Converts an IPv6 link-local address to a MAC address

ipv6-to-mac.py

#!/usr/bin/env python3

'''
thanks https://ben.akrin.com/?p=1347
To convert from an IPv6 link-local address to a MAC address:
- the 1st 64 bits go away (the fe80:: part)
- bit 71 gets flipped (after fe80:: is gone this would be the 7th bit)
- bits 89 through 112 go away (the ff:fe)
'''

import re
from sys import argv

def run_tests():
    test("fe80::5074:f2ff:feb1:a87f", "52:74:f2:b1:a8:7f")
    test("fe80::12c3:7bff:fef1:be70", "10:c3:7b:f1:be:70")
    test("fe80::8286:88ff:fe3e:7313", "80:86:88:3e:73:13")
    test("FE80::5074:F2FF:FEB1:A87F", "52:74:f2:b1:a8:7f")
    test("fe80::5074:f2ff:feb1:f", "52:74:f2:b1:00:0f")
    test("fe80::200:ff:fe00:0", "00:00:00:00:00:00")
    test("fe80::0:ff:fe00:0", "02:00:00:00:00:00")
    test("fe80::ff:fe00:0", "02:00:00:00:00:00")

def test(addr, expected):
    print("testing input", addr, "expecting", expected)
    result = addr_to_mac(addr)
    if result == expected:
        print("PASS\n")
    else:
        print("FAIL", result, "\n")

def addr_to_mac(addr):
    # make a list like: ['5074', 'f2ff', 'feb1', '7f']
    arr = addr.split(":")[-4:]

    # fill in omitted zeros:
    arr = list(map(lambda s: s.zfill(4), arr))

    # flip the 7th bit of the 1st octect
    flipped = hex(int(arr[0][:2], 16) ^ 2)[2:].zfill(2)
    arr[0] = flipped + arr[0][2:]
    # now arr is like : ['5274', 'f2ff', 'feb1', '007f']

    # get rid of the fffe in the middle:
    arr[1] = arr[1][:2]
    arr[2] = arr[2][2:]
    # now arr is like : ['5274', 'f2', 'fe', '007f']

    mac = "".join(arr)
    return ':'.join([mac[i:i+2] for i in range(0, len(mac), 2)]).lower()


if __name__ == "__main__":
    if len(argv) < 2:
        print("Usage: %s ADDRESS" % argv[0])
        exit(1)

    if argv[1] == "test":
        run_tests()
        exit(0)

    addr = argv[1][:argv[1].rfind("/")]
    if not re.match("fe80:(:[a-f0-9]{1,4}){3,4}$", addr, re.IGNORECASE):
        print("Error, expected input in a format like \"fe80::5074:f2ff:feb1:a87f\"")
        exit(1)

    mac = addr_to_mac(addr)
    print(mac)