Iptables Command Line Tool

Internal

• iptables

Overview

iptables is a Linux userspace command line tool that manipulates the IPv4 filtering rules. The IPv6 equivalent is ip6tables. The tools can be used to list the rules loaded in memory and add and remove rules. All the changes only reflect in memory and won't survive a reboot, unless the associated service's configuration files are correspondingly updated.

iptables always targets the "filter" table by default. In order to change the target table, use the -t <table-name> options.

Commands

List Rules

iptables -L -nv --line-numbers

The command list all rules in the selected chain. If no chain is selected, all chains are listed.

The command applies to the specified table ("filter" is the default). If you need to list rules from a table other than "filter", use -t (example -t nat).

The command is often used with the -n option, in order to avoid long reverse DNS lookups.

Add a Rule to the Default Table

iptables -A <chain> -i <interface> -p <protocol> -s <source>  [-m <module> <module-config>] -j <target>

The rule will only be added in memory

The rule will only be added in memory and won't survive a reboot. In order to make the rule permanent TODO.

The rule will be added at the bottom of the chain

By default, the rule will be added at the bottom of the chain, after the last existent rule. This means that all existent rules will be evaluated before a packet reaches the newly added rule, so if there's a previous rule that discards the packet, the newly added rule might never be exercised. For that, you may want to consider adding the rule at a specific position in the chain.

The rule will be added to the "filter" table

The rule will be added to the "filter" table. If you need to add the rule to a different table, use -t.

Chain

The target chain is specified with -A. It can be one of INPUT, OUTPUT, FORWARD, PREROUTING and POSTROUTING.

Interface

The network interface to apply the rule to is specified with -i. It must be one of your existing network interfaces (example "eth0"). If -i is not specified, the rule applies to all interfaces.

Protocol

The protocol to apply the rule to is specified with -p. It must be one of the known protocols, such as "tcp" or "udp". If -p is not specified, the rule applies to all protocols.

Source

TODO

Module Configuration

-m stands for module and it is used to specify module-specific configuration.

Module examples:

• "state" [iptables state Module]| reads the connection state
• "mac" - reads the MAC address
• "tcp" . "Interesting" modules:

TODO

Target

-j stands for jump and it is used to specify the rule's target: ACCEPT, REJECT, DROP, etc.

Example of allowing external HTTP access on a specific interface:

iptables -A INPUT -i enp0s25 -p tcp -j ACCEPT