Re: Generating new IDs for cloning

[Michael Stone <mstone@debian.org>]

On Thu, Aug 08, 2019 at 06:16:31PM +0200, Marc Haber wrote:
> On Thu, 8 Aug 2019 11:50:07 -0400, Michael Stone <mstone@debian.org>
> wrote:
> > On Thu, Aug 08, 2019 at 11:41:52AM -0400, Marvin Renich wrote:
> > > * Michael Stone <mstone@debian.org> [190808 08:42]:
> > > > On Thu, Aug 08, 2019 at 08:37:16AM -0400, Marvin Renich wrote:
> > > > > Does anyone know what applications use this file for what purpose?  Is
> > > > > this a systemd-ism?
> > > >
> > > > man machine-id
> > >
> > > The man page says what it is (a unique, random ID for the machine) and
> > > how to initialize it, but says nothing about why it exists. What
> > > applications expect it to be there?
> >
> > > From the man page:
> >
> >       The machine ID does not change based on local or network configuration
> >       or when hardware is replaced. Due to this and its greater length, it
> >       is a more useful replacement for the gethostid(3) call that POSIX
> >       specifies.
> > [...]
> >       When a machine is booted with systemd(1) the ID of the machine will be
> >       established. If systemd.machine_id= or --machine-id= options (see
> >       first section) are specified, this value will be used. Otherwise, the
> >       value in /etc/machine-id will be used. If this file is empty or
> >       missing, systemd will attempt to use the D-Bus machine ID from
> >       /var/lib/dbus/machine-id, the value of the kernel command line option
> >       container_uuid, the KVM DMI product_uuid (on KVM systems), and finally
> >       a randomly generated UUID.
>
> What Marvin says: That doesn't explain anything.

I guess I need you to say what's confusing. It's an ID that doesn't 
change. You can look up gethostid to get a little more background. If 
you need an ID for a host it's useful. If you don't need an ID for a 
host, then it's not so useful. Once upon a time (gethostid(3) alludes to 
this) it was thought that an IPv4 would be a good unique ID, but with 
the rise of NAT it quickly became useless and hence the search for an 
alternative. The second paragraph is pretty clear that machine-id is one 
of a number of possible seeds for systemd to set the machine's ID. The 
rest of the file talks about how applications can use an API to get a 
constant ID based on the machine ID and that applications should 
generally not use the contents of the machine-id file directly. So to 
find out what applications use the file you'd have to look for 
references to the various APIs (such as 
sd_id128_get_machine_app_specific or dbus-uuidgen) which themselves use 
/etc/machine-id