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Re: Naming of network devices - how to improve it in buster




On Tue, 11 Jul 2017 at 13:45:25 +0200, Bjørn Mork wrote:
> I got to ask: Why?  We do not have stable names for e.g. disks.  Why do
> we need it for network devices?

We do have stable names for disks: look in /dev/disk/by-* and you'll see
a bewildering variety of ways to refer to the same disk or partition.

The thing that is different for network devices is that network
devices are not files (device nodes), so udev cannot create a symlink
/dev/network/by-mac/01:23:45:67:89:ab -> /dev/network/eth0 or whatever.
Network devices are (as far as I know) the only class of device managed by
udev that is not backed by a device node, which means udev cannot provide
multiple equivalent names for the same device, and is forced to choose
exactly one of those names, and rename the device if the chosen name
is not the kernel-generated one. That is why naming network devices is,
and has always been, more controversial than naming disks: they are the
one device class in Linux that violates the Unix design rule-of-thumb
"everything is a file".

If network devices were files, udev wouldn't have a configurable
NamePolicy to rename them: it would just provide symlinks for all the
possible naming policies, and let the sysadmin use any, all or none of
those names when configuring tools like ifupdown. Unfortunately, that
isn't possible.

Relatedly, network device name lengths are limited to the length of some
arbitrarily-sized struct field in the kernel ABI, which is why some of
the names udev can generate are so cryptic - they have to pack enough
information to uniquely identify a device into a very small space. Again,
compare with disks, where the names can be as long as you like.

As long as the kernel represents network devices as some unique network
thing that is not a device node in the filesystem, how to name them is
always going to be an ugly compromise, and someone is always going to be
upset by the default.

Happily, there is enough configuration that those with Strong Opinions
can name their network devices in any way of their choice, with one
notable exception: names from the kernel's various namespaces (eth0,
wlan1, wwan2) can only be reliable if no renaming at all is taking
place (which is one of the options available in configuration).

> For static network configurations on systems with multiple interfaces,
> the correct and only logical place for the device association is with
> the rest of the network configuration. If you use NetworkManager, then
> it is up to NetworkManager to match it with a specific network device -
> if required.  The rest of the system does not need to care.

This seems a good plan. NetworkManager and systemd-networkd (what I
personally use on laptops/desktops and servers, respectively) both have
approximately this approach.

> The remaining problem is to make ifupdown do device matching on other
> (and hopefully more stable) attributes than the device name.

... or alternatively, not use ifupdown, which was designed for a
rather different era of networking, in situations where it's unsuitable?

ifupdown served us well for many years as a competitor to Red Hat
sysconfig and SUSE YAST, but those other distributions have moved
on and perhaps so should we. Maybe it's time to admit that using a
Debian-specific network configuration format/infrastructure by default[1]
has become more bug than feature?

    S

[1] For desktop systems, ifupdown is already only arguably the default:
    if debian-installer is installing NetworkManager, it configures
    ifupdown to only bring up lo so that NetworkManager can do the rest;
    but our default init system (systemd) would bring up lo anyway,
    so ifupdown is redundant. Subsequently replacing pid 1 with sysvinit
    is of course supported, but anyone who is contemplating doing so
    had better know what they are doing, to an extent where defaults are
    not entirely relevant.