aircrack-ng : airmon-ng stop working after updating to 15.04 ( ioctl(SIOCSIWMODE) failed: Device or resource busy )

Solution 1:

I finally found a nice trick to have airmon-ng working as it is supposed to; without any need to "check kill" or turn off completely network manager. By the way It is network manager that is the cause of the problem since the version 0.9.8.10.

1. Open your network manager configuration file as root in order to edit it

sudo gedit /etc/NetworkManager/NetworkManager.conf

2. Add the following lines to prevent network manager from interfering with airmon-ng

[keyfile]
unmanaged-devices=interface-name:wlan0mon;interface-name:wlan1mon;interface-name:wlan2mon;interface-name:wlan3mon;interface-name:wlan4mon;interface-name:wlan5mon;interface-name:wlan6mon;interface-name:wlan7mon;interface-name:wlan8mon;interface-name:wlan9mon;interface-name:wlan10mon;interface-name:wlan11mon;interface-name:wlan12mon    #  avoid conflicts with airmon-ng

The wlanXmon interfaces are just for monitor mode... So there is no problems if network manager can't manage them : It shouldn't!
You can add as many interfaces as you want, 12 should be enough unless you are a addicted to buy wireless USB. Use the separator ; between every interfaces.

3. Saves the changes. Effects are immediate: you can now use mode monitor on one interface while you are connected to the Internet through network manager with another interface.

Solution previously published in aircrack-ng forum > Edit NewtorkManager.conf to prevent conflicts with airmon-ng (no check kill)