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256986 Description of the Microsoft Windows Registry
When you connect through a caching ISA Proxy that has anonymous access, and you try to perform a NT LAN Manager (NTLM) Authentication with an IIS4 or IIS5 Web server, Microsoft Internet Explorer 6 may not automatically authenticate with NTLM pre-authorization (PreAuth). Instead, you may be continually prompted to authenticate.
A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.
If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.
Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site:
Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.
The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.
Date Time Version Size File name ------------------------------------------------------- 06-Aug-2003 13:15 6.0.2800.1243 586,240 Wininet.dll
Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
This hotfix uses the following registry information to determine how to treat the socket during a 401 Authentication. To use the hotfix, set the registry value to 0. To turn off the hotfix and revert to the original behavior, set the value to 1. By default, the installation of this hotfix package sets the value to 0.
To set the registry value to 0, follow these steps:
Click Start, click Run, type regedit, and then click OK.
Locate and then click either of the following keys in the registry:
On the Edit menu, point to New, and then click DWORD.
Type ReleaseSocketDuring401Auth, and then press ENTER.
On the Edit menu, click Modify.
Type 0, and then click OK.
To work around this problem, edit all the "401" response pages in your IIS deployment so they are larger than 1460 bytes. 1460 bytes is the size of one TCP packet.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
If the "401" response page content is smaller than 1460 bytes, Internet Explorer closes the current connection and tries to re-use a previous connection from the keep-alive pool for that server. Because ISA (or any other proxy) has closed the related connection between ISA and the IIS server, the re-use of the Internet Explorer connection does not work. As a result, Internet Explorer produces an authentication prompt.
When the "401" response page is larger than 1460 bytes, Internet Explorer must maintain the current connection so that it can drain the socket of all pending data. This causes Internet Explorer to re-use this socket for the later NTLM authentication traffic. As a result, the connection works seamlessly.