description = [[ Sends a DHCPDISCOVER request to a host on UDP port 67. The response comes back to UDP port 68, and is read using pcap (due to the inability for a script to choose its source port at the moment). DHCPDISCOVER is a DHCP request that returns useful information from a DHCP server. 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 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. DHCPINFORM is another type of DHCP request that requests the same information, but doesn't reserve an address. Unfortunately, because many home routers simply ignore DHCPINFORM requests, we opted to use DHCPDISCOVER instead. 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, -- DHCPDISCOVER 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. -- @args randomize_mac Set to true or 1 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). -- @args fake_requests Set to an integer to make that many fake requests -- before the real one(s). This could be useful, for example, if you -- also use randomize_mac and you want to try exhausting -- all addresses. -- @args timeout Set to an integer to use it for a timeout. My router -- responds to fake_requests rate limited, at about 1 -- response/second. Therefore, timeout has to be at least -- fake_requests * 1000. Default: 5000. -- -- @output -- Interesting ports on 192.168.1.1: -- PORT STATE SERVICE -- 67/udp open dhcps -- | dhcp-discover: -- | | IP Offered: 192.168.1.101 -- | | 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: 208.81.7.10, 208.81.7.14 -- -- The functions for creating and parsing DHCP requests are general, and -- should be able to create and parse any DHCP request and response. If -- other scripts require DHCP support, dhcp_build and dhcp_parse, with -- their related functions, can easily be abstracted into a NSELib. author = "Ron Bowes" license = "Same as Nmap--See http://nmap.org/book/man-legal.html" categories = {"default", "discovery", "intrusive"} require 'bin' require 'bit' require 'ipOps' require 'shortport' require 'stdnse' local request_types = { DHCPDISCOVER = 1, DHCPOFFER = 2, DHCPREQUEST = 3, DHCPDECLINE = 4, DHCPACK = 5, DHCPNAK = 6, DHCPRELEASE = 7, DHCPINFORM = 8 } local request_types_str = {} request_types_str[1] = "DHCPDISCOVER" request_types_str[2] = "DHCPOFFER" request_types_str[3] = "DHCPREQUEST" request_types_str[4] = "DHCPDECLINE" request_types_str[5] = "DHCPACK" request_types_str[6] = "DHCPNAK" request_types_str[7] = "DHCPRELEASE" request_types_str[8] = "DHCPINFORM" portrule = shortport.portnumber(67, "udp") callback = function(packetsz, layer2, layer3) return string.sub(layer3, 33, 36) end ---Read an IP address or a list of IP addresses. Print an error if the length isn't a multiple of 4. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_ip(data, pos, length) if(length ~= 4) then if((length % 4) ~= 0) then stdnse.print_debug(1, "dhcp-discover: Invalid length for an ip address (%d)", length) pos = pos + length return pos, nil else local results = {} for i=1, length, 4 do local value pos, value = bin.unpack("request_types_str -- table. Print an error if the length isn't 1. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_message_type(data, pos, length) local value pos, value = read_1_byte(data, pos, length) if(value == nil) then stdnse.print_debug(1, "dhcp-discover: Couldn't read the 1-byte message type") return pos, nil end return pos, request_types_str[value] end ---Read a single byte, and return 'false' if it's 0, or 'true' if it's non-zero. Print an error if the -- length isn't 1. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_boolean(data, pos, length) local result pos, result = read_1_byte(data, pos, length) if(result == nil) then stdnse.print_debug(1, "dhcp-discover: Couldn't read the 1-byte boolean") return pos, nil elseif(result == 0) then return pos, "false" else return pos, "true" end end ---Read a 2-byte unsigned little endian value. Print an error if the length isn't 2. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_2_bytes(data, pos, length) if(length ~= 2) then stdnse.print_debug(1, "dhcp-discover: Invalid length for data (%d; should be %d)", length, 2) pos = pos + length return pos, nil end return bin.unpack(">S", data, pos) end ---Read a list of 2-byte unsigned little endian values. Print an error if the length isn't a multiple -- of 2. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_2_bytes_list(data, pos, length) if((length % 2) ~= 0) then stdnse.print_debug(1, "dhcp-discover: Invalid length for data (%d; should be multiple of %d)", length, 2) pos = pos + length return pos, nil else local results = {} for i=1, length, 2 do local value pos, value = bin.unpack(">S", data, pos) table.insert(results, value) end return pos, results end end ---Read a 4-byte unsigned little endian value. Print an error if the length isn't 4. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_4_bytes(data, pos, length) if(length ~= 4) then stdnse.print_debug(1, "dhcp-discover: Invalid length for data (%d; should be %d)", length, 4) pos = pos + length return pos, nil end return bin.unpack(">I", data, pos) end ---Read a 4-byte unsigned little endian value, and interpret it as a time offset value. Print an -- error if the length isn't 4. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_time(data, pos, length) local result if(length ~= 4) then stdnse.print_debug(1, "dhcp-discover: Invalid length for data (%d; should be %d)", length, 4) pos = pos + length return pos, nil end pos, result = bin.unpack(">I", data, pos) -- This code was mostly taken from snmp-sysdescr.nse. It should probably be abstracted into stdnse.lua [TODO] local days, hours, minutes, seconds, htime, mtime, stime days = math.floor(result / 86400) htime = math.fmod(result, 86400) hours = math.floor(htime / 3600) mtime = math.fmod(htime, 3600) minutes = math.floor(mtime / 60) seconds = math.fmod(mtime, 60) local dayLabel if days == 1 then dayLabel = "day" else dayLabel = "days" end return pos, string.format("%d %s, %d:%02d:%02d", days, dayLabel, hours, minutes, seconds) end ---Read a list of static routes. Each of them are a pair of IP addresses, a destination and a -- router. Print an error if the length isn't a multiple of 8. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_static_route(data, pos, length) if((length % 8) ~= 0) then stdnse.print_debug(1, "dhcp-discover: Invalid length for data (%d; should be multiple of %d)", length, 8) pos = pos + length return pos, nil else local results = {} for i=1, length, 8 do local destination, router pos, destination = read_ip(data, pos, 4) pos, router = read_ip(data, pos, 4) table.insert(results, {destination=destination, router=router}) end return pos, results end end ---Read a list of policy filters. Each of them are a pair of IP addresses, an address and a -- mask. Print an error if the length isn't a multiple of 8. -- --@param data The packet. --@param pos The position in the packet. --@param length The length that the server claims the field is. --@return The new position (will always be pos + length, no matter what we think it should be) --@return The value of the field, or nil if the field length was wrong. local function read_policy_filter(data, pos, length) if((length % 8) ~= 0) then stdnse.print_debug(1, "dhcp-discover: Invalid length for data (%d; should be multiple of %d)", length, 8) pos = pos + length return pos, nil else local results = {} for i=1, length, 8 do local address, router pos, address = read_ip(data, pos, 4) pos, mask = read_ip(data, pos, 4) table.insert(results, {address=address, mask=mask}) end return pos, results end end ---These are the different fields for DHCP local actions = {} actions[1] = {name="Subnet Mask", func=read_ip, default=true} actions[2] = {name="Time Offset", func=read_4_bytes, default=false} actions[3] = {name="Router", func=read_ip, default=true} actions[4] = {name="Time Server", func=read_ip, default=true} actions[5] = {name="Name Server", func=read_ip, default=true} actions[6] = {name="Domain Name Server", func=read_ip, default=true} actions[7] = {name="Log Server", func=read_ip, default=true} actions[8] = {name="Cookie Server", func=read_ip, default=true} actions[9] = {name="LPR Server", func=read_ip, default=true} actions[10] = {name="Impress Server", func=read_ip, default=true} actions[11] = {name="Resource Location Server", func=read_ip, default=true} actions[12] = {name="Hostname", func=read_string, default=true} actions[13] = {name="Boot File Size", func=read_2_bytes, default=false} actions[14] = {name="Merit Dump File", func=read_string, default=false} actions[15] = {name="Domain Name", func=read_string, default=true} actions[16] = {name="Swap Server", func=read_ip, default=true} actions[17] = {name="Root Path", func=read_string, default=false} actions[18] = {name="Extensions Path", func=read_string, default=false} actions[19] = {name="IP Forwarding", func=read_boolean, default=false} actions[20] = {name="Non-local Source Routing", func=read_boolean, default=true} actions[21] = {name="Policy Filter", func=read_policy_filter, default=false} actions[22] = {name="Maximum Datagram Reassembly Size",func=read_2_bytes, default=false} actions[23] = {name="Default IP TTL", func=read_1_byte, default=false} actions[24] = {name="Path MTU Aging Timeout", func=read_time, default=false} actions[25] = {name="Path MTU Plateau", func=read_2_bytes_list, default=false} actions[26] = {name="Interface MTU", func=read_2_bytes, default=false} actions[27] = {name="All Subnets are Local", func=read_boolean, default=false} actions[28] = {name="Broadcast Address", func=read_ip, default=true} actions[29] = {name="Perform Mask Discovery", func=read_boolean, default=false} actions[30] = {name="Mask Supplier", func=read_boolean, default=false} actions[31] = {name="Perform Router Discovery", func=read_boolean, default=false} actions[32] = {name="Router Solicitation Address", func=read_ip, default=true} actions[33] = {name="Static Route", func=read_static_route, default=true} actions[34] = {name="Trailer Encapsulation", func=read_boolean, default=false} actions[35] = {name="ARP Cache Timeout", func=read_time, default=false} actions[36] = {name="Ethernet Encapsulation", func=read_boolean, default=false} actions[37] = {name="TCP Default TTL", func=read_1_byte, default=false} actions[38] = {name="TCP Keepalive Interval", func=read_4_bytes, default=false} actions[39] = {name="TCP Keepalive Garbage", func=read_boolean, default=false} actions[40] = {name="NIS Domain", func=read_string, default=true} actions[41] = {name="NIS Servers", func=read_ip, default=true} actions[42] = {name="NTP Servers", func=read_ip, default=true} actions[43] = {name="Vendor Specific Information", func=read_string, default=false} actions[44] = {name="NetBIOS Name Server", func=read_ip, default=true} actions[45] = {name="NetBIOS Datagram Server", func=read_ip, default=true} actions[46] = {name="NetBIOS Node Type", func=read_1_byte, default=false} actions[47] = {name="NetBIOS Scope", func=read_string, default=false} actions[48] = {name="X Window Font Server", func=read_ip, default=true} actions[49] = {name="X Window Display Manager", func=read_ip, default=true} actions[50] = {name="Requested IP Address (client)", func=read_ip, default=false} actions[51] = {name="IP Address Lease Time", func=read_time, default=false} actions[52] = {name="Option Overload", func=read_1_byte, default=false} actions[53] = {name="DHCP Message Type", func=read_message_type, default=false} actions[54] = {name="Server Identifier", func=read_ip, default=true} actions[55] = {name="Parameter Request List (client)", func=read_string, default=false} actions[56] = {name="Error Message", func=read_string, default=true} actions[57] = {name="Maximum DHCP Message Size", func=read_2_bytes, default=false} actions[58] = {name="Renewal Time Value", func=read_time, default=false} actions[59] = {name="Rebinding Time Value", func=read_time, default=false} actions[60] = {name="Class Identifier", func=read_string, default=false} actions[61] = {name="Client Identifier (client)", func=read_string, default=false} ---Build a DHCP packet. -- --@param request_type The type of request (such as DHCPINFORM). See the request_types -- table. --@param ip_address The ip address (as a 4-byte string) where the server will send the response. -- Generally, it'll be host.bin_ip_src or 255.255.255.255. --@param mac_address The mac address (as a string no more than 16 bytes) where the server will -- send the response. Generally, this will be host.mac_addr_src or -- simply a blank string (""). The field will be padded to 16 bytes with null bytes. --@param request_options [optional] The options to request from the server, as a string of bytes where each -- byte represents a single option. For the types of options, see rfc2132. Some DHCP -- servers (such as my Linksys WRT54g) will ignore this list and send whichever options -- it wants. Options won't necessarily be honoured, it's up to the server what it sends -- back. By default, all options (1..61) are requested. --@param overrides [optional] A table of overrides. If a field in the table matches a field in the DHCP -- packet (see rfc2131 section 2 for a list of possible fields. Or, just look at the -- code. --@param lease_time [optional] The lease time for which to request an IP. Default: 1 second. --@return The packet, as a string. It should be sent to the server on UDP/67. local function dhcp_build(request_type, ip_address, mac_address, request_options, overrides, lease_time) local packet = '' if(overrides == nil) then overrides = {} end if(request_options == nil) then -- Request the defaults, or there's no verbosity; otherwise, request everything! request_options = '' for i = 1, 61, 1 do if(nmap.verbosity() > 0) then request_options = request_options .. string.char(i) else if(actions[i] and actions[i].default) then request_options = request_options .. string.char(i) end end end end -- Header packet = packet .. bin.pack(">CCCC", overrides['op'] or 1, overrides['htype'] or 1, overrides['hlen'] or 6, overrides['hops'] or 0) -- BOOTREQUEST, 10mb ethernet, 6 bytes long, 0 hops packet = packet .. bin.pack(">I", overrides['xid'] or 0x4e4d4150) -- Transaction ID packet = packet .. bin.pack(">SS", overrides['secs'] or 0, overrides['flags'] or 0x0000) -- Secs, flags packet = packet .. bin.pack("A", ip_address) -- Client address packet = packet .. bin.pack("I", overrides['cookie'] or 0x63825363) -- Magic cookie -- Options packet = packet .. bin.pack(">CCC", 0x35, 1, request_type) -- Request type packet = packet .. bin.pack(">CCA", 0x37, #request_options, request_options) -- Request options packet = packet .. bin.pack(">CCI", 0x33, 4, lease_time or 1) -- Lease time packet = packet .. bin.pack(">C", 0xFF) -- Termination return packet end ---Parse a DHCP packet (either a request or a response) and return the results as a table. The -- table at the top of this function (actions) defines the name of each field, as -- laid out in rfc2132, and the function that parses it. -- -- In theory, this should be able to parse any valid DHCP packet. -- --@param data The DHCP packet data. Any padding at the end of the packet will be ignored (by default, -- DHCP packets are padded with \x00 bytes). local function dhcp_parse(data) local pos = 1 local result = {} -- Receive the first bit and make sure we got the correct operation back pos, result['op'], result['htype'], result['hlen'], result['hops'] = bin.unpack(">CCCC", data, pos) if(result['op'] ~= 2) then return false, string.format("DHCP server returned invalid reply ('op' wasn't BOOTREPLY (it was 0x%02x))", result['op']) end -- Confirm the transaction id pos, result['xid'] = bin.unpack(">I", data, pos) if(result['xid'] ~= 0x4e4d4150) then return false, string.format("DHCP server returned invalid reply (transaction id didn't match (0x%08x != ))", result['xid'], 0x4e4d4150) end -- Unpack the secs, flags, addresses, sname, and file pos, result['secs'], result['flags'] = bin.unpack(">SS", data, pos) pos, result['ciaddr'] = bin.unpack("I", data, pos) if(result['cookie'] ~= 0x63825363) then return false, "DHCP server returned invalid reply (the magic cookie was invalid)" end -- Parse the options result['options'] = {} while true do local option, length pos, option, length = bin.unpack(">CC", data, pos) -- Check for termination condition if(option == 0xFF) then break; end -- Get the action from the array, based on the code local action = actions[option] -- Verify we got a valid code (if we didn't, we're probably in big trouble) if(action == nil) then stdnse.print_debug(1, "dhcp-discover: Unknown option: %d", option) pos = pos + length else -- Call the function to parse the option, and insert the result into our results table local value stdnse.print_debug(2, "dhcp-discover: Attempting to parse %s", action['name']) pos, value = action['func'](data, pos, length) if(nmap.verbosity() == 0 and action.default == false) then stdnse.print_debug(1, "dhcp-discover: Server returned unrequested option (%s => %s)", action['name'], value) else if(value) then table.insert(result['options'], {name=action['name'], value=value}) else stdnse.print_debug(1, "dhcp-discover: Couldn't determine value for %s", action['name']); end end end -- Handle the 'Option Overload' option specially -- if it's set, it tells us to use the file and/or sname values after we -- run out of data. if(option == 52) then if(value == 1) then data = data .. result['file'] elseif(value == 2) then data = data .. result['sname'] elseif(value == 3) then data = data .. result['file'] .. result['sname'] else stdnse.print_debug(1, "dhcp-discover: Warning: 'Option Overload' gave an unsupported value: %d", value) end end end return true, result end local function go(host, port) local pcap, socket local status, err, data local result local results = {} local timeout = 5000 if(nmap.registry.args.timeout) then timeout = tonumber(nmap.registry.args.timeout) end local requests = 1 if(nmap.registry.args.requests) then requests = tonumber(nmap.registry.args.requests) end -- Verify we have a IPv4 address if(string.len(host.bin_ip_src) ~= 4) then return false, "Sorry, dhcp-discover only supports IPv4!" end -- Verify we have a MAC address if(string.len(host.mac_addr_src) ~= 6) then return false, "Sorry, dhcp-discover only supports Ethernet!" end -- Create a pcap socket to listen for the response (this is a HUGE hack. TODO: Fix once I can set the source port) pcap = nmap.new_socket() pcap:pcap_open(host.interface, 590, 0, callback, "udp port 68") stdnse.print_debug(1, "dhcp-discover: Setting socket timeout to %ds", timeout) pcap:set_timeout(timeout) -- Create the UDP socket socket = nmap.new_socket() status, err = socket:connect(host.ip, port.number, "udp") if(status == false) then return false, "Couldn't create socket: " .. err end -- We're going to need some low quality random numbers math.randomseed(os.time()) -- Create fake requests if the user asked to. These are fired and forgotten, we ignore the responses. if(nmap.registry.args.fake_requests) then for i=1, tonumber(nmap.registry.args.fake_requests), 1 do -- Build and send a DHCP request using the specified request type, or DHCPDISCOVER local request_type = request_types[nmap.registry.args.dhcptype or "DHCPDISCOVER"] if(request_type == nil) then return false, "Valid request types: " .. stdnse.strjoin(", ", 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 -- Build and send the packet local response = dhcp_build(request_type, host.bin_ip_src, mac_addr, nil, {xid=i}) socket:send(response) end end -- Build and send a DHCP request using the specified request type, or DHCPDISCOVER for i = 1, requests, 1 do -- Register the packet cap pcap:pcap_register("NMAP") -- Decide which type of request to make local request_type = request_types[nmap.registry.args.dhcptype or "DHCPDISCOVER"] if(request_type == nil) then return false, "Valid request types: " .. stdnse.strjoin(", ", 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 -- Build and send the packet stdnse.print_debug(2, "dhcp-discover: Sending DHCP request #%d", i) local response = dhcp_build(request_type, host.bin_ip_src, mac_addr) socket:send(response) -- Receive the result status, err, _, data = pcap:pcap_receive() if(status == false) then stdnse.print_debug(1, "dhcp-discover: Error calling pcap_receive(): %s", err) return false, "Error calling pcap_receive(): " .. err end -- If no data was captured (ie, a timeout), return what, if anything, we have if(data == nil) then stdnse.print_debug(1, "dhcp-discover: Error calling pcap_receive(): TIMEOUT") if(#results > 0) then return true, results else return false, "Error calling pcap_receive(): TIMEOUT" end end -- Cut off the address/transport headers data = string.sub(data, 29) -- I doubt this is the right way to do this, but since we're only supporting IPv4 + UDP, maybe it'll work? -- Parse the result status, result = dhcp_parse(data) if(status == false) then stdnse.print_debug(1, "dhcp-discover: Couldn't parse DHCP packet: %s", result) return false, "Couldn't parse DHCP packet: " .. result end table.insert(results, result) end socket:close() -- Done! return true, results end action = function(host, port) local status, results = go(host, port) if(status == false) then return stdnse.format_output(false, results) end if(results == nil) then return nil end -- Set the port state to open nmap.set_port_state(host, port, "open") local response = {} -- Display the results for i, result in ipairs(results) do if(#results ~= 1) then table.insert(response, string.format("Result %d", i)) end table.insert(response, string.format("IP Offered: %s", result.yiaddr_str)) for _, v in ipairs(result.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