Otdfbt

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!

myserver% perl -e 'for ($i=0;$i<6;$i++)@m[$i]=int(rand(256)); printf "%X:%X:%X:%X:%X:%Xn",@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:%Xn",@m;'
02:8E:94:A3:47:26

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'

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.

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

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.

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/')

I use:

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