Installable File System (IFS) Drivers
Installable File System (IFS) drivers, also known as file system filter drivers, are a type of file system driver that can be inserted into the Windows file system stack. These drivers intercept file system operations at a specific level, allowing them to monitor, modify, or block I/O requests before they reach the underlying file system or the disk driver.
Overview
IFS drivers are powerful tools for a variety of purposes, including:
- Data Protection and Security: Implementing real-time virus scanning, encryption, or access control policies.
- Performance Optimization: Caching frequently accessed data or optimizing I/O patterns.
- Data Management: Implementing features like deduplication, compression, or snapshotting.
- Troubleshooting and Monitoring: Logging file system activity for auditing or diagnostic purposes.
Architecture
IFS drivers typically operate by attaching themselves to an existing file system stack. When an I/O request is made to a file or directory, it first passes through the IFS driver. The driver can then:
- Pass the request down to the next driver in the stack (usually the original file system).
- Complete the request itself.
- Modify the request before passing it down.
- Block the request entirely.
This interception mechanism allows for fine-grained control over file system operations.
Types of IFS Drivers
While the general principle is interception, IFS drivers can be categorized by their function:
- File System Filter Drivers: These are the most common type, designed to filter I/O requests.
- Mini-Filter Drivers: A more modern and recommended approach, part of the Filter Manager framework, which simplifies development and improves stability.
Developing IFS Drivers
Developing IFS drivers requires a deep understanding of the Windows kernel and its I/O subsystem. Key components and concepts include:
- I/O Request Packets (IRPs): The fundamental data structure used for I/O operations in Windows.
- Filter Manager: A kernel-mode service that simplifies the development of file system filter drivers. It manages the attachment of filters to file systems and handles many low-level details.
- Callback Routines: Drivers register callback routines that are invoked by the Filter Manager for specific I/O operations.
- Driver Entry Points: Standard kernel-mode driver entry points like
DriverEntry.
Key APIs and Structures:
FltCreateFile,FltReadFile,FltWriteFile: Functions for interacting with files via the Filter Manager.FLT_REGISTRATION: Structure used to register a mini-filter driver.PFLT_PRE_OPERATION_CALLBACK,PFLT_POST_OPERATION_CALLBACK: Types for pre- and post-operation callback routines.
For detailed guidance, refer to the official Microsoft documentation on File System Filter Drivers and the Filter Manager APIs.