Generating Random MAC Addresses with Python

README.md

The mini-tool has a CLI-Interface with the following options:

• Unicast or Multicast? Default: Unicast
• Locally Administered or Globally Unique? Default: Locally Administered
• Prescribe specific OUI (overwrites the above two)

TODO

• Add an option to generate a number of MACs without collisions.

Resources

• MAC Adress on Wikipedia
• redhat's virtualization guide on Generating a new unique MAC address
• A Perl Script that does the same: randmac.pl
• The python module netaddr can help you work with MAC addresses.

randmac.py

#!/usr/bin/env python

import random

def random_bytes(num=6):
    return [random.randrange(256) for _ in range(num)]

def generate_mac(uaa=False, multicast=False, oui=None, separator=':', byte_fmt='%02x'):
    mac = random_bytes()
    if oui:
        if type(oui) == str:
            oui = [int(chunk) for chunk in oui.split(separator)]
        mac = oui + random_bytes(num=6-len(oui))
    else:
        if multicast:
            mac[0] |= 1 # set bit 0
        else:
            mac[0] &= ~1 # clear bit 0
        if uaa:
            mac[0] &= ~(1 << 1) # clear bit 1
        else:
            mac[0] |= 1 << 1 # set bit 1
    return separator.join(byte_fmt % b for b in mac)

def main():
    import argparse
    parser = argparse.ArgumentParser()
    parser.add_argument('--uaa', action='store_true', help='generates a universally administered address (instead of LAA otherwise)')
    parser.add_argument('--multicast', action='store_true', help='generates a multicast MAC (instead of unicast otherwise)')
    parser.add_argument('--oui', help='enforces a specific organizationally unique identifier (like 00:60:2f for Cisco)')
    parser.add_argument('--byte-fmt', default='%02x', help='The byte format. Set to %02X for uppercase hex formatting.')
    parser.add_argument('--separator', default=':', help="The byte separator character. Defaults to ':'.")
    args = parser.parse_args()
    print(generate_mac(oui=args.oui, uaa=args.uaa, multicast=args.multicast, separator=args.separator, byte_fmt=args.byte_fmt))

if __name__ == '__main__':
    main()

A non-random version for generating a MAC address from a string:

def generate_mac(text: str, multicast=False, universally_administered=False) -> str:
    from hashlib import sha256

    h = sha256(text.encode("utf-8")).digest()

    # https://en.wikipedia.org/wiki/MAC_address#Unicast_vs._multicast_(I/G_bit)
    # https://gist.github.com/pklaus/9638536
    bit = h[0]
    if multicast:
        bit |= 1  # set bit 0
    else:
        bit &= ~1  # clear bit 0
    if universally_administered:
        bit &= ~(1 << 1)  # clear bit 1
    else:
        bit |= 1 << 1  # set bit 1

    m0 = int.from_bytes([bit], "little")
    m1 = int.from_bytes([h[4]], "little")
    m2 = int.from_bytes([h[8]], "little")
    m3 = int.from_bytes([h[8]], "little")
    m4 = int.from_bytes([h[16]], "little")
    m5 = int.from_bytes([h[20]], "little")

    return f"{m0:02x}:{m1:02x}:{m2:02x}:{m3:02x}:{m4:02x}:{m5:02x}"

It might have a high hash collision rate, but it's still pretty useful to me. I use it to generate a MAC from a hostname to stop my router from delivering an old IP address from my recreated virtual machines.