How to disable proxy PAC caching in Internet Explorer [RESOLVED]

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 Automatic Proxy Result Cache is a performance enhancement that Microsoft 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. 

Internet Explorer Proxy Settings

Note : When you deactivate Automatic Proxy Caching, the browser will follow client-side processing, which could speed up  page loading in the browser. Thus, this change will impact browser performance, depending on the logic of the Automatic Proxy Configuration Script used and its size. 

Method 1: Modify the registry

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:322756 How to back up and restore the registry in Windows

You can disable the Automatic Proxy Result Cache by using the following registry key:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\CurrentVersion\Internet Settings

Value: EnableAutoproxyResultCache
Data value: 0 = disable caching; 1 (or key not present) = enable automatic proxy caching (this is the default behavior)If the registry key is not present, you can create the registry key by using the following registry file:
Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\CurrentVersion\Internet Settings]”EnableAutoProxyResultCache”=dword:00000000″

Method 2: Modify Group Policy settings

  1. Click Start, click Run, type gpedit.msc, and then click OK.
  2. In Group Policy Object Editor, double-click User Configuration\Administrative Templates\Windows Components\Internet Explorer.
  3. Double-click Disable caching of Auto-Proxy scripts.
  4. Click Enable, and then click OK.

You may have to use this registry key if you connect by using a proxy server that is handling secure and non-secure requests on the same server. One example of this behaviour is the SQUID Proxy Server software. Because Internet Explorer typically caches port information, it may not send a secure request over the correct port number when it tries to send secure and non-secure responses to the same server, but on different port numbers.

Duncan Newell

Duncan is a technology professional with over 20 years experience of working in various IT roles. He has a interest in cyber security, and has a wide range of other skills in radio, electronics and telecommunications.

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