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Diffstat (limited to 'test/lua/dissector.lua')
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diff --git a/test/lua/dissector.lua b/test/lua/dissector.lua new file mode 100644 index 0000000..836aa7b --- /dev/null +++ b/test/lua/dissector.lua @@ -0,0 +1,659 @@ +---------------------------------------- +-- script-name: dns_dissector.lua +-- +-- author: Hadriel Kaplan <hadrielk at yahoo dot com> +-- Copyright (c) 2014, Hadriel Kaplan +-- This code is in the Public Domain, or the BSD (3 clause) license if Public Domain does not apply +-- in your country. +-- +-- Version: 2.1 +-- +-- Changes since 2.0: +-- * fixed a bug with default settings +-- * added ability for command-line to overide defaults +-- +-- Changes since 1.0: +-- * made it use the new ProtoExpert class model for expert info +-- * add a protocol column with the proto name +-- * added heuristic dissector support +-- * added preferences settings +-- * removed byteArray2String(), and uses the new ByteArray:raw() method instead +-- +-- BACKGROUND: +-- This is an example Lua script for a protocol dissector. The purpose of this script is two-fold: +-- * To provide a reference tutorial for others writing Wireshark dissectors in Lua +-- * To test various functions being called in various ways, so this script can be used in the test-suites +-- I've tried to meet both of those goals, but it wasn't easy. No doubt some folks will wonder why some +-- functions are called some way, or differently than previous invocations of the same function. I'm trying to +-- to show both that it can be done numerous ways, but also I'm trying to test those numerous ways, and my more +-- immediate need is for test coverage rather than tutorial guide. (the Lua API is sorely lacking in test scripts) +-- +-- OVERVIEW: +-- This script creates an elementary dissector for DNS. It's neither comprehensive nor error-free with regards +-- to the DNS protocol. That's OK. The goal isn't to fully dissect DNS properly - Wireshark already has a good +-- DNS dissector built-in. We don't need another one. We also have other example Lua scripts, but I don't think +-- they do a good job of explaining things, and the nice thing about this one is getting capture files to +-- run it against is trivial. (plus I uploaded one) +-- +-- HOW TO RUN THIS SCRIPT: +-- Wireshark and Tshark support multiple ways of loading Lua scripts: through a dofile() call in init.lua, +-- through the file being in either the global or personal plugins directories, or via the command line. +-- See the Wireshark User's Guide chapter on Lua (https://www.wireshark.org/docs/wsdg_html_chunked/wsluarm_modules.html). +-- Once the script is loaded, it creates a new protocol named "MyDNS" (or "MYDNS" in some places). If you have +-- a capture file with DNS packets in it, simply select one in the Packet List pane, right-click on it, and +-- select "Decode As ...", and then in the dialog box that shows up scroll down the list of protocols to one +-- called "MYDNS", select that and click the "ok" or "apply" button. Voila`, you're now decoding DNS packets +-- using the simplistic dissector in this script. Another way is to download the capture file made for +-- this script, and open that - since the DNS packets in it use UDP port 65333 (instead of the default 53), +-- and since the MyDNS protocol in this script has been set to automatically decode UDP port 65333, it will +-- automagically do it without doing "Decode As ...". +-- +---------------------------------------- +-- do not modify this table +local debug_level = { + DISABLED = 0, + LEVEL_1 = 1, + LEVEL_2 = 2 +} + +-- set this DEBUG to debug_level.LEVEL_1 to enable printing debug_level info +-- set it to debug_level.LEVEL_2 to enable really verbose printing +-- note: this will be overridden by user's preference settings +local DEBUG = debug_level.LEVEL_1 + +local default_settings = +{ + debug_level = DEBUG, + port = 65333, + heur_enabled = true, + heur_regmode = 1, +} + +-- for testing purposes, we want to be able to pass in changes to the defaults +-- from the command line; because you can't set lua preferences from the command +-- line using the '-o' switch (the preferences don't exist until this script is +-- loaded, so the command line thinks they're invalid preferences being set) +-- so we pass them in as command arguments insetad, and handle it here: +local args={...} -- get passed-in args +if args and #args > 0 then + for _, arg in ipairs(args) do + local name, value = arg:match("(.+)=(.+)") + if name and value then + if tonumber(value) then + value = tonumber(value) + elseif value == "true" or value == "TRUE" then + value = true + elseif value == "false" or value == "FALSE" then + value = false + elseif value == "DISABLED" then + value = debug_level.DISABLED + elseif value == "LEVEL_1" then + value = debug_level.LEVEL_1 + elseif value == "LEVEL_2" then + value = debug_level.LEVEL_2 + else + error("invalid commandline argument value") + end + else + error("invalid commandline argument syntax") + end + + default_settings[name] = value + end +end + +local dprint = function() end +local dprint2 = function() end +local function reset_debug_level() + if default_settings.debug_level > debug_level.DISABLED then + dprint = function(...) + print(table.concat({"Lua:", ...}," ")) + end + + if default_settings.debug_level > debug_level.LEVEL_1 then + dprint2 = dprint + end + end +end +-- call it now +reset_debug_level() + +dprint2("Wireshark version = ", get_version()) +dprint2("Lua version = ", _VERSION) + +---------------------------------------- +-- Unfortunately, the older Wireshark/Tshark versions have bugs, and part of the point +-- of this script is to test those bugs are now fixed. So we need to check the version +-- end error out if it's too old. +local major, minor, micro = get_version():match("(%d+)%.(%d+)%.(%d+)") +if major and tonumber(major) <= 1 and ((tonumber(minor) <= 10) or (tonumber(minor) == 11 and tonumber(micro) < 3)) then + error( "Sorry, but your Wireshark/Tshark version ("..get_version()..") is too old for this script!\n".. + "This script needs Wireshark/Tshark version 1.11.3 or higher.\n" ) +end + +-- more sanity checking +-- verify we have the ProtoExpert class in wireshark, as that's the newest thing this file uses +assert(ProtoExpert.new, "Wireshark does not have the ProtoExpert class, so it's too old - get the latest 1.11.3 or higher") + +---------------------------------------- + + +---------------------------------------- +-- creates a Proto object, but doesn't register it yet +local dns = Proto("mydns","MyDNS Protocol") + +---------------------------------------- +-- multiple ways to do the same thing: create a protocol field (but not register it yet) +-- the abbreviation should always have "<myproto>." before the specific abbreviation, to avoid collisions +local pf_trasaction_id = ProtoField.new ("Transaction ID", "mydns.trans_id", ftypes.UINT16) +local pf_flags = ProtoField.new ("Flags", "mydns.flags", ftypes.UINT16, nil, base.HEX) +local pf_num_questions = ProtoField.uint16("mydns.num_questions", "Number of Questions") +local pf_num_answers = ProtoField.uint16("mydns.num_answers", "Number of Answer RRs") +local pf_num_authority_rr = ProtoField.uint16("mydns.num_authority_rr", "Number of Authority RRs") +local pf_num_additional_rr = ProtoField.uint16("mydns.num_additional_rr", "Number of Additional RRs") + +-- within the flags field, we want to parse/show the bits separately +-- note the "base" argument becomes the size of the bitmask'ed field when ftypes.BOOLEAN is used +-- the "mask" argument is which bits we want to use for this field (e.g., base=16 and mask=0x8000 means we want the top bit of a 16-bit field) +-- again the following shows different ways of doing the same thing basically +local pf_flag_response = ProtoField.new ("Response", "mydns.flags.response", ftypes.BOOLEAN, {"this is a response","this is a query"}, 16, 0x8000, "is the message a response?") +local pf_flag_opcode = ProtoField.new ("Opcode", "mydns.flags.opcode", ftypes.UINT16, nil, base.DEC, 0x7800, "operation code") +local pf_flag_authoritative = ProtoField.new ("Authoritative", "mydns.flags.authoritative", ftypes.BOOLEAN, nil, 16, 0x0400, "is the response authoritative?") +local pf_flag_truncated = ProtoField.bool ("mydns.flags.truncated", "Truncated", 16, nil, 0x0200, "is the message truncated?") +local pf_flag_recursion_desired = ProtoField.bool ("mydns.flags.recursion_desired", "Recursion desired", 16, {"yes","no"}, 0x0100, "do the query recursivley?") +local pf_flag_recursion_available = ProtoField.bool ("mydns.flags.recursion_available", "Recursion available", 16, nil, 0x0080, "does the server support recursion?") +local pf_flag_z = ProtoField.uint16("mydns.flags.z", "World War Z - Reserved for future use", base.HEX, nil, 0x0040, "when is it the future?") +local pf_flag_authenticated = ProtoField.bool ("mydns.flags.authenticated", "Authenticated", 16, {"yes","no"}, 0x0020, "did the server DNSSEC authenticate?") +local pf_flag_checking_disabled = ProtoField.bool ("mydns.flags.checking_disabled", "Checking disabled", 16, nil, 0x0010) + +-- no, these aren't all the DNS response codes - this is just an example +local rcodes = { + [0] = "No Error", + [1] = "Format Error", + [2] = "Server Failure", + [3] = "Non-Existent Domain", + [9] = "Server Not Authoritative for zone" +} +-- the above rcodes table is used in this next ProtoField +local pf_flag_rcode = ProtoField.uint16("mydns.flags.rcode", "Response code", base.DEC, rcodes, 0x000F) +local pf_query = ProtoField.new("Query", "mydns.query", ftypes.BYTES) +local pf_query_name = ProtoField.new("Name", "mydns.query.name", ftypes.STRING) +local pf_query_name_len = ProtoField.new("Name Length", "mydns.query.name.len", ftypes.UINT8) +local pf_query_label_count = ProtoField.new("Label Count", "mydns.query.label.count", ftypes.UINT8) +local rrtypes = { [1] = "A (IPv4 host address)", [2] = "NS (authoritative name server)", [28] = "AAAA (for geeks only)" } +local pf_query_type = ProtoField.uint16("mydns.query.type", "Type", base.DEC, rrtypes) +-- again, not all class types are listed here +local classes = { + [0] = "Reserved", + [1] = "IN (Internet)", + [2] = "The 1%", + [5] = "First class", + [6] = "Business class", + [65535] = "Cattle class" +} +local pf_query_class = ProtoField.uint16("mydns.query.class", "Class", base.DEC, classes, nil, "keep it classy folks") + +---------------------------------------- +-- this actually registers the ProtoFields above, into our new Protocol +-- in a real script I wouldn't do it this way; I'd build a table of fields programmatically +-- and then set dns.fields to it, so as to avoid forgetting a field +dns.fields = { pf_trasaction_id, pf_flags, + pf_num_questions, pf_num_answers, pf_num_authority_rr, pf_num_additional_rr, + pf_flag_response, pf_flag_opcode, pf_flag_authoritative, + pf_flag_truncated, pf_flag_recursion_desired, pf_flag_recursion_available, + pf_flag_z, pf_flag_authenticated, pf_flag_checking_disabled, pf_flag_rcode, + pf_query, pf_query_name, pf_query_name_len, pf_query_label_count, pf_query_type, pf_query_class } + +---------------------------------------- +-- create some expert info fields (this is new functionality in 1.11.3) +-- Expert info fields are very similar to proto fields: they're tied to our protocol, +-- they're created in a similar way, and registered by setting a 'experts' field to +-- a table of them just as proto fields were put into the 'dns.fields' above +-- The old way of creating expert info was to just add it to the tree, but that +-- didn't let the expert info be filterable in wireshark, whereas this way does +local ef_query = ProtoExpert.new("mydns.query.expert", "DNS query message", + expert.group.REQUEST_CODE, expert.severity.CHAT) +local ef_response = ProtoExpert.new("mydns.response.expert", "DNS response message", + expert.group.RESPONSE_CODE, expert.severity.CHAT) +local ef_ultimate = ProtoExpert.new("mydns.response.ultimate.expert", "DNS answer to life, the universe, and everything", + expert.group.COMMENTS_GROUP, expert.severity.NOTE) +-- some error expert info's +local ef_too_short = ProtoExpert.new("mydns.too_short.expert", "DNS message too short", + expert.group.MALFORMED, expert.severity.ERROR) +local ef_bad_query = ProtoExpert.new("mydns.query.missing.expert", "DNS query missing or malformed", + expert.group.MALFORMED, expert.severity.WARN) + +-- register them +dns.experts = { ef_query, ef_too_short, ef_bad_query, ef_response, ef_ultimate } + +---------------------------------------- +-- we don't just want to display our protocol's fields, we want to access the value of some of them too! +-- There are several ways to do that. One is to just parse the buffer contents in Lua code to find +-- the values. But since ProtoFields actually do the parsing for us, and can be retrieved using Field +-- objects, it's kinda cool to do it that way. So let's create some Fields to extract the values. +-- The following creates the Field objects, but they're not 'registered' until after this script is loaded. +-- Also, these lines can't be before the 'dns.fields = ...' line above, because the Field.new() here is +-- referencing fields we're creating, and they're not "created" until that line above. +-- Furthermore, you cannot put these 'Field.new()' lines inside the dissector function. +-- Before Wireshark version 1.11, you couldn't even do this concept (of using fields you just created). +local questions_field = Field.new("mydns.num_questions") +local query_type_field = Field.new("mydns.query.type") +local query_class_field = Field.new("mydns.query.class") +local response_field = Field.new("mydns.flags.response") + +-- here's a little helper function to access the response_field value later. +-- Like any Field retrieval, you can't retrieve a field's value until its value has been +-- set, which won't happen until we actually use our ProtoFields in TreeItem:add() calls. +-- So this isResponse() function can't be used until after the pf_flag_response ProtoField +-- has been used inside the dissector. +-- Note that calling the Field object returns a FieldInfo object, and calling that +-- returns the value of the field - in this case a boolean true/false, since we set the +-- "mydns.flags.response" ProtoField to ftype.BOOLEAN way earlier when we created the +-- pf_flag_response ProtoField. Clear as mud? +-- +-- A shorter version of this function would be: +-- local function isResponse() return response_field()() end +-- but I though the below is easier to understand. +local function isResponse() + local response_fieldinfo = response_field() + return response_fieldinfo() +end + +-------------------------------------------------------------------------------- +-- preferences handling stuff +-------------------------------------------------------------------------------- + +-- a "enum" table for our enum pref, as required by Pref.enum() +-- having the "index" number makes ZERO sense, and is completely illogical +-- but it's what the code has expected it to be for a long time. Ugh. +local debug_pref_enum = { + { 1, "Disabled", debug_level.DISABLED }, + { 2, "Level 1", debug_level.LEVEL_1 }, + { 3, "Level 2", debug_level.LEVEL_2 }, +} + +dns.prefs.debug = Pref.enum("Debug", default_settings.debug_level, + "The debug printing level", debug_pref_enum) + +dns.prefs.port = Pref.uint("Port number", default_settings.port, + "The UDP port number for MyDNS") + +dns.prefs.heur = Pref.bool("Heuristic enabled", default_settings.heur_enabled, + "Whether heuristic dissection is enabled or not") + +---------------------------------------- +-- a function for handling prefs being changed +function dns.prefs_changed() + dprint2("prefs_changed called") + + default_settings.debug_level = dns.prefs.debug + reset_debug_level() + + default_settings.heur_enabled = dns.prefs.heur + + if default_settings.port ~= dns.prefs.port then + -- remove old one, if not 0 + if default_settings.port ~= 0 then + dprint2("removing MyDNS from port",default_settings.port) + DissectorTable.get("udp.port"):remove(default_settings.port, dns) + end + -- set our new default + default_settings.port = dns.prefs.port + -- add new one, if not 0 + if default_settings.port ~= 0 then + dprint2("adding MyDNS to port",default_settings.port) + DissectorTable.get("udp.port"):add(default_settings.port, dns) + end + end + +end + +dprint2("MyDNS Prefs registered") + + +---------------------------------------- +---- some constants for later use ---- +-- the DNS header size +local DNS_HDR_LEN = 12 + +-- the smallest possible DNS query field size +-- has to be at least a label null terminator, 2-bytes type and 2-bytes class +local MIN_QUERY_LEN = 5 + +---------------------------------------- +-- some forward "declarations" of helper functions we use in the dissector +-- I don't usually use this trick, but it'll help reading/grok'ing this script I think +-- if we don't focus on them. +local getQueryName + + +---------------------------------------- +-- The following creates the callback function for the dissector. +-- It's the same as doing "dns.dissector = function (tvbuf,pkt,root)" +-- The 'tvbuf' is a Tvb object, 'pktinfo' is a Pinfo object, and 'root' is a TreeItem object. +-- Whenever Wireshark dissects a packet that our Proto is hooked into, it will call +-- this function and pass it these arguments for the packet it's dissecting. +function dns.dissector(tvbuf,pktinfo,root) + dprint2("dns.dissector called") + + -- set the protocol column to show our protocol name + pktinfo.cols.protocol:set("MYDNS") + + -- We want to check that the packet size is rational during dissection, so let's get the length of the + -- packet buffer (Tvb). + -- Because DNS has no additional payload data other than itself, and it rides on UDP without padding, + -- we can use tvb:len() or tvb:reported_len() here; but I prefer tvb:reported_length_remaining() as it's safer. + local pktlen = tvbuf:reported_length_remaining() + + -- We start by adding our protocol to the dissection display tree. + -- A call to tree:add() returns the child created, so we can add more "under" it using that return value. + -- The second argument is how much of the buffer/packet this added tree item covers/represents - in this + -- case (DNS protocol) that's the remainder of the packet. + local tree = root:add(dns, tvbuf:range(0,pktlen)) + + -- now let's check it's not too short + if pktlen < DNS_HDR_LEN then + -- since we're going to add this protocol to a specific UDP port, we're going to + -- assume packets in this port are our protocol, so the packet being too short is an error + -- the old way: tree:add_expert_info(PI_MALFORMED, PI_ERROR, "packet too short") + -- the correct way now: + tree:add_proto_expert_info(ef_too_short) + dprint("packet length",pktlen,"too short") + return + end + + -- Now let's add our transaction id under our dns protocol tree we just created. + -- The transaction id starts at offset 0, for 2 bytes length. + tree:add(pf_trasaction_id, tvbuf:range(0,2)) + + -- We'd like to put the transaction id number in the GUI row for this packet, in its + -- INFO column/cell. First we need the transaction id value, though. Since we just + -- dissected it with the previous code line, we could now get it using a Field's + -- FieldInfo extractor, but instead we'll get it directly from the TvbRange just + -- to show how to do that. We'll use Field/FieldInfo extractors later on... + local transid = tvbuf:range(0,2):uint() + pktinfo.cols.info:set("(".. transid ..")") + + -- now let's add the flags, which are all in the packet bytes at offset 2 of length 2 + -- instead of calling this again and again, let's just use a variable + local flagrange = tvbuf:range(2,2) + + -- for our flags field, we want a sub-tree + local flag_tree = tree:add(pf_flags, flagrange) + -- I'm indenting this for clarity, because it's adding to the flag's child-tree + + -- let's add the type of message (query vs. response) + local query_flag_tree = flag_tree:add(pf_flag_response, flagrange) + + -- let's also add an expert info about it + if isResponse() then + query_flag_tree:add_proto_expert_info(ef_response, "It's a response!") + if transid == 42 then + tree:add_tvb_expert_info(ef_ultimate, tvbuf:range(0,2)) + end + else + query_flag_tree:add_proto_expert_info(ef_query) + end + + -- we now know if it's a response or query, so let's put that in the + -- GUI packet row, in the INFO column cell + -- this line of code uses a Lua trick for doing something similar to + -- the C/C++ 'test ? true : false' shorthand + pktinfo.cols.info:prepend(isResponse() and "Response " or "Query ") + + flag_tree:add(pf_flag_opcode, flagrange) + + if isResponse() then + flag_tree:add(pf_flag_authoritative, flagrange) + end + + flag_tree:add(pf_flag_truncated, flagrange) + + if isResponse() then + flag_tree:add(pf_flag_recursion_available, flagrange) + else + flag_tree:add(pf_flag_recursion_desired, flagrange) + end + + flag_tree:add(pf_flag_z, flagrange) + + if isResponse() then + flag_tree:add(pf_flag_authenticated, flagrange) + flag_tree:add(pf_flag_rcode, flagrange) + end + + flag_tree:add(pf_flag_checking_disabled, flagrange) + + -- now add more to the main mydns tree + tree:add(pf_num_questions, tvbuf:range(4,2)) + tree:add(pf_num_answers, tvbuf:range(6,2)) + -- another way to get a TvbRange is just to call the Tvb like this + tree:add(pf_num_authority_rr, tvbuf(8,2)) + -- or if we're crazy, we can create a sub-TvbRange, from a sub-TvbRange of the TvbRange + tree:add(pf_num_additional_rr, tvbuf:range(10,2):range()()) + + local num_queries = questions_field()() + local pos = DNS_HDR_LEN + + if num_queries > 0 then + -- let's create a sub-tree, using a plain text description (not a field from the packet) + local queries_tree = tree:add("Queries") + + local pktlen_remaining = pktlen - pos + + -- multiple questions in one query hasn't been used for a long time, but just in case, let's loop + while num_queries > 0 and pktlen_remaining > 0 do + if pktlen_remaining < MIN_QUERY_LEN then + -- old way: queries_tree:add_expert_info(PI_MALFORMED, PI_ERROR, "query field missing or too short") + queries_tree:add_proto_expert_info(ef_bad_query) + return + end + + -- we don't know how long this query field in total is, so we have to parse it first before + -- adding it to the tree, because we want to identify the correct bytes it covers + local label_count, name, name_len = getQueryName(tvbuf:range(pos,pktlen_remaining)) + if not label_count then + queries_tree:add_expert_info(PI_MALFORMED, PI_ERROR, name) + return + end + + -- now add the first query to the 'Queries' child tree we just created + -- we're going to change the string generated by this later, after we figure out the subsequent fields. + -- the whole query field is the query name field length we just got, plus 2-byte type and 2-byte class. + local q_tree = queries_tree:add(pf_query, tvbuf:range(pos, name_len + 4)) + + q_tree:add(pf_query_name, tvbuf:range(pos, name_len), name) + pos = pos + name_len + + pktinfo.cols.info:append(" "..name) + + -- the following tree items are generated by us, not encoded in the packet per se, so mark them as such + q_tree:add(pf_query_name_len, name_len):set_generated() + q_tree:add(pf_query_label_count, label_count):set_generated() + + q_tree:add(pf_query_type, tvbuf:range(pos, 2)) + q_tree:add(pf_query_class, tvbuf:range(pos + 2, 2)) + pos = pos + 4 + + -- now change the query text + -- calling a Field returns a multival of one FieldInfo object for + -- each value, so we select() only the most recent one + q_tree:set_text(name..": type "..select(-1, query_type_field()).display + ..", class "..select(-1, query_class_field()).display) + + pktlen_remaining = pktlen_remaining - (name_len + 4) + num_queries = num_queries - 1 + end -- end of while loop + + if num_queries > 0 then + -- we didn't process them all + queries_tree:add_expert_info(PI_MALFORMED, PI_ERROR, num_queries .. " query field(s) missing") + return + end + end + + -- parsing answers, authority RRs, and additional RRs is up to you! + + dprint2("dns.dissector returning",pos) + + -- tell wireshark how much of tvbuff we dissected + return pos +end + +---------------------------------------- +-- we want to have our protocol dissection invoked for a specific UDP port, +-- so get the udp dissector table and add our protocol to it +DissectorTable.get("udp.port"):add(default_settings.port, dns) + +---------------------------------------- +-- we also want to add the heuristic dissector, for any UDP protocol +-- first we need a heuristic dissection function +-- this is that function - when wireshark invokes this, it will pass in the same +-- things it passes in to the "dissector" function, but we only want to actually +-- dissect it if it's for us, and we need to return true if it's for us, or else false +-- figuring out if it's for us or not is not easy +-- we need to try as hard as possible, or else we'll think it's for us when it's +-- not and block other heuristic dissectors from getting their chance +-- +-- in practice, you'd never set a dissector like this to be heuristic, because there +-- just isn't enough information to safely detect if it's DNS or not +-- but I'm doing it to show how it would be done +-- +-- Note: this heuristic stuff is new in 1.11.3 +local function heur_dissect_dns(tvbuf,pktinfo,root) + dprint2("heur_dissect_dns called") + + -- if our preferences tell us not to do this, return false + if not default_settings.heur_enabled then + return false + end + + if tvbuf:len() < DNS_HDR_LEN then + dprint("heur_dissect_dns: tvb shorter than DNS_HDR_LEN of:",DNS_HDR_LEN) + return false + end + + local tvbr = tvbuf:range(0,DNS_HDR_LEN) + + -- the first 2 bytes are transaction id, which can be anything so no point in checking those + -- the next 2 bytes contain flags, a couple of which have some values we can check against + + -- the opcode has to be 0, 1, 2, 4 or 5 + -- the opcode field starts at bit offset 17 (in C-indexing), for 4 bits in length + local check = tvbr:bitfield(17,4) + if check == 3 or check > 5 then + dprint("heur_dissect_dns: invalid opcode:",check) + return false + end + + -- the rcode has to be 0-10, 16-22 (we're ignoring private use rcodes here) + -- the rcode field starts at bit offset 28 (in C-indexing), for 4 bits in length + check = tvbr:bitfield(28,4) + if check > 22 or (check > 10 and check < 16) then + dprint("heur_dissect_dns: invalid rcode:",check) + return false + end + + dprint2("heur_dissect_dns checking questions/answers") + + -- now let's verify the number of questions/answers are reasonable + check = tvbr:range(4,2):uint() -- num questions + if check > 100 then return false end + check = tvbr:range(6,2):uint() -- num answers + if check > 100 then return false end + check = tvbr:range(8,2):uint() -- num authority + if check > 100 then return false end + check = tvbr:range(10,2):uint() -- num additional + if check > 100 then return false end + + dprint2("heur_dissect_dns: everything looks good calling the real dissector") + + -- don't do this line in your script - I'm just doing it so our test-suite can + -- verify this script + root:add("Heuristic dissector used"):set_generated() + + -- ok, looks like it's ours, so go dissect it + -- note: calling the dissector directly like this is new in 1.11.3 + -- also note that calling a Dissector object, as this does, means we don't + -- get back the return value of the dissector function we created previously + -- so it might be better to just call the function directly instead of doing + -- this, but this script is used for testing and this tests the call() function + dns.dissector(tvbuf,pktinfo,root) + + -- since this is over a transport protocol, such as UDP, we can set the + -- conversation to make it sticky for our dissector, so that all future + -- packets to/from the same address:port pair will just call our dissector + -- function directly instead of this heuristic function + -- this is a new attribute of pinfo in 1.11.3 + pktinfo.conversation = dns + + return true +end + +-- now register that heuristic dissector into the udp heuristic list +if default_settings.heur_regmode == 1 then + -- this is the "normal" way to register a heuristic: using a lua function + dns:register_heuristic("udp",heur_dissect_dns) +elseif default_settings.heur_regmode == 2 then + -- this is to test the fix for bug 10695: + dns:register_heuristic("udp",dns.dissector) +elseif default_settings.heur_regmode == 3 then + -- and this too is to test the fix for bug 10695: + dns:register_heuristic("udp", function (...) return dns.dissector(...); end ) +end + +-- We're done! +-- our protocol (Proto) gets automatically registered after this script finishes loading +---------------------------------------- + +---------------------------------------- +-- DNS query names are not just null-terminated strings; they're actually a sequence of +-- 'labels', with a length octet before each one. So "foobar.com" is actually the +-- string "\06foobar\03com\00". We could create a ProtoField for label_length and label_name +-- or whatever, but since this is an example script I'll show how to do it in raw code. +-- This function is given the TvbRange object from the dissector() function, and needs to +-- parse it. +-- On success, it returns three things: the number of labels, the name string, and how +-- many bytes it covered of the buffer. +-- On failure, it returns nil and the error message. +getQueryName = function (tvbr) + local label_count = 0 + local name = "" + local name_len = 0 + + local len_remaining = tvbr:len() + if len_remaining < 2 then + -- it's too short + return nil, "invalid name" + end + + local barray = tvbr:bytes() -- gets a ByteArray of the TvbRange + local pos = 0 -- unlike Lua, ByteArray uses 0-based indexing + + repeat + local label_len = barray:get_index(pos) + if label_len >= len_remaining then + return nil, "invalid label length of "..label_len + end + pos = pos + 1 -- move past label length octet + if label_len > 0 then + -- append the label and a dot to name string + -- note: this uses the new method of ByteArray:raw(), added in 1.11.3 + name = name .. barray:raw(pos, label_len) .. "." + label_count = label_count + 1 + pos = pos + label_len -- move past label + end + name_len = name_len + label_len + 1 + len_remaining = len_remaining - (label_len + 1) -- subtract label and its length octet + until label_len == 0 + + -- we appended an extra dot, so get rid of it + name = name:sub(1, -2) + + if name == "" then + -- this is the root zone (.) + name = "<Root>" + end + + return label_count, name, name_len +end |