Writing Analyzers with Spicy

Spicy is a parser generator that makes it easy to create robust C++ parsers for network protocols, file formats, and more. Zeek supports integrating Spicy analyzers so that one can create Zeek protocol, packet and file analyzers. This section digs into how that integration works. We begin with a short “Getting Started” guide showing you the basics of using Spicy with Zeek, followed by an in-depth tutorial on adding a complete protocol analyzer to Zeek. The final part consists of a reference section documenting everything the Spicy integration supports.

While this documentation walks through all the bits and pieces that an analyzer consists of, there’s an easy way to get started when writing a new analyzer from scratch: the Zeek package manager can create analyzer scaffolding for you that includes an initial Spicy grammar (*.spicy), Zeek integration glue code (*.evt; see below) and a corresponding CMake build setup. To create that scaffolding, use the package managers create command and pass one of --features=spicy-protocol-analyzer, --features=spicy-packet-analyzer, or --features=spicy-file-analyzer to create a Zeek protocol, packet, or file analyzer, respectively. See the tutorial for more on this.

Note that Zeek itself installs the grammars of its builtin Spicy analyzers for potential reuse. For example, the Finger grammar gets installed to <PREFIX>/share/spicy/finger/finger.spicy. It can be used in custom code by importing it with import Finger from finger;.


This documentation focusses on writing external Spicy analyzers that you can load into Zeek at startup. Zeek also comes with the infrastructure to build Spicy analyzers directly into the executable itself, just like traditional built-in analyzers. We will document this more as we’re converting more of Zeek’s built-in analyzers over to Spicy. For now, we recommend locking at one of the existing built-in Spicy analyzers (Syslog, Finger) as examples.


A word on terminology: In Zeek, the term “analyzer” generally refers to a component that processes a particular protocol (“protocol analyzer”), file format (“file analyzer”), or low-level packet structure (“packet analyzer”). “Processing” here means more than just parsing content: An analyzer controls when it wants to be used (e.g., with connections on specific ports, or with files of a specific MIME type); what events to generate for Zeek’s scripting layer; and how to handle any errors occurring during parsing. While Spicy itself focuses just on the parsing part, Spicy makes it possible to provide the remaining pieces to Zeek, turning a Spicy parser into a full Zeek analyzer. That’s what we refer to as a “Spicy (protocol/file/packet) analyzer” for Zeek.