Difference between systemd wpa_supplicant.service and wpa_supplicant@wlan0.service?

Question

Using Arch Linux on a Raspberry Pi and a USB Wi-Fi dongle, I can get Wi-Fi working by enabling the wpa_supplicant@wlan0 systemd service. This uses the /etc/wpa_supplicant/wpa_supplicant@wlan0.conf file and specifies the wlan0 interface.

I understand the wpa_supplicant systemd service does the same thing, except it autodetects the wireless interface, but starting this service instead doesn't work, even when I have a valid /etc/wpa_supplicant/wpa_supplicant.conf file.

How would I use the wpa_supplicant systemd service to autodetect the wireless interface and setup my Wi-Fi?

If autodetection is not what the wpa_supplicant service does, what is the point of the wpa_supplicant service vs the wpa_supplicant@interface service?

Answer 1

As stated on here, the wpa_supplicant.service file is for DBus aware services, so it will be autodetected by NetworkManager for example. As I imagine you are not using NetworkManager, either you have to write a program/script that will use DBus information to lift the interface or use the specific interface version wpa_supplicant@.service. Quoting:

The wpa_supplicant package provides multiple systemd service files:

wpa_supplicant.service - uses D-Bus, recommended for NetworkManager users.

wpa_supplicant@.service - accepts the interface name as an argument and starts the wpa_supplicant daemon for this interface. It reads a /etc/wpa_supplicant/wpa_supplicant-interface.conf configuration file.

wpa_supplicant-nl80211@.service - also interface specific, but explicitly forces the nl80211 driver (see below). The configuration file path is /etc/wpa_supplicant/wpa_supplicant-nl80211-interface.conf.

wpa_supplicant-wired@.service - also interface specific, uses the wired driver. The configuration file path is /etc/wpa_supplicant/wpa_supplicant-wired-interface.conf.

Answer 2

The wpa_supplicant@.service installed with the wpa_supplicant package from pacman at /usr/lib/systemd/system/wpa_supplicant@.service is a dynamic service file which allows a service such as wpa_supplicant@wlan0 to be managed. It will look for a wpa_supplicant configuration file at /etc/wpa_supplicant/wpa_supplicant-wlan0.conf, where the wlan0 in the filename corresponds to the wlan0 in the service name.

If your nic uses the nl80211 driver, you may use the wpa_supplicant-nl80211@.service in the same way. The configuration file it looks for is /etc/wpa_supplicant/wpa_supplicant-nl80211-<interface>.conf