60e4fb4240
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.
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