Hacking_and_Penetration_Testing_with_Low_Power_Devices

186 CHAPTER 7 Building an army of devices
The SP parameter can be used to set the cyclic sleep period between 0 and 268 seconds
in 10 millisecond increments. For coordinators, the SP value determines how
long stored packets for indirect connections are retained. Packets are kept for a time
equal to 2.5 times SP.
The ST parameter establishes the inactivity time before a modem in a cyclic sleep
mode goes to sleep. The permissible range for ST is 0 to 65.535 seconds in one millisecond
increments. The ST parameter must be set to the same value on end devices
and coordinators.
For our purposes, a cyclic sleep mode seems most appropriate. Sleep mode 4 does not
require the use of GPIO pins. If this mode is used and the drone has data to send, sleep can
bedisabledbysettingSM tozero.The sleepmodecanbe resetafterthe dataare transferred.
In most cases, the command console will initiate communications, so the sleep mode can
be left at 4. Sleep mode 5 permits the SM parameter to be set to a consistent value.
The following code snippet demonstrates how parameters such as SM, SP, and
ST can be read and set up programmatically. In addition, it shows how the sleep line
can be toggled:
import serial
from xbee import XBee
# read a value from an XBee modem
def readXbeeParameter(xb, param):
xb.send(’at’, frame_id¼’A’, command¼param)
resp ¼ xb.wait_read_frame()
return resp[’parameter’]
# write a Xbee modem parameter
def setXbeeParameter(xb, param, value):
xb.send(’at’, frame_id¼’A’, command¼param, parameter¼value)
# cause an Xbee modem configured for pin sleep to sleep\
by asserting pin
def xbeePinSleep():
with open(’/sys/class/gpio/gpio69/value’, ’w’) as f:
f.write(’1’)
# wake up a Xbee modem from pin sleep by deasserting sleep pin
def xbeePinAwake():
with open(’/sys/class/gpio/gpio69/value’, ’w’) as f:
f.write(’0’)
ADDING SECURITY
It is likely that many potential penetration testing targets are not using XBee networking.
For these organizations, the command and control traffic is out of band
and likely to go undetected. Even in cases when the client uses XBee, traffic using