Changing the ethX to Ethernet Device Mapping in EL6 and Fedora 12 to 14

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 AN!Wiki :: How To :: Changing the ethX to Ethernet Device Mapping in EL6 and Fedora 12 to 14

Template note icon.png
Note: This applies up to Fedora 14. Fedora 15 works quite differently now that it uses biosdevname. Please follow this tutorial for Fedora 15 and newer.

When you have two or more ethernet devices in one machine, the EL6 or Fedora install may not install them in the order you want. This document will show you how to change the ethX to physical ethernet device mapping.

This is desirable in cases where specific network cards need to be used on specific networks, and you are interested in maintaining common ethX names across servers. A practical example is found in the 2-Node Red Hat KVM Cluster Tutorial.

Contents

For the Impatient

Assuming that NetworkManager is already removed. This example will swap eth0 and eth1.

# Stop the network daemon
/etc/init.d/network stop
cd /etc/sysconfig/network-scripts
# Move your ifcfg files to the device names you desire
mv ifcfg-eth0 ifcfg-temp
mv ifcfg-eth1 ifcfg-eth0
mv ifcfg-temp ifcfg eth1
# Modify DEVICE lines to match new file names
vim /etc/sysconfig/network-scripts/ifcfg-eth*
# Make changes as needed
# Remove udev rules file
rm /etc/udev/rules.d/70-persistent-net.rules
start_udev
/etc/init.d/network start

Thanks for fenris02 and mutk

Example

I find it easier to follow instructions when I have an example to follow, so let me provide one here:

I've got the same hardware in the same configuration in both of my storage nodes. To keep things simple, I want to make sure that both nodes use the same ethX name for the network devices on each of the given networks.

Desired Mapping

Let's assume that you want this:

  • eth0
    • MAC: 90:E6:BA:71:82:EA
  • eth1
    • MAC: 00:21:91:19:96:53
  • eth2
    • MAC: 00:0E:0C:59:46:E4

Initial Mapping

When the Fedora install finished, you may have gotten this:

  • eth0
    • MAC: 00:21:91:19:96:53
  • eth1
    • MAC: 90:E6:BA:71:82:EA
  • eth2
    • MAC: 00:0E:0C:59:46:E4

The Problem

In the above example, eth2 is where we want it, so we leave it alone. The problem is that eth0 and eth1 are reversed.

The Fix

What we need to do is modify the DEVICE=... line in each ifcfg-ethX file to reflect the order we want.

To avoid confusion later on, we should also rename the files to reflect the new device names.

Stopping the Network

First, stop the network. This is important because if you change the MAC address mapping while the network is still up, the init.d script will fail to bring down the network devices and you will need to reboot for the changes to take effect.

/etc/init.d/network stop

Interface Configuration

Go to the /etc/sysconfig/network-scripts/ directory and then use cat to see the contents of the ifcfg-eth* network configuration files:

Template note icon.png
Note: The cat output below only shows the lines we're interested in. You will likely see several more lines not shown below.
cd /etc/sysconfig/network-scripts
cat ifcfg-eth0
DEVICE="eth0"
HWADDR="00:21:91:19:96:53"
cat ifcfg-eth1
DEVICE="eth1"
HWADDR="90:E6:BA:71:82:EA"
cat ifcfg-eth2
DEVICE="eth2"
HWADDR="00:0E:0C:59:46:E4"

The important lines are the DEVICE=... and HWADDR=... lines. The rest of the lines will very likely differ from the output above. It is outside the scope of this tutorial to cover the various settings. For an exhaustive list of available options, please see the Red Hat EL6 or the Fedora 12 documentation.

First, change the file names to reflect the device order that you want. In this example we need to swap eth0 and eth1.

cd /etc/sysconfig/network-scripts
mv ifcfg-eth0 ifcfg-temp
mv ifcfg-eth1 ifcfg-eth0
mv ifcfg-temp ifcfg-eth1

Using your editor of choice, open each file and change the DEVICE=... to match the new filename.

Here is the updated files with updated network settings:

cd /etc/sysconfig/network-scripts
cat ifcfg-eth0
DEVICE="eth0"
HWADDR="90:E6:BA:71:82:EA"
cat ifcfg-eth1
DEVICE="eth1"
HWADDR="00:21:91:19:96:53"
cat ifcfg-eth2
DEVICE="eth2"
HWADDR="00:0E:0C:59:46:E4"

'udev' Configuration

The user-space device manager, udev, maintains a list of configuration files that help ensure that hardware to /dev (software) mappings stay consistent, even when hardware is hot swapped or moved between physical connections. For networking hardware, this mapping is recorded in the 70-persistent-net.rules.

Simply delete this file.

rm /etc/udev/rules.d/70-persistent-net.rules

Making the Changes Take Effect

First, you need to reload udev so that it updates the mapping in memory. This is done using the start_udev program:

start_udev
Starting udev:                                             [  OK  ]

With udev updated, the last step is to restart the network daemon:

/etc/init.d/network start
Bringing up loopback interface:                            [  OK  ]
Bringing up interface eth0:                                [  OK  ]
Bringing up interface eth1:                                [  OK  ]
Bringing up interface eth2:                                [  OK  ]

Done! use ifconfig to verify that the network cards are properly assigned to the specified ethX names.

Caveat!

NOTE: If you are using a vlan, the device facing the vlan can not have the HWADDR=... value set! Set the others and leave this commented out. Otherwise, because of how the vlan loads, the OS will not see the physical device and will fail to bring up the interface at all.

 

Any questions, feedback, advice, complaints or meanderings are welcome.
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