How to write a systemd service unit file so it waits until a specific interface is up before starting?

Question

I am trying to write a unit file for hostapd on Debian.

The hostapd daemon seems to need the interface for the associated Wifi interface to be up before it is run or else it SEGV's.

I've tried network-online.target, but still get the SEGV. I think this is because network-online.target only needs to see ANY interface up, and other interfaces comes up before the Wifi interface is ready enough for hostapd to work.

What is the best way to have a service wait for a specific interface to be up before starting?

Answer 1

Since systemd 219, you may use systemd's systemd-networkd-wait-online tool directly, with an --interface parameter to specify the interface to wait for. For example:

ExecStartPre=/usr/lib/systemd/systemd-networkd-wait-online --interface=$IFACE

However, there is a caveat. This tool needs systemd-networkd to be running, even if it does nothing in your configuration. On the other hand, starting with the same systemd 219, systemd-networkd is socket-activatable and automatically exits on idle. So everything you need is to pull in its socket.

Putting it all together, modifications to unit file will look like this (with $IFACE substituted with your desired interface):

[Unit]
Requires=systemd-networkd.socket
After=systemd-networkd.socket
...

[Service]
ExecStartPre=/usr/lib/systemd/systemd-networkd-wait-online --interface=$IFACE
ExecStart=/usr/bin/hostapd ...
...