DDoS attacks come in three broad families: volumetric floods that saturate your pipe, protocol attacks that exhaust connection state, and application-layer floods that look like legitimate traffic. Each one exhausts a different resource, so each one needs a different defence, and no single layer stops all three. This guide maps the common attack forms to the resource they target and the mitigation that actually works against them, explains what an on-host firewall can and cannot stop, and ends with a practical incident-response runbook for when your server is under fire.

A fresh Linux server ships with no firewall rules at all. Every listener you start is instantly reachable from the whole internet. The fix is a short, well-understood baseline: default-deny inbound, allow established traffic, and an explicit allow-list for the services you actually run. This guide builds that baseline in nftables rule by rule, explains the parts everyone gets wrong (ICMP and IPv6), shows the same policy in ufw and firewalld, and covers how not to lock yourself out, plus the Docker port-publishing trap.

If your server runs a current Debian, Ubuntu, or RHEL-family distro, your "iptables" rules are almost certainly already executing inside nftables. The iptables command has been a compatibility shim since 2019. The real question isn't which one wins; it's whether to keep writing rules in a legacy syntax on top of the new engine. This guide covers what actually changed architecturally, a side-by-side syntax comparison, what the honest performance data says, and a migration path that won't break Docker or lock you out.