Initialize ProtoField table inside protocol init function.

edit

It would help a bit if you could say exactly what it is that "doesn't seem to be working."

The fields table is left unmodified.

Are you actually creating the protocol somewhere? You don't in the sample code. Do you have lines like

local my_protocol = Proto("myprotocol", "MyProtocol")

How do you know that the fields table is left unmodified? Note that if you're trying to test it by printing in the Lua Console (as opposed to adding it to the tree), then if you define the protocol as a local, as above, then it will be nil in the Lua Console.

Using your basic code as above but creating a protocol and dissector, and registering it to a port:

function my_protocol.dissector(buf, pkt, tree)

    local subtree = tree:add(my_protocol, buf(0, 2))
    subtree:add(my_protocol.fields[1], buf(0, 1))
end

local udp_table = DissectorTable.get("udp.port")
udp_table:add(123, my_protocol)

I can definitely add fields from the protocol: Note that the Lua tables are 1-indexed.

The point was based on fields initialization in prototocol init function. What I mean by "left unmodified" is that all changes to fieds table inside protocol init function is not seen by the WireShark UI search filter.

Discussion of .init functions: 15907: Lua needs to support running some dissector code at "dissector registration" time and other dissector code at "dissector handoff registration" time