Linux iptables Command

The iptables command in Linux is used to configure, maintain, and inspect the tables of IP packet filter rules in the Linux kernel firewall. It’s a powerful tool used for network traffic filtering and Network Address Translation (NAT).

🔰 Basic Syntax

$ iptables [OPTIONS] ACTION CHAIN RULE_SPECIFICATION

• ACTION – -A (append), -I (insert), -D (delete), -L (list), -F (flush)
• CHAIN – INPUT, OUTPUT, FORWARD, PREROUTING, POSTROUTING
• RULE_SPECIFICATION – Matches and targets like source, destination, protocol, ports, etc.

🔧 Commonly Used Chains

Chain	Description
INPUT	Incoming packets to the local system
OUTPUT	Outgoing packets from the local system
FORWARD	Packets being routed through the system
PREROUTING	Used to alter packets as they arrive
POSTROUTING	Used to alter packets before they leave

✅ Common iptables Examples

List All Rules

$ sudo iptables -L -v -n

Allow Incoming SSH (Port 22)

iptables -A INPUT -p tcp --dport 22 -j ACCEPT

Block an IP Address

iptables -A INPUT -s 192.168.1.100 -j DROP

Allow Traffic from a Specific IP

iptables -A INPUT -s 10.0.0.5 -j ACCEPT

NAT Example (Masquerading for Internet Sharing)

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Flush All Rules

iptables -F

🔒 Save iptables Rules

For Debian/Ubuntu:

sudo iptables-save > /etc/iptables/rules.v4

For RHEL/CentOS:

service iptables save

🛑 Important Notes

• Rules are checked top-down until a match is found.
• iptables rules do not persist after reboot unless saved.
• Consider using iptables-save and iptables-restore for persistence.