Scanning for SSID's at Runtime

Hi all - I’m new here so thanks in advance for the help!

I have been trying to make a container on a Raspberry Pi that can set-up it’s own WiFi at runtime.

I came across this example which has pointed me down the correct road for configuring the Host NetworkManager via dbus.

My question though - does anyone know if it is possible to do a WiFi Scan for the Access Points it can see from the container? I normally use iwlist to do this on a normal debian instance but obviously I can’t do this from the container.

I noticed NetworkManager’s libnm has a function nm_device_wifi_request_scan but I don’t see this exposed via dbus.

Anyone any tips?

You may call the RequestScan method on the org.freedesktop.NetworkManager.Device.Wireless interface. Please note that this will return immediately - before the scanning results are available.

You may subscribe to the completion of the scan request, but I do not remember the exact details. Checking the nmcli could reveal how it is done there. I think it was the last-scan property which was introduced in NM 1.12.

Alternatively you may do some heuristic blocking solution that could be easier to implement.

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Thanks for the info @majorz - I will try that this week!

Thanks, please let me know how that went for you when you have a chance to try it out.

Hey, I got this to work using the method you outlined.

For now I am just blocking with a sleep while we scan but I imagine that I will try and use the signals later if this gets in the way.

I was also unsure of how to interpret the parameter list for RequestScan:

IN a{sv} options: Options of scan. Currently 'ssids' option with value of "aay" type is supported.

This may be a scan filter? Unsure, but sending an empty dict seems to find everything.

import time
import dbus

bus = dbus.SystemBus()

# Get a proxy for the base NetworkManager object
proxy = bus.get_object("org.freedesktop.NetworkManager", "/org/freedesktop/NetworkManager")
manager = dbus.Interface(proxy, "org.freedesktop.NetworkManager")

all_aps = []

# Get all network devices
devices = manager.GetDevices()
for d in devices:
    dev_proxy = bus.get_object("org.freedesktop.NetworkManager", d)
    prop_iface = dbus.Interface(dev_proxy, "org.freedesktop.DBus.Properties")

    # Make sure the device is enabled before we try to use it
    state = prop_iface.Get("org.freedesktop.NetworkManager.Device", "State")
    if state <= 2:
        continue

    # Get device's type; we only want wifi devices
    iface = prop_iface.Get("org.freedesktop.NetworkManager.Device", "Interface")
    dtype = prop_iface.Get("org.freedesktop.NetworkManager.Device", "DeviceType")
    if dtype == 2:   # WiFi
        # Get a proxy for the wifi interface
        wifi_iface = dbus.Interface(dev_proxy, "org.freedesktop.NetworkManager.Device.Wireless")
        wifi_prop_iface = dbus.Interface(dev_proxy, "org.freedesktop.DBus.Properties")

        opt = dbus.Dictionary({
        })

        # Get all APs the card can see
        wifi_iface.RequestScan(options=opt)
        
        time.sleep(1)
        
        aps = wifi_iface.GetAllAccessPoints()
        for path in aps:
            ap_proxy = bus.get_object("org.freedesktop.NetworkManager", path)
            ap_prop_iface = dbus.Interface(ap_proxy, "org.freedesktop.DBus.Properties")
            ssid = bytearray(ap_prop_iface.Get("org.freedesktop.NetworkManager.AccessPoint", "Ssid")).decode()

            # Cache the BSSID
            if not ssid in all_aps:
                all_aps.append(ssid)

# and print out all APs the wifi devices can see
print("\nFound APs:")
for bssid in all_aps:
    print(bssid)

Empty dict should do the job for the scan options. This is a scan filter indeed for scanning for particular networks.

I think you may increase the sleep value a bit, since one second will not be enough for the scan operation to complete. It usually takes a couple of seconds from what I remember. I think 3 or even 5 will be a safe bet.

You may put some protection at your SSID decode call, since if by any chance there is a nearby network, which SSID cannot be decoded, the method call will throw an exception.

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Thanks for the help!