How to set Linux MAC address

Last post I wrote about a embedded device running Linux where a MAC address is stored in the EEPROM. The first six bytes are reserved for the MAC address which (ideally) only should be written once.

To apply the MAC address the numbers need to be hexadecimal formatted and separated with a colon(:) symbol. It is possible to dynamically change the MAC address of Ethernet, beware that you need to be root and that the ethernet interface is not up.

ifconfig eth0 hw ether 00:01:02:03:04:05

Or, you could permanently change the MAC address by setting it into the hwaddress in the /etc/network/interfaces configuration file.

auto eth0
iface eth0 inet dhcp
    hwaddress ether 00:01:02:03:04:05

Now we know how and where to set the MAC address. Next, we can read the first six bytes using dd and pipe it to hexdump. Fortunately, hexdump already provides API for formatting the input.

The -e '1/1' %02x parameter will format each space followed by a byte into a hexadecimal number with two digits and a leading zero if needed.

DELIMITER=":"
MAC=$(dd if=$EEPROM bs=6 count=1 | hexdump -ve '1/1 "%02x'"$DELIMITER"'"')
echo $MAC # output 00:01:02:03:04:05:

Finnaly the trailing colon : needs to be removed. There are multiple ways to do this

1. Pipe the output to sed and replace the last : OR
2. Use Bash Substring Expansion to remove the character from a string.

Solution 1:

MAC=$(dd if=$EEPROM bs=6 count=1 | hexdump -ve '1/1 "%02x'"$DELIMITER"'"' | sed s/:$//g)

Solution 2:

MAC=$(dd if=$EEPROM bs=6 count=1 | hexdump -ve '1/1 "%02x'"$DELIMITER"'"')
MAC=${MAC::-1} # Remove last character ':'

Personally I prefer solution 2 since I find that sed harder to read and I often have to explain it to other engineers who are reviewing it. Therefore solution 2 (with a comment) is clearer. If you are a system expert and love one-lines, go for solution 1.