Discovering device names

Find Nemo!

Article from ADMIN 14/2013

By Thorsten Scherf

Ethernet devices in Linux have always been called eth0 and nothing else. All of a sudden, this universal truth has lost its validity, and Linux administrators need to understand why and how.

Recently, a customer asked me what was going on with his system. All of a sudden, he no longer had an eth0; instead, he was seeing strange names like em1 or p3p1 at the console. He wanted to know exactly what was happening.

The explanation is fairly simple. The rule-based udev subsystem is responsible for naming. If all criteria for a rule match a network device found at boot time, the device is typically given the defined name, such as eth<X>. Here's an example:

# grep eth0 /etc/udev/rules.d/70-persistent-net.rules
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="f0:de:f1:d5:c1:25", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"

The rule says that a network device with the stated MAC address should be given a device name of eth0 and thus be the first device visible on the system. The name in the file is arbitrary. If you like, you can call the card public or private. This principle works to a certain extent. The downside is that you don't know which card exactly this is on the system. The only thing that is clear is that it is the card with the stated MAC address; whether this is also the first network device on the system remains unclear. For system administrators, this situation has always been a big worry. If you've ever stood in front of a server with a dozen network ports looking for eth0 , you will know exactly what I mean.

Danger of Confusion

The situation is aggravated in network installations where the TFTP boot request is sent on eth0, but the installer later sends the DHCPDISCOVER message via a different network card identified as eth0. This situation is really fun to troubleshoot.

I would like the name I see on the system to be

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