Working with MAC addresses

Posted by Steve on Thu 22 Feb 2007 at 16:47

MAC addresses are often taken for granted, things that nobody thinks about. However there are times when you do need to worry about them. Here we'll demonstrate how to view and change the MAC address of your Debian system.

Recently I had to deal with a machine which was having a strange networking problem:

One booted the machine couldn't be contacted remotely.
However the machine became contactable once it made a single outgoing network connection.

It took a while to figure out what was going wrong, but eventually we discovered that the machine was acquiring a different MAC address each time it booted. This explained the symptoms:

On booting the machine would get a new MAC address - but the routers ahead of it would have the old one cached so no traffic would reach it.
Once the machine made a single outgoing connection the router(s) would see the new MAC address and update themselves - so incoming connections worked.

The following messages in the system logs show the problem explicitly, once we knew to look there:

0000:00:07.0: Invalid Mac address detected: b1:8e:66:ea:0f:00
Please complain to your hardware vendor. Switching to a random MAC.
eth0: forcedeth.c: subsystem: 01458:e000 bound to 0000:00:07.0

Thankfully the solution to this problem was very simple - just allocate a static MAC address to the machine, so when it reboots it doesn't generate a random one of its own.

Find your MAC address

To view the MAC address of your machine you can use the ifconfig command. To see all interfaces run:
skx@mine:~$ ifconfig
eth0      Link encap:Ethernet  HWaddr 00:17:31:56:BC:2D
          inet addr:192.168.1.20  Bcast:192.168.1.255  Mask:255.255.255.0
..
..

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0

..
..
Here the MAC address is displayed after HWaddr, so the MAC address of this system is "00:17:31:56:BC:2D".

If you just want the MAC address of an interface you could run something similar to this:
skx@mine:~$ ifconfig eth0 | grep HWaddr | awk '{print $5}'
00:17:31:56:BC:2D

Set your MAC address

If you wish to explicitly set your MAC address upon a Debian system you can do so by adding an entry to your /etc/network/interfaces file.

Beneath each interface you wish to setup add "hwaddress ether xx:xxx..". For example:
auto eth0 static
iface eth0 inet static
  address 1.2.3.4
  gateway ...
  broadcast ...
  hwaddress ether 02:01:02:03:04:08

This article can be found online at the Debian Administration website at the following bookmarkable URL:

http://www.debian-administration.org/articles/501