[systemd-devel] Waiting for (transient) hostname configuration

[Lennart Poettering]

On Di, 19.04.22 11:14, Alessio Igor Bogani (alessio.bogani at elettra.eu) wrote:

> Dear systemd developers,
>
> Sorry for my very bad english.

Not bad at all.

> We use the hostname provided by DHCP to start the right subset of our
> applications on each of our machines (in other words the transient
> hostname is used as predictable machine identifier).
>
> Make it works on the ancient SysV is straightforward but I can't find
> a (reasonable) way to achieve the same goal with systemd. We already
> use both "Requires" and "After" with "network-online.target
> nss-lookup.target" into our units but it isn't enough. When our units
> start they obtain "localhost" from %H specifier. We don't find a way
> to wait for (transient) hostname to be configured. The only solution
> that it seems to work is to replace our binary in ExecStart with a
> script which make a busy-loop until `hostname` output doesn't differ
> from "localhost".
>
> Is there a better way?

First of all: % specifiers (including %H) are resolved at the moment
the unit files are *loaded*, i.e. typically at earliest boot, long
before they are actually *started*. Thus, if you change the hostname
and want %H to resolve to that, you need to issue a reload at the
right moment, i.e. issue `systemctl daemon-reload` once the hostname
is acquired.

If your hostname is set via DHCP you need to wait for DHCP to be
acquired. How that's done, depends on the networking solution you
use. If you use systemd-networkd, then the
sytemd-network-wait-online.service is what you want to use. If you
enable that then network-online.target should be the point where DHCP
is acquired and thus also the hostname in effect.

Lennart

--
Lennart Poettering, Berlin