Firewall rules inside a sandbox - boat

You have root, so you can add any firewall rule. Boat reaches the sandbox in two ways. A rule that blocks either one marks the sandbox degraded.

What	Protocol	Runs as
Boat agent (commands, snapshots, health)	TCP port 8911	`user`
Tunnel for HTTPS URLs and the agent	WireGuard, outgoing UDP to the address in `sudo wg show wg0 endpoints`	kernel, but carries packets from `user` processes

Block outgoing UDP for `user`

Linux tags a tunnel packet with the process that sent the data inside it. So an owner rule on user also blocks the tunnel. To block outgoing UDP for user and keep Boat working, allow the tunnel endpoint first.

WG=$(sudo wg show wg0 endpoints | awk '{print $2}' | cut -d: -f1)
sudo iptables -I OUTPUT 1 -m owner --uid-owner user -p udp ! -d 127.0.0.0/8 -j REJECT
sudo iptables -I OUTPUT 1 -p udp -d "$WG" -j ACCEPT

What happens to a blocked sandbox

Boat never stops a sandbox for this, or marks it error, if it answers on any route. Instead:

The sandbox shows "health": "degraded" with a reason.
Boat sends a sandbox.degraded webhook.