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Linux Networking Technology

Discovering DHCP Servers with NMAP

I was working at a client site where a device would constantly receive a new IP address via DHCP nearly every second. It was the only device on the network that had this issue but I decided to test for rogue DHCP servers. If someone knows of a GUI tool to do this let me know in the comments. I utilized the command line utility NMAP to scan the network.

sudo nmap --script broadcast-dhcp-discover

The output should look something like:

Starting Nmap 7.70 ( https://nmap.org ) at 2019-11-25 15:52 EST
Pre-scan script results:
| broadcast-dhcp-discover:
| Response 1 of 1:
| IP Offered: 172.20.1.82
| DHCP Message Type: DHCPOFFER
| Server Identifier: 172.20.1.2
| IP Address Lease Time: 7d00h00m00s
| Subnet Mask: 255.255.255.0
| Time Offset: 4294949296
| Router: 172.20.1.2
| Domain Name Server: 8.8.8.8
| Renewal Time Value: 3d12h00m00s
|_ Rebinding Time Value: 6d03h00m00s

This was the test that ran on my local network verifying only one DHCP server. If there were multiple, we would see another response.

Ultimately this was not the issue at my client site but this is a new function of NMAP that I had not used.

Let me know your experiences with rogue DHCP in the comments!

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