Windows 95 Denial of Service

IGMP Query Times Out in Windows 95 with Winsock2
Update on this Denial of Service Attack

Reported November 14, 1997

Systems Affected

Last reviewed: December 3, 1997
Article ID: Q174095

The information in this article applies to:
  • Microsoft Windows 95
  • Microsoft Windows 95 OEM Service Release 2


On a Windows 95-based computer with the Winsock2 Update for Windows 95 installed, IGMP queries may time out prematurely. This may result in computers that are members of a multicast group being dropped from group membership.


The TCP/IP driver in the Winsock2 Update for Windows 95 does not properly implement the Max Response Time field as documented in the IGMP version 2 draft. This behavior may cause clients participating in a multicast group to not respond to IGMP requests from a router within the timeout period. When this occurs, the router no longer forwards multicast packets to the expired group member.


This issue is resolved by the following updated file for Windows 95 and Windows 95 OEM Service Release 2 (OSR2):

   Vip.386 version 4.10.1512 (dated 7/29/97) and later

To install this update, use the following steps.

NOTE: This update should be applied only to Windows 95-based computers on which the Winsock2 Update has been installed.

  1. Download the Vipup20.exe file from the Microsoft Software Library to an empty folder.
  2. In My Computer or Windows Explorer, double-click the Vipup20.exe file you downloaded in step 1.
  3. Follow the instructions on the screen.

The following file is available for download from the Microsoft Software Library:

 ~ Vipup20.exe (size: 176312 bytes) 

    For more information about downloading files from the Microsoft Software Library, please see the following article in the Microsoft Knowledge Base:

       ARTICLE-ID: Q119591
       TITLE     : How to Obtain Microsoft Support Files from Online Services

    The following files are installed by Vipup20.exe:

       File name     Version    Date/Time        Size     Destination folder
       Vip.386       4.10.1512  7/29/97 3:12p    71,754   Windows\System


    The Winsock2 Update for Windows 95 is included in the Winsock2 Software Development Kit (SDK), available from the following location on the Microsoft FTP site:

    This issue is known to occur with the Vip.386 file version 4.10.1423 included in the Ws295sdk.exe file dated 6/23/97 from the FTP location listed above. Future revisions to this package may not exhibit this symptom.

    Keywords          : win95 kbfile kbnetwork kbtool kbfixlist
    Version           : WINDOWS:95
    Platform          : WINDOWS
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