ufw: Simple Firewalling on Debian and Ubuntu
Enable ufw without losing your SSH session, learn the rule grammar, and see exactly which iptables chains it builds underneath.
Netfilter
The packet filtering framework built into the Linux kernel. Articles tagged Netfilter cover iptables, nftables and ufw: tables, chains, hooks, connection tracking, sets and the rules each front end actually writes.
Latest posts
nftables: The Modern Linux Firewall
Your iptables rules already run on nftables. Handles, named sets, inet tables and atomic ruleset loads, all proven with real output from a live lab.
iptables Fundamentals: Tables, Chains, Real Rules
Tables, chains and rule order, demonstrated with real captures from a Debian host in a live lab: DROP against REJECT timing, conntrack, logging and NAT.