dhcp-discover
NSE detection rule by Ron Bowes · Same as Nmap--See https://nmap.org/book/man-legal.html
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
local dhcp = require "dhcp"
local rand = require "rand"
local nmap = require "nmap"
local outlib = require "outlib"
local shortport = require "shortport"
local stdnse = require "stdnse"
local string = require "string"
local table = require "table"
local ipOps = require "ipOps"
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
]]
---
-- @see broadcast-dhcp6-discover.nse
-- @see broadcast-dhcp-discover.nse
--
-- @args dhcp-discover.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 dhcp-discover.mac Set to <code>native</code> (default) or
-- <code>random</code> or a specific client MAC address in the DHCP
-- request. Keep in mind that you may not see the response if
-- a non-native address is used. Setting it to <code>random</code> will
-- possibly cause the DHCP server to reserve a new IP address each time.
-- @args dhcp-discover.clientid Client identifier to use in DHCP option 61.
-- The value is a string, while hardware type 0, appropriate for FQDNs,
-- is assumed. Example: clientid=kurtz is equivalent to specifying
-- clientid-hex=00:6b:75:72:74:7a (see below).
-- @args dhcp-discover.clientid-hex Client identifier to use in DHCP option 61.
-- The value is a hexadecimal string, where the first octet is
-- the hardware type.
-- @args dhcp-discover.requests Set to an integer to make up to that many
-- requests (and display the results).
--
-- @usage
-- nmap -sU -p 67 --script=dhcp-discover <target>
-- @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
--
-- @xmloutput
-- <elem key="DHCP Message Type">DHCPACK</elem>
-- <elem key="Server Identifier">192.168.1.1</elem>
-- <elem key="IP Address Lease Time">1 day, 0:00:00</elem>
-- <elem key="Subnet Mask">255.255.255.0</elem>
-- <elem key="Router">192.168.1.1</elem>
-- <table key="Domain Name Server">
-- <elem>208.81.7.10</elem>
-- <elem>208.81.7.14</elem>
-- </table>
--
--
-- 2022-04-22 - Revised by nnposter
-- o Implemented script arguments "clientid" and "clientid-hex" to allow
-- passing a specific client identifier (option 61)
--
-- 2020-01-14 - Revised by nnposter
-- o Added script argument "mac" to prescribe a specific MAC address
-- o Deprecated argument "randomize_mac" in favor of "mac=random"
--
-- 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 https://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.debug1("is IPv4 compatible only.")
return false
end
return shortport.portnumber(67, "udp")(host, port)
end
action = function(host, port)
local dhcptype = (stdnse.get_script_args(SCRIPT_NAME .. ".dhcptype") or "DHCPINFORM"):upper()
local dhcptypeid = dhcp.request_types[dhcptype]
if not dhcptypeid then
return stdnse.format_output(false, "Invalid request type (use "
.. table.concat(dhcp.request_types_str, " / ")
.. ")")
end
local reqcount = tonumber(stdnse.get_script_args(SCRIPT_NAME .. ".requests") or 1)
if not reqcount then
return stdnse.format_output(false, "Invalid request count")
end
local iface, err = nmap.get_interface_info(host.interface)
if not (iface and iface.address) then
return stdnse.format_output(false, "Couldn't determine local IP for interface: " .. host.interface)
end
local options = {}
local overrides = {}
local macaddr = (stdnse.get_script_args(SCRIPT_NAME .. ".mac") or "native"):lower()
-- Support for legacy argument "randomize_mac"
local randomize = (stdnse.get_script_args(SCRIPT_NAME .. ".randomize_mac") or "false"):lower()
if randomize == "true" or randomize == "1" then
stdnse.debug1("Use %s.mac=random instead of %s.randomize_mac=%s", SCRIPT_NAME, SCRIPT_NAME, randomize)
macaddr = "random"
end
if macaddr ~= "native" then
-- Set the scanner as a relay agent
overrides.giaddr = string.unpack("<I4", ipOps.ip_to_str(iface.address))
end
local macaddr_iter
if macaddr:find("^ra?nd") then
macaddr_iter = function () return rand.random_string(6) end
else
if macaddr == "native" then
macaddr = host.mac_addr_src
else
macaddr = macaddr:gsub(":", "")
if not (#macaddr == 12 and macaddr:find("^%x+$")) then
return stdnse.format_output(false, "Invalid MAC address")
end
macaddr = stdnse.fromhex(macaddr)
end
macaddr_iter = function () return macaddr end
end
local clientid = stdnse.get_script_args(SCRIPT_NAME .. ".clientid")
if clientid then
clientid = "\x00" .. clientid -- hardware type 0 presumed
else
clientid = stdnse.get_script_args(SCRIPT_NAME .. ".clientid-hex")
if clientid then
clientid = clientid:gsub(":", "")
if not clientid:find("^%x+$") then
return stdnse.format_output(false, "Invalid hexadecimal client ID")
end
clientid = stdnse.fromhex(clientid)
end
end
if clientid then
if #clientid == 0 or #clientid > 255 then
return stdnse.format_output(false, "Client ID must be between 1 and 255 characters long")
end
table.insert(options, {number = 61, type = "string", value = clientid })
end
local results = {}
for i = 1, reqcount do
local macaddr = macaddr_iter()
stdnse.debug1("Client MAC address: %s", stdnse.tohex(macaddr, {separator = ":"}))
local status, result = dhcp.make_request(host.ip, dhcptypeid, iface.address, macaddr, options, nil, overrides)
if not status then
return stdnse.format_output(false, "Couldn't send DHCP request: " .. result)
end
table.insert(results, result)
end
if #results == 0 then
return nil
end
nmap.set_port_state(host, port, "open")
local response = stdnse.output_table()
-- Display the results
for i, result in ipairs(results) do
local result_table = stdnse.output_table()
if dhcptype ~= "DHCPINFORM" then
result_table["IP Offered"] = result.yiaddr_str
end
for _, v in ipairs(result.options) do
if type(v.value) == 'table' then
outlib.list_sep(v.value)
end
result_table[ v.name ] = v.value
end
if(#results == 1) then
response = result_table
else
response[string.format("Response %d of %d", i, #results)] = result_table
end
end
return response
end