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Introduction to Trisul Scripting for Bro IDS users
A key feature of the popular open source network analysis platform Bro IDS is the custom BRO language. It allows you to write scripts to enhance the functionality of the platform. Trisul Network Analytics is also a platform that can be extended by writing scripts. This page introduces the Trisul Scripting API for those who are already familiar with Bro IDS scripting.
Trisul scripting
There are two differences between the Bro and Trisul approaches. Lets sort them out before diving deeper.
First, Trisul integrates the packet processing as well as the database.So you directly work with database objects like metrics , resources, flows, documents, and graphs.This can be a bit confusing to Bro scripters who focus on generating logs and notices.
A second difference is, You can script either the packet processing stream or the analytics stream. We call these two streams the Frontend (Fastpath) or the Backend (slowpath). They communicate using a messaging API. This can be a bit odd to newcomers because the backend scripts have a longer time budget to execute.
Comparison
| Feature | Bro | Trisul |
|---|---|---|
| language | .bro language | LuaJIT |
| docs | Bro Scripting | Trisul LUA API |
| protocol decoding | Bro framework provides fine grained events representing protocol fields to your script. | Trisul framework provides a lower level access to the payload itself, or for some common protocols the results of Trisul's built in dissection. Decoding a payload isnt as hard as it sounds, we released the open source BITMAUL library to dissect protocols to the depth you want. |
| events | fine grained “typed” events. For example dns_A6_reply(..) event contains parsed fields for the DNS AAAA reply record | loose documents in a canonical text format. In Trisul, DNS Resource is a text dump of a DNS transaction in a canonical DIG format. You can pick the fields you want using Regex. This means you have a dramatically lower number of events to deal with and are free to decode packets to the depth you want. |
| extending | you can write C code and integrate it to your Bro scripting using a *.bif file. This involves a binary compile process | leverages LuaJIT FFI to directly call library functions |
| time budget | packet drops if script is slow | frontend scripts have to execute fast to avoid packet drops, backend scripts have a more relaxed time budget of about 0-30 seconds. |
| deployment | place script files in directory | similar place scripts in directory on probe or put it on the hub node for automatic deployment to all probes |
| debugger | manual | built in LUA debugger. just call dbg() to drop into an interactive debugger |
| threading | single with load balanced workers | multithreaded with load balanced threads, this allows for very fast state sharing between multiple threads using message passing. During development and debugging you can turn it into a single threaded system |
| async | yes | yes - you can have a deferred execution of a LUA code block |
| intel | intel framework | you can choose your own framework. We like to dump all threat intel into a LevelDB database using LuaJIT FFI to access LevelDB. You can choose any other system. |
| packaging | Yes - Bro packages | Yes - Trisul APPs |
| example | JA3 TLS Fingerprint written in Bro | JA3 written in Trisul notice how in Trisul we parse the TLS record manually, while in BRO we use the typed events like ssl_client_hello() , ssl_extensions() etc which are supplied by Bro. With Trisul, you have slightly more work to do with the parsing the protocol, but you are independent of what the framework supplies. The Trisul code is longer because we are adding a lot of metrics and graph analytics in the script |
| disadvantage | - | LuaJIT has a 2GB limit on total memory use, therefore your scripts cant allocate too much memory. Use Trisul aggregations instead of building large lookup tables or use LevelDB to store data. |
Types of Trisul Scripts
There are 16 different types of Trisul scripts depending on what you want to accomplish.
- 6 Frontend scripts that work on Packets and Reassembly : Input Filter , Simple Counter, Reassembly Handler, File Extraction , PCAP storage , Protocol Handler,
- 7 Backend scripts that work on metrics, documents, resources : Engine Monitor, Alert Monitor, Counter Monitor, Session Monitor, Resource Monitor, FTS Monitor , Flow Tracker
- 3 Definition scripts : New Counter Group, New Alert Group, New Resource Group
For more on this refer to Script selector cheat sheet
To conclude, this was a quick introduction to Trisul scripting. We will be diving deeper into each of the areas in the coming days.