Firewall Issues: Diagnostics & Troubleshooting

How to just let the desired traffic pass

A common problem related to network security is a misconfigured firewall, which might either block necessary traffic or, on the contrary, let through traffic that should be blocked. This problem, if not addressed, can cause disruptions in services or make your server vulnerable to attacks.

Problem Explanation

A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It establishes a barrier between a trusted internal network and untrusted external network, like the Internet.

Misconfiguration of the firewall rules can lead to various issues like denial of services, inability to connect to the server, slow connections, or even security breaches. For instance, if the iptables firewall rules are too restrictive, they can block important traffic which can lead to service disruptions. On the other hand, if the rules are too loose, they can allow malicious traffic which can lead to security vulnerabilities.

Common Causes

The two most frequent causes of firewall issues on a Linux server are:

  1. Incorrect rules: This could be due to a syntax error in the rule definition or a misunderstanding of the rule functionality.
  2. Inappropriate rule order: iptables processes rules in a top-down manner, so an incorrect order can result in some rules being ignored.


Diagnosing a misconfigured firewall involves checking the current firewall rules and the server's network activity. The iptables -L -n -v command will show you the current rules in place, whereas the netstat -tuln command can help you understand the server's network activity.

iptables -L -n -v
netstat -tuln


If you have identified that a firewall issue is causing disruptions, follow these steps to troubleshoot it:

  1. Backup current rules: Before making any changes, it's a good practice to backup existing rules using the command iptables-save > iptables_backup.

    iptables-save > iptables_backup

  2. Analyze the rules: Check the list of rules and look for any that seem out of place or overly restrictive/broad.

  3. Modify the rules: Using the iptables command, modify the rules as necessary. Remember, changes are not persistent across reboots unless you make them so.

  4. Test: After modifying rules, test your services to ensure they're working as expected.

Relevant Applications

The most common application that can cause firewall issues is iptables, but others like ufw or firewalld can also be involved depending on your server setup.


Firewall issues can cause big headaches, but they can be diagnosed and fixed with a bit of patience and understanding of how firewall rules work. Always remember to backup your current rules before making any changes, and always test after making modifications to ensure everything is working as expected.

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