iptables

iptables is a user-space utility that allows a system administrator to configure the IP packet filter rules of the Linux kernel firewall, implemented as a set of Netfilter hooks. For many years, it has been the standard tool for firewalling on Linux.

iptables works by inspecting packets as they traverse the network stack and applying a set of rules in a specific order.

Common `iptables` Commands

Here are some of the most frequently used commands for managing iptables rules:

List All Rules (Verbose):
```
sudo iptables -L -v
```
List Rules for a Specific Chain:
```
sudo iptables -L INPUT -v
```
Append a Rule (adds to the end of the chain): This example allows incoming TCP traffic to the web server on port 80.
```
sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT
```
Insert a Rule (adds to a specific position): This inserts a rule at the beginning of the INPUT chain to allow SSH access.
```
sudo iptables -I INPUT 1 -p tcp --dport 22 -j ACCEPT
```

Delete a Rule: You can delete a rule by specifying the same rule definition or by using the line number (which you can see with --line-numbers).

# By rule definition
sudo iptables -D INPUT -p tcp --dport 80 -j ACCEPT

# By line number (e.g., delete rule number 3 in the INPUT chain)
sudo iptables -D INPUT 3

Flush Rules (Clear all rules in a chain):
```
sudo iptables -F INPUT
```
Flush All Chains and Delete Custom Chains:
```
sudo iptables -F
sudo iptables -X
```
Save and Restore Rules: The method for making iptables rules persistent across reboots can vary by distribution. A common method is using the iptables-save and iptables-restore commands.
```
# Save current rules
sudo iptables-save > /etc/iptables/rules.v4

# Restore rules from a file
sudo iptables-restore < /etc/iptables/rules.v4
```

While still widely used, iptables is gradually being succeeded by nftables, which offers a more modern and efficient syntax.

Common iptables Commands​

Common `iptables` Commands