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Save to your "Personal Lua Plugins" folder as a ".lua" file.

-- 241222_data_strings.lua - change non-printable data.data characters to "."
-- Based on EASYPOST.lua

-- Step 1 - document as you go. See header above and set_plugin_info().
local easypost_info =
{
    version = "1.0.0",
    author = "Good Coder",
    description = "Important EASYPOST stuff",
    repository = "Floppy in top drawer"
}

set_plugin_info(easypost_info)

-- Step 2 - create a protocol to attach new fields to
local easypost_p = Proto.new("easypost","Important EASYPOST Protocol")

-- Step 3 - add some field(s) to Step 2 protocol
local pf = { payload = ProtoField.string("easypost.payload", "EASYPOST string") }

easypost_p.fields = pf

-- Step 4 - create a Field extractor to copy packet field data.
easypost_payload_f = Field.new("data.data")

-- Step 5 - create the postdissector function that will run on each frame/packet
function easypost_p.dissector(tvb,pinfo,tree)
    local subtree = nil

    -- copy existing field(s) into table for processing
    finfo = { easypost_payload_f() }

    if (#finfo > 0) then
        if not subtree then
            subtree = tree:add(easypost_p)
        end
        for k, v in pairs(finfo) do
            -- process data and add results to the tree
            local field_data = v.range:bytes():raw()
            local field_str = string.gsub(field_data, '[%z]', ".")
            subtree:add(pf.payload, field_str)
        end
    end
end

-- Step 6 - register the new protocol as a postdissector
register_postdissector(easypost_p)

Frame 1: 101 bytes on wire (808 bits), 101 bytes captured (808 bits) on interface Fake IF, Import from Hex Dump, id 0
Ethernet II, Src: Send_00 (20:53:45:4e:44:00), Dst: Receive_00 (20:52:45:43:56:00)
Internet Protocol Version 4, Src: 10.1.1.1, Dst: 10.2.2.2
User Datagram Protocol, Src Port: 1234, Dst Port: 0
Data (59 bytes)
    Data: 370000000353454c45435420416472657373652c20474b2046524f4d206175737765697364617465692057484552452041647265737365203d2039
    [Length: 59]
Important EASYPOST Protocol
    EASYPOST string: 7...\x03SELECT Adresse, GK FROM ausweisdatei WHERE Adresse = 9