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Finding the Rogue DHCP Server With Wireshark (by Tony Fortunato)

 

I am surprised that this exercise we do in class still proves to be helpful as well as quite popular.

There are many utilities out there to help find rogue servers, but why bother when you already have Wireshark installed. When you get comfortable with this exercise you can save some steps by creating a capture filter for just DHCP packets, or better yet, just DHCP server packets.  As always with protocol analysis, there are many ways to do this exercise and this is just my preference since it forces me and the attendees to review the DHCP process as they go through the packets.

Rogue DHCP servers are becoming more common these days since a DHCP server can simply be a part of an application loaded on your computer.  The introduction of tablets and smart phones that can provide hotspot support, are also DHCP servers. I even see more applications out there that turns your laptop into a WiFi hotspot so you can tether it to your tablet or smart phone.

Don’t worry, I haven’t forgotten the classic example of an employee who wants wireless access in a nearby conference room and simply connects the LAN port of his wireless router at his desk and starts dishing out IP addresses.

I like the added twist where I ask people to identify the legitimate DHCP based on paying attention to the story, not the packets.  I can’t tell you how many times I figure out a problem by going back to the user and having a conversation rather than going over the trace a million times.

I think people forget that Wireshark and protocol analysis is an exercise in forensics and you need a story for context and to make sense of the packets.

I have said many times that many times the answer comes from the story, not the packets.

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