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Day 18 of #BashSecurity: iptables firewall basics

The safety net trick that saved me from lockouts:

```bash
sudo at now + 5 minutes <<< 'iptables -F; iptables -P INPUT ACCEPT'
# apply restrictive rules...
# if SSH still works, cancel the revert
sudo atrm $(atq | awk '{print $1}' | tail -1)
```

Schedules an automatic rule flush if you accidentally block yourself. Console access at 2am is not fun.

Also: always use -n flag with iptables -L.…