Wandering Thoughts archives

What controls Red Hat Enterprise's ethN device names

Since I just went digging for this the other day, here's what I know about what controls what Ethernet devices get named on Red Hat Enteprise (and probably also on Fedora, but I haven't looked at my Fedora systems in this level of detail).

• if kudzu is enabled, it uses /etc/sysconfig/hwconf to name everything. If there is no such file or the data in it doesn't match current reality, various bad things happen.

  (You can probably hand-edit the file if necessary.)

• otherwise, interface naming is controlled by the HWADDR setting in the ifcfg-* files in /etc/sysconfig/network-scripts. If there is no Ethernet address specified, you get no renaming.

The ifcfg files are used by both udev and the ifup scripts that actually bring interfaces up and so on. When udev detects a new network device (including at boot, I believe), it runs /lib/udev/rename_device, which searches the ifcfg-* files for a HWADDR that matches the new device and uses the DEVICE setting from that file to give a name to the new interface.

(A network device that is hotplugged after system boot also winds up running /etc/sysconfig/network-scripts/net.hotplug.)

During boot, the order of operation is udev first, then kudzu, and finally the network init script winds up ifup'ing all of the interfaces that are supposed to be running, potentially undoing any damage kudzu did (if kudzu left the ifcfg configuration files along, which is unlikely).

(You may gather that I have a pretty low opinion of kudzu; in fact, I have been turning it off on most of my systems for years. It was left enabled on this RHEL system mostly because I hadn't taken the time to audit what init scripts were getting run.)

My problem with Ethernet naming on Red Hat Enterprise 5

Here's my problem: I have a bunch of identical 1U servers (SunFire X2100 M2s) with four onboard Ethernet ports, driven by two different chipsets (two nVidia ones, two Broadcom ones). I want to configure our RHEL installs so that no matter which physical unit I stuff the system disks into, the Ethernet ports come up with consistent names that match the ports on the back of the server; eth0 should always be the port labeled 'port 0' and so on.

(Since they have hotswap drive bays, we want to be able to easily swap drives between units in case of hardware failure or the like. It also simplifies general administration a bunch if the Ethernet naming matches the hardware naming.)

In the good old days, this was simple; just set up /etc/modprobe.conf to alias eth0 and eth1 to the tg3 driver and eth1 and eth2 to the forcedeth driver, and everything usually worked.

In the new world of udev, not so much; much like with Ubuntu, everything really wants to name things based on known Ethernet addresses, and there seems to be no way to control what order modules are loaded in. The furthest I've gotten is a configuration that does nothing with any 'new' Ethernet ports, so you have to log in on the console and change all of the HWADDR values in the ifcfg files to have the correct Ethernet addresses.

(To do this, you have to turn off kudzu with 'chkconfig --del kudzu'. If you leave it enabled, it will helpfully configure any 'new' Ethernet ports to do DHCP on boot, and in the process it will replace your working ifcfg files with new ones. Yes, it leaves the old files around with .bak extensions, but I am pretty sure that if you swap hardware twice you will lose them entirely.)