GUIDE: Reliably wait for network when booting Jessie

[fvisagie]

Superseded - posted an improved implementation (controlled by Wait for Network) to the Raspbian forum.

This guide uses Jessie's systemd init system to implement simple and reliable detection of network availability for mounting remote drives, etc.

Guide

1. Change to super-user

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pi@fv-rpi3b:~ $ sudo su
root@fv-rpi3b:/home/pi#

2. In /lib/systemd/system, create network-wait-online.service
(no special permissions needed)

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root@fv-rpi3b:/home/pi# nano /lib/systemd/system/network-wait-online.service

Paste in the following:

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#
# Uses 'hostname --all-fqdns' to confirm that both: IP address[es] assigned, and DNS operational
#

[Unit]
Description=Wait for Network to be Online
Documentation=man:systemd.service(5) man:systemd.special(7)
Conflicts=shutdown.target
After=network.target
Before=network-online.target

[Service]
Type=oneshot
ExecStart=/bin/bash -c 'while [ -z $(hostname --all-fqdns) ]; do sleep 1; done'
TimeoutStartSec=1min 30s

[Install]
WantedBy=network-online.target

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Ctrl-O

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Ctrl-X

to write the file and exit

3. Enable network-wait-online.service

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root@fv-rpi3b:/home/pi# systemctl enable network-wait-online.service
Created symlink from /etc/systemd/system/network-online.target.wants/network-wait-online.service to /lib/systemd/system/network-wait-online.service.

4. Reboot

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root@fv-rpi3b:/home/pi# reboot now

Testing

In addition to normal usage testing, the following steps help create long network availability delays for testing purposes:

1. Configure your Pi for Wi-Fi networking
2. Configure the Wi-Fi access point to hide its SSID (“hidden network”)
3. In /etc/wpa_supplicant/wpa_supplicant.conf, disable scanning (disable or remove ‘ap_scan’ and ‘scan_ssid’ entries)
4. Reboot or re-acquire the network

Why I prefer this approach

It provides reliable detection

When configured to Wait for Network, the Jessie systemd init process detects dhcpcd obtaining either static or leased interface configuration/s, before it proceeds to network.target and network-online.target. At that exact point, however, interface address/es are not yet configured, and name resolution comes later still.

When not configured to Wait for Network (or when dhcpcd reaches its timeout while waiting), dhcpcd changes to a background daemon and the init process proceeds immediately. This means network.target and network-online.target are reached even without the availability of interface configuration/s.

Compounding the problem further is the fact that - as standard - network.target and network-online.target do no further checking. network-online.target is therefore often reached before interface configurations have been applied, and in cases even before they have been obtained.

This approach adds the necessary checking to network-online.target, testing for both IP configuration and DNS operation.

It is deterministic

Delay-based approaches may waste boot time or not wait long enough. This approach waits as long as necessary and not any longer to ensure reliable network availability.

It is future-proof?

By integrating into the Jessie systemd init process in a standard way, the configuration for this solution will hopefully survive OS upgrades .

Other approaches

Boot or user login delays:
Fixed delays usually either waste time by waiting too long, or are unreliable because they do not necessarily wait long enough.

fstab noauto, x-systemd.automount options:
This delays mounting until first access. Similarly to above it is not guaranteed to introduce a long enough delay (and in practice usually doesn't).

fstab _netdev or x-systemd.requires=network-online.target options:
These are redundant with a standard Jessie systemd installation -

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pi@fv-rpi3b:~ $ man systemd.special

…
network-online.target
…
All mount units for remote network file systems automatically pull in this unit, and order themselves after it.
…

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pi@fv-rpi3b:~ $ systemctl list-dependencies mnt-media.mount
mnt-media.mount
● ├─-.mount
● ├─system.slice
● └─network-online.target

As mentioned above, reaching network-online.target on a standard configuration does not guarantee anything. Once network-wait-online.service has been enabled:

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mnt-media.mount
● ├─-.mount
● ├─system.slice
● └─network-online.target
●   └─network-wait-online.service

[Glasairman]

Are you sure about this?

I have read that any diversion from the standard systemd settings should be in /etc/systemd/system/xxx.service NOT in /lib/systemd.

/etc/systemd won't be overwritten by the next upgrade and comes after /lib/systemd.. and therefore takes priority .

Of course systemctl enable etc. is also needed

[fvisagie]

You may well be right, I didn't manage to find much authoritative information at the time.

I have read

I'd be interested in your source, if you wouldn't mind sharing it?