Monitor mode and FCS

Jouni Malinen jkmaline
Wed Jan 15 20:38:20 PST 2003

On Wed, Jan 15, 2003 at 03:43:27PM -0500, Bichot Guillaume (Princeton) wrote:

> We are working in monitor mode with Hostap-2002-10-12 and a linksys WPC11
> pccard v3. We tried the set-up the card in order to receive the frame with a
> wrong FCS (prism2_param wlan0 monitor_allow_fcserr 1).

> In order to create some wrong frames, we move away from the access point.
> What we observed is that wlansniff display only good (status : 0000) frames
> although our RTP sequence number was not anymore continuous.

Like Jean wrote earlier, errors might occur during the frame
preamble and there is not much that can be done in such cases since the
receiver does not really know what kind of encoding it should expect to
receive. Moving the sniffer away from the AP could produce errors
during the preamble and the frame is dropped completely.

> We got the same result with different prism2x cards.
> QUESTION : Does prism2_param wlan0 monitor_allow_fcserr works with any card
> ? Did we missed something in our configuration ?

Yes, it does work at least with the cards I use in testing. One of the
easiest tests for this is to change the sniffer to incorrect channel
(e.g., 5 when AP is using channel 6) and compare the number of received
frames when allow_fcserr is 0 and 1.

However, even the frames with FCS errors seem to come with "status
0000" in wlansniff.. This could be a bug in the driver or wlansniff, but
I did not notice anything wrong in this code path during a quick look..

Jouni Malinen                                            PGP id EFC895FA

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