Stop error 0xD1 occurs when code sets the SIO_FASTPATH_LOOP_BACK flag

Applies to: Windows 8.1Windows 8.1 EnterpriseWindows Server 2012 R2 Datacenter


You have a third-party legacy Transport Driver Interface (TDI) filter driver installed on a computer. If you run code that sets the SIO_FASTPATH_LOOP_BACK flag for a connection, you receive Stop error message 0xD1.


This problem occurs because TDI filter drivers implicitly set the SO_CONDITIONAL_ACCEPT flag for all TCP connections that are opened on that computer. After you set the SIO_FASTPATH_LOOP_BACK option for a loopback connection, a race condition occurs.

TDI filter drivers may be used by some antivirus and VPN software.


Hotfix information

A supported hotfix is available from Microsoft Support. 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 the problem described in this article. 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, go to the following Microsoft website: 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.


To apply this hotfix, you must be running Windows 8.1 or Windows Server 2012 R2.

Additionally, you must first install update 2919355 on Windows 8.1 or Windows Server 2012 R2.

Restart requirement

You must restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any previously released hotfix.


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.


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