Auto-connect to saved network after stopping and starting wpa_supplicant.service

Question

Board: Raspberry Pi 3 B+

OS: Raspian Stretch (with desktop and recommended software) 2018-11-13

I'm building an application that requires me to temporarily disable the wpa_supplicant service, while I update the wifi remotely.

Steps

• I run 'sudo systemctl stop wpa_supplicant.service'
• I append a wifi network to the wpa_supplicant.conf using a python script
• I run 'sudo systemctl start wpa_supplicant.service'
• I then have to run 'sudo systemctl restart dhcpcd.service', to see a list of available wireless connections again.

The problem is, even though I have a valid network in my wpa_supplicant.conf, it doesn't automatically try to connect at this point. I have to restart the Pi, and then it properly connects to my saved network.

I have tried 'sudo ifconfig wlan0 down' and then 'sudo ifconfig wlan0 up', but that doesn't do the trick.

Answer 1

The problem is that default Raspbian uses dhcpcd for networking. It manages wpa_supplicant its own way as a hook and does not respect default service management. So using systemctl for wpa_supplicant has no meaning when using dhcpcd. systemctl status wpa_supplicant.service will mostly show you that it is disabled.

But to manage wpa_supplicant online you can use wpa_cli. Here are some steps to show how it could be integrated into your application. I have configured wpa_supplicant with my default network, but that is just not in range:

rpi ~$ wpa_cli -iwlan0 list_networks
network id / ssid / bssid / flags
0       default_Net     any

rpi ~$  iw dev wlan0 link
Not connected.

Then add the current network that is just available, to /etc/wpa_supplicant/wpa_supplicant.conf. I use bash instead of python:

rpi ~$ sudo bash -c 'cat >> /etc/wpa_supplicant/wpa_supplicant.conf <<EOF
network={
    ssid="current_Net"
    psk="secretPassword"
}
EOF'

Now reconfigure wpa_supplicant:

rpi ~$ wpa_cli -iwlan0 reconfigure
OK

rpi ~$ wpa_cli -iwlan0 list_networks
network id / ssid / bssid / flags
0       default_Net     any
1       current_Net     any     [CURRENT]

rpi ~$  iw dev wlan0 link
Connected to b8:27:eb:86:d0:2d (on wlan0)
        SSID: current_Net
        freq: 2437
        RX: 1746 bytes (12 packets)
        TX: 5134 bytes (35 packets)
        signal: -54 dBm
        tx bitrate: 1.0 MBit/s

        bss flags:      short-slot-time
        dtim period:    2
        beacon int:     100

So for your application you only need two steps:

• append a wifi network to the wpa_supplicant.conf using a python script
• run '/sbin/wpa_cli -iwlan0 reconfigure'

Answer 2

Raspbian Stretch does not normally use wpa_supplicant.service as shown by systemctl status wpa_supplicant.service

● wpa_supplicant.service - WPA supplicant
   Loaded: loaded (/lib/systemd/system/wpa_supplicant.service; disabled; vendor preset: enabled)
   Active: inactive (dead)

That is not to say it doesn't use wpa_supplicant as shown by systemctl status dhcpcd.service

● dhcpcd.service - dhcpcd on all interfaces
   Loaded: loaded (/lib/systemd/system/dhcpcd.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2019-03-04 17:23:49 AEDT; 4 days ago
  Process: 354 ExecStart=/usr/lib/dhcpcd5/dhcpcd -q -b (code=exited, status=0/SUCCESS)
 Main PID: 361 (dhcpcd)
   CGroup: /system.slice/dhcpcd.service
           ├─361 /sbin/dhcpcd -q -b
           └─477 wpa_supplicant -B -c/etc/wpa_supplicant/wpa_supplicant.conf -iwlan0 -Dnl80211,wext

dhcpcd starts wpa_supplicant with /lib/dhcpcd/dhcpcd-hooks/10-wpa_supplicant It is instructive to read the comments in this file:-

# Start, reconfigure and stop wpa_supplicant per wireless interface.
# This is needed because wpa_supplicant lacks hotplugging of any kind

Raspbian Jessie started wpa_supplicant in /etc/network/interfaces (although this was done by wpa-conf and I am not sure wpa_supplicant.service was ever invoked).

Normally if you make any changes to wpa_supplicant.conf you would restart dhcpcd.

NOTE is is possible (and common) to have multiple instances of wpa_supplicant running.

Hopefully this answers some of your questions, but from your Comments you are doing something else, which was not in your Question.