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I need the ability to generate random MAC addresses, so I wrote up a little function that does that:

>>> random_mac()
'7C:93:B7:AF:BA:AE'
>>> random_mac()
'D8:D8:A0:D4:A5:3F'
>>> random_mac(unicast=False, universal=True)
'55:47:C6:EE:C6:2B'
>>> random_mac(unicast=True, universal=False)
'FE:A1:4B:98:76:B6'

I decided to allow the user to pick if it's unicast/multicast and universally/locally administered; even though I'll only need unicast/universal. This caused me headaches though because I'm still not great at dealing with bits. The LSB of the first octet indicates uni/multicast, and the second LSB of the octet indicates universal/local, so these bits can't be random.

After playing around with a few failed ideas (generating all random bits, then "fixing" the two bits later), I finally decided to generate a random number between 0 and 63 inclusive, left shift it twice, than add the two bits on after. It works, but it's ugly and looks suboptimal.

It's not a lot of code, but I'd like a few things reviewed:

  • Is there a better approach? It feels hacky generating it as two pieces then adding them together. I tried explicitly setting the bits, but the code to decide between |, and & and ~ got messier than what I have now, so I went with this way.
  • The number constants are bugging me too. The numbers kind of sit on a border of self-explanatory and magic, so I decided to name them to be safe. LAST_SIX_BITS_VALUE feels off though.
  • Is treating a boolean value as a number during bitwise operation idiomatic? Is it clear as I have it now?
  • Attaching the first octet to the rest is suboptimal as well. Speed isn't a huge concern, but I'm curious if there's a cleaner way that I'm missing.

from random import randint, randrange

N_MAC_OCTETS = 6
OCTET_VALUE = 256
LAST_SIX_BITS_VALUE = 63

def random_mac(unicast: bool = True, universal: bool = True) -> str:
    least_two_bits = (not unicast) + ((not universal) << 1)
    first_octet = least_two_bits + (randint(0, LAST_SIX_BITS_VALUE) << 2)
    octets = [first_octet] + [randrange(OCTET_VALUE) for _ in range(N_MAC_OCTETS - 1)]
    return ":".join(f"{octet:02X}" for octet in octets)

Examples of the bits for the first octet for different inputs:

def display(mac):
    print(mac, f"{int(mac.split(':')[0], 16):08b}")

# Unicast, Universal
>>> display(random_mac(True, True))
04:27:DE:9A:1B:D7 00000100  # Ends with 0,0

# Unicast, Local
>>> display(random_mac(True, False))
72:FB:49:43:D5:F2 01110010  # 1,0

# Multicast, Universal
>>> display(random_mac(False, True))
7D:BF:03:4E:E5:2A 01111101  # 0,1

# Multicast, Local
>>> display(random_mac(False, False))
2F:73:52:12:8C:50 00101111  # 1,1
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    \$\begingroup\$ small suggestion, it might help to look at how the library randmac does it. randmac Python 3 \$\endgroup\$
    – Parekh
    Oct 11 '20 at 17:46
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    \$\begingroup\$ would it help that you deal with nibbles instead of octet? \$\endgroup\$
    – hjpotter92
    Oct 11 '20 at 18:32
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    \$\begingroup\$ @AryanParekh That's a good idea. For some reason I never even considered that there would be a library for this. I'll check it out. \$\endgroup\$ Oct 11 '20 at 18:47
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    \$\begingroup\$ @hjpotter92 I don't really care too much about the implementation (except for readability) providing the end result is the same. It may ease it a bit though to deal with the "special nibble" on its own, then add it in to the rest of the octet. That would make some of the magic constants simpler numbers. \$\endgroup\$ Oct 11 '20 at 18:49
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    \$\begingroup\$ @Carcigenicate that's cool, I assume that you might have wanted to create your own method rather than using a library, if that's not the case, prefer using the library as the class from the library is really useful, with methods that allow you to use different formats, \$\endgroup\$
    – Parekh
    Oct 11 '20 at 19:03
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  • Negating an argument is somewhat counterintuitive. Consider passing them as multicast and local instead.

  • I would seriously consider defining

      UNIVERSAL = 0x01
      MULTICAST = 0x02
    

    and pass them as a single argument, is in

          random_mac(UNIVERSAL | MULTICAST)
    
  • Using both randint and randrange feels odd. I would stick with randrange.

  • First octet needs a special treatment anyway. That said, consider

      def random_mac(special_bits = 0):
          octets = [randrange(OCTET_VALUE) for _ in range(N_MAC_OCTETS)]
          octets[0] = fix_octet(octet[0], special_bits)
          return ":".join(f"{octet:02X}" for octet in octets)
    

    with

      def fix_octet(octet, special_bits):
          return (octet & ~0x03) | special_bits
    
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5
  • \$\begingroup\$ @Carcigenicate: fix_octet just clears the low 2 bits, then ORs in what should be there. Looks very comprehensible to me, and exactly what I would have suggested if there wasn't already an answer doing that. (Generating all 6 octets the same way and modifying 1 is also what I was going to suggest). Getting the caller to pass ORed bitflags is great; it simplifies the implementation and makes the call-site more self-documenting. (And is a common idiom in C for system calls like mmap(..., MAP_PRIVATE | MAP_ANONYMOUS | ...), or for ORing together permission bit flags, or lots of other things.) \$\endgroup\$ Oct 12 '20 at 2:42
  • \$\begingroup\$ @PeterCordes I may be misunderstanding something, but I was just saying that this currently seems to give an incorrect result. I think it's because the flags need to be MULTICAST = 0x01; LOCAL = 0x02 instead of what they currently are. \$\endgroup\$ Oct 12 '20 at 3:12
  • \$\begingroup\$ @Carcigenicate: I was replying to your first comment. You second comment didn't point out what the problem was or give an example, and I didn't notice the bit-flags definitions were backwards. But yes, 0x1 is the LSB, 0x2 is the second bit, so that matches your question (and wikipedia). \$\endgroup\$ Oct 12 '20 at 3:14
  • \$\begingroup\$ @PeterCordes OK, now this makes sense. Thank you. \$\endgroup\$ Oct 12 '20 at 3:17
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    \$\begingroup\$ After changing the flags to MULTICAST = 0x01; LOCAL = 0x02, this does indeed give the correct output. Thank you for the suggestions. \$\endgroup\$ Oct 12 '20 at 16:02
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Some observations on the API

Naming

The IEEE strongly discourages use of the name MAC or MAC-48. These names should only be used as an obsolete label for EUI-48.

It is also imprecise, since not all MAC addresses are EUI-48 addresses. For example, FireWire MAC addresses are EUI-64.

So, your function should probably be named random_eui48 instead.

Keyword-only arguments

Having two boolean parameters can lead to confusion. I would make them keyword-only arguments so that the caller is always forced to name them:

def random_eui48(*, unicast: bool = True, universal: bool = True) -> str:

Defaults

I agree with the choice of making Unicast the default. It is probably what users will usually need more often. However, I disagree with making universally administered addresses the default. In fact, I find it highly dubious to randomly generate UAAs at all. At most, you should randomly generate addresses within an OUI you own.

So, I would very much prefer to make LAAs the default.

Choice of parameters

I would choose the parameters such that they are "off-by-default" (False) and ca be "turned on" by the caller:

def random_eui48(*, multicast: bool = False, universal: bool = False) -> str:

API extension: supply OUI

It really only makes sense to generate a UAA within an OUI you own. Therefore, your API should provide for passing in a OUI to generate an addresses for. Make sure you take care of both the MAC-S and MAC-L registries!

Implementation

An EUI-48 is a 48 bit number. I find it strange to treat it as a conglomerate of 5 8 bit and one 6 bit number.

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    \$\begingroup\$ Thank you. Regarding "EUI-48", is "MAC" simply not used in industry, or should it only be avoided when you're referring to a physical address of a certain length? I've actually never heard of "EUI-48" before; beyond the mention on the Wiki page. And as for why I'm randomizing the OUI as well: I'm writing a malicious DHCP client and need random MACs so that I can continue requesting new addresses while keeping the previous addresses on lease. \$\endgroup\$ Oct 12 '20 at 13:53
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    \$\begingroup\$ Oh, MAC is being used all over the place, it's just that the IEEE strongly advises against it. If you really are talking about MAC addresses, I think it makes sense to use the term "MAC", but be aware that, depending on the protocol, MAC addresses need not be 48 bit. And conversely, EUIs are used for addresses other than MAC addresses. E.g. the Clock ID in the Precision Type Protocol is an EUI, albeit an EUI-64. \$\endgroup\$ Oct 13 '20 at 5:44
  • \$\begingroup\$ I'm not sure the IEEE disapproves of the use of "MAC" as a general term. Rather, my understanding is that the term "MAC-48" is deprecated in favour of "EUI-48". "MAC address" usually refers to how the identifier is used, where as "[MAC/EUI]-48" refers to the numbering scheme. \$\endgroup\$
    – Tom Wright
    Oct 13 '20 at 16:21
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random.randrange() takes start, stop, and step arguments just like range(). To select the first octet, start is based on the unicast and universal flags, end is 256, and step is 4 (four possible combinations of unicast and universal).

N_MAC_OCTETS = 6
OCTET_VALUE = 256
LAST_SIX_BITS_VALUE = 63

def random_mac(unicast: bool = True, universal: bool = True) -> str:
    first_octet = randrange(3 ^ (universal*2 + unicast), OCTET_VALUE, 4)
    octets = [first_octet] + [randrange(OCTET_VALUE) for _ in range(N_MAC_OCTETS - 1)]
    return ":".join(f"{octet:02X}" for octet in octets)

or better:

UNICAST = 0
MULTICASE = 1

UNIVERSAL = 0
LOCAL = 2

def random_mac(flags: int = UNICAST | UNIVERSAL) -> str:
    first_octet = randrange(flags, OCTET_VALUE, 4)
    octets = [first_octet] + [randrange(OCTET_VALUE) for _ in range(N_MAC_OCTETS - 1)]
    return ":".join(f"{octet:02X}" for octet in octets)

Called like:

random_mac(LOCAL | MULTICAST)
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    \$\begingroup\$ Using universal = True for unicast is the opposite of the bit value. That is, when unicast it true the bit value is false. Similarly for universal. This smells like a source of bugs. I'd suggest changing the parameters to multicast and local so they match the bit values or, as vnp suggested, define some constants that can be or-ed. \$\endgroup\$
    – RootTwo
    Oct 11 '20 at 20:59
2
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While still using @vnp's fix_octet() function, an alternate approach might be

def random_mac(special_bits = 0):
    return ':'.join('%02x'%randint(0,255) if i != 0 else '%02x'%fix_octet(randint(0,255),special_bits) for i in range(6))
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2
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(not unicast) + ((not universal) << 1)
  • When manipulating bits or bitfields, use |, not +. Even though the result will be the same here, the semantic is different.
  • You read left-to-right. So handle the bits left to right.
  • I do agree with the fact that negations quickly mess with your head.

I'd rather write:

(local << 1) | multicast

Going one step further, I'd replace:

least_two_bits = (not unicast) + ((not universal) << 1)
first_octet = least_two_bits + (randint(0, LAST_SIX_BITS_VALUE) << 2)

With

first_octet = (randint(0, LAST_SIX_BITS_VALUE) << 2) | (local << 1) | multicast

You could define LAST_SIX_BITS_VALUE as ((1 << 6)-1) to make it more explicit that its value comes from the need for 6 bits. One step further would be to define

FIRST_OCTET_RANDOM_BITS_NUMBER = 6
FIRST_OCTET_RANDOM_BITS_MAX_VALUE = (1 << FIRST_OCTET_RANDOM_BITS_NUMBER) - 1

I agree that mixing randint (where the top value is inclusive) and randrange (where it isn't) is confusing.

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