Third task: Calculate the size of a directory and its contents

The real miss here is in trying to do it without googling but not bothering to search manpages. They're there for a reason. If you're unsure of what to look for, "man -k <keyword>" to search.

For filtering networks, netfilter has a "limit" module that can be used to, for example, automatically limit all remote IPs to a set number of connections, rather than trying to piecemeal block bad actors. You can also do things like rate limit new connections, etc. Because it works off the chain-based approach, any known-good systems can be separately allowed through first (or allowed unlimited connections) before the limiting rule is applied.