systemd
`DEFAULT_HOSTNAME=` in os-release is not set when `Hostname=` is set
mkosi commit the issue has been seen with
main
Used host distribution
Fedora 40
Used target distribution
Fedora 40
Linux kernel version used
6.8.7-300.fc40.x86_64
CPU architectures issue was seen on
x86_64
Unexpected behaviour you saw
On a usr/-only build the hostname reverted to "fedora" after losing a DHCP lease which assigned a hostname instead of the hostname set during image build by mkosi. See https://github.com/systemd/systemd/issues/32541
As there is only one level of transient hostname "storage" there is no way to get the original hostname back afterwards. Therefore the only fallback possible is to use the default hostname from os-release (or the one compiled during systemd build). mkosi should therefore also update that variable to ensure the expected behavior.
Used mkosi config
No response
mkosi output
No response
Reading the linked issue I'm not sure setting the DEFAULT_HOSTNAME is something we should always do unconditionally, since it then would put the burden on people do remove that from the os-release file, if they don't want it and it doesn't seem like something that warrants an extra option. It might be something that's best left documented as is with a hint to use a script to change the os-release file.
I see no scenario where people would have to remove that line. If people are already setting Hostname= they expect the machine to have a certain hostname. So if the hostname is reset after losing a DHCP lease it makes way more sense to have that set as the Fallback instead of whatever the distribution usually ships
