Lua is a powerful light-weight programming language designed for extending applications. Lua is designed and implemented by a team at PUC-Rio, the Pontifical Catholic University of Rio de Janeiro in Brazil. Lua was born and raised at Tecgraf, the Computer Graphics Technology Group of PUC-Rio, and is now housed at Lua.org. Both Tecgraf and Lua.org are laboratories of the Department of Computer Science.
Lua's been added to Wireshark as a language for prototyping and scripting.
Beware the GPL
Wireshark is released under GPL so every derivative work based on Wireshark must be released under the terms of the GPL.
Even if the code you write in Lua does not need to be GPL'ed. The code written in Lua that uses bindings to Wireshark must be distributed under the GPL terms. see the GPL FAQ for more info
There is at least one Wireshark author that will not allow to distribute derivative work under different terms. To distribute Lua code that uses Wireshark's bindings under different terms would be a clear violation of the GPL.
If it isn't clear to you what the GPL is and how it works please consult your lawyer.
Lua in Wireshark
Although it's possible to write dissectors in Lua, Wireshark dissectors are written in C, as C is several times faster than Lua. Lua is ok for prototyping dissectors, during Reverse Engineering you can use your time for finding out how things work instead of compiling and debugging your C dissector.
Post-dissectors are dissectors meant to run after every other dissector has run. They can add items the dissection tree so they can be used to create your own extensions to the filtering mechanism.
/Taps are used to collect information after the packet has been dissected.
Lua has shipped with the Windows version of Wireshark since 0.99.4. Availability on other platforms varies. To see if your version of Wireshark supports Lua, go to Help→About Wireshark and look for Lua in the "Compiled with" paragraph.
In some older versions Lua was available as a plugin.
To test Lua on your system, do the following:
Make sure Lua is enabled in the global configuration as described below in How Lua Fits Into Wireshark
- Create a simple Lua script such as:
-- hello.lua -- Lua's implementation of D. Ritchie's hello world program. print("hello world!")
Name this script hello.lua and place it in the current directory.
Run tshark -X lua_script:hello.lua from the command prompt. You should see something like:
$ tshark -X lua_script:hello.lua hello world! Capturing on en0 1 0.000000 126.96.36.199 -> 188.8.131.52 NBNS Name query NB XXX.COM<00>
If you can read "hello world!" in the first line after you run tshark Lua is ready to go!
Please note: On Windows, you may not see any output when running Lua scripts in Wireshark. If the console window is enabled it will be opened after the lua engine is loaded. This does not affect TShark, since it is a console program.
How Lua fits into Wireshark
Every time wireshark starts it will search for a script called init.lua located in the global configuration directory of Wireshark. If Wireshark finds this file it will run the script.
Once <global config dir>/init.lua has run that there are two variables that tell wireshark whether to continue looking for scripts.
If the first init script sets the variable disable_lua to true Wireshark will stop reading scripts and shut down the lua engine right after the script was run.
If Wireshark is running suexec (i.e. as root but launched by another user) it will check if the variable run_user_scripts_when_superuser is set to true before loading any further scripts.
Once this first script was run Wireshark will continue running <personal config dir>/init.lua and then all scripts passed with the -X lua_script:xxx.lua command line option in the given order.
All these scripts will be run before packets are read, at the end of the dissector registration process. So, what you have to do is to register a series of functions that will be called while processing packets.
Wireshark's Lua API
Examples of generic Lua code can be found in The Sample Code page of Lua-Users wiki.
Introducing Lua at O'Reilly (onlamp.com)