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docs: say how to delay network-online.target

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Could be related to https://access.redhat.com/solutions/4879291.
Unfortunately I can't access this page, but the title seems
relevant.
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Zbigniew Jędrzejewski-Szmek 2022-05-17 12:25:52 +02:00
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commit 45b1321653
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docs/NETWORK_ONLINE.md
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@ -224,3 +224,38 @@ it does not delay boot. It is usually easier to write such a service in a
be (re-)established, but is instead started when the network has connectivity,
and if the network goes away, it fails and relies on the system manager to
restart it if appropriate.
## Modyfing the meaning of `network-online.target`
As described above, the meaning of this target is defined first by which
implementing services are enabled (`NetworkManager-wait-online.service`,
`systemd-networkd-wait-online.service`, …), and second by the configuration
specific to those services.
For example, `systemd-networkd-wait-online.service` will wait until all
interfaces that are present and managed by
[systemd-networkd.service(8)](http://www.freedesktop.org/software/systemd/man/systemd-networkd.service.html).
are fully configured or failed and at least one link is online; see
[systemd-networkd-wait-online.service(8)](http://www.freedesktop.org/software/systemd/man/systemd-networkd-wait-online.service.html)
for details. Those conditions are affected by the presence of configuration
that matches various links, but also by settings like
`Unmanaged=`, `RequiredForOnline=`, `RequiredFamilyForOnline=`; see
[systemd.network(5)](http://www.freedesktop.org/software/systemd/man/systemd.socket.html).
It is also possible to plug in additional checks for network state. For
example, to delay `network-online.target` until some a specific host is
reachable (the name can be resolved over DNS and the appropriate route has been
established), the following simple service could be used:
```ini
[Unit]
DefaultDependencies=no
After=nss-lookup.target
Before=network-online.target
[Service]
ExecStart=sh -c 'while ! ping -c 1 example.com; do sleep 1; done'
[Install]
WantedBy=network-online.target
```