal firewall program. From their FAQ http://www.agnitum.com/support/outpostfaq.html

Why do I see 'Field blocked by Outpost' in my web server logs?
If you see 'Field blocked by Outpost' in your site statistics, it means those visitors have Outpost Firewall installed on their own computers. They have set Outpost to block 'referrers' to protect their privacy and not disclose the site they visited prior to yours. Please note that these users still will be able to browse your website. You can ask your visitors to enable Referrers in their Active Content plug-in to see how they arrived at your site.


This is reportedly done by Zone Alarm Pro; some have reported that Symantec's Norton Internet Security uses this string as well.

Because of the way this is implemented, the referring URL is often logged by AXS as the URL folder following by that string, i.e. "http://www.xav.com/perl/Hidden-Referrer".

    "Software and strings used in blocking the referring URL"