Detection of Rogue Access Points

Jason Antman jason at jasonantman.com
Thu Oct 18 13:15:18 UTC 2012


Some very good points were made in the thread. I've dealt with this 
problem a few times. I'll admit, the only perfect solution I've found is 
to install a Internet-only (its own router interface or VLAN, firewalled 
off from everything else) AP for people to use because, frankly, 
consumer-grade APs are just too easy to install.

On the technical side, to reiterate what others have suggested:
- Scan MACs from the router ARP table or DHCP logs, flag anything from a 
common wireless vendor
- Script to pull new DHCP leases, check each with NMAP, alert on 
anything suspicious
- Port security, if you can
- Scanning the air

The only *real* way to detect rogue APs is to actually scan the 
airwaves. There's a bunch of vendors who sell hardware/software 
solutions for this, and there are also a lot of APs that support it, 
especially if you can deal with something manual. Ubiquiti Networks 
sells some sub-$100 USD access points that do a nice "site survey" as 
well as a spectrum analyzer, and could be used to get this info. Of 
course that becomes more of a burden if there are multiple other 
wireless networks within range of you (should be fine if your branches 
are on their own property, could be a problem if they're in 
commercial/professional buildings). I don't know if the Ubiquiti 
products are easily scriptable, but they *do* offer a SDK and with some 
amount of effort, it would probably be possible to pull this data via a 
script.

The times that I've done this, we've just grabbed a bunch of 
decommissioned corporate laptops with wireless & wired ethernet 
interfaces, put Linux on them, and written a script that scans for 
visible wireless networks every 5-30 minutes, and emails any changes to 
us. Laptops were configured for DHCP, and just plugged in and nestled 
somewhere in the wiring closet. Net cost $0 (well maybe some patch 
cables), and worked fine for us.

-Jason

On 10/14/2012 04:59 PM, Jonathan Rogers wrote:
> Gentlemen,
>
> An issue has come up in my organization recently with rogue access points.
> So far it has manifested itself two ways:
>
> 1. A WAP that was set up specifically to be transparent and provided
> unprotected wireless access to our network.
>
> 2. A consumer-grade wireless router that was plugged in and "just worked"
> because it got an address from DHCP and then handed out addresses on its
> own little network.
>
> These are at remote sites that are on their own subnets (10.100.x.0/24;
> about 130 of them so far). Each site has a decent Cisco router at the
> demarc that we control. The edge is relatively low-quality managed layer 2
> switches that we could turn off ports on if we needed to, but we have to
> know where to look, first.
>
> I'm looking for innovative ideas on how to find such a rogue device,
> ideally as soon as it is plugged in to the network. With situation #2 we
> may be able to detect NAT going on that should not be there. Situation #1
> is much more difficult, although I've seen some research material on how
> frames that originate from 802.11 networks look different from regular
> ethernet frames. Installation of an advanced monitoring device at each site
> is not really practical, but we may be able to run some software on a
> Windows PC in each office. One idea put forth was checking for NTP traffic
> that was not going to our authorized NTP server, but NTP isn't necessarily
> turned on by default, especially on consumer-grade hardware.
>
> Any ideas?
>
> Thank you for your time,
>
> Jonathan Rogers




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