Iptables tutorial: securing VPS with Linux firewall
Iptables is a firewall program for Linux
- What is Iptables?
- How does iptables work?
- How to install and use iptables Linux firewall
Install iptables
Define chain rules
Persist changes
Iptables is a firewall program for Linux. It will monitor traffic from and to your server using tables. These tables contain sets of rules, called chains, that will filter incoming and outgoing data packets.
- Tables are categories of rules, each designed for specific tasks:
- A chain is the name of a rule’s chain. The three built-in chains in iptables are INPUT, OUTPUT and FORWARD
- Rules. Iptables rules are user-defined commands that control incoming and outgoing network traffic on a Linux system
- Targets. A target is a decision of what to do with a packet. The packet is either accepted, dropped, or rejected.
Step 1: Chains and Tables
iptables operates using tables and chains:
Tables
Linux firewall iptables have four default tables that manage different rule chains:
- Filter- The default packet filtering table. used for packet filtering (ex ,
ACCEPT, DROP) - NAT- network address translation, Handles network address translation. It is used for packets that require alterations (e.g. modifying source/destination IPs).
- Mangle- Adjusts the IP header properties of packets. (e.g. modifying
headers). - Raw- Raw is used only for configuring packets so that they are exempt from connection tracking
Chain
In this iptables tutorial, we are going to work with one of the default tables, called filter. It consists of three chains:
- INPUT– controls incoming packets to the server.
- FORWARD– filters incoming packets that will be forwarded somewhere else.
- OUTPUT– filter packets that are going out from your server.
Match criteria:
This match is used to check for certain protocols
- Source/Dsination IP
- Protocol (TCP, UDP, ICMP)
- Port numbers
- Interfaces
Targets:
The target/jumps tells the rule what to do with a packet that is a perfect match with the match section of the rule
- ACCEPT– Allow the traffic through.
- DROP– Block the traffic
- REJECT– Discard the packet and send an error response
Step 2: How to Install and use iptables Linux firewall
iptables is installed by default on most Linux distributions. To confirm that
iptables is installed, run:
