I recently had a case where a few of my [very] remote Raspberry Pis dropped off the network upon reboot (power failure, etc.).  I run the 'testing' release of Raspbian & Debian on my Linux boxes so I do an apt-get dist-upgrade rather often.  The RPis that dropped off the network had indeed been upgraded recently so I had one shipped back to me for investigation.

It turns out that eth0 and wlan0 had been renamed to enx$MAC and looked like this:

(I had the RPi plugged into my TV)

I do know about predictable network interface names, which is supposed to prevent interface names changing when hardware is swapped, among other things.  The enx$MAC naming scheme is used for USB devices.  Up until now I've been content to disable this feature by adding net.ifnames=0 to the kernel command-line.  Indeed, I had this flag in /boot/cmdline.txt on the Pis but it just isn't working anymore.

It turns out that this feature is set to be deprecated in Debian 10, along with the automatically-generated /etc/udev/rules.d/70-persistent-net.rules file, which is appended when a new NIC is added to the system.  This is documented in /usr/share/doc/udev/README.Debian.gz, apparently.

Unfortunately for me, the RPis never had 70-persistent-net.rules written and apparently udev 229-5 already ignores net.ifnames=0.  One of my dist-upgrades bumped udev from 229-4 to 229-5.  This is why none of the interface names that were defined in /etc/network/interfaces were found on boot and the RPis booted up with no enabled interfaces.

/usr/share/doc/udev/README.Debian.gz goes on to say that if custom interface names are needed, use 76-netnames.rules with the following content, which looks similar to what was previously written out in 70-persistent-net.rules automatically:

SUBSYSTEM=="net", ACTION=="add", ATTR{address}=="00:50:56:1a:aa:ab", NAME="eth0"

There's no mention of this method of interface naming being deprecated so I'm going to use it for now.

This is just the latest annoyance, for me, when it comes to Linux's interface naming.  Several years back the right way to setup predictable interface names was to use /etc/iftab and ifrename(8)!

What I haven't figured out is how this is going to impact VMs and virtio.  My own VMs with VirtualBox have static MAC addresses assigned to the vbox file so using 76-netnames.rules will work fine.  However, I'm not sure if EC2 and other Xen-based VPS providers always provide a constant MAC address.  One of my EC2 instances has 53-ec2-network-interfaces.rules, which handles some sort of hotplugging.  I'm not sure if that script will stop working as well after some time and udev upgrades but it would seem the solution is to use some ATTR other than 'addresss'.

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