Taken [from upstream][1].

With `LogsDirectory=eturnal`, eturnal currently logs to
`/var/log/eturnal/eturnal.log`.

[1]: https://github.com/processone/eturnal/blob/master/config/eturnal.service

With `ETURNAL_ETC_PREFIX=/project/eturnal` eturnal uses
`/project/eturnal/etc/eturnal.yml`.

From [issues #9 (*Document ERL_EPMD_ADDRESS and ERL_EPMD_PORT*)][1]:

> === ERL_EPMD_ADDRESS ===
>
> On eturnal startup, an Erlang Port Mapper Daemon (epmd) service is spawned (if
> not running already). This service listens on all available interfaces by
> default. Setting the `ERL_EPMD_ADDRESS' environment variable tells epmd to
> listen on the specified comma-seperated list of IP addresses instead. Note that
> the IPv4 and IPv6 loopback addresses are implicitly added to this list if not
> specified. It's usually recommended to set `ERL_EPMD_ADDRESS' to `127.0.0.1'
> (e.g., by calling `systemctl edit eturnal' and adding an
> `Environment="ERL_EPMD_ADDRESS=127.0.0.1"' line to the `[Service]' section).

[1]: https://github.com/processone/eturnal/issues/9

This reverts commit f95afb4.

Despite `net.ipv6.conf.all.disable_ipv6 = 1`, epmd tries to bind to the
IPv6 loopback device, [which fails][1].

    eturnalctl[91982]: Protocol 'inet_tcp': register/listen error: econnrefused

So, remove `ERL_EPMD_ADDRESS=127.0.0.1`, which is no problem, as our
firewall blocks access to epmd from the internet.

[1]: https://github.com/processone/eturnal/issues/11

`LogsDirectory=` does not support absolute paths:

    Nov 03 13:23:09 pitti.molgen.mpg.de systemd[1]: /etc/systemd/system/eturnal.service:20: LogsDirectory= path is absolute, ignoring: /project/eturnal/log

Therefore, [do not set it all][1]:

> If this environment variable is unset as well, a `log' directory is
> created within the installation prefix.

The directory `log` is a symbolic link to `/project/eturnal/log`

[1]: https://github.com/processone/eturnal/blob/f9168eb70d2a1b8f7f110f4022985adb65175e55/doc/overview.edoc#L178-L188

…ation

We use mxstartupctl to start the service.

[`ProtectSystem=strict` mounts more directories as read-only.][1]

[The upstream service unit only sets it to `full` to support older
systemd releases.][2] systemd 242, currently shipped by MarIuX, support
`strict`, so use it.

[1]: https://www.freedesktop.org/software/systemd/man/systemd.exec.html#ProtectSystem=
[2]: https://github.com/processone/eturnal/issues/14#issuecomment-843021648