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Professional ASP.NET 2.0 Special Edition
by Bill Evjen, Scott Hanselman, Devin Rader, Farhan Muhammad, Srinivasa Sivakumar
October 2006, Hardcover


Giving ASP.NET Proper Permissions to Upload Files

You might receive errors when your end users upload files to your Web server through the FileUpload control in your application. These might occur because the destination folder on the server is not writable for the account used by ASP.NET. If ASP.NET is not enabled to write to the folder you want, you can enable it using the folder's properties.

First, right-click on the folder where the ASP.NET files should be uploaded and select Properties from the provided menu. The Properties dialog for the selected folder opens. Click the Security tab to make sure the ASP.NET Machine Account is included in the list and has the proper permissions to write to disk. If it is enabled, you see something similar to what is presented in Figure 2.

Professional ASP.NET 2.0 Special Edition - Figure 2
Figure 2

If you don't see the ASP.NET Machine Account in the list of users allowed to access the folder, add ASP.NET by clicking the Add button and entering ASPNET (without the period) in the text area provided (see Figure 3).

Professional ASP.NET 2.0 Special Edition - Figure 3
Figure 3

Click OK, and you can then click the appropriate check boxes to provide the permissions needed for your application.

Understanding File Size Limitations

Your end users might never encounter an issue with the file upload process in your application, but you should be aware that some limitations exist. When users work through the process of uploading files, a size restriction is actually sent to the server for uploading. The default size limitation is 4MB (4096kb); the transfer fails if a user tries to upload a file that is larger than 4096kb.

A size restriction protects your application. You want to prevent malicious users from uploading numerous large files to your Web server in an attempt to tie up all the available processes on the server. Such an occurrence is called a denial of service attack. It ties up the Web server's resources so that legitimate users are denied responses from the server.

One of the great things about .NET, however, is that it usually provides a way around limitations. You can usually change the default settings that are in place. To change the limit on the allowable upload file size, you make some changes in either the web.config.comments (found in the ASP.NET 2.0 configuration folder at C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG) or in your application's web.config file.

In the web.config.comments file, find a node called <httpRuntime>. In this file, you see that the default allowable file size is dictated by the actual request size permitted to the Web server (4096KB). The <httpRuntime> section of the web.config.comments file is shown in Listing 2.

Listing 2: Changing the file-size limitation setting in the web.config file
<httpRuntime 
 executionTimeout="110" 
 maxRequestLength="4096" 
 requestLengthDiskThreshold="80" 
 useFullyQualifiedRedirectUrl="false" 
 minFreeThreads="8" 
 minLocalRequestFreeThreads="4" 
 appRequestQueueLimit="5000" 
 enableKernelOutputCache="true" 
 enableVersionHeader="true" 
 requireRootedSaveAsPath="true" 
 enable="true" 
 shutdownTimeout="90" 
 delayNotificationTimeout="5" 
 waitChangeNotification="0" 
 maxWaitChangeNotification="0" 
 enableHeaderChecking="true" 
 sendCacheControlHeader="true" 
 apartmentThreading="false" />

You can do a lot with the <httpRuntime> section of the web.config file, but two properties — the maxRequestLength and executionTimeout properties — are especially interesting.

The maxRequestLength property is the setting that dictates the size of the request made to the Web server. When you upload files, the file is included in the request; you alter the size allowed to be uploaded by changing the value of this property. The value presented is in kilobytes. To allow files larger than the default of 4MB, change the maxRequestLength property as in the following:

maxRequestLength="11000"

This example changes the maxRequestLength property's value to 11,000KB (around 10MB). With this setting in place, your end users can upload 10MB files to the server. When changing the maxRequestLength property, be aware of the setting provided for the executionTimeout property. This property sets the time (in seconds) for a request to attempt to execute to the server before ASP.NET shuts down the request (whether or not it is finished). The default setting is 90 seconds. The end user receives a timeout error notification in the browser if the time limit is exceeded. If you are going to permit larger requests, remember that they take longer to execute than smaller ones. If you increase the size of the maxRequestLength property, you should examine whether to increase the executionTimeout property as well.

If you are working with smaller files, it's advisable to reduce the size allotted for the request to the Web server by decreasing the value of the maxRequestLength property. This helps safeguard your application from a denial of service attack.

Making these changes in the web.config.comments file applies this setting to all the applications that are on the server. If you want to apply this only to the application you are working with, apply the <httpRuntime> node to the web.config file of your application, overriding any setting that is in the web.config.comments file. Make sure this node resides between the <system.web> nodes in the configuration file.