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Q189099: RRAS Could Forward IP Fragments Without Filtering

Article: Q189099
Product(s): Microsoft Windows NT
Version(s): WinNT:4.0
Operating System(s): 
Keyword(s): 
Last Modified: 09-AUG-2001

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The information in this article applies to:

- Microsoft Routing and Remote Access Service Update for Windows NT Server version 4.0 
- Microsoft Windows NT Server version 4.0 
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IMPORTANT: This article contains information about editing the registry. 
Before you edit the registry, make sure you understand how to restore it if
a problem occurs. For information about how to do this, view the "Restoring 
the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help 
topic in Regedt32.exe.

SYMPTOMS
========

If you enable packet filtering on an interface in the Microsoft Routing and
Remote Access Service Update for Windows NT (RRAS), IP fragments could be
forwarded without being filtered.

CAUSE
=====

RRAS filters are "stateless." Therefore, no matter how the filter is defined, it
is possible that an unwanted fragment will be forwarded. For RRAS to drop
unwanted fragments, it must keep track of the protocol state, from packet to
packet, for each routed conversation.

RESOLUTION
==========

To resolve this problem, obtain the latest service pack for Windows NT 4.0. For
additional information, please see the following article in the Microsoft
Knowledge Base:

  Q152734 How to Obtain the Latest Windows NT 4.0 Service Pack


After you apply the service pack, make the following registry changes to use the
new filter capabilities:

WARNING: Using Registry Editor incorrectly can cause serious problems that may
require you to reinstall your operating system. Microsoft cannot guarantee that
problems resulting from the incorrect use of Registry Editor can be solved. Use
Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys and
Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete
Information in the Registry" and "Edit Registry Data" Help topics in
Regedt32.exe. Note that you should back up the registry before you edit it. If
you are running Windows NT or Windows 2000, you should also update your
Emergency Repair Disk (ERD).

  HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IPFilterDriver
  \Parameters

NOTE: The above registry key is one path; it has been wrapped for readability.

Add the following values to the above Registry key:

- DefaultForwardFragments - REG_DWORD = 0 (Decimal)

  Setting this to "0" causes the filterdriver to drop fragments if they don't
  match a previous header.

- EnableFragmentChecking - REG_DWORD = 1 (Decimal)

  Setting this to "1" enables fragment checks.

NOTE: Even with this new filter update properly installed, it is possible for
unwanted fragments to be forwarded through the router.


Additional query words: ntrouter

======================================================================
Keywords          :  
Technology        : kbWinNTsearch kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbAudDeveloper kbRRASNTSearch kbRRASNT400
Version           : WinNT:4.0
Hardware          : ALPHA x86
Issue type        : kbbug

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