You need to use a kernel version>= 2.6.10:

ifconfig wlan0 down
rmmod acx
wget http://www.cmartin.tk/acx/acx-20070101.tar.bz2
tar -xjf acx-20070101.tar.bz2
cd acx-20070101
wget http://patches.aircrack-ng.org/acx-20070101.patch
patch -Np1 -i acx-20070101.patch
make -C /lib/modules/`uname -r`/build/ M=`pwd` modules
make -C /lib/modules/`uname -r`/build/ M=`pwd` modules_install
modprobe acx

note: if the code doesn't work, Ubuntu users will want to change the following lines:

make -C /lib/modules/`uname -r`/build/ M=`pwd` modules
make -C /lib/modules/`uname -r`/build/ M=`pwd` modules_install

and replace `uname -r` by the name of their current kernel, followed by generic. The reason for this is that the actual modules directory for the current kernel (which is what the /lib/modules/`uname -r`/ commande will return) does not contain the build symbolic link, and therefore no makefile will be found for the make command. Only the /lib/modules/2.6.20-16-generic/ contains a simlink called “build” that will ensure the proper makefile is found.

ie: as of 26th of June 2007, under Ubuntu 7.04 “Feisty Fawn”, the correct lines would be:

make -C /lib/modules/2.6.20-16-generic/build/ M=`pwd` modules
make -C /lib/modules/2.6.20-16-generic/build/ M=`pwd` modules_install

You get kernel messagess similar to: Jul 3 00:44:12 ubuntop kernel: [ 736.008000] wlan0: FAILED to free any of the many full tx buffers. Switching to emergency freeing. Please report! Jul 3 00:44:12 ubuntop kernel: [ 736.008000] wlan0: tx timeout!

From the author of the driver patch:

That's a problem with the driver. I saw it several times while writing the acx111 patch. It works like this:

Every packet you transmit allocates a tx buffer, so it will start filling up the whole buffer space until its freed again and that's the point. It just doesn't get freed in time. Once you reached the limit and have eaten up all available buffers, it won't work again until you reload the driver. Its waiting for a TX_COMPLETE message…

Possible workarounds (don't know for sure right now, just try it):

• This happens when you use the unpatched driver, double and trible check that you're infact using a patched driver: look at the buildtime, remove all acx modules, make sure your custom module is in the correct /lib/modules/`uname -r`/ path…
• Use aireplay-ng 0.8 or higher, as they incorporate indirect support for acx injection (sending more ack frames)
• Change your hardware mac to the fakemac used for “-h”