How to disable automatic proxy caching in Internet Explorer

Summary

When you configure Internet Explorer to use an automatic proxy configuration script, it caches the proxy that is returned by the FindProxyForURL call. The caching mechanism (Automatic Proxy Result Cache) is performed on a host basis (that is, not on an URL basis). This prevents you from using different proxies to gain access to the same Web server. The following sample scenario demonstrates this behavior:


 

• Configure a Web server that is named "Webserver" with two virtual folders: "/login" and "/test".

• Create the following PAC file:

  function FindProxyForURL (url, host)
  {
  if shExpMatch (url, "http://webserver/login") return "DIRECT";
  else return "PROXY myproxy:80; PROXY mybackupproxy:80";
  }

• Configure Internet Explorer to use this PAC file.

• Start Internet Explorer and type the following Web address:

  http://webserver/loginInternet Explorer loads the PAC file and correctly uses a direct connection.

• When the logon page appears, type http://webserver/test. If you use Network Monitor, you can observe that access to "http://webserver/test" still uses a direct connection instead of one of the returned proxies.

More Information

The Automatic Proxy Result Cache is a performance enhancement that was added to Internet Explorer 5.5 and later. The purpose of the cache is to reduce the client-side processing of the automatic proxy configuration script. When you connect to an Internet site, the FindProxyForURL function is used to determine whether a proxy should be used and which proxy to use. Internet Explorer 5.5 and later first checks the Automatic Proxy Result Cache to determine whether a proxy was used to connect to the host on previous attempts. If this checks fails, it indicates that this is the first attempt to connect to the host during the current session and the normal proxy detection logic applies.

If an automatic proxy configuration script is configured to be used and Internet Explorer is able to retrieve it from the network (either if the Automatically Detect Settings option or the Use automatic configuration script are enabled), the Automatic Proxy Result Cache is updated with the hostname being accessed and the complete set of proxy servers returned by parsing the script. In the example, any hostname that is determined to use a proxy server will have "PROXY myproxy:80; PROXY mybackupproxy:80" added to the Automatic Proxy Result Cache.

In this scenario, “myproxy” is used for any subsequent access to a cached hostname and the automatic proxy configuration script is not re-processed. If “myproxy” is suddenly inaccessible, the following happens:
 

• The “myproxy” proxy is added to the list of bad proxy servers.

• The “mybackupproxy” proxy is selected to serve the request for the hostname in question, taken from the Automatic Proxy Result Cache.

• If the connection is successful, “mybackupproxy” is used in subsequent requests to this hostname during the same browsing session.

If all of the proxy servers returned from the automatic proxy configuration script are inaccessible, then you receive a "Page Cannot Be Displayed" error message in Internet Explorer (which would happen if a static proxy server is suddenly inaccessible).

This allows proxy failover to still occur even if the Automatic Proxy Result Cache is enabled but it does not allow you to specify a different proxy server for the same hostname but, for instances, for different URL’s, as illustrated in the example. If this is a requirement, then you may want to disable the Automatic Proxy Result Cache feature. This will result in client-side processing of every GET request that is issued by Internet Explorer. As a result, Internet Explorer performance may be impacted depending on the logic of the Automatic Proxy Configuration Script and its size. To disable the Automatic Proxy Result Cache, use one of the following methods.

Note If you disable automatic proxy caching, Internet Explorer performance may be affected.