Thursday, July 14, 2011

Persistent MAC Address on the PandaBoard

The PandaBoard doesn't have an EEPROM to store a MAC address for the on-board NIC, and as a result, a new random MAC address is generated on boot.  This can be a problem for DHCP lease pools, firewall rules, and more.

I mentioned in an earlier post that Ubuntu 11.04 is somehow keeping the NIC's MAC address persistent despite the lack of an EEPROM.  I investigated a bit and found it was a simple combination of (1) setting a MAC address in the NetworkManager settings and (2) this driver patch going into the latest upstream kernels: smsc95xx: generate random MAC address once, not every ifup.

Back in Fedora 13, I upgraded to David Marlin's latest 2.6.39-1.01.fc13.armv7l kernel from — which includes the above patch — and booted the PandaBoard.

The first thing I noticed was that the NIC is named eth0 now instead of usb0 (that's a nice touch) and, as expected, it has a new random MAC address:
[root@fedora-arm ~]# ifconfig -a
eth0      Link encap:Ethernet  HWaddr 4E:19:68:14:3C:F9
          BROADCAST MULTICAST  MTU:1492  Metric:1

I changed the MAC address using the standard tools and it appeared to stick:
[root@fedora-arm ~]# ifconfig eth0 hw ether CA:BC:15:4D:B4:49
[root@fedora-arm ~]# ifconfig eth0
eth0      Link encap:Ethernet HWaddr CA:BC:15:4D:B4:49
          BROADCAST MULTICAST MTU:1492 Metric:1

I wanted to make this survive a reboot, so I looked at the /etc/sysconfig/network-scripts/ifup-eth script and found this chunk:
# this isn't the same as the MAC in the configuration filename. It is
# available as a configuration option in the config file, forcing the kernel
# to think an ethernet card has a different MAC address than it really has.
if [ -n "${MACADDR}" ]; then
   ip link set dev ${DEVICE} address ${MACADDR}

(The ip command is preferred over the older ifconfig command.)

So, I edited /etc/sysconfig/network-scripts/ifcfg-eth0 and removed the HWADDR variable and set MACADDR instead:

I rebooted the PandaBoard and it worked:
                Welcome to Fedora
                Press 'I' to enter interactive startup.
Starting udev: [  OK  ]
Setting hostname fedora-arm:  [  OK  ]
fedora-arm login: root
Last login: Thu Jul 14 16:48:49 on console
[root@fedora-arm ~]# ifconfig eth0 | grep HWaddr
eth0      Link encap:Ethernet HWaddr CA:BC:15:4D:B4:49

Hooray for persistent MAC addresses!


  1. Actually, the ubuntu kernel uses the die id from the soc to generate a unique mac address. So far, it has been consistent and mostly unique in my setup of 6 pandas (one appears to have a duplicate die id of a prerelease panda).

    And now, so does u-boot with pxe support. I now have my pool of pandas booting w/o any SD cards (use omap4boot to push u-boot.bin).

  2. Looking at the Natty kernel source, I don't see where it's generating the MAC off the die id.

    I found this upstream patch that adds a new omap_get_die_id() function:;a=commit;h=2e130fc3d4fd5b38ee5d3c0a5c7f9fb85bce698e

    And I see the new omap_get_die_id() function is in the Natty kernel:;a=blob;f=arch/arm/mach-omap2/id.c;h=5f9086c65e48262835a39d3c7c86b000eba543dd;hb=HEAD#l108

    However, I do not see any calls to the omap_get_die_id() function in the smsc95xx.c driver:;a=blob;f=drivers/net/usb/smsc95xx.c;h=bc86f4b6ecc218289bad5889ffce90e6e57c27cd;hb=HEAD

    There was a proposed patch for the BeagleBoard xM to use the omap_get_die_id() function when generating a MAC address, but this patch does not appear to have been accepted upstream:

    Can you point to where it's using the die id to generate the MAC address? Maybe I'm looking in the wrong place.

  3. It appears that this issue was additional to the omap4 kernel (see;a=commit;h=10f38b455e75b85f72e98786e5518cf7b0324634).So you\'ll force mac address at boot-time by adding the smsc95xx.macaddr=xx:xx:xx:xx:xx:xx parameter to the kernel statement.

    Silvester Norman

    Change Mac Address