Friday, August 27, 2010

Fix out of order network interfaces in Linux

I removed two old NICs and assigned two news NICs for a Vmware VM (SLES 10) and I expect the interface name to be eth0, eth1, but they appear as eth2, eth3. 
dmesg output revealed that eth0 was renamed to eth2 and eth1 was renamed to eth3 at some stage, It turned out udev rules renamed it.


30-net_persistent_names.rules had four entries, the first 2 recorded MAC address of two old NICs ; the last 2 entries recorded MAC address of current two NICs. Upon matching current MAC address, the udev rule renamed the interfaces to eth2 and eth3.
$cat /etc/udev/rules.d/30-net_persistent_names.rules
# This rules are autogenerated from /lib/udev/rename_netiface.
# But you can modify them, but make sure that you don't use an interface name
# twice. Also add such interface name rules only in this rules file. Otherwise
# rename_netiface will create wrong rules for new interfaces.
# It is safe to delete a rule, as long as you did not disable automatic rule
# generation. Only if all interfaces get a rule the renaming will work
# flawlessly. See also /etc/udev/rules.d/31-net_create_names.rules.
# Read /usr/share/doc/packages/sysconfig/README.Persistent_Interface_Names for
# further information.
# Use only a-z, A-Z and 0-9 for interface names!
SUBSYSTEM=="net", ACTION=="add", SYSFS{address}=="00:50:56:b7:6d:df", IMPORT="/lib/udev/rename_netiface %k eth0"
SUBSYSTEM=="net", ACTION=="add", SYSFS{address}=="00:50:56:b7:0b:2c", IMPORT="/lib/udev/rename_netiface %k eth1"
SUBSYSTEM=="net", ACTION=="add", SYSFS{address}=="00:50:56:b7:1a:26", IMPORT="/lib/udev/rename_netiface %k eth2"
SUBSYSTEM=="net", ACTION=="add", SYSFS{address}=="00:50:56:b7:14:a6", IMPORT="/lib/udev/rename_netiface %k eth3"

How to fix it?

The fix is easy you can delete all four entries then reboot, the file will be populated with correct entries automatically. if you don't want to reboot, edit the file with correct entries then run “/lib/udev/rename_netiface oldname newname” manually.

Further discussion.

udev rule makes device naming very easy. you can ensure interfaces are named according to PCI order, for example, you want to name onboard NIC as eth0 and name PCI NIC as eth1. ( some times the order is reversed).
 Check if the NIC name follows PCI order.

$ ls -l /sys/class/net/eth*/device
lrwxrwxrwx 1 root root 0 2010-08-26 09:46 /sys/class/net/eth0/device -> ../../../devices/pci0000:00/0000:00:11.0/0000:02:00.0
lrwxrwxrwx 1 root root 0 2010-08-26 09:46 /sys/class/net/eth1/device -> ../../../devices/pci0000:00/0000:00:11.0/0000:02:01.0

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