root@OpenWrt:~# ifconfig wl0 down | iwconfig wl0 mode monitor | ifconfig wl0 up
root@OpenWrt:~# iwconfig wl0
wl0       IEEE 802.11-DS  ESSID:""
          Mode:Monitor  Frequency:2.437 GHz  Tx-Power:19 dBm
          RTS thr:2347 B   Fragment thr:2346 B
          Encryption key:off