how to make wpa_supplicant exit when the desired SSID is not found?

Question

I have a device (e.g. raspberry) running a minimal version of linux built with YOCTO and with not network manager available. At a given time a network configuration may be present in the file /home/root/wpa.conf, but it is not guaranteed that the corresponding wifi network can be detected when the configuration is present (the device can move). I want to write a service that constantly tries to connect to the wifi and, retries when it fails. The problem is that wpa_supplicant does not exit when the SSID is not found.

This is my service:

[Unit]
After=network.target

[Service]
ExecStart=/home/root/connect_to_wifi.sh
Restart=always # this would be useful if wpa_supplicant exited on fail
StandardOutput=file:/home/root/connect_to_wifi.log
StandardError=file:/home/root/connect_to_wifi_error.log

[Install]
WantedBy=default.target

This is the script:

#!/bin/bash
ifconfig wlan0 up
udhcpc -i wlan0 -b
wpa_supplicant -i wlan0  -c /home/root/wpa.conf

I omitted the -B in wpa_supplicant because the service would exit after running the command, killing wpa_supplicant. I tried to set type=forking in the service but it didn't work. However, I don't think this is the problem in this case.

How can i modify my service and script in order to continuously attempt to connect?

Answer 1

wpa supplicant is not supposed to exit; you got a misconception how an SSID "is found": you need to wait for a beacon to appear if you want to look for SSIDs. If you know the SSID and BSSID of the AP, you can just try to send a registration packet to the AP, but that will have to be done in multiple attempts, too, because a single failure to respond might be totally normal – this is a shared medium after all, and you don't know the RF environment of the access point.

So, what you want to implement is actually the default observable behaviour for practically all managers of network connections I'm aware of, that only being systemd's NetworkManager and wicd, the Android connection manager, the windows connection manager and the thing that iOs has.

So, really, I don't think you're actually solving anything that's not already solved; "connect to this network when available" is what everyone already offers.

So, the right solution would be to configure the network configuration manager (on the raspberry pi, chances are that's the NetworkManager) to look for that network - and that's it.