Introduction
IPTables is a user-space utility in Linux used to configure and manage the Netfilter firewall framework. It helps administrators control incoming and outgoing network traffic based on defined security rules. Checking opened ports in IPTables is an essential task for system administrators to verify firewall configurations, troubleshoot connectivity issues, and ensure that only required services are accessible from external networks.
This guide explains how to check opened ports configured through IPTables on a Linux server
Prerequisites
Before proceeding, ensure the following requirements are met:
- A Linux server with IPTables installed and configured
- Root or sudo access to the server
- Basic knowledge of Linux command-line operations
- SSH access to the server terminal
You can verify whether IPTables is installed by running:
iptables --versionIMPLEMENTATION
netstat -plan |grep LISTEN
Steps to open a new port in IP tables
Before opening a port, you should be aware of why are you opening the port and what is the port being used for.
Save the existing IP tables.
root@server [~]# /etc/init.d/iptables save
Saving firewall rules to /etc/sysconfig/iptables: [ OK ]
To open a port, execute the commands below.
iptables -A allowed -p tcp –dport portno -s IPaddress -j ACCEPT
iptables -A allowed -p udp –dport portno -s IPaddress -j ACCEPT
To allow all IP address,
iptables -A INPUT -i eth0 -p tcp -m tcp –dport portno -j ACCEPT
Check whether the Port is allowed.
root@server [~]# iptables -nL | grep :3306
ACCEPT tcp — 0.0.0.0/0 0.0.0.0/0 tcp dpt:3306
ACCEPT tcp — 0.0.0.0/0 0.0.0.0/0 tcp dpt:3306
CONCLUSION
Checking opened ports in IPTables is an important step in maintaining Linux server security and troubleshooting network access issues. By reviewing INPUT chain rules, filtering ACCEPT entries, and verifying active listening services, administrators can ensure that only necessary ports are exposed to external traffic. Regular firewall audits help reduce security risks and maintain proper server access control.