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Showing posts from January, 2014

WLAN Packet Capture - Filtering Out Bad FCS Frames

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Often when looking through a wireless capture file, there may be a number of frames which have been corrupted, but Wireshark has attempted to decode it as best it can.

When a frame is corrupted, the frame check sequence of the frame will fail, indicating that some part (or parts) of the frame have errored during transit.

When reviewing a trace, it can be very easy to miss the fact that the FCS is wrong and that you are essentially looking at a corrupt frame. This will often manifest itself bizarre frame types and field values which can lead you completely astray in your diagnosis efforts.

There are a couple of ways to get around this.

Firstly, you can add a display filter to remove all of the frames with a bad FCS (wlan.fcs_bad == 1), but use this option with care (see note below):



The drawback to this approach is that just because some frames fail the FCS, the actual frame that arrived at the destination station may have been OK. It depends on where your analyser is physica…

DHCP Option 43 for Meru APs (using a Cisco Router/Switch DHCP Server)

Meru access points, like many other controller-based APs, can use DHCP option 43 to acquire the address of the wireless LAN controller that they need to join.
In brief, controller-based APs need to find their way back to a controller to obtain their operating code and parameters. Out-of-the-box, they don't know the IP address of the controller they should be talking to. When they are hooked to a wired network, they will request an IP address via DHCP. The DHCP process can also be used to pass them the address of the wireless LAN controller they should be speaking with (in addition to the usual parameters such as IP address, mask, default gateway etc.).
DHCP option 43 is used to pass the WLC IP address information to the AP. However,  figuring out the format of the information that should be put in to the option 43 field can be something of a challenge, depending on the DHCP server you are using.
I recently had to set up a DHCP server on a Cisco switch to provide IP addressing and…