Introduction
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Introduction

Bro is an intrusion detection system that works by passively watching traffic seen on a network link. It is built around an event engine that pieces network packets into events that reflect different types of activity. Some events are quite low-level, such as the monitor seeing a connection attempt; some are specific to a particular network protocol, such as an FTP request or reply; and some reflect fairly high-level notions, such as a user having successfully authenticated during a login session.

Bro runs the events produced by the event engine through a policy script, which you (the Bro administrator) supply, though in general you will do so by using large portions of the scripts (``analyzers''; see below) that come with the Bro distribution.

You write policy scripts in ``Bro'', a specialized language geared towards network analysis in general and security analysis in particular. Bro scripts are made up of event handlers that specify what to do whenever a given event occurs. Event handlers can maintain and update global state information, write arbitrary information to disk files, generate new events, call functions (either user-defined or predefined), generate alerts that produce syslog messages, and invoke arbitrary shell commands. These latter might terminate a running connection or talk to your border router to install an ACL prohibiting traffic from a particular host, for example.

The Bro language is strongly typed and includes a bunch of types designed to aid analyzing network traffic. It also supports implicit typing, meaning that often you don't need to explicitly indicate a variable's type because Bro can figure it out from context. This feature makes the strong typing a bit less of a pain, while retaining its bug-finding benefits.

For high performance, Bro relies on use of an efficient packet filter to capture only a (hopefully small) subset of the traffic that transits the link it monitors. Related to this, Bro comes with a set of analyzers, that is, scripts for analyzing different protocols and different types of activity. In general you can pick and choose among these for which types of analysis you want to enable, and Bro will only capture traffic relating to the analyzers you choose. Thus, you can control how much work Bro has to do by the analyzers you designate, a potentially major consideration if the monitored link has a high volume of traffic.

Experience has shown that the policy scripts often require tailoring to each environment in which they're used; but if the tailoring is done by editing the analyzers supplied with the Bro distribution, you wind up with multiple copies of the analyzers, all slightly different, such that when you want to make a general change to all of them, it takes careful (and tedious) editing to correctly apply the change to all of the copies.

Consequently, Bro emphasizes the use of tables and sets of values as ways to codify policy particulars such as which hosts should generate alerts if seen engaged in various types of connections, which usernames are sensitive and should trigger alerts when used, and so on. The various analyzers are written such that you can (often) customize them by simply changing variables associated with the analyzer. Furthermore, Bro supports a notion of refining the initialization of a variable, so that, in a separate file from the one defining an analyzer, you can either (i) redefine the variable's initial value, (ii) add new elements to a given table, set or pattern, or (iii) remove elements from a given table or set. In a nutshell, refinement allows you to specify particular policies in terms of their differences from existing policies, rather than in their entirety.

You can find an overview of Bro in the paper ``Bro: A System for Detecting Network Intruders in Real-Time,'' Proceedings of the 1998 USENIX Security Symposium [Pa98], and a revised version in Computer Networks [Pa99]. A copy of the latter is included in the Bro distribution.

Using this manual. This manual is intended to provide full documentation for users of Bro, both those who wish to write Bro scripts to use Bro's existing analyzers, and those who wish to implement event engine support for new Bro analyzers. The current version of the manual is incomplete; in particular, it does not discuss the internals of the event engines, and a number of other topics have only placeholders.

The manual is organized not as a tutorial, but rather closer to a reference manual. In particular, the intent is for the index to be highly comprehensive, and to serve as one of the main tools to help you navigate through Bro's numerous features and capabilities. Accordingly, the index contains many ``redundant'' entries, that is, the same information indexed in multiple ways, to try to make it particularly easy to look up information. For example, you'll find a list of all of the predefined functions under ``predefined functions'', and also under ``functions''. There are similar entries for ``events'' and ``variables''.

The manual also includes Note:'s and Deficiency:'s that emphasize points that may be subtle or counter-intuitive, or that reflect bugs of some form. The general delineation between the two is that Note:'s discuss facets of Bro not likely to change, while Deficiency:'s will (should) eventually get fixed.

I'm very interested in feedback on whether the manual in general and the index in particular is effective, what should be added or removed from it to improve it, any errors found in the index or (of course) elsewhere in the manual, and what topics you would give the highest priority for the next revision of the manual. In addition, any contributions to the manual will be highly welcome! You'll find the source for the manual in doc/manual-src/.

The current version of the manual is organized as follows. We begin with an overview of how to get started using Bro: building and installing it, running it interactively and on live and prerecorded network traffic, and the helper utilities (scripts and programs) included in the distribution (Chapter 2).

Chapter 3 then discusses the different types, values, and constants that Bro supports. The intent is to provide you with some of the flavor of the language. In addition, later chapters use these concepts to explain things like the types associated with the arguments passed to different event handlers.

Chapter 6 lists the different variables and functions that Bro predefines. The variables generally reflect particular values that control the behavior of the event engine or reflect its status, and the functions are for the most part utilities to aid in the writing of Bro scripts.

Chapter 7 discusses the different analyzers that Bro provides. It is far and away the longest chapter, since there are a good number of analyzers, and some of them are quite rich in their analysis.

Finally, Chapter 8 briefly lists different aspects of Bro that have not yet been documented (in addition to the event engine and the Bro language itself).

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