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
- DNS Servers
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.
Set to an integer to make up to that many requests (and display the results).
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.
nmap -sU -p 67 --script=dhcp-discover <target>
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: 188.8.131.52, 184.108.40.206
License: Same as Nmap--See https://nmap.org/book/man-legal.html