You might want to implement a function to handle this, something like so?
local function dissect_RuPortId_F(tree, buffer)
local t = {}
local mask = 2^my_protocol.prefs.ru_port_id_width - 1
local val = bit.band(buffer(4, 2):uint(), mask)
for i = 15, 0, -1 do
if bit.band(bit.rshift(mask, i), 1) == 1 then
table.insert(t, (bit.band(bit.rshift(val, i), 1) == 1 and '1') or
'0')
else
table.insert(t, '.')
end
if (i % 4) == 0 then
table.insert(t, ' ')
end
end
tree:add(RuPortId_F, buffer(4, 2)):set_text(table.concat(t) .. " = Ru Port
ID: " .. val)
end -- dissect_RuPortId_F()
…
dissect_RuPortId_F(tree, buffer)
If the offset isn’t fixed at 4, then you can pass that as an argument too, and
if you want to be able to add any information to the Info column, then pass
pinfo also.
And you probably want to limit the bit width to only values between 1-16. One
way to do this is to use an enum preference instead of a uint preference, e.g.:
local ru_port_id_width_enums = {
{1, "1", 1}, {2, "2", 2}, {3, "3", 3}, {4, "4", 4},
{5, "5", 5}, {6, "6", 6}, {7, "7", 7}, {8, "8", 8},
{9, "9", 9}, {10, "10", 10}, {11, "11", 11}, {12, "12", 12},
{13, "13", 13}, {14, "14", 14}, {15, "15", 15}, {16, "16", 16}
}
my_protocol.prefs.ru_port_id_width = Pref.enum("RU Port ID width", 4, "The RU
Port ID width, in bits (1-16)", ru_port_id_width_enums, false)
You *may* be able to still error check the preference if declared as a uint
using a my_protocol.prefs_changed() function[1], but the enum type seems ok
here, and anyway, I’m actually not entirely sure how to do it since the
following does not work:
function my_protocol.prefs_changed()
if my_protocol.prefs.ru_port_id_width < 1 then
my_protocol.prefs.ru_port_id_width = 1
end
if my_protocol.prefs.ru_port_id_width > 16 then
my_protocol.prefs.ru_port_id_width = 16
end
end -- my_protocol.prefs_changed()
Here, you end up with an error of the sort, “Lua: Error During execution of
prefs apply callback: [string
“C:\Users\cmaynard\AppData\Roaming\Wireshark\p…”:51: bad argument #3 to
'__newindex' (userdata expected, got number)”. As far as I can tell, there
seems to be no way to limit preferences to a valid range in Lua. If this is
the case, then it seems to be a limitation worthy of an enhancement bug report.
Hope it helps.
- Chris
[1]: See 11.6.4.6. proto.prefs_changed at
https://www.wireshark.org/docs/wsdg_html_chunked/lua_module_Proto.html
From: Wireshark-dev [mailto:wireshark-dev-boun...@wireshark.org] On Behalf Of
David Aldrich
Sent: Monday, September 3, 2018 11:32 AM
To: wireshark-dev@wireshark.org
Subject: [Wireshark-dev] Lua dissector: How to set sub-field bit widths using
preferences?
Our protocol includes a 16-bit field which is sub-divided into 4 sub-fields.
The width of those sub-fields is variable so I want to specify the widths using
Wireshark preferences. I understand how to create and read Wireshark
preferences but I am unsure of how to apply them in this circumstance.
My code structure looks like this:
my_protocol = Proto("...", "...")
-- Create a preference
my_protocol.prefs.ru_port_id_width = Pref.uint( "RU_Port_ID width ", 4 )
-- Read the preference
ru_port_id_width = my_protocol.prefs.ru_port_id_width
-- Specify a field using the preference
RuPortId_F = ProtoField.uint16("...", "...", base.HEX, NULL,
((2^ru_port_id_width)-1))
-- Create a fields table and add the field to it
xran_protocol.fields = {RuPortId_F}
-- Then specify the dissector function
function my_protocol.dissector(buffer, pinfo,tree)
-- Show the field in a subtree
local RtcidPcid_range = buffer(4,2) -- the 16-bit field
local ecpriRtcidPcid = ecpriRtcidPcid_range:uint()
subtree:add(RuPortId_F, RtcidPcid_range, RtcidPcid) -- the variable width
sub-field
The trouble with this is that the preference is only read at startup. A new
value can't be specified by the user without restarting Wireshark to apply it.
Is there a better sequence of code that I code use to overcome this limitation?
Best regards
David
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