[systemd-devel] Antw: [EXT] Re: [systemd‑devel] Waiting for (transient) hostname configuration

[Ulrich Windl]

>>> Lennart Poettering <lennart at poettering.net> schrieb am 19.04.2022 um 11:41
in
Nachricht <Yl6DumuBrCyJzmYR at gardel-login>:
> 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

That's quite some surprise: I thought they are evaluated when they are
executed (as in shell scripts).
Is there a way to "reload" a specific unit file?

> 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