Marcus made some suggestions, via email, as to how to trace the cause of these problems. One of the suggestions was to test with qemu. I've just tried, but can't get past an error. I did this:
sudo ip link add br0 type bridge
sudo ip addr add 10.0.2.1/16
sudo ip link set br0 up
sudo ip link set br0 promisc on
<restart vblade configured to use br0>
qemu-system-x86_64 -m 1024 -boot order=n -netdev bridge,id=net0,br=br0
-device virtio-net-pci,netdev=net0 -kernel /tftpboot/boot/linux -initrd
/tftpboot/boot/initrd -append "rd.kiwi.live.pxe
The error I see is "-netdev bridge,id=net0,br=br0: bridge helper failed"
I tried adding "allow br0" to /etc/qemu/bridge.conf but that didn't help.
The commands I'm using could be completely wrong. I totally failed to find any instructions that I could understand. These command were established in a long back and forth conversation with ChatGPT.