My first Wireshark LUA dissector (UDP): dissecting James Forshaw's SuperFunkyChat (https://github.com/tyranid/ExampleChatApplication) network protocol.

udp-superfunkychat-dissector.lua

--
-- SuperFunkyChat (https://github.com/tyranid/ExampleChatApplication) by James Forshaw - Wireshark UDP dissector
--
-- On Fedora one has to install wireshark-devel first:
--	$ sudo dnf install wireshark-devel -y
--
-- Then it can be used as following:
--	$ wireshark -X lua_script:udp-superfunkychat-dissector.lua -r capture.pcap -Y "udp.port == 12345"
--
-- Wrriten by: Mariusz B. / mgeeky, '18
-- 

SUPERFUNKYCHAT = Proto ("chat", "SuperFunkyChat protocol dissector")

local fields = SUPERFUNKYCHAT.fields
fields.chksum = ProtoField.uint32("chat.chksum", "Checksum", base.HEX)
fields.command = ProtoField.uint8("chat.command", "Command")
fields.data = ProtoField.bytes("chat.data", "Data")

commands = {
	[0] = "New user joined",
	[1] = "Server accepted new client",
	[2] = "/quit command",
	[3] = "Message",
	[5] = "Private Message",
	[6] = "User requested /list",
	[7] = "/list result",
}

-- Helps us read string preceded with it's length.
function read_string(buffer, start)
	local length = buffer(start, 1):uint()
	local data = buffer(start + 1, length):string()
	return data, (length + 1)
end


function parse_0(datatree, data)
	local info = ""
	local username, pos0 = read_string(data, 0)	
	local hostname, pos1 = read_string(data, pos0)

	datatree:add(SUPERFUNKYCHAT, data(0, pos0), "Username: " .. username)
	datatree:add(SUPERFUNKYCHAT, data(pos0, pos1), "Hostname: '" .. hostname .. "'")

	info = "New user joined: " .. username .. ", hostname: '" .. hostname .. "'"
	return info
end

function parse_1(datatree, data)
	local info = "Server accepted new incoming client."
	datatree:add(SUPERFUNKYCHAT, data(0, 1), info)
	return info
end

function parse_2(datatree, data)
	local info = "/quit '"
	local msg, len = read_string(data, 0)

	info = info .. msg .. "'"
	datatree:add(SUPERFUNKYCHAT, data(0, len), info)
	return info
end

function parse_3(datatree, data)
	local info = ""
	local username, pos0 = read_string(data, 0)	
	local message, pos1 = read_string(data, pos0)

	datatree:add(SUPERFUNKYCHAT, data(0, pos0), "Username: " .. username)
	datatree:add(SUPERFUNKYCHAT, data(pos0, pos1), "Message: '" .. message .. "'")

	info = "Username: " .. username .. ", Message: '" .. message .. "'"
	return info
end

function parse_5(datatree, data)
	local info = ""

	local toUser, pos0 = read_string(data, 0)
	local unknown = data(pos0, 4):uint()
	local username, pos1 = read_string(data, pos0 + 4)
	local message, pos2 = read_string(data, pos0 + 4 + pos1)

	info = "Private Message from: " .. username .. ", to: " .. toUser .. ". Message: '" .. message .. "'"
	datatree:add(SUPERFUNKYCHAT, data(0, pos0 + 4 + pos1 + pos2), info)
	return info
end

function parse_6(datatree, data)
	local info = "User requested /list of other users."
	datatree:add(SUPERFUNKYCHAT, data(0, 1), info)
	return info
end

function parse_7(datatree, data)
	local info = ""
	local numOfUsers = data(0, 4):uint()
	local offset = 4

	info = "Returned list of " .. numOfUsers .. " users."
	datatree:add(SUPERFUNKYCHAT, data(0, pos0 + 4 + pos1 + pos2), info)

	for i=1, numOfUsers do
		local offset0 = offset
		local username, pos0 = read_string(data, offset)
		offset = offset + pos0
		local hostname, pos1 = read_string(data, offset)
		offset = offset + pos1

		local info1 = "User: " .. username .. ", hostname: " .. hostname
		datatree:add(SUPERFUNKYCHAT, data(offset0, offset), info1)
	end
	return info
end

function calc_checksum(tag, data)
	local checksum = tag
	local datastr = data:string()
	for i=1, #datastr do
		checksum = checksum + string.byte(datastr, i)
	end
	return checksum
end


-- Dissecting main function
--	* buffer - input packet buffer, UDP as "Testy Virtual Buffer" (TVB)
-- 	* pinfo - packet info
--	* tree - protocol dissecting tree's handle
function SUPERFUNKYCHAT.dissector (buffer, pinfo, tree)
	-- Set the name in the "Protocol" column
	pinfo.cols.protocol = "CHAT"

	-- Create subtree with protocol dissection
	local subtree = tree:add(SUPERFUNKYCHAT, buffer(), "SuperFunkyChat protocol data")

	local checksum = buffer(0, 4):uint()
	local command = buffer(4, 1)
	local data = buffer(5):tvb()

	local calculated_checksum = calc_checksum(command:uint(), buffer(5))
	local checksumtext = ""

	if calculated_checksum == checksum then
		checksumtext = "Checksum fine"
		subtree:add_expert_info(PI_CHECKSUM, PI_INFO, checksumtext)
	else
		checksumtext = "Checksum incorrect, should be: " .. calculated_checksum .. ", instead of: " .. checksum .. ""
		subtree:add_expert_info(PI_CHECKSUM, PI_WARN, checksumtext)
	end

	subtree:add(SUPERFUNKYCHAT.fields.chksum, checksum)
	subtree:add(SUPERFUNKYCHAT.fields.command, command)

	command = command:uint()

	subtree:append_text(", Command: " .. command .. " (" .. commands[command] .. ")")
	pinfo.cols.info = "Command: " .. command .. " (" .. commands[command] .. ")"

	-- Parse the packet further.
	local datatree = subtree:add(SUPERFUNKYCHAT.fields.data, data())

	local info = ""
	if command == 0 then
		info = parse_0(datatree, data)
	elseif command == 1 then
		info = parse_1(datatree, data)
	elseif command == 2 then
		info = parse_2(datatree, data)
	elseif command == 3 then
		info = parse_3(datatree, data)
	elseif command == 5 then
		info = parse_5(datatree, data)
	elseif command == 6 then
		info = parse_6(datatree, data)
	elseif command == 7 then
		info = parse_7(datatree, data)
	end

	pinfo.cols.info:append("; " .. info)
end

udp_table = DissectorTable.get("udp.port")
udp_table:add(12345, SUPERFUNKYCHAT)