nmap/scripts/broadcast-dhcp-discover.nse

description = [[
Sends a DHCP request to the broadcast address (255.255.255.255) and reports
the results. The script uses a static MAC address (DE:AD:CO:DE:CA:FE) while
doing so in order to prevent scope exhaustion.

The script reads the response using pcap by opening a listening pcap socket
on all available ethernet interfaces that are reported up. If no response
has been received before the timeout has been reached (default 10 seconds)
the script will abort execution.

The script needs to be run as a privileged user, typically root.
]]

---
-- @usage
-- sudo nmap --script broadcast-dhcp-discover
-- 
-- @output
-- | broadcast-dhcp-discover: 
-- |   IP Offered: 192.168.1.114
-- |   DHCP Message Type: DHCPOFFER
-- |   Server Identifier: 192.168.1.1
-- |   IP Address Lease Time: 1 day, 0:00:00
-- |   Subnet Mask: 255.255.255.0
-- |   Router: 192.168.1.1
-- |   Domain Name Server: 192.168.1.1
-- |_  Domain Name: localdomain
--
-- @args broadcast-dhcp-discover.timeout time in seconds to wait for a response
--       (default: 10s)
--

-- Version 0.1
-- Created 07/14/2011 - v0.1 - created by Patrik Karlsson

author = "Patrik Karlsson"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
categories = {"broadcast", "safe"}

prerule = function() return not( nmap.address_family() == "inet6") end

require 'dhcp'
require 'ipOps'
require 'packet'

-- Creates a random MAC address
--
-- @return mac_addr string containing a random MAC
local function randomizeMAC()
	local mac_addr = ""
	for j=1, 6 do
		mac_addr = mac_addr .. string.char(math.random(1, 255))
	end	
	return mac_addr
end

-- Gets a list of available interfaces based on link and up filters
--
-- @param link string containing the link type to filter
-- @param up string containing the interface status to filter
-- @return result table containing the matching interfaces
local function getInterfaces(link, up)
	if( not(nmap.list_interfaces) ) then return end
	local interfaces, err = nmap.list_interfaces()
	local result
	if ( not(err) ) then
		for _, iface in ipairs(interfaces) do
			if ( iface.link == link and iface.up == up ) then
				result = result or {}
				result[iface.device] = true
			end
		end
	end
	return result
end	

-- Listens for an incoming dhcp response
--
-- @param iface string with the name of the interface to listen to
-- @param timeout number of ms to wait for a response
-- @param xid the DHCP transaction id
-- @param result a table to which the result is written
local function dhcp_listener(iface, timeout, xid, result)
	local sock = nmap.new_socket()
	local condvar = nmap.condvar(result)
	
	sock:set_timeout(100)
	sock:pcap_open(iface, 1500, false, "ip && udp && port 68")
	
	local start_time = nmap.clock_ms()
	while( nmap.clock_ms() - start_time < timeout ) do
		local status, _, _, data = sock:pcap_receive()
		-- abort, once another thread has picked up our response
		if ( #result > 0 ) then
			sock:close()
			condvar "signal"
			return
		end
	
		if ( status ) then
			local p = packet.Packet:new( data, #data )
			if ( p and p.udp_dport ) then
				local data = data:sub(p.udp_offset + 9)
				local status, response = dhcp.dhcp_parse(data, xid)
				if ( status ) then
					table.insert( result, response )
					sock:close()
					condvar "signal"
					return
				end
			end
		end
 	end
	sock:close()
	condvar "signal"
end


action = function()

	if not nmap.is_privileged() then
		return ("\n  ERROR: %s needs to be run as a privileged user (root)."):format(SCRIPT_NAME)
	end

	local host, port = "255.255.255.255", 67
	local timeout = stdnse.get_script_args("broadcast-dhcp-discover.timeout")
	timeout = tonumber(timeout) or 10

	-- convert from seconds to ms
	timeout = timeout * 1000

	-- randomizing the MAC could exhaust dhcp servers with small scopes
	-- if ran multiple times, so we should probably refrain from doing
	-- this?
	local mac = string.char(0xDE,0xAD,0xC0,0xDE,0xCA,0xFE)--randomizeMAC()
	
	local interfaces
	
	-- first check if the user supplied an interface
	if ( nmap.get_interface() ) then
		interfaces = { [nmap.get_interface()] = true }
	else
		-- As the response will be sent to the "offered" ip address we need
		-- to use pcap to pick it up. However, we don't know what interface
		-- our packet went out on, so lets get a list of all interfaces and
		-- run pcap on all of them, if they're a) up and b) ethernet.
		interfaces = getInterfaces("ethernet", "up")
	end

	if( not(interfaces) ) then return "\n  ERROR: Failed to retrieve interfaces (try setting one explicitly using -e)" end
	
	local transaction_id = bin.pack("<I", math.random(0, 0x7FFFFFFF))
	local request_type = dhcp.request_types["DHCPDISCOVER"]
	local ip_address = bin.pack(">I", ipOps.todword("0.0.0.0"))

	-- we nead to set the flags to broadcast
	local request_options, overrides, lease_time = nil, { flags = 0x8000 }, nil
	local status, packet = dhcp.dhcp_build(request_type, ip_address, mac, nil, request_options, overrides, lease_time, transaction_id)
	if (not(status)) then return "\n  ERROR: Failed to build packet" end

	local socket = nmap.new_socket("udp")
	socket:bind(nil, 68)
	socket:sendto( host, port, packet )
	socket:close()

	local threads = {}
	local result = {}
	local condvar = nmap.condvar(result)
	
	-- start a listening thread for each interface
	for iface, _ in pairs(interfaces) do
		local co = stdnse.new_thread( dhcp_listener, iface, timeout, transaction_id, result )
		threads[co] = true
	end
	
	-- wait until all threads are done
	repeat 
		condvar "wait"
		for thread in pairs(threads) do
			if coroutine.status(thread) == "dead" then threads[thread] = nil end
		end
	until next(threads) == nil
	
	local response = {}
	-- Display the results
	for i, r in ipairs(result) do
		table.insert(response, string.format("IP Offered: %s", r.yiaddr_str))
		for _, v in ipairs(r.options) do
			if(type(v['value']) == 'table') then
				table.insert(response, string.format("%s: %s", v['name'], stdnse.strjoin(", ", v['value'])))
			else
				table.insert(response, string.format("%s: %s\n", v['name'], v['value']))
			end
		end
	end
	return stdnse.format_output(true, response)		
end