description = [[
Classifies a host's IP ID sequence (test for susceptibility to idle
scan).

Sends six probes to obtain IP IDs from the target and classifies them
similiarly to Nmap's method.  This is useful for finding suitable zombies
for Nmap's idle scan (<code>-sI</code>) as Nmap itself doesn't provide a way to scan
for these hosts.
]]

---
-- @usage 
-- nmap --script ipidseq [--script-args probeport=port] target
-- @args probeport Set destination port to probe
-- @output
-- Host script results:
-- |_ipidseq: Incremental! [used port 80]

-- I also implemented this in Metasploit as auxiliary/scanner/ip/ipidseq, but
-- this NSE script was actually written first (unfortunately it only worked
-- with vanilla Nmap using dnet ethernet sending.. ugh)
--
-- Originally written 05/24/2008; revived 01/24/2010

author = "Kris Katterjohn"

license = "Same as Nmap--See http://nmap.org/book/man-legal.html"

categories = {"safe", "discovery"}

require 'bin'
require 'packet'
require 'nmap'
require 'stdnse'

local NUMPROBES = 6

local ipidseqport

--- Pcap check function
-- @return Destination and source IP addresses and TCP ports
local check = function(layer3)
	local ip = packet.Packet:new(layer3, layer3:len())
	return bin.pack('AA=S=S', ip.ip_bin_dst, ip.ip_bin_src, ip.tcp_dport, ip.tcp_sport)
end

--- Updates a TCP Packet object
-- @param tcp The TCP object
local updatepkt = function(tcp)
	tcp:tcp_set_sport(math.random(0x401, 0xffff))
	tcp:tcp_set_seq(math.random(1, 0x7fffffff))
	tcp:tcp_count_checksum(tcp.ip_len)
	tcp:ip_count_checksum()
end

--- Create a TCP Packet object
-- @param host Host object
-- @param port Port number
-- @return TCP Packet object
local genericpkt = function(host, port)
	local pkt = bin.pack("H",
		"4500 002c 55d1 0000 8006 0000 0000 0000" ..
		"0000 0000 0000 0000 0000 0000 0000 0000" ..
		"6002 0c00 0000 0000 0204 05b4"
	)

	local tcp = packet.Packet:new(pkt, pkt:len())

	tcp:ip_set_bin_src(host.bin_ip_src)
	tcp:ip_set_bin_dst(host.bin_ip)
	tcp:tcp_set_dport(port)

	updatepkt(tcp)

	return tcp
end

--- Classifies a series of IP ID numbers like get_ipid_sequence() in osscan2.cc
-- @param ipids Table of IP IDs
local ipidseqclass = function(ipids)
	local diffs = {}
	local allzeros = true
	local allsame = true
	local mul256 = true
	local inc = true

	if #ipids < 2 then
		return "Unknown"
	end

	local i = 2

	while i <= #ipids do
		if ipids[i-1] ~= 0 or ipids[i] ~= 0 then
			allzeros = false
		end

		if ipids[i-1] <= ipids[i] then
			diffs[i-1] = ipids[i] - ipids[i-1]
		else
			diffs[i-1] = ipids[i] - ipids[i-1] + 65536
		end

		if #ipids > 2 and diffs[i-1] > 20000 then
			return "Randomized"
		end

		i = i + 1
	end

	if allzeros then
		return "All zeros"
	end

	i = 1

	while i <= #diffs do
		if diffs[i] ~= 0 then
			allsame = false
		end

		if (diffs[i] > 1000) and ((diffs[i] % 256) ~= 0 or
		   ((diffs[i] % 256) == 0 and diffs[i] > 25600)) then
			return "Random Positive Increments"
		end

		if diffs[i] > 5120 or (diffs[i] % 256) ~= 0 then
			mul256 = false
		end

		if diffs[i] >= 10 then
			inc = false
		end

		i = i + 1
	end

	if allsame then
		return "Constant"
	end

	if mul256 then
		return "Broken incremental!"
	end

	if inc then
		return "Incremental!"
	end

	return "Unknown"
end

--- Determines what port to probe
-- @param host Host object
local getport = function(host)
	for _, k in ipairs({"ipidseq.probeport", "probeport"}) do
		if nmap.registry.args[k] then
			return tonumber(nmap.registry.args[k])
		end
	end

	--local states = { "open", "closed", "unfiltered", "open|filtered", "closed|filtered" }
	local states = { "open", "closed" }
	local port = nil

	for _, s in ipairs(states) do
		port = nmap.get_ports(host, nil, "tcp", s)
		if port then
			break
		end
	end

	if not port then
		return nil
	end

	return port.number
end

hostrule = function(host)
	if not nmap.is_privileged() then
		nmap.registry[SCRIPT_NAME] = nmap.registry[SCRIPT_NAME] or {}
		if not nmap.registry[SCRIPT_NAME].rootfail then
			stdnse.print_verbose("%s not running for lack of privileges.", SCRIPT_NAME)
		end
		nmap.registry[SCRIPT_NAME].rootfail = true
		return nil
	end

	if nmap.address_family() ~= 'inet' then
		stdnse.print_debug("%s is IPv4 compatible only.", SCRIPT_NAME)
		return false
	end
	if not host.interface then
		return false
	end
	ipidseqport = getport(host)
	return (ipidseqport ~= nil)
end

action = function(host)
	local i = 1
	local ipids = {}
	local sock = nmap.new_dnet()
	local pcap = nmap.new_socket()
	local saddr = packet.toip(host.bin_ip_src)
	local daddr = packet.toip(host.bin_ip)
	local try = nmap.new_try()

	try(sock:ip_open())

	try = nmap.new_try(function() sock:ip_close() end)

	pcap:pcap_open(host.interface, 104, false, "tcp and dst host " .. saddr .. " and src host " .. daddr .. " and src port " .. ipidseqport)

	pcap:set_timeout(host.times.timeout * 1000)

	local tcp = genericpkt(host, ipidseqport)

	while i <= NUMPROBES do
		try(sock:ip_send(tcp.buf))

		local status, len, _, layer3 = pcap:pcap_receive()
		local test = bin.pack('AA=S=S', tcp.ip_bin_src, tcp.ip_bin_dst, tcp.tcp_sport, tcp.tcp_dport)
		while status and test ~= check(layer3) do
			status, len, _, layer3 = pcap:pcap_receive()
		end

		if status then
			table.insert(ipids, packet.u16(layer3, 4))
		end

		updatepkt(tcp)

		i = i + 1
	end

	pcap:close()
	sock:ip_close()

	local output = ipidseqclass(ipids)

	if nmap.debugging() > 0 then
		output = output .. " [used port " .. ipidseqport .. "]"
	end

	return output
end