use dummy UUIDs for NM; since those would change every boot

[Mathieu Trudel-Lapierre]

Use dummy UUIDs:

00000000-0000-0000-0000-000000000001

^ would do just fine; since that's unlikely to end in a collision and a generated UUID would never be the same across reboot.

[Steve Langasek (vorlon)]

The underlying justification here is that currently, we are wasting (possibly scarce) entropy at each boot to generate a UUID whose global uniqueness provides no particular value to us because it doesn't persist across reboots (therefore you can't usefully refer to it from elsewhere anyway).

We have two ways to generate a UUID with libuuid: one is to use the system mac address plus the current time, the other is to use a true RNG. The first method presents a chicken-and-egg problem for netplan, because the libuuid implementation only looks at mac addresses of "up" interfaces, which we don't have at the time netplan runs at boot. So we fall back to an RNG, which means each time netplan generates NM config, it gets a different identifier for the connection config.

Using serial ids (00000000-0000-0000-0000-000000000001, 00000000-0000-0000-0000-000000000002, ...) may not be optimal because things that *do* generate references to these connections on the system will potentially be referring to different connections with the same ID across reboot.

Another option is for netplan to do its own implementation of guaranteed-not-to-collide and *idempotent* UUIDs, so that the same config repeatably produces the same identifier. Suggestion for input to the algorithm:

 - machine UUID (if available early enough on first boot), or mac address of corresponding network device
 - mtime of the netplan config file
 - name of the device (==netplan yaml key)

[Mathieu Trudel-Lapierre (cyphermox)]

For now part of this is "fixed" as netplan only generates UUIDs for the interfaces that really need them: only for NM, and only for VLAN devices.

nplan (0.32) bionic; urgency=medium

  * src/nm.c: better handle the UUID generation; the order of iterating
    through interaces may affect things here. Also make sure the tests catch
    a null UUID.

 -- Mathieu Trudel-Lapierre <email address hidden> Tue, 14 Nov 2017 08:53:51 -0500

[Lukas Märdian (slyon)]

FTR, the commit which "fixed" this issue partially: https://github.com/canonical/netplan/commit/365b7e5321720e3ebbf9a985f82c1475b219932b

Moving this bug report into the "netplan.io" component, as "nplan" is deprecated.