In hostnamed this is exposed as a dbus property, and in the logs in both
places.
This is of interest to network management software and such: if the fallback
hostname is used, it's not as useful as the real configured thing. Right now
various programs try to guess the source of hostname by looking at the string.
E.g. "localhost" is assumed to be not the real hostname, but "fedora" is. Any
such attempts are bound to fail, because we cannot distinguish "fedora" (a
fallback value set by a distro), from "fedora" (received from reverse dns),
from "fedora" read from /etc/hostname.
/run/systemd/fallback-hostname is written with the fallback hostname when
either pid1 or hostnamed sets the kernel hostname to the fallback value. Why
remember the fallback value and not the transient hostname in /run/hostname
instead?
We have three hostname types: "static", "transient", fallback".
– Distinguishing "static" is easy: the hostname that is set matches what
is in /etc/hostname.
– Distingiushing "transient" and "fallback" is not easy. And the
"transient" hostname may be set outside of pid1+hostnamed. In particular,
it may be set by container manager, some non-systemd tool in the initramfs,
or even by a direct call. All those mechanisms count as "transient". Trying
to get those cases to write /run/hostname is futile. It is much easier to
isolate the "fallback" case which is mostly under our control.
And since the file is only used as a flag to mark the hostname as fallback,
it can be hidden inside of our /run/systemd directory.
For https://bugzilla.redhat.com/show_bug.cgi?id=1892235.
gethostname(3) says it's unspecified whether the string is properly terminated
when the hostname is too long. We created a buffer with one extra byte, and it
seems the intent was to let that byte serve as terminator even if we get an
unterminated string from gethostname().
No functional change, just moving a bunch of things around. Before
we needed a rather complicated setup to test hostname_setup(), because
the code was in src/core/. When things are moved to src/shared/
we can just test it as any function.
The test is still "unsafe" because hostname_setup() may modify the
hostname.
Zbigniew Jędrzejewski-Szmek