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hostname: consider systemd.hostname= static and give it precedence #25158

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hostname: consider systemd.hostname= static and give it precedence #25158

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Commits on Jan 11, 2024

  1. hostname: consider systemd.hostname= static and give it precedence 

    The systemd.hostname= kernel commandline can for example be used to set a
unique hostname on embedded devices with read-only root file systems,
by letting the bootloader generate it from factory data.

The current semantics result in somewhat unexpected behaviour when doing so.
Clean up the semantics of the systemd.hostname= kernel parameter to make it
more useful and predictable.

The previous semantics were:

 - systemd.hostname= takes precedence over /etc/hostname during boot,
   even though it is technically considered a transient hostname.
 - In systemd-hostnamed however /etc/hostname takes precedence over
   systemd.hostname= as it is considered transient again and the normal
   priorities (static over transient over default) are followed.
   This means if a new transient hostname is set, the hostname of the system
   will be sethostname()'d to the one in /etc/hostname (if present) or
   the new transient hostname.
 - If /etc/hostname does not exist, systemd-hostnamed does not report a static
   hostname via dbus to consumers like the NetworkManager dhcp client.
   In the case of NetworkManager this results in no hostname being set in dhcp
   requests.

The new semantics are:

 - systemd.hostname= is considered a static hostname during boot and in
   systemd-hostnamed and as such takes precedence over transient or default
   hostnames.
 - systemd.hostname= takes precedence over /etc/hostname.
    @hnez
    hnez committed Jan 11, 2024
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  2. hostname: use hostname= kernel command line parameter as default host… 

    …name

The hostname= kernel commandline parameter is interpreted by the Linux
kernel and used as the initial hostname of the system.

As of now systemd (when running as PID 1) will behave as follows with
respect to the hostname set via the kernel command line:

 - If a static hostname is configured via systemd.hostname= or /etc/hostname
   it will be set regardless of what was set via hostname=.
 - If no static hostname is configured but there is already _some_ hostname
   set (e.g. via the hostname= kernel command line), that hostname will be
   kept.
 - If no static hostname is configured and none is configured e.g. via
   hostname= a generic default is used based on e.g. the OS release.

If later on a new transient hostname is set via the `SetHostname` DBus call
to systemd-hostnamed, it will take precedence over the hostname provided via
the hostname= kernel command line, but not over the static hostname.
This means the hostname= parameter behaves like a default hostname.

There is however an inconsistency: when unsetting the transient hostname
with an empty `SetHostname` method call without a static hostname set.
In this case the hostname will not revert to the one set via the hostname=
parameter, but instead to a generic default hostname.

Fix this by considering hostname= as an actual default hostname when no
static or transient hostname is set.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
    @hnez
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  3. man/hostnamed: document the various hostname sources and their priori… 

    …ties

The way a hostname for the system is chosen has become quite complex
with three classes (static, transient, default) of hostnames and
multiple options per class, each with their own priorities w.r.t.
which hostname will be prefered over which other hostname.

Document the different hostname sources in the systemd-hostnamed man page.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
    @hnez
    hnez committed Jan 11, 2024
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