Deep Packet Inspection Using Layer 7 Traffic Shaping

Deep packet inspection

The Layer 7 tab in the Traffic Shaper in pfSense 2.1.

For quite a while, traffic shaping has been considered an integral part of any good firewall. This necessitates some means of classifying the traffic so the traffic can then be policed. The traditional method of traffic shaping centered on classifying traffic based on network and transport data fields, not using deep packet inspection. This usually centered around the following elements:

  • Service class marks
  • Source and/or destination IP addresses
  • Ports

However, these methods are not always effective in traffic classification. This is especially the case with P2P traffic, which often uses random, non-default ports. An HTTP server utilizing port hopping and encrypted traffic may also defy level 3 (network) and level 4 (transport) classification.

Enter Layer 7 Deep Packet Inspection

One possible solution to the shortcomings of network and transport level classification is layer 7 (L7) classification, which involves deep packet inspection. In L7 classification, user traffic can be identified based on an application pattern, which is a sort of signature used by an application during its communications. All applications either use a specific application pattern or may share the pattern with other applications.

Deep packet inspection

Configuring P2P options in the wizard.

IPCop is a good example of utilizing L7 deep packet inspection and classification. IPCop is a Linux-based firewall that was originally a fork of the SmoothWall firewall. Although not an official part of IPCop, an advanced QoS (Quality of Service) add-on is available. But while IPCop can support classification by application protocol, it does not allow the definition of shaping policies. Rather, it can only block such traffic, which greatly limits the use of this feature.

pfSense, however, has fully incorporated L7 deep packet inspection and classification into its traffic shaper. Traffic shaping is achieved in pfSense through AltQ, which makes available Class Based Queueing (CBQ), Priority Queueing (PRIQ) and Hierarchical Fair Service Curve (HFSC). All of these can be configured automatically through the use of a wizard. Beginning with pfSense 2.0, an additional shaping mechanism called Dummynet became available. Dummynet was originally designed for the ipfw firewall, and has a related application called ipfw-classifyd. This application is able to produce blocking rules for incoming traffic or perform traffic shaping by assigning IP packets to an AltQ queue or a Dummynet pipe or queue. It was modified to work with the pf firewall and is the component responsible for L7 classification. It also allows different types of operations to be applied to an identified application protocol, usually either blocking it or assigning it to a limiter or queue.

In order to invoke ipfw-classifyd, pf uses divert sockets. Essentially, it interrupts the normal flow of packets and sends them to a listening socket (ipfw-classifyd). Overhead is kept to a minimum by teaching pf about the actions to be taken ahead of time and by limitng the number of packets that are diverted from the kernel to the application. All of this is controlled via a graphical interface, in which the user must specify at least one protocol (but may specify more than one). The user can create L7 rules groups containing one or more L7 rules. The user can take any one of the created rules groups and assign it a firewall rule.

But with pfSense, the user does not have to explicitly create L7 rules groups. This is because the Traffic Shaper Wizard in versions 2.0 and newer invokes L7 classification in the Peer-to-Peer and Network Games sections. In both sections, the select box on the top of the page can be enabled, and the related protocols or applications can be blocked one by one. Finally, the user can extend the functionality of L7 packet inspection by uploading new application patterns to the system. This feature is important when the user wants to block an application that uses a protocol pattern that is not defined in the system. If such a pattern is uploaded to the system, it only appears in the list of protocols when a container is created or modified. It does not affect the Traffic Shaper Wizard, which remains unchanged.

Other Articles in This Series:

Traffic Shaping in pfSense: What it Does
Traffic Shaping Wizard: Introduction
Queue Configuration in pfSense 2.1
Traffic Shaping Rules in pfSense 2.1
Traffic Shaping Rules in pfSense 2.1
Layer 7 Rules Groups in pfSense 2.1
Bandwidth Limiting with the pfSense Limiter

External Links:

L7 Classification and Policing in the pfSense Platform – a scholarly paper about the addition of layer 7 deep packet inspection to pfSense 2.0.

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