How does "biosdevname" really work?

Question

I know the purpose of "biosdevname" feature in Linux, but I'm not sure how exactly it works.

I tested it with Ubuntu 14.04 and Ubuntu 14.10 (both 64-bit server editions) and it looks like they enable it by default - right after system startup my network interface has a name such as p4p1 instead of eth0, no customization is needed. As I understood it, in order for biosdevname to be enabled, BOTH of these two conditions must be met:

• a boot option biosdevname=1 must be passed to a kernel
• biosdevname package must be installed

As I already mentioned, both Ubuntu 14.04 and 14.10 seem to offer biosdevname as a default feature: they come with biosdevname package already installed, I didn't need to modify grub.cfg either - GRUB_CMDLINE_LINUX_DEFAULT has no parameters and my network interface still has a BIOS name (p*p*) instead of a kernel name (eth*.)

Later I wanted to restore the old style device naming and that's where the interesting part begins. I decided to experiment a bit while trying to disable the biosdevname feature. Since it requires biosdevname package to work (or so I read here and there), I assumed removing it would be enough to disable the feature, so I typed:

sudo apt-get purge biosdevname

To my surprise, after reboot my network interface was still p4p1, so biosdevname clearly still worked even though biosdevname package had been wiped out.

As a next step, I applied appropriate changes to /etc/network/interfaces in order to restore the old name of my network interface (removed entry for p4p1 and added entry for eth0). As a result, after another reboot, ifconfig reported neither eth0 nor p4p1 which was another proof that OS still understood BIOS names instead of kernel names.

It turned out that I also had to explicitly change GRUB entry to GRUB_CMDLINE_LINUX_DEFAULT=biosdevname=0 and update GRUB to get the expected result (biosdevname disabled and old name of network interface restored).

My question is: how could biosdevname work without biosdevname package? Is it not required after all? If so, what exactly provides the biosdevname functionality and how does it work?

Answer 1

The reason biosdevname keeps annoying you even after you uninstall the package, is that it installed itself in the initrd 'initial ramdisk' file as well.

When uninstalling, the /usr/share/initramfs-tools/hooks/biosdevname is removed, but there is no postrm script in the package so update-initramfs is not executed and biosdevname is still present in the /boot/initrd... file used in the first stage of system startup.

You can fully get rid of it like this:

$ sudo update-initramfs -u