The drill takes the household's internet down, which means anything driving it
step-by-step stops being able to act at exactly the moment it matters -- an
agent needs the internet to think, so it would freeze mid-failover with the
levers half-thrown. This script needs only the LAN, and its cleanup runs from
an EXIT trap so both force-fault levers are cleared on any exit path, including
being killed.
Belt and braces around it: the watchdog on vyos002 stands the box down by
itself after 150s holding the VIP with no WAN, so even if the script dies the
internet comes back. Refuses to start if the internet is already down or if
vyos001 is not the current holder.
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