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Detection of Rogue Access Points


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