Re: lua decoder accessing info from layers above

[John Thacker <johnthacker () gmail com>]

On Mon, Oct 12, 2020, 8:00 PM Fulko Hew <fulko.hew () gmail com> wrote:

> I'm trying to update/improve someone else's decoder written in Lua.
> It's for a simple UDP (and TCP) based protocol.
> But I need to be able to get access to the upper layer
> to be able to decode it easily.
>
> Can someone provide an example of how to determine if the higher layer was
> UDP or TCP?

If all you need to know is whether it was called from TCP or UDP, then a
typical approach is to create slightly different dissector functions, one
for TCP and one for UDP (they can then call a common function, setting a
parameter) and register the TCP dissector with TCP and the UDP dissector
with UDP. That approach for C dissectors is demonstrated, for example, here:

https://gitlab.com/wireshark/wireshark/-/blob/master/doc/README.heuristic

And it's pretty similar for Lua dissectors using some of the examples
linked from here:

https://gitlab.com/wireshark/wireshark/-/wikis/Lua

John Thacker

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