Microsoft KB Archive/260694

= Description of the MaxClientRequestBuffer Registry Value =

Article ID: 260694

Article Last Modified on 11/21/2006

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APPLIES TO


 * Microsoft Internet Information Services 5.0
 * Microsoft Internet Information Server 4.0

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This article was previously published under Q260694



We strongly recommend that all users upgrade to Microsoft Internet Information Services (IIS) version 6.0 running on Microsoft Windows Server 2003. IIS 6.0 significantly increases Web infrastructure security. For more information about IIS security-related topics, visit the following Microsoft Web site:

http://www.microsoft.com/technet/security/prodtech/IIS.mspx



IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows Registry



SUMMARY
You can use the new Internet Information Server (IIS) registry value MaxClientRequestBuffer to limit the cumulative bytes size of the request line and header fields sent in a request to IIS.

NOTE: In IIS 4.0, MaxClientRequestBuffer is only recognized when the program runs on Windows NT 4.0 Server Service Pack 5 or later.



Reason for MaxClientRequestBuffer
The MaxClientRequestBuffer registry value was added to IIS as a security measure, so that the maximum size of the request line and header fields could be set by the IIS administrator in order to reduce the likelihood of an attack caused by requests containing large amounts of data in the request line or headers.

NOTE: The HTTP specifications do not specify the maximum size of the request line or header fields.

For more information about a security problem that may occur when very large URLs that contain escape characters are used, see the security bulletin MS00-023 (http://www.microsoft.com/TechNet/security/bulletin/ms00-023.asp).

Default Sizes in IIS 4 and 5
In IIS 4.0, the default maximum size of request line and header fields is 2 megabytes (MB).

In IIS 5.0, this is reduced to 128 kilobytes (KB).

In IIS 5.0 with Windows 2000 Service Pack 4 installed, this is reduced to 16 KB.

Details of the Request Structure
The data limited by MaxClientRequestBuffer (the response line and headers), consists of all data from the first byte of the request through the last byte before the entity body. This includes the method, the URL, any additional path information, the query string, the HTTP version, all the headers, and the characters that delimit all parts of the request. In other words, this includes all data that is not part of the request's entity body.

Use POST Instead of GET to Send Large Amounts of Data in Request
GET requests use the URL portion of the request line to send information to IIS. The POST method, however, sends the information in the request's entity body instead. Therefore, you should consider using the POST method to send a large amount of data in the request.

MaxClientRequestBuffer Limit Is Affected by Buffers
Because IIS reads the blocks of the request data into buffers, the limit set by MaxClientRequestBuffer is not extremely precise. Therefore, after you use MaxClientRequestBuffer, you may have to test to see if the actual limit on the size of the client request and header fields is what you expect.

Side Effect on SF_STATUS_REQ_READ_NEXT
The use of MaxClientRequestBuffer can affect Internet Server Application Programming Interface (ISAPI) filters. If a filter attempts to use the SF_STATUS_REQ_READ_NEXT feature to read in a complete request before it would normally happen, it can encounter this limit, which causes requests to fail.

For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

255574 INFO: IIS Reports an Error with Filters Using the SF_STATUS_REQ_READ_NEXT Return Value.

Add the MaxClientRequestBuffer Registry Value
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.

To change the maximum size of a URL request, perform the following to add the MaxClientRequestBuffer registry value:  Run the Registry Editor (Regedt32.exe). Locate the following key in the registry:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\w3svc\parameters

 From the Edit menu, select Add Value, and then add the following registry value:

Value Name: MaxClientRequestBuffer

Data Type: REG_DWORD

 In the DWORD Editor dialog box, under Radix, select Decimal. In the Data text box, type the number of bytes for the maximum URL request size.

Note You must restart the IIS service for the changes to take effect.

(c) Microsoft Corporation 2000, All Rights Reserved. Contributions by Kevin Zollman, Microsoft Corporation.

Additional query words: iis 5 4 get request sp sp5 sp6 http 1.0 1.1 start-line header-fields entity-body entity-fields hack vulnerability akz

Keywords: kbinfo kbwin2ksp4fix KB260694

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