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The 15 best practices for securing Linux with Iptables

In this article, we'll provide you with the 15 best practices for managing iptables rules effectively and efficiently, helping you to secure your Linux-based systems and prevent unwanted network traffic.

by Abraham
best practices for securing linux with iptables

iptables is a robust network traffic management application for Linux computers. It regulates incoming and outgoing network traffic and defines rules and policies to safeguard your system from harmful behavior. This post will review the top fifteen recommended practices for utilizing iptables to safeguard your Linux system. We will go through issues including building a default policy, implementing rules for specific services, and monitoring traffic via logging. Following these recommended practices will keep your system secure and safe from harmful activities.

Anyone who has used iptables has, at some point, locked themselves out of a remote server. It’s simple to prevent, yet it’s commonly overlooked. I hope this article will help you overcome this rampant obstacle.

Best iptables Practices

Below is a list of the best practices for iptables firewalls. Follow it to the latter to avoid falling into avoidable circumstances in the future.

1. Keep the rules short and straightforward.

iptables is a strong tool, and it is easy to become overburdened with complicated rules. The more complex your rules, however, the more difficult they will be to debug if something goes wrong.

It’s also critical to maintain your iptables rules well-organized. This entails putting relevant rules together and properly naming them so you know what each rule accomplishes. Also, comment on any rules you aren’t presently using since this will help decrease clutter and simplify identifying the rules you want when you need them.

2. Keep track of lost packets.

Dropped packets can be used to monitor your system for harmful behavior. It also assists you in identifying any potential security weaknesses in your network.

iptables makes logging lost packets simple. Simply include the “-j LOG” option in your rule configuration. This will record all dropped packets, their source/destination addresses, and other pertinent information such as protocol type and packet size.

You can rapidly detect suspicious behavior on your network and take relevant action by tracking lost packets. It’s also a good idea to read over these logs regularly to confirm that your firewall rules are running correctly.

3. By default, block everything.

iptables will permit all traffic to flow through by default. As a result, any malicious traffic can simply enter your system and inflict damage.

To avoid this, you should set iptables to block all outgoing and inbound traffic by default. Then, you may write rules that allow just the traffic required by your apps or services. In this manner, you can ensure that no unsolicited traffic enters or exits your system.

4. Update your system regularly.

iptables is a firewall that guards your system against harmful assaults. iptables must be updated when new threats develop to guarantee that they can be detected and blocked.

To keep your system secure, check for updates regularly and apply any applicable patches or security fixes. This will assist in guaranteeing that your system is up to date with the most recent security measures and can efficiently defend against any attacks.

5. Check that your firewall is operational.

A firewall is a crucial part of network security, and it’s critical to ensure that all the rules you’ve put up are enforced.

To do this, you should examine your iptables logs regularly for any unusual behavior or banned connections. You may also use programs such as Nmap to scan your network from the outside to discover whether your firewall is blocking any ports or services. Additionally, it would be best to examine your iptables rules regularly to verify they are still valid and relevant.

6. Separate chains should be used for various sorts of traffic.

You can manage and regulate traffic flow by using distinct chains. For example, if you have an incoming traffic chain, you may create rules that allow or reject specific types of data from reaching your network.

You may also utilize distinct chains for incoming and outgoing traffic, letting you select which services have access to the internet. This is especially significant for security since it allows you to intercept harmful traffic before it reaches its target. You may also design more detailed rules that are easier to manage and debug by utilizing distinct chains.

7. Before making any modifications, put them to the test.

iptables is a useful tool for configuring your firewall but is also prone to errors. If you make changes without testing them, you risk locking yourself out of the server or triggering security vulnerabilities.

Always validate your iptables rules before applying them to avoid this. You may test the consequences of your modifications using tools like iptables-apply to ensure they perform as intended. In this manner, you can ensure that your adjustments will not result in unanticipated problems.

8. Permit only what you require.

Enabling only the required traffic reduces your attack surface and the likelihood of a successful assault.

If you don’t need to accept inbound SSH connections from outside your system, for example, don’t open that port. Close that port if you don’t need to allow outgoing SMTP connections. You may dramatically lower the danger of an attacker obtaining access to your system by restricting what is permitted in and out of your network.

9. Create a copy of your configuration files.

iptables is a powerful tool, and making mistakes when defining your firewall rules is simple. If you don’t have a copy of your configuration files, any changes you make might lock you out of your system or expose it to attack.

Back up your iptables configuration files regularly, especially after making significant changes. If something goes wrong, you may rapidly restore the older versions of your configuration file and be up and running again in no time.

10. Don’t overlook IPv6.

IPv6 is the next version of IP addressing and is gaining popularity. As a result, you must ensure that your firewall rules are current and incorporate IPv6 traffic.

iptables may be used to control both IPv4 and IPv6 traffic. However, the two protocols have certain peculiarities. Because IPv6 has a bigger address space than IPv4, you’ll need more detailed rules to filter IPv6 traffic. Furthermore, IPv6 packets have unique header fields than IPv4 packets. Therefore your rules must be adjusted correspondingly. Finally, IPv6 permits multicast traffic, which necessitates the implementation of extra rules to guarantee that only permitted traffic is let through.

11. Do not flush iptables rules haphazardly.

Always verify the default policy of each chain before running iptables -F. If the INPUT chain is configured to DROP, you must change it to ACCEPT if you want to connect to the server after the rules have been flushed. When you clarify the rules, keep the security ramifications of your network in mind. Any disguising or NAT rules will be eliminated, and your services will be entirely exposed.

12. Separate complex rule groups into separate chains.

Even if you’re the sole systems administrator on your network, it’s critical to maintain your iptables rules under control. If you have a very intricate set of rules, try separating them into their own chain. Simply add a jump to that chain from your normal set of chains.

13. Use REJECT until you are certain that your rules are operating properly.

When developing iptables rules, you’ll most likely be testing them frequently. Using the REJECT target instead of the DROP target can help speed up that procedure. Instead of worrying if your packet is being lost or if it ever makes it to your server, you’ll get an instant denial (a TCP reset). When you’re through testing, you may change the rules from REJECT to DROP.

This is a big aid throughout the test for individuals working towards their RHCE. When you’re worried and in a hurry, the instant package rejection is a relief.

14. Don’t make DROP the default policy.

A default policy is set for all iptables chains. If a packet does not fit any rules in a pertinent chain, it will be processed according to the default policy. Several users set their primary policy to DROP, which might have unforeseen repercussions.

Consider the following scenario: your INPUT chain has several rules that accept traffic, and you’ve configured the default policy to DROP. Later, another administrator gets into the server and flushes the rules (which is also not recommended). I’ve encountered several competent system administrators ignorant of the default policy for iptables chains. Your server will become inoperable instantly. Because they fit the chain’s default policy, all of the packets will be discarded.

Rather than utilizing the default policy, I usually recommend adding an explicit DROP/REJECT rule at the end of your chain that matches everything. You may keep your default policy to ACCEPT, reducing the likelihood of banning all server access.

15. Always save your rules

Most distributions allow you to store your iptables rules and have them persist between reboots. This is a good practice as it will help you retain your rules after configuration. Besides, it saves you the stress of re-writing the rules. Therefore, always ensure you save your rules after making any modifications on the server.


iptables is a command-line interface for configuring and maintaining tables for the Linux kernel’s Netfilter firewall for IPv4. The firewall compares packets to the rules described in these tables and executes the desired action if a match is found. A set of chains is referred to as tables. The iptables program provides a comprehensive interface to your local Linux firewall. Through a simple syntax, it gives millions of network traffic control choices. This article has provided the best practices you must follow when using iptables. I hope you found it helpful. If yes, let me know via the comments section below.

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