I am a developer who has very limited knowledge of server administration; I learn as I go and mainly deal with web servers.
Is a firewall like CSF needed on something like EC2 given that AWS already provides a basic firewall?
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Research the concept of defense in depth.
Is this needed? Maybe not. But it's sure nice to have two levels of firewall to rely on in case you accidentally mess up your security groups at some point.
As a rule, I always use network-level firewalls and host firewalls on all of my systems, regardless of where they're deployed. I recommend others do the same. There's very little additional overhead in maintenance, performance, etc.