Wednesday, April 29, 2015

Snort++ Inspection Events

One of the goals of Snort++ is to provide a more flexible framework for packet processing by implementing an event-driven approach.  Another is to produce data only when needed, to minimize expensive normalizations.  To help explain these concepts, let's start by examining how Snort processes packets.  The key steps are given in the following figure:

Snort 2X Packet Processing
The preprocess step is highly configurable.  Arbitrary preprocessors can be loaded dynamically at startup, configured in snort.conf, and then executed at runtime.  Basically, the preprocessors are put into a list which is iterated for each packet.  Recent versions have tweaked the list handling
some, but the same basic architecture has allowed Snort to grow from a sniffer, with no preprocessing, to a full-fledged IPS, with lots of preprocessing.

While this "list of plugins" approach has considerable flexibility, it hampers future development where the flow of data from one preprocessor to the next depends on traffic conditions, a common situation with advanced features like application identification.  In this case, a preprocessor like HTTP may be extracting and normalizing data that ultimately is not used, or app ID may be repeatedly checking for data that is just not available.

Callbacks help break out of the preprocess straightjacket.  This is where one preprocessor supplies another with a function to call when certain data is available.  Snort has started to take this approach to pass some HTTP and SIP preprocessor data to app ID.  However, it remains a peripheral feature and still requires the production of data that may not be consumed.

The basic processing steps Snort++ takes are similar to Snort's as seen in the following diagram.  The preprocess step employs specific inspector types instead of a generalized list, but the basic procedure includes stateless packet decoding, TCP stream reassembly, and service specific analysis in both cases.  (Snort++ provides hooks for arbitrary inspectors, but they are not central to basic flow processing and are not shown.)

Snort 3X Packet Processing

However, Snort++ also provides a more flexible mechanism than callback functions.  By using inspection events, it is possible for an inspector to supply data that other inspectors can process.  This is known as the observer pattern or publish-subscribe pattern.

Note that the normalized data is not actually published.  Instead, access to the data is published, and that means that subscribers can access the raw or normalized version(s) as needed.  Normalizations are done only on the first access, and subsequent accesses get the previously normalized data.  This results in just in time (JIT) processing.

A basic example of this in action is provided by the extra data_log plugin. It is a passive inspector, ie it does nothing until it receives the data it subscribed for ('other' in the above diagram).  By adding the following to your snort.lua configuration, you will get a simple URI logger:

data_log = { key = 'http_raw_uri' }

Inspection events coupled with pluggable inspectors provide a very flexible framework for implementing new features.  And JIT buffer stuffers allow Snort++ to work smarter, not harder.  These capabilities will be leveraged more and more as Snort++ development continues.  Look for weekly updates on github (snortadmin/snort3) and monthly updates on snort.org.

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