dhcp-discover.nse

local dhcp = require "dhcp"
local math = require "math"
local nmap = require "nmap"
local shortport = require "shortport"
local stdnse = require "stdnse"
local string = require "string"
local table = require "table"

description = [[
Sends a DHCPINFORM request to a host on UDP port 67 to obtain all the local configuration parameters
without allocating a new address.

DHCPINFORM is a DHCP request that returns useful information from a DHCP server, without allocating an IP
address. The request sends a list of which fields it wants to know (a handful by default, every field if
verbosity is turned on), and the server responds with the fields that were requested. It should be noted
that the server doesn't have to return every field, nor does it have to return them in the same order,
or honour the request at all. A Linksys WRT54g, for example, completely ignores the list of requested
fields and returns a few standard ones. This script displays every field it receives. 

With script arguments, the type of DHCP request can be changed, which can lead to interesting results. 
Additionally, the MAC address can be randomized, which in should override the cache on the DHCP server and
assign a new IP address. Extra requests can also be sent to exhaust the IP address range more quickly. 

Some of the more useful fields:
* DHCP Server (the address of the server that responded)
* Subnet Mask
* Router
* DNS Servers
* Hostname
]]

---
-- @args dhcptype The type of DHCP request to make. By default,  DHCPINFORM is sent, but this
--                argument can change it to DHCPOFFER,  DHCPREQUEST, DHCPDECLINE, DHCPACK, DHCPNAK, 
--                DHCPRELEASE or DHCPINFORM. Not all types will evoke a response from all servers,
--                and many require different fields to contain specific values. 
-- @args randomize_mac Set to <code>true</code> or <code>1</code> to  send a random MAC address with
--                the request (keep in mind that you may  not see the response). This should 
--                cause the router to reserve a new  IP address each time.
-- @args requests Set to an integer to make up to  that many requests (and display the results). 
--
-- @output
-- Interesting ports on 192.168.1.1:
-- PORT   STATE SERVICE
-- 67/udp open  dhcps
-- |  dhcp-discover:
-- |  |  DHCP Message Type: DHCPACK
-- |  |  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: 208.81.7.10, 208.81.7.14
-- 

--
-- 2011-12-28 - Revised by Patrik Karlsson <patrik@cqure.net>
--   o Removed DoS code and placed script into discovery and safe categories
--
-- 2011-12-27 - Revised by Patrik Karlsson <patrik@cqure.net>
--   o Changed script to use DHCPINFORM instead of DHCPDISCOVER
--


author = "Ron Bowes"

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

categories = {"discovery", "safe"}


-- We want to run against a specific host if UDP/67 is open
function portrule(host, port)
	if nmap.address_family() ~= 'inet' then
		stdnse.print_debug("%s is IPv4 compatible only.", SCRIPT_NAME)
		return false
	end

	return shortport.portnumber(67, "udp")(host, port)
end

local function go(host, port)

	-- Build and send a DHCP request using the specified request type, or DHCPINFORM
	local requests = tonumber(nmap.registry.args.requests or 1)
	local results = {}
	for i = 1, requests, 1 do
		-- Decide which type of request to make
		local request_type = dhcp.request_types[nmap.registry.args.dhcptype or "DHCPINFORM"]
		if(request_type == nil) then
			return false, "Valid request types: " .. stdnse.strjoin(", ", dhcp.request_types_str)
		end
	
		-- Generate the MAC address, if it's random
		local mac_addr = host.mac_addr_src
		if(nmap.registry.args.randomize_mac == 'true' or nmap.registry.args.randomize_mac == '1') then
			stdnse.print_debug(2, "dhcp-discover: Generating a random MAC address")
			mac_addr = ""
			for j=1, 6, 1 do
				mac_addr = mac_addr .. string.char(math.random(1, 255))
			end
		end
		
		local iface, err = nmap.get_interface_info(host.interface)
		if ( not(iface) or not(iface.address) ) then
			return false, "Couldn't determine local ip for interface: " .. host.interface
		end
		
		local status, result = dhcp.make_request(host.ip, request_type, iface.address, mac_addr)
		if( not(status) ) then
			stdnse.print_debug(1, "dhcp-discover: Couldn't send DHCP request: %s", result)
			return false, result
		end

		table.insert(results, result)
	end

	-- Done!
	return true, results
end

action = function(host, port)
	local status, results = go(host, port)


	if(not(status)) then
		return stdnse.format_output(false, results)
	end

	if(not(results)) then
		return nil
	end

	-- Set the port state to open
	if(host) then
		nmap.set_port_state(host, port, "open")
	end

	local response = {}

	-- Display the results
	for i, result in ipairs(results) do
		local result_table = {}

		if ( nmap.registry.args.dhcptype and 
			 "DHCPINFORM" ~= nmap.registry.args.dhcptype ) then
			table.insert(result_table, string.format("IP Offered: %s", result.yiaddr_str))
		end
		for _, v in ipairs(result.options) do
			if(type(v['value']) == 'table') then
				table.insert(result_table, string.format("%s: %s", v['name'], stdnse.strjoin(", ", v['value'])))
			else
				table.insert(result_table, string.format("%s: %s\n", v['name'], v['value']))
			end
		end

		if(#results == 1) then
			response = result_table
		else
			result_table['name'] = string.format("Result %d of %d", i, #results)
			table.insert(response, result_table)
		end
	end

	return stdnse.format_output(true, response)
end