What's a 'Proxy' ?

A 'Proxy' is an intermediate computer that 'intercepts' a request made for an Internet resource, then reissues that request on behalf of the intercepted computer.

For example — as happened here just now — your computer's web browser requested a resource (this page) from the grc.com web site. But your browser's request was intercepted by a 'transparent' intermediate proxying computer — probably belonging to your Internet service provider. That proxying computer, in turn, forwarded the request to grc.com's web server.

Our security testing technology detected that this was happening and responded to the proxy with this special 'intercept' page so that the proxy would, in turn, return it to your web browser.

Because none of this is usually very important for normal web surfing, such proxies are termed 'transparent'. But the problem is that they are not transparent enough to allow reliable security testing. Since we were connected to the proxy, rather than to your computer, we are unable to accurately analyze the security of your computer's connection to the Internet. (We could tell you all about the proxy's security, but that's a subject for another page.)

Internet service providers often use proxies to improve the subjective performance of their network for their customers. By locally storing copies of often-requested web resources (like all the various bits and pieces of Amazon's web site) web pages assemble much more quickly. Most people are never aware of their existence.

The ShieldsUP! system incorporates technology designed to circumvent many common web browser proxies so that most of our visitors never encounter this special interception page. This built-in circumvention technology is one of several reasons why ShieldsUP!'s tests are often more accurate than other web-based online security tests.

However, in this case, it appears that our automatic proxy circumvention system has failed to determine your machine's true IP address, so the results of further tests would not be trustworthy.

The worrisome header contained in your request is:

The presence of this header is indicative of an intermediate proxy, as discussed above. We are unable to bypass this proxy, since even your browser's secure SSL connections are being intercepted. If you are able to disable your browser's use of this proxy we'll be able to check your system, but until then we are unable to proceed.