[systemd-devel] systemd-timesyncd with read-only root filesystem

[Michael Chapman]

On Fri, 9 Dec 2016, André Hartmann wrote:
> Hi Michael,
>
> Am 09.12.2016 um 12:43 schrieb Michael Chapman:
>>  On Fri, 9 Dec 2016, Michael Chapman wrote:
>>  [...]
>> >  You will need to use the .service extension on at least the first of
>> >  those links. systemd will only consider links in that directory that
>> >  have valid unit names. (I'm pretty sure the intermediate link's name
>> >  doesn't matter, but I wouldn't want to rely on that -- best just to
>> >  use valid full unit names everywhere.)
>>
>>  Oh, something else I thought of... I'm pretty sure it doesn't make sense
>>  having a symlink to /dev/null inside a .wants directory. Really, only
>>  the name of those links are considered. The links aren't actually
>>  dereferenced.
>
> I have to disagree on this, the name does not matter, the link target does 
> matter (see log below, the relevant status is marked with <---).

Your log only shows that it's the presence of the link that matters, not 
what the link is pointing to.

Here, I'll demonstrate:

   # cat >/etc/systemd/system/example.service <<EOF
   [Unit]
   Description=Example

   [Service]
   ExecStart=/bin/true

   [Install]
   WantedBy=multi-user.target
   EOF

Enabling this creates the symlink:

   # systemctl daemon-reload
   # systemctl is-enabled example.service
   disabled
   # systemctl enable example.service
   Created symlink from /etc/systemd/system/multi-user.target.wants/example.service to /etc/systemd/system/example.service.
   # systemctl is-enabled example.service
   enabled

But I can make the symlink's target whatever I want... even a dangling 
symlink:

   # ln -sf the-target-does-not-matter /etc/systemd/system/multi-user.target.wants/example.service
   # systemctl daemon-reload
   # systemctl is-enabled example.service
   enabled

/dev/null is just fine, it's still a valid link:

   # ln -sf /dev/null /etc/systemd/system/multi-user.target.wants/example.service
   # systemctl daemon-reload
   # systemctl is-enabled example.service
   enabled

Only the name of the link is significant:

   # mv /etc/systemd/system/multi-user.target.wants/{example,somethingelse}.service
   # systemctl daemon-reload
   # systemctl is-enabled example.service
   disabled

Put simply, you can't enable and disable services the way you're trying 
to do so.

- Michael