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Linux Port Opening Guide: Port Management with Firewall

The basic way to control network traffic on Linux servers is port management.
In this guide, you will learn to list open ports on your Linux system, open a new port, and manage these settings securely.

What Will You Learn in This Guide?​

  • List open ports in Linux
  • Open ports with ufw, firewalld and iptables
  • Testing whether the opened ports are working or not
  • Making port rules permanent

What is a Port?​

A port is a communication endpoint on the network.
In Linux, each service receives or sends data through a specific port.

Port ranges:

  • 0–1023 → Known ports (SSH 22, HTTP 80, HTTPS 443)
  • 1024–49151 → User ports
  • 49152–65535 → Dynamic ports

In this guide, the 4000/TCP port is used as an example.

Prerequisites​

  • Basic command of terminal usage
  • Root or sudo authority

1. List Open Ports​

You should check existing ports before opening a new port.

View open ports with netstat​

netstat -lntu
  • This command lists TCP and UDP listening sockets.

1. view open ports with ss


ss -lntu
  • This command is faster and more up to date than netstat.

2. Opening a Port in Linux​

Ubuntu/Debian (ufw)​


sudo ufw allow 4000
  • This command allows TCP port 4000.

CentOS/RHEL (firewalld)​


sudo firewall-cmd --add-port=4000/tcp
  • This command opens the port temporarily.

Public Method (iptables)​


sudo iptables -A INPUT -p tcp --dport 4000 -j ACCEPT
  • This command adds a new permission to the system's packet filtering rules.

3. Testing the Opened Port​

  • Even if a port is open, if there is no service listening on that port, a connection cannot be established.

Port listening with netcat


ls | nc -l -p 4000
  • This command starts listening on port 4000.
  • connection test with telnet

telnet localhost 4000
  • This command checks whether the port accepts connections.

4. Making Port Rules Persistent​

  • ufw

  • Rules are persistent by default.

firewalld​


sudo firewall-cmd --permanent --add-port=4000/tcp
sudo firewall-cmd --reload
iptables
  • Rules must be registered with the iptables-persistent package.
VehicleEase of UseDistributionExample Command
ufwVery EasyUbuntu/Debianufw allow 80
firewalldMediumCentOS/RHELfirewall-cmd --add-port=80/tcp
iptablesDifficultAll Linuxiptables -A INPUT -p tcp --dport 80 -j ACCEPT

Frequently Asked Questions (FAQ)​

1. Is it safe to open ports? Unnecessary open ports increase the attack surface. You should only open the ports that are needed.

2. The port is open but the connection cannot be established, why? There is no active service listening to the port.

3. Can I open a port range? Yes.


sudo ufw allow 5000:6000/tcp

4. Is it useful to change the SSH port? Yes. Significantly reduces brute-force attacks.

Result​

Port management in Linux is the basis of system security. A properly configured firewall increases both performance and security.

You can immediately try high-performance and secure Linux servers on the GenixNode infrastructure.