Re: Capturing packets with bad FCS in monitor mode

[Qasim Javed <qasimj@gmail.com>]

Hi Daniel,

I am using the same card and the ath9k driver. I was also not able to
receive frames with CRC errors by setting the fcsfail flag while
creating the monitor mode interface. I looked up the code and found
that ath9k ignores frames with CRC errors in monitor mode.

The ath9k_rx_accept function in common.c has the following condition

if (ah->opmode == NL80211_IFTYPE_MONITOR) {
        if (rx_stats->rs_status &
                ~(ATH9K_RXERR_DECRYPT | ATH9K_RXERR_MIC |
                ATH9K_RXERR_CRC))
        return false;

Clearly, this function will return false in case of a CRC error while
receiving on a monitor interface.

For your info, this function is called by ath9k_rx_skb_preprocess
which in turn is invoked in ath9k_rx_tasklet.

In order to receive frames with CRC errors in monitor mode, I just
removed ATH9K_RXERR_CRC from the above snippet, recompiled and
reloaded ath9k and was able to receive frames with CRC errors.

I hope that helps.

-Qasim

On Thu, Aug 5, 2010 at 9:14 AM, Daniel Haid <d.haid@gogi.tv> wrote:
>
> Hello,
>
> is it possible to receive packets with bad FCS in monitor mode (ath9k driver)?
> There is a comment in ath9k/common.c which says that a bad checksum is
> ignored in monitor mode, but I have so far never been able to receive a
> packet with bad FCS (I do not know how to generate such packets,
> so I might just be too lucky that all packets are received unimpaired).
>
> Does ath9k hardware drop packets with bad FCS directly? If so, can
> it be disabled?
>
> Best regards.
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