Defaulting to MAC-based names for network interfaces

[Felix Lechner]

Hi everyone,

Upon personal reflection, a declarative operating system like Guix probably
ought to use only predictable interface names.

More details about this proposal, including the text below, are
available in Bug#63508.

While shorter names like 'eno1' offer an indisputable convenience and beauty
when typing on the command line, administrators in Guix are unlikely to do so
due to the declarative configuration system.

Some system services may explicitly refer to interface names in their
configuration. They would also benefit from the predictable and constant
nature of MAC-based names.

The latter is particularly relevant on multi-homed machines, i.e. those with
more than one network connection.

A MAC-based interface name as issued by 'eudev' looks like this:

  enx0123456789af   (fictitious)

The commit in Bug#63508 was deployed on two production machines. The
migration to MAC-based interface names took place without issues. A
second reconfiguration was used to add the new interface name in
services tha needed it. The second step can be skipped, since the name
is known with certainty in advance.

The current naming scheme is less desirable because some services may silently
refuse to start after equipment was added or removed. A removal may take
place, for example, when something broke or when equipment was sold.

The device enumeration may also change when a CMOS battery fails and system
options are lost. In the author's option, Guix should not depend on BIOS
enumeration for device names.

In the author's case, the name of the sole network interface changed from
enp3s0 to enp4s0 when a PCIe disk controller (a SAS host-based adapter) was
installed. As a result, OpenSMTPd silently failed to start.

This commit switches 'eudev' from the standard naming order

  ID_NET_NAME_ONBOARD
  ID_NET_NAME_SLOT
  ID_NET_NAME_PATH

to ID_NET_NAME_MAC, which is always available. [1]

The author initially attempted to achieve the same result via

(udev-rules-service 'net-name-mac
  (udev-rule
    "01-net-name-mac.rules"
    "SUBSYSTEM==\"net\", ACTION==\"add\", NAME=\"$env{ID_NET_NAME_MAC}\"
")))

but that did not work. While the situation was not examined exhaustively, it
was not clear that udevadm can currently work because the standard command to
test udev setups: [2]

  $ udevadm --debug test /sys/class/net/*

did not find the script installed via the 'udev-service-type'.

A review of the 'eudev' sources indicated that the path to find rules [3] is
hard-coded to the store location during installation. An attempt to set the
path to /etc/udev/rules.d yielded a build error because that target folder
outside the store was understandably not writable.

The manual page for udevadm did not offer a way to select the runtime location
of the udev/rules.d folder via environment variables or a command-line option.

Anyone for whom such a setup is working properly should please contact the
author. Thank you!

This commit may result in some loss of privacy, although it is presently not
clear how meaningful that is. With this commit, anyone using privacy-enhanced
IPv6 addresses risks having their MAC exposed when they publish their
configuration files in Git or post a well-meant sample in a chat rooms,
because that configuration may mention the MAC address.

Moreover, the compatibility with schemes to generate fake one-time MAC
addresses upon boot should be evaluated. One concern is that the explicit
reference to a network interface in a configuration file would likely force
the use of a single and constant MAC address for that interface.

This commit was tested in production and is currently being used.

The change here resulted in the recompilation of several seemingly unrelated
packages such as Emacs and GTK. Perhaps those dependency relationships should
be examined.

[1] 
https://wiki.debian.org/NetworkInterfaceNames#How_to_migrate_to_this_scheme_on_upgraded_systems
[2] https://wiki.archlinux.org/title/Udev#Testing_rules_before_loading
[3] 
https://github.com/eudev-project/eudev/blob/39979ddf46e75d1b75bf381e1c73914c226c4302/configure.ac#L180
[4] https://en.wikipedia.org/wiki/IPv6_address#Temporary_addresses