On Sun, Aug 20, 2017 at 03:45:08PM +0100, Rowland Penny wrote:
> On Sun, 20 Aug 2017 16:27:26 +0200
> Adam Borowski <kilobyte@???> wrote:
[my snip:]
> > * any renames to "eth0"/"wlan0" are a losing idea, as a new interface
> > can appear at any moment, clashing with what you just tried to rename
> > to. Several approaches to avoid this race have been tried, none worked
> > reliably. Thus, any sane renaming should use a new namespace. Of
> > what was proposed, it looks like people liked "en0"/"wl0" the most
> > (yeah, it's purely an aesthethic thing). My idea "e0"/"w0" is too
> > short to imply out of context you're talking about interface names,
> > etc.

> What is the difference between eth0/wlan0 and en0/wl0 or even e0/w0 ?
> They are just variations on a theme.

You can't safely rename to eth0/wlan0. At bootup, when an interface is
being cold/hot-plugged, an *udev script is run. When that script decides
that the interface that it's been called for should be "eth1", it is
possible (during boot-up, likely) that another interface pops up, and the
kernel gives it the next name in sequence, ie, "eth1". And the result is
not pretty.

It's a natural thing to favour "eth*", ie, like things used to be in the
past. Too bad, this fails due to the above race. In theory, the obvious
thing would be "if rename fails, try again" -- but none of implementations
so far hasn't done this right. Using your own namespace avoids this problem
entirely.

> The systemd way of doing things is, in my opinion, stupid and doesn't
> actually help.

It's main downside is that you never know what interfaces a new machine has.
Without systemd-udev, you can 100% predict that a machine with only one
wired interface will have eth0. With systemd "predictable" names, you never
know. Even worse, that name is not predictable over major kernel upgrades,
a slight change of qemu's command line, etc.

> If you have more than one ethernet (or wireless) device, then yes,
> they need to be consistently named, but as most people will not be
> adding (or removing) devices often, could they be issued with a
> name based on their MAC and store this somewhere and then use this
> to rename the network devices once they are all up ?

But how can you know they're all up? Even if you disregard hotplug (ie,
attaching a new (typically USB) network card at runtime), some hardware
takes a surprisingly long time to get ready. Waiting for userspace to
upload required firmware (which needs the filesystem to be up) means the
hardware's initialization only _starts_ at the time you'd expect it to
be all done.

Even an answer "I expect as many interfaces as there were last boot" won't
work if the admin has just installed a shiny new network card. And this
happens even on regular desktops -- my on-board PHY decided to become a
nyetwork card a few months ago.

Meow!
-- 
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⣾⠁⢰⠒⠀⣿⡁ Vat kind uf sufficiently advanced technology iz dis!?
⢿⡄⠘⠷⠚⠋⠀                                        -- Genghis Ht'rok'din
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