How to Use Managed Code (C#) to Create an FTP Provider that Prevents Leeching

by Robert McMurray

[This documentation is preliminary and is subject to change.]

Note: The FTP 8.0 service ships as a feature for IIS 8.0 in Windows 8 and Windows 8 Server.

Microsoft has created a new FTP 8.0 service for Windows Server® 2012 that builds upon the rich set of features that were introduced in FTP 7.0 and FTP 7.5. In addition, this new FTP service extends the list of Application Programming Interfaces (APIs) for FTP with new extensibility features like support for custom authorization and simple event processing.

With that in mind, this walkthrough will lead you through the steps to use managed code to create a simple FTP provider that prevents users from downloading more files per-session than you want to allow, which is a behavior that is known that is known in Internet-speak as "leeching." The FTP provider in this walkthrough implements the IFtpPreprocessProvider.HandlePreprocess() and IFtpPostprocessProvider.HandlePostprocess() methods in order to restrict a user to a maximum number of downloads that you specify in your provider's entry in the ApplicationHost.config file.

The following items are required to complete the procedures in this article:

1. IIS 8.0 must be installed on your Windows Server 2012 server, and the Internet Information Services (IIS) Manager must also be installed.
2. The new FTP 8.0 service must be installed.
3. You need to install Visual Studio.

Note: This walkthrough was written by using Visual Studio 2010. You may use other versions of Visual Studio, but some of the steps in this walkthrough may not be exact.

In this step, you will create a project in Visual Studio 2010 for the demo provider.

1. Open Microsoft Visual Studio 2010.

2. Click the File menu, then New, then Project.

3. In the New Project dialog box:

   • Choose Visual C# as the project type.
   • Choose Class Library as the template.
   • Type FtpLeechPrevention as the name of the project.
   • Click OK.

4. Add a strong name key to the project:

   • Click Project, and then click FtpLeechPrevention Properties.
   • Click the Signing tab.
   • Check the Sign the assembly check box.
   • Choose <New...> from the strong key name drop-down box.
   • Enter FtpLeechPreventionKey for the key file name.
   • If desired, enter a password for the key file; otherwise, clear the Protect my key file with a password check box.
   • Click OK.

5. Add a custom build event to add the DLL automatically to the Global Assembly Cache (GAC) on your development computer:

   • Click Project, and then click FtpLeechPrevention Properties.

   • Click the Build Events tab.

   • Enter the following in the Post-build event command line dialog box:

     net stop ftpsvc
     call "%VS100COMNTOOLS%\vsvars32.bat">nul
     gacutil.exe /if "$(TargetPath)"
     net start ftpsvc
     

6. Save the project.

In this step, you will implement the extensibility interface for the demo provider.

1. Add a reference to the FTP extensibility library for the project:

   • Click Project, and then click Add Reference...
   • Click the Browse tab.
   • Navigate to your %ProgramFiles(x86)%\Reference Assemblies\Microsoft\IIS folder.
   • Click Microsoft.Web.FtpServer and then click OK.

2. Add the code for the authentication class:

   • In Solution Explorer, double-click the Class1.cs file.

   • Remove the existing code.

   • Paste the following code into the editor:

     using System;
     using System.Collections;
     using System.Collections.Generic;
     using System.Collections.Specialized;
     using Microsoft.Web.FtpServer;
     
     public class FtpLeechPrevention :
       BaseProvider,
       IFtpPreprocessProvider,
       IFtpPostprocessProvider,
       IFtpLogProvider
     {
       private static Dictionary<string, int> _sessionList = null;
       private static int _maxDownloads = 1;
     
       protected override void Initialize(StringDictionary config)
       {
         // Test if the dictionary has been created.
         if (_sessionList == null)
         {
           // Create the dictionary if it hasn't been created.
           _sessionList = new Dictionary<string, int>();
         }
         // Retrieve the max downloads from configuration.
         if (string.IsNullOrEmpty(config["MaxDownloads"]) == false)
           _maxDownloads = Convert.ToInt16(config["MaxDownloads"]);
       }
     
       public FtpProcessStatus HandlePreprocess(
         FtpPreprocessParameters preProcessParameters)
       {
         // Test if the command was a RETR command.
         if (preProcessParameters.Command.Equals(
           "RETR", StringComparison.InvariantCultureIgnoreCase))
         {
           // Test if the max download count was exceeded.
           if (_sessionList[preProcessParameters.SessionId].Equals(_maxDownloads))
           {
             // Reject the command if the max download count was exceeded.
             return FtpProcessStatus.FtpProcessRejectCommand;
           }
         }
         return FtpProcessStatus.FtpProcessContinue;
       }
     
       public FtpProcessStatus HandlePostprocess(
         FtpPostprocessParameters postProcessParameters)
       {
         // Test if the command was a successful RETR command.
         if (postProcessParameters.Command.Equals(
           "RETR", StringComparison.InvariantCultureIgnoreCase) &&
           postProcessParameters.FtpStatus == 226)
         {
           // Increment the download count.
           _sessionList[postProcessParameters.SessionId]++;
         }
         return FtpProcessStatus.FtpProcessContinue;
       }
     
       public void Log(FtpLogEntry logEntry)
       {
         // Test if the control channel was opened.
         if (logEntry.Command.Equals(
           "ControlChannelOpened",
           StringComparison.InvariantCultureIgnoreCase))
         {
           // Add the current session to the dictionary.
           _sessionList.Add(logEntry.SessionId, 0);
         }
         // Test if the control channel was closed.
         if (logEntry.Command.Equals(
           "ControlChannelClosed",
           StringComparison.InvariantCultureIgnoreCase))
         {
           // Remove the current session from the dictionary.
           _sessionList.Remove(logEntry.SessionId);
         }
       }
     }
     

3. Save and compile the project.

In this step, you will add the demo provider to your FTP service and an FTP site.

1. Determine the assembly information for the extensibility provider:

   • Open a command prompt and enter the following commands:

     call "%VS100COMNTOOLS%\vsvars32.bat"
     
     gacutil.exe /l FtpLeechPrevention
     

   • Copy the information for the FtpLeechPrevention assembly; for example:

     FtpLeechPrevention, Version=1.0.0.0, Culture=neutral, PublicKeyToken=426f62526f636b73
     

   • Close the .

2. Add the extensibility provider to the global list of FTP providers:

   • At the moment there is no UI that enables you to add a custom provider with its related configuration settings to IIS, so you will have to use command line syntax like the following example:

     cd /d "%WinDir%\System32\Inetsrv"
     
     appcmd.exe set config -section:system.ftpServer/providerDefinitions /+"[name='FtpLeechPrevention',type='FtpLeechPrevention,FtpLeechPrevention,version=1.0.0.0,Culture=neutral,PublicKeyToken=426f62526f636b73']" /commit:apphost
     
     appcmd.exe set config -section:system.ftpServer/providerDefinitions /+"activation.[name='FtpLeechPrevention']" /commit:apphost
     
     appcmd.exe set config -section:system.ftpServer/providerDefinitions /+"activation.[name='FtpLeechPrevention'].[key='MaxDownloads',value='5']" /commit:apphost
     

     Note

     You need to update the above syntax using the managed type information for your provider and your desired number of downloads.

3. Add the custom provider to a site:

   • At the moment there is no UI that enables you to add custom features to a site, so you will have to use command line syntax like the following example that configures the Default Web Site:

     cd /d "%WinDir%\System32\Inetsrv"
     
     appcmd.exe set site "Default Web Site" /+ftpServer.customFeatures.providers.[name='FtpLeechPrevention',enabled='true'] /commit:apphost
     

In this walkthrough you learned how to:

• Create a project in Visual Studio 2010 for a custom FTP provider.
• Implement the extensibility interface for custom FTP functionality.
• Add a custom provider to your FTP service.

When an FTP client connects to the FTP service, the FTP provider will count the number of times that an FTP client downloads a file, and prevent the user from downloading more files per-session than you have allowed in your configuration settings.