File dhcp-discover
Download: http://nmap.org/svn/scripts/dhcp-discover.nse
User Summary
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
Script Arguments
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).
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.
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.
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.
Example Usage
nmap -sV -sC <target>
Script 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
Requires
Categories: default discovery intrusive
Author: Ron Bowes
License: Same as Nmap--See http://nmap.org/book/man-legal.html