A Step-by-Step Guide to Configuring Linux Firewall (iptables)

This article introduces the configuration of Linux firewall (iptables) with the core objective of protecting server security. iptables is a packet filtering tool that manages traffic through tables (primarily filter), chains (INPUT/OUTPUT/FORWARD), and rules (match conditions + actions). Before configuration, existing rules should be checked (`iptables -L -n`) and cleared (`-F`/`-X`). Key steps include: allowing traffic on the local loopback interface (lo), setting default policies (DROP for inbound, ACCEPT for outbound), opening necessary ports (e.g., SSH, 80/443 for web services), and finally saving the rules (using `service iptables save` for CentOS and installing `iptables-persistent` for Ubuntu). Security considerations: prioritize rule order, apply the principle of least privilege, avoid directly exposing port 22 to the public internet, and regularly audit rules. Common operations include viewing, deleting, and clearing rules. By following these steps, a basic firewall can be quickly configured to meet the security needs of most servers.