The Bible study laptop had working internet and a panel that said there
was no connection. Both were true, and one thing caused both.
Measured on the machine:
nmcli device status wlp2s0 wifi unmanaged
nmcli networking connectivity none while curl fetched fine
nmcli device wifi list nothing, with the radio enabled
/run/network/ifstate wlp2s0=wlp2s0
a second wpa_supplicant, -i wlp2s0, with no -c config file
dhcpcd holding the lease
/etc/network/interfaces 0600 root:root
Debian's installer had been given the Wi-Fi, so it wrote it into
/etc/network/interfaces and ifupdown brings it up. NetworkManager ships
with plugins=ifupdown and [ifupdown] managed=false, so an interface named
in that file is UNMANAGED — which is why the applet had nothing to report
and why scanning found nothing. An unmanaged device is not a radio
NetworkManager will scan with. rfkill was ruled out separately: the radio
is enabled.
managed=false stays; it is what keeps a tower's static Ethernet where an
administrator put it. Only a wireless interface is moved.
classroom-wifi grows --check, --adopt (with --dry-run) and --rollback.
--adopt builds a NetworkManager SYSTEM connection from the SSID and key
already in that root-only file, so the password never leaves the machine
and never reaches a human or a log; it lands in a 0600 root file with
psk-flags=0, the same shape 0.2.4 established, and the polkit rules
protecting it are untouched.
Deliberately NOT run by the upgrade. It moves a machine's only link
between managers and the machine it could strand is a laptop in a locked
room, so classroom-migrate only looks and prints what it found. An
administrator runs it over SSH while watching.
Two things this learned from testing the rollback rather than from
reading the code, and both are why the rollback is worth testing:
* ifdown was missing in the lab, subprocess raised FileNotFoundError,
and the exception escaped AFTER the stanza was commented out. No
rollback ran. Every hand-over command now goes through a runner that
tolerates a missing binary, and the whole hand-over is inside a guard
that restores the file on any failure at all.
* it now refuses to start when ifdown or ifup are absent, because
without them the hand-over would be a one-way door.
Success also requires the interface itself to be carrying the connection,
not merely that the machine is online — a machine with a second link
would otherwise report a failed migration as a success.
Verified in the lab against real NetworkManager with a real ifupdown
file: --check and --dry-run leave it byte-identical, a failed --adopt
rolls back byte-identically, and --rollback restores a half-migrated
machine byte-identically and exits 0. The one thing no lab can do is the
hand-over succeeding, which needs the laptop's radio.
Co-Authored-By: Claude Opus 5 <[email protected]>
