Re: lua decoder accessing info from layers above

["Maynard, Chris via Wireshark-dev" <wireshark-dev () wireshark org>]

> From: Wireshark-dev <wireshark-dev-bounces () wireshark org> On Behalf Of Fulko Hew
> Sent: Monday, October 12, 2020 8:00 PM
> To: Developer support list for Wireshark <wireshark-dev () wireshark org>
> Subject: [Wireshark-dev] lua decoder accessing info from layers above
>
> 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?
>
> Thanks
> Fulko

I was going to suggest a possible alternative solution which would be to grab "frame.protocols" and scan it for udp or 
tcp, but unfortunately this method doesn't work.  For example:

local f_frame_protocols = Field.new("frame.protocols")

function p_foo.dissector(buf, pinfo, tree)
    print("Protocols: " .. (f_frame_protocols() and f_frame_protocols().value or "Unknown"))
end

I suspect this has something to do with it being a generated field, but this looks like a Wireshark bug to me, unless 
someone can explain why this doesn't and shouldn't be expected to work.
- Chris









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