3

You probably know that MAC address can be generated, for instance, in the following way:

macaddr=$(dd if=/dev/random bs=1024 count=1 2>/dev/null|md5sum|sed 's/^\(..\)\(..\)\(..\)\(..\)\(..\)\(..\).*$/\1:\2:\3:\4:\5:\6/')
echo $macaddr

But this method can result in MAC address that looks like this one: 07:d4:51:9f:50:6c . You simply can't use the address . If you tried, you would get this error:

# ip link set dev wlan0 address $macaddr
RTNETLINK answers: Cannot assign requested address

So the line above should be rewritten. And the question is how it should look like so the MAC address always was valid?

3
  1. extract the first byte (e.g. 07 from your example)
  2. bitwise AND it with decimal 254 (11111110 - all bits except zeroth bit set)
  3. bitwise OR it with decimal 2 (00000010 - only 1st bit set)
  4. combine the first byte back with the last five bytes

e.g.

#! /bin/sh

mac='07:d4:51:9f:50:6c'

lastfive=$( echo "$mac" | cut -d: -f 2-6 )
firstbyte=$( echo "$mac" | cut -d: -f 1 )

# make sure bit 0 (broadcast) of $firstbyte is not set,
# and bit 1 (local) is set.
# i.e. via bitwise AND with 254 and bitwise OR with 2.

firstbyte=$( printf '%02x' $(( 0x$firstbyte & 254 | 2)) )

mac="$firstbyte:$lastfive"

echo "$mac"

Output:

06:d4:51:9f:50:6c

07 hex is 00000111 binary. bitwise AND-ing it with 254 (11111110 binary) results in 00000110 binary (6 decimal). bitwise OR-ing it with 2 (00000010 binary) results in no change because bit 1 is already set. Final result is 06 hex.

2
  • I just tested and it looks like that it works just fine. Apr 29 '16 at 4:52
  • thanks for the edit. i didn't even notice that i missed the last byte.
    – cas
    Apr 29 '16 at 4:56

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