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

[Stef Walter]

On 30.04.20 10:43, Zbigniew Jędrzejewski-Szmek wrote:
> 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?

I'd support that. Obviously I should have included a docs patch in my
pull request :S

Stef