Pi Zero W configuration to run AP + Client Mode on Raspbian Buster lite

Question

I have a project in which I need to configure a Raspberry Pi Zero W to run both wireless AP and Client mode at the same time.

I've seen the tutorials and methods others have used on Raspbian Stretch, but they don't seem to work on Raspbian Buster.

I'm following this tutorial at the moment on Buster.

When I follow the steps in that tutorial I get an error saying that Device "ap0" doesn't exist

sudo ifdown --force ap0
Cannot find device "ap0"
Device "ap0" does not exist.
Cannot find device "ap0"

I'm thinking that it's because the tutorial is trying to create a virtual AP device using udev by modifying /etc/udev/rules.d/70-persistent-net.rules.

However, Raspbian Buster doesn't seem to have that file, so I created it.

But still ap0 isn't created / can't be found. The wireless client mode connection works however. The AP mode isn't.

Questions:

1. Is there a better technique to use to enable AP + client mode on RPi Zero W using Buster?
2. Is there a modification to the tutorial that will make this work on Buster?
3. There must be some networking setup/configuration difference between Buster and Stretch that cause this tutorial to not work, correct?

Answer 1

There are many quirks with creating the virtual interface ap0. You have to follow exactly a sequence, otherwise it won't work. If other applications bind to the wifi port it cannot be set.

1. create a virtual interface ap0 for the access point
2. start access point daemon hostapd using interface ap0
3. start wpa_supplicant for wifi client using interface wlan0

Many tutorials I have seen does not respect this and will fail with error messages like yours, in particular when systemd is used. Then the order of starting services without dependencies isn't defined and may vary from installation to installation. To respect this you should also use systemd-networkd. How to do it you can look at Access point as WiFi router/repeater, optional with bridge. This will also answer all your three questions.