Tool for Finding Rogue DHCP

black3dynamite

https://www.question-defense.com/2012/10/02/sniff-all-broadcast-traffic-using-tcpdump
sudo tcpdump -i eth0 ether broadcast and ether multicast

stacksofplates

I took a library someone wrote and made a small program to print out DHCP message info. Let me know what platform you want it for and I'll build it for that.

StorageNinja

@Pete-S said in Tool for Finding Rogue DHCP:

Capture with tcpdump, analyze with wireshark on whatever machine you want.

Turn on DHCP snooping on your switches and have it suppress DHCP servers on access ports.

1337

This is what it looks like with tcpdump when you have two dhcp servers:

Captured on the DHCP client when the interface goes up.

DHCP server 1 is x.x.x.251
DHCP server 2 is x.x.x.252
Client IP becomes x.x.x.150

If you add the option -v for verbose you can see all the info inside the DHCP offer/ack.
For instance mac-address, lease-time, domain, ntp server etc.

When you have only one DHCP server and everything is working you will just see the IP from the one DHCP server. If you look closer at the communication you'll see that the first line will be the offer from the DHCP server and the second will be the acknowledge.

jmoore

@Pete-S tcpdump is what I was thinking of too.

1337

@jmoore said in Tool for Finding Rogue DHCP:

@Pete-S tcpdump is what I was thinking of too.

I think it's a good tool. Prior to this I've only used it to capture and then run wireshark to analyze but this thread showed up at the right time.

The dual DHCP servers in my post above was a real scenario. Two DHCP servers in a failover configuration that I suspected didn't work as it should. And it didn't because both DHCP servers would send out an IP address but since they where synced the addresses didn't collide. I could verify and correct it with the help of tcpdump.

wrx7m

Ran into this about 12 years ago. A guy on the dev team decided to setup his own DHCP server. Screwed up all sorts of stuff. Can't remember for sure what we did, but I think after we realized that it wasn't actually an issue with our known DHCP servers, we decided to talk to the dev team and found out that is what he had done.

DustinB3403

@wrx7m What happened with the dev?

wrx7m

@DustinB3403 said in Tool for Finding Rogue DHCP:

@wrx7m What happened with the dev?

He was reprimanded but not fired.

Dashrender

@wrx7m said in Tool for Finding Rogue DHCP:

Ran into this about 12 years ago. A guy on the dev team decided to setup his own DHCP server. Screwed up all sorts of stuff. Can't remember for sure what we did, but I think after we realized that it wasn't actually an issue with our known DHCP servers, we decided to talk to the dev team and found out that is what he had done.

It amazes me how many people just don't think about it - they have a problem, they think they know how to solve it, and just slap something onto the network.