In my earlier commit
manual: Don't suggest exposing VM port to local network.
I made a side change titled
Use `127.0.0.1` also on the VM side, otherwise connections to
services that, in the VM, bind to `127.0.0.1` only
(doing the safe approach) do not work.
Unfortunately, that was wrong:
QEMU inside the VM always communicates via the virtualised
Ethernet interface, not via the VM's loopback interface.
So trying to connect to `127.0.0.1` on the VM's side cannot work.
The setting
QEMU_NET_OPTS="hostfwd=tcp::2222-:22"
caused the VM's port 2222 to be advertised on the host as
`0.0.0.0:2222`, thus anybody in the local network of the host
could SSH into the VM.
Instead, port-forward to localhost only.
Use `127.0.0.1` also on the VM side, otherwise connections to
services that, in the VM, bind to `127.0.0.1` only
(doing the safe approach) do not work.
See e.g. https://github.com/NixOS/nixpkgs/issues/100192
for more info why localhost listening is the best default.
