Raspberry PI: If pin 21 is pulled to ground (just connect the two pins at the USB end of the GPIO header, preferrably put a jumper over them) write MAC addresses to file

showmac.py

import fcntl, socket, struct
import RPi.GPIO as GPIO


def get_mac(interface):
    try:
        s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        info = fcntl.ioctl(s.fileno(), 0x8927,  struct.pack('256s', interface[:15]))
        return ':'.join(['%02x' % ord(char) for char in info[18:24]])
    except(IOError):
        return ""


GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)

GPIO.setup(21, GPIO.IN, GPIO.PUD_UP)

if GPIO.input(21) == False: #False == LOW, i.e. ACTIVE!
    f = open('/home/pi/jukebox/static/help_local.html', 'w')
    f.write("eth0: " + get_mac("eth0") + "<br>")
    f.write("wlan0: " + get_mac("wlan0"))
    f.close()

GPIO.cleanup()

License: CC0