Network traffic Filtering techniques for Windows, either in user-mode or kernel-mode, falls into one of two categories: stream and packet methods. This document presents useful techniques to build robust security software products such as personal firewalls and VPN clients for Windows 2000 or higher.
Before going further with this article, I would personally recommend WPF for Vista and higher, and TDI filters + NDIS Hook for earlier versions to build a combined stream and packet filtering solutions.
Winsock Layered Service Provider
A Winsock Layered Service Provider (LSP) is a DLL that operates on the Winsock functions to inspect, modify and intercept the inbound and outbound Internet traffic as streams and not as packets. LSP also runs in the workspace of the process it intercepts making easy to filter streams based on caller PID, short name or full path.
LSP can be chained and are useful tool for data-monitoring, content filtering, stream based sniffers, Quality of Service (QoS), authentication, encryption … LSP technology is often exploited by spyware and adware programs to bombard users with advertisements and email spam.
There is one known limitation and one common issue with LSPs. On some Windows versions, LSP can be bypassed by calling TCP/IP stack directly via TDI making useless, for instance, Trojan or virus protections at this level. A bogus LSP or improper LSP removal/unregistration operation may break the whole TCP/IP stack or leave the machine without working network connection.
Windows 2000/XP Filter Hook Driver
A Filter Hook driver is supported on Windows 2000/XP only and is implemented as a kernel mode driver. It operates by registering a callback with the IP Filter Driver that gets called when sending a receiving a packet. Filtering rules are limited to pass, drop or forward decision based on IP addresses and ports information.
The callback registration process uses an IRP with IOCTL_PF_SET_EXTENSION_POINTER as an IO control code and a PF_SET_EXTENSION_HOOK_INFO structure filled with a pointer to the callback routine.
A Filter Hook driver is simple to implement but has three serious limitations. Only one callback routine can be installed each time on the system. It is not possible to filter Ethernet frames. Outgoing packets cannot be modified.
Windows 2000/XP Firewall Hook Driver
A Firewall Hook driver is very similar to a Filter-Hook driver but installs a callback in the IP driver. The callback registration process uses an IRP with IOCTL_IP_SET_FIREWALL_HOOK as an IO control code and an IP_SET_FIREWALL_HOOK_INFO structure filled with a pointer to the callback routine.
Although it is not well documented, writing a Firewall Hook driver requires few lines of code. The main limitation is the support of Windows 2000 and XP only.
NDIS Hook Driver
There are two approved techniques to write an NDIS Hook driver. The first one is based on interception of some NDIS wrapper functions at runtime by writing a kernel mode driver that patches NDIS.sys in memory to replace the addresses of NdisRegisterProtocol, NdisDeregisterProtocol, NdisOpenAdapter and NdisCloseAdapter functions with internal ones.
The second one is based on registering a fake NDIS Protocol driver just to get a pointer to an internal NDIS structure NDIS_PROTOCOL_BLOCK.
At this level, both methods have enough information to substitute all protocols and adapters handlers to getting full control over all network traffic.
Although these approaches use sophisticated hacking techniques and require good understanding of different NDIS versions internals, an NDIS Hook driver is easy to install and able to filter, inject or modify packets. Several security software products including personal firewalls and VPN clients use these techniques.
This approach is discouraged for Windows Vista and higher.
NDIS Intermediate Driver
An NDIS intermediate driver, also called NDIS IM driver, is inserted just above miniport drivers and just below transport protocols in the overall networking protocol stack allowing incoming and outgoing packets filtering, inspection or modification. An NDIS Intermediate driver is a documented alternative to NDIS Hook drivers and offers the same functionalities.
NDIS intermediate drivers should be digitally signed at Microsoft to allow silent installations. This technology is replaced by NDIS Lightweight Filter drivers on Vista and higher.
NDIS Lightweight Filter Driver
NDIS Lightweight Filter drivers (LWF drivers) are introduced in Windows Vista and higher to replace NDIS Intermediate driver technology. They offer the same packets filtering, inspection or modification capabilities.
NDIS Lightweight Filter drivers are easier to implement and are designed to improve overall performances.
TDI Filter Driver
The Transport Driver Interface (TDI) defines a kernel mode network interface that is exposed at the upper edge of all transport protocol stacks. TDI also provides standard methods for protocol addressing, sending and receiving datagrams, writing and reading streams, initiating connections, detecting disconnects making it the only socket interface in the kernel.
TDI Filter drivers sit between TDI clients (such as AFD.sys, NETBT.sys) and TDI transports (such as TCPIP.sys) and intercept the communication between them. In case of TCP/IP filtering, the technique consists in writing a kernel-mode driver that layers itself over devices created by TCPIP.sys driver (\Device\RawIp, \Device\Udp, \Device\Tcp, \Device\Ip and \Device\MULTICAST) using IoAttachDevice routine. A good understanding of how to handle and interact with IRPs is required.
It is recommended to stop using TDI filters and move to Windows Filtering Platform (WFP) on Vista and later platforms. Windows makes it possible for TDI filters to see TCP/IP traffic is just for compatibility reasons and it does not yield good performance.
Windows Filtering Platform
Windows Filtering Platform (WPF) is a new architecture available in Windows Vista and higher that was built to replace all existing packet filtering technologies such as Winsock LSP, TDI filter and NDIS Intermediate driver and to provide better performance and less development complexities. Callout drivers, Filter Engine, Base Filtering Engine and Shims are components of the WPF architecture.
The WFP API consists of a user-mode API and a kernel-mode API that interacts with the packet processing that takes place at several layers in the networking stack. With WFP, incoming and outgoing packets can be filtered and modified before they reach their destinations, making this architecture ideal for implementing various filtering applications or solutions (such as personal firewalls, intrusion detection systems, antivirus programs, network monitoring tools, and parental controls). WFP arbitration rules also minimize the risk that software components get affected by any future Service Pack release.
WPF is highly recommended for developing security related solutions on Vista and higher.