USB WiFi dongle ad-hoc criteria

Question

I'm trying to setup an ad-hoc network (intranet-only, no internet) of RPi4s (Buster 2019-09-26) using dongles with detachable antennas. I can successfully

• setup ad-hoc modes on the internal WiFi (both 2GHz and 5GHz) and add babeld on top of it.
• get internet from a WiFi USB dongle (used MrEngman scripts to install drivers).

But I can't get ad-hoc to work on the dongles though - I set things up same as for wlan0 but packets don't go through. Searching around indicates it's probably the dongle that can't do ad-hoc.

My question is: what is the attribute I should look for in a dongle? So far all were using the rtl8822bu, is that the culprit? How can I check ahead of time if a chipset will work? I don't mind scouring the internet looking for the right dongle, but I'm not sure what makes the right one right or why.

Current setup process in case it's not a hardware issue:

wlan0

Setup interfaces

sudo systemctl stop dhcpcd.service
sudo ifconfig wlan0 down
sudo nano /etc/network/interfaces

Make it look like

# interfaces(5) file used by ifup(8) and ifdown(8)

# Please note that this file is written to be used with dhcpcd
# For static IP, consult /etc/dhcpcd.conf and 'man dhcpcd.conf'

# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

auto lo
iface lo inet loopback

iface eth0 inet dhcp

auto wlan0
iface wlan0 inet static
    address xxx.xxx.xx.xx
    netmask 255.255.255.0
    wireless-channel 44
    wireless-essid my_net
    wireless-mode ad-hoc

Then reboot

sudo ifconfig wlan0 up
sudo reboot

Do this for each Pi, changing the static IP, and you have an ad-hoc network!

wlan1

Connect to the internet using wlan0, plug dongle in, then run

sudo wget http://www.fars-robotics.net/install-wifi -O /usr/bin/install-wifi
sudo chmod +x /usr/bin/install-wifi
sudo install-wifi

You can now connect to the internet through your dongle! Now follow the wlan0 steps but set it up for wlan1 (and add iface wlan0 inet dhcp to the interfaces file).

---

EDIT

Output from sudo iw list | grep -A 12 -i "valid interface combinations"

    Supported interface modes
        * IBSS
        * managed
        * AP
        * P2P-client
        * P2P-GO
    Band 1:
        Capabilities: 0x1963
            RX LDPC
            HT20/HT40
            Static SM Power Save
            RX HT20 GI
            RX HT40 SGI
--
    Supported interface modes
        * IBSS
        * managed
        * AP
        * P2P-client
        * P2P-GO
        * P2P-device
    Band 1:
        Capabilities: 0x1022
            HT20/HT40
            Static SM Power Save
            RX HT20 GI
            No RX STBC

Answer 1

Every device that conform to IEEE 802.11 must support a wireless ad hoc network. In the specification it is called independent basic service set (IBSS). The supported interface modes of your WiFi USB dongle show that it also supports it of course.

So IBSS is the keyword you have to look for your ad hoc network.

You are using old style deprecated Debian ifupdown managed in /etc/network/interfaces that somehow have to coexist with default dhcpcd. This may work with standard well known managed WiFi networks but I haven't found a working IBSS configuration with it. I have looked at IBSS with the straight forward systemd-networkd network management. Maybe you may find some ideas at How to setup an unprotected Ad Hoc (IBSS) Network and if possible with WPA encryption?. At section ♦ For developers and for troubleshooting you will also find a manual setup for testing how it works.