IIS web server, ISAPI extensions and form posting default caching size of 48K

If you are developing especially native code applications for Microsoft's IIS web server, you might in certain cases run into a difficult to debug issue with IIS' default upload/form processing. By default, IIS has a property called UploadReadAheadSize which has the value of 49152, or 48K. This setting affects how ISAPI extensions process data that is uploaded to the server, either as file attachments or (large) form fields.

If this setting is too small for your application, then you might see missing upload information, truncated form data, or similar. This can lead you to think it is a bug in your application, while instead it could be a simple issue of reconfiguring IIS.

This setting is documented on MSDN, and can be changed for example using an ADSI script. Alternatively, if you are using IIS 7 or 7.5 (Windows Server 2008 and Windows 7), then you can also change this setting using the management console. Go to the application's configuration editor settings, and navigate to the path System.WebServer -> Server Runtime -> uploadReadAheadSize. Change the value to something larger (for example 200K), and hit Apply. You're all set!