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How do I generate a random MAC address from the Linux command line?

I search for a solution that only requires standard tools commonly found on the Linux command line.

The MAC address will be used for a guest KVM.

11 Answers 11

48

I use

macaddr=$(echo $FQDN|md5sum|sed 's/^\(..\)\(..\)\(..\)\(..\)\(..\).*$/02:\1:\2:\3:\4:\5/')

The benefit of this method, over a completely random number, is that it's possible to reliably reproduce the MAC address based on the FQDN of the machine, which I find useful sometimes. The 02 for the first octet just sets the "locally assigned" bit, which makes it obvious that it's not a vendor-provided MAC address, and guarantees that you won't collide with a real NIC's MAC address.

If you need to generate multiple MAC addresses per host, I used to concatenate the FQDN with the name of the bridge to connect the interface to; this did a good job of spreading things out for different NICs.

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  • +1 for the reproducibility; for certain applications, that makes it a much superior method to mine. – MadHatter Aug 10 '11 at 8:18
  • Thanks a lot, I like the idea of having it reproducible. – Erik Sjölund Aug 10 '11 at 11:58
  • Plus no mac address conflicts in the unlikely event you'd randomly generate the same mac twice – Petter H Sep 25 '14 at 8:06
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    As alternative you can use tr -dc A-F0-9 < /dev/urandom | head -c 10 | sed -r 's/(..)/\1:/g;s/:$//;s/^/02:/' – ALex_hha May 26 '16 at 8:59
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    It's only an "alternative" in the sense that it produces a completely different end result to what my snippet does... – womble May 30 '16 at 6:10
8

The posted scripts are good, but I want to add a warning: Mind the Birthday (paradoxon)!

It comes from the fact that even if you have just 23 people, the chance is already 50% that 2 of them have birthday on the same day.

It depends on your scenario how you use it, but if you generate the MACS randomly, at approx 1 million your chance for a mac number clash is 40% at 2 million it is already 87%!

If you need just a couple this is ok, but when you maintain a server farm with hundreds of servers, each of them hosting tens of virtual machines, or if you use the macs as index in some db for bookkeeping and you need uniques be careful!

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  • Thanks, for the warning about the Birthday paradox! In my case I will take the risk as I will generate around 20 MAC addresses. – Erik Sjölund Aug 10 '11 at 12:22
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    If you're running hundreds of servers each hosting tens of virtual machines all on the same broadcast domain, you've got bigger problems than MAC address collision risk. – womble Sep 25 '14 at 23:28
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    "It comes from the fact that even if you have just 23 people, the chance is already 50% that 2 of them have birthday on the same day." That's not even remotely true. There is about a 50% chance that two of 23 people have the same birthday anniversary, not the same birthday. – Ron Maupin Jul 29 '16 at 21:45
  • If you choose 40 of the 48 bits randomly, as suggested in womble's answer, you need 100,000 servers to have a 0.5% chance of any of them conflicting. You can use instacalc.com/28845 to calculate this. So it's not hundreds of servers, it's millions of servers. – Arnout Apr 24 at 10:34
6

These variants work as well.

longer:

openssl rand -hex 6 | sed 's/\(..\)\(..\)\(..\)\(..\)\(..\)\(..\)/\1:\2:\3:\4:\5:\6/'

or shorter:

openssl rand -hex 6 | sed 's/\(..\)/\1:/g; s/:$//'

The load consumption of both variants is very similar according to quick measuring with time.

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  • Hi Anthony, I see no other variant combining openssl rand and sed here, so this is unique solution in this topic. – Jaroslav Kucera Sep 26 '16 at 11:27
  • That's true. He/she used fold -w2|paste -sd: - instead of sed. The sed solution is probably easier to remember as it uses a more familiar tool – though I learned more from his/her answer. – Anthony Geoghegan Sep 26 '16 at 13:21
  • I think the first command won't work because it does not set the first bit to be even! – amrx Jul 26 '18 at 21:49
  • Hi @amrx, are you sure the first bit of MAC must be even? I have NIC in one of my servers, which begins with ec so 11101100 in binary... – Jaroslav Kucera Jul 27 '18 at 11:56
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    Hi @JaroslavKucera, Unicast MAC addresses must never set the 1's place bit in the first byte. That's the "group" (multicast/broadcast) bit. If you make up your own MAC address, you're supposed to set the 2's place bit (the "locally administered" bit) in the first byte, to differentiate it from a guaranteed globally unique MAC address. – amrx Jul 28 '18 at 21:52
5
myserver% perl -e 'for ($i=0;$i<6;$i++){@m[$i]=int(rand(256));} printf "%X:%X:%X:%X:%X:%X\n",@m;'
55:C2:A5:FA:17:74

Ah, the ol' Swiss Army Chainsaw rides again. And by way of version 0.2, I'm unashamedly stealing womble's excellent point about the first octet being 02:

myserver% perl -e 'for ($i=0;$i<5;$i++){@m[$i]=int(rand(256));} printf "02:%X:%X:%X:%X:%X\n",@m;'
02:8E:94:A3:47:26
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  • Thanks MadHatter, I tried your second variant and it worked. Very nice! – Erik Sjölund Aug 11 '11 at 12:47
4

I know this post is old, but for future visitors, if you want a cryptographically secure pseudorandom MAC address, without being limited to 0x02 as the OUI, here is a fast mostly platform agnostic generator:

$ printf '%02x' $((0x$(od /dev/urandom -N1 -t x1 -An | cut -c 2-) & 0xFE | 0x02)); od /dev/urandom -N5 -t x1 -An | sed 's/ /:/g'
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2

Here's another one, based on wombie's answer:

macaddr=$(dd if=/dev/urandom bs=1024 count=1 2>/dev/null|md5sum|sed 's/^\(..\)\(..\)\(..\)\(..\)\(..\)\(..\).*$/\1:\2:\3:\4:\5:\6/')
echo $macaddr
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  • There's no need to run urandom output through md5sum; you can just use od as per Aaron Toponce's answer. – womble Sep 25 '14 at 23:29
2

Here are five other options, all of which use random bits for the least significant bit of the most significant byte that indicates if the address is unicast or multicast and for the second-least significant bit of the most significant byte that indicates if the address is universally or locally administered.

jot -w%02X -s: -r 6 1 256
openssl rand -hex 6|fold -w2|paste -sd: -
od -N6 -tx1 -An /dev/random|awk '$1=$1'|tr \  :
god -N6 -tx1 -An /dev/random|cut -c2-|tr \  :
hexdump -n6 -e'/1 ":%02X"' /dev/random|cut -c2-

jot comes with OS X and BSDs but not with most Linux distributions. In jot -w changes the format, -s changes the separator, and -r generates random numbers.

od is in POSIX but hexdump is not.

OS X's od (/usr/bin/od below) uses a different output format than GNU od:

$ /usr/bin/od -N6 -tx1 -An /dev/random|tr ' ' :
:::::::::::d9::b9::d7::da::5f::96::::::::::::::::::::::::::::::::::::::::
$ god -N6 -tx1 -An /dev/random|tr ' ' :
:f5:6d:0a:3b:39:f9

In OS X's od options placed after an argument for an input file are treated as the names of input files, so the command in the answer by Aaron Toponce reads from /dev/urandom indefinitely with OS X's od.

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1

I use:

echo -n 02; od -t x1 -An -N 5 /dev/urandom | tr ' ' ':'
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1

You could just add a $RANDOM after $FQDN and this would give you random mac addresses every time you run it. This is especially helpful for poeple who want to create backup vms using snapshots or clones of vms.

macaddr=$(echo $FQDN$RANDOM|md5sum|sed 's/^\(..\)\(..\)\(..\)\(..\)\(..\).*$/02:\1:\2:\3:\4:\5/')
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  • 1
    Note that $RANDOM is available in bash, but may not be available in other shells. – Michael Hampton Oct 31 '18 at 15:27
0

Python one-liner:

python3 -c 'import os; print(":".join(["{:02x}".format(x) for x in b"\02x" + os.urandom(5)]))'
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0

Just for fun, here is a pure bash version, tested against Bash 4.4.12(1)-release:

read -N6 b </dev/urandom
LC_ALL=C printf "%02x:%02x:%02x:%02x:%02x:%02x\n" "'${b:0:1}" "'${b:1:1}" "'${b:2:1}" "'${b:3:1}" "'${b:4:1}" "'${b:5:1}"

First line reads 6 characters from /dev/urandom; then using the C character set print the 0-filled hex value of each character separated with a colon (the newline is optional but useful to print out the value).

Since you can print directly to a variable with printf -v myvar there is no fork/subshell needed to capture the result.

Extracting the value of a character using printf is defined in POSIX printf documentation:

If the leading character is a single-quote or double-quote, the value shall be the numeric value in the underlying codeset of the character following the single-quote or double-quote.

NB: read will not block on /dev/urandom, so it may return early causing the last bytes of the MAC to be all 0's. This will be particularly noticeable if run in a thigh loop or if another application is doing heavy reads at the same time from /dev/urandom.

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