[systemd-devel] systemd-hostnamed/hostnamectl and transient hostname change

[Zbigniew Jędrzejewski-Szmek]

On Mon, Apr 27, 2020 at 12:00:36PM +0200, Thomas HUMMEL wrote:
> On 4/27/20 11:51 AM, Mantas Mikulėnas wrote:
> 
> Hello, thanks for your answer.
> 
> >On Mon, Apr 20, 2020 at 6:17 PM Thomas HUMMEL
> ><thomas.hummel at pasteur.fr <mailto:thomas.hummel at pasteur.fr>>
> >wrote:
> >
> >    1. why does the transient hostname change while I stated --static only
> >    while running hostnamectl ?
> >
> >    2. why does the change take some time to appear on dbus ?
> >
> >
> 
> >Hostnamed does not implement receiving hostname change
> >notifications from the kernel, so it always reports you the same
> >hostname that it has seen on startup.
> 
> That was my understanding as well.

Lennart opened a PR to remove the caching:
https://github.com/systemd/systemd/pull/15624.

> >You're only seeing changes because hostnamed /exits when idle/ --
> >the next time you're actually talking to a brand new instance of
> >hostnamed, which has seen the new hostname.
> 
> But this does not explain why the transient hostname is changed as I
> only changed the static one, does it ? Unless this new instance sets
> it from the static one when it starts ? I mean something has to call
> sethostname(2) to set the transient to the new static one, right ?

The documentation is wrong. The code in hostnamed sets the kernel
hostname when setting the static one. This was changed in
https://github.com/systemd/systemd/commit/c779a44222:

commit c779a44222161155c039a7fd2fd304c006590ac7
Author: Stef Walter <stef at thewalter.net>
Date:   Wed Feb 12 09:46:31 2014 +0100

   hostnamed: Fix the way that static and transient host names interact
   
   It is almost always incorrect to allow DHCP or other sources of
   transient host names to override an explicitly configured static host
   name.
   
   This commit changes things so that if a static host name is set, this
   will override the transient host name (eg: provided via DHCP). Transient
   host names can still be used to provide host names for machines that have
   not been explicitly configured with a static host name.
   
   The exception to this rule is if the static host name is set to
   "localhost". In those cases we act as if no
   static host name has been explicitly set.

We need to reconcile the code and the docs. I'd go for updating the docs
to match the code, because this is a long-standing behaviour and people
haven't been complaining about it. (I'm assuming you're not unhappy, just
confused by the unexpected results...). Opinions?

Zbyszek