Linux Open Port: Step-by-Step Guide to Managing Firewall Ports

Introduction

A port is a communication endpoint. Within an operating system, a port is opened or closed to data packets for specific processes or network services.

Typically, ports identify a specific network service assigned to them. This can be changed by manually configuring the service to use a different port, but in general, the defaults can be used.

The first 1024 ports (port numbers 0 to 1023) are referred to as well-known port numbers and are reserved for the most commonly used services. These include SSH (port 22), HTTP (port 80), HTTPS (port 443).

Port numbers above 1024 are referred to as ephemeral ports.

• Port numbers 1024 to 49151 are called the registered/user ports.
• Port numbers 49152 to 65535 are called the dynamic/private ports.

In this tutorial, you will open an ephemeral port on Linux, since the most common services use the well-known ports.

Prerequisites

To complete this tutorial, you will need:

• Familiarity with using the terminal.

List All Open Ports

Before opening a port on Linux, you must check the list of all open ports, and choose an ephemeral port to open that is not on that list.

Use the netstat command to list all open ports, including TCP and UDP, which are the most common protocols for packet transmission in the network layer.

netstat -lntu

This will print:

• all listening sockets (-l)
• the port number (-n)
• TCP ports (-t)
• UDP ports (-u)

Output
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address    Foreign Address  State
tcp        0      0 127.0.0.1:5432   0.0.0.0:*        LISTEN
tcp        0      0 127.0.0.1:27017  0.0.0.0:*        LISTEN
tcp        0      0 127.0.0.1:6379   0.0.0.0:*        LISTEN
tcp        0      0 127.0.0.53:53    0.0.0.0:*        LISTEN
tcp        0      0 0.0.0.0:22       0.0.0.0:*        LISTEN
tcp6       0      0 ::1:5432         :::*             LISTEN
tcp6       0      0 ::1:6379         :::*             LISTEN
tcp6       0      0 :::22            :::*             LISTEN
udp        0      0 127.0.0.53:53    0.0.0.0:*        LISTEN

Verify that you are receiving consistent outputs using the ss command to list listening sockets with an open port:

ss -lntu

This will print:

Output
Netid  State   Recv-Q  Send-Q    Local Address:Port   Peer Address:Port
udp    UNCONN  0       0         127.0.0.53%lo:53          0.0.0.0:*
tcp    LISTEN  0       128           127.0.0.1:5432        0.0.0.0:*
tcp    LISTEN  0       128           127.0.0.1:27017       0.0.0.0:*
tcp    LISTEN  0       128           127.0.0.1:6379        0.0.0.0:*
tcp    LISTEN  0       128       127.0.0.53%lo:53          0.0.0.0:*
tcp    LISTEN  0       128             0.0.0.0:22          0.0.0.0:*
tcp    LISTEN  0       128               [::1]:5432        0.0.0.0:*
tcp    LISTEN  0       128               [::1]:6379        0.0.0.0:*
tcp    LISTEN  0       128                [::]:22          0.0.0.0:*

This gives more or less the same open ports as netstat.

Opening a Port on Linux to Allow TCP Connections

Now, open a closed port and make it listen for TCP connections.

For the purposes of this tutorial, you will be opening port 4000. However, if that port is not open in your system, feel free to choose another closed port. Just make sure that it’s greater than 1023.

Ensure that port 4000 is not used using the netstat command:

netstat -na | grep :4000

Or the ss command:

ss -na | grep :4000

The output must remain blank, thus verifying that it is not currently used, so that you can add the port rules manually to the system iptables firewall.

For Ubuntu Users and ufw-based Systems

Use ufw - the command line client for the Uncomplicated Firewall.

Your commands will resemble:

sudo ufw allow 4000

Refer to How to Setup a ufw Firewall on Ubuntu for more details.

For CentOS and firewalld-based Systems

Use firewall-cmd - the command line client for the firewalld daemon.

Your commands will resemble:

firewall-cmd --add-port=4000/tcp

Refer to How to Set Up firewalld on CentOS for more details.

For Other Linux Distributions

Use iptables to change the system IPv4 packet filter rules.

iptables -A INPUT -p tcp --dport 4000 -j ACCEPT

Refer to How To Set Up A Firewall Using iptables for more details.

Test the Newly Opened Port for TCP Connections

Now that you have successfully opened a new TCP port, it is time to test it.

First, start netcat (nc) and listen (-l) on port (-p) 4000, while sending the output of ls to any connected client:

ls | nc -l -p 4000

Now, after a client has opened a TCP connection on port 4000, they will receive the output of ls. Leave this session alone for now.

Open another terminal session on the same machine.

Since you opened a TCP port, use telnet to check for TCP Connectivity. If the command doesn’t exist, install it using your package manager.

Input your server IP and the port number (4000 in this example) and run this command:

telnet localhost 4000

This command tries to open a TCP connection on localhost on port 4000.

You’ll get an output similar to this, indicating that a connection has been established with the listening program (nc):

Output
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
while.sh

The output of ls (while.sh, in this example) has also been sent to the client, indicating a successful TCP Connection.

Use nmap to check if the port (-p) is open:

nmap localhost -p 4000

This command will check the open port:

Output
Starting Nmap 7.60 ( https://nmap.org ) at 2020-01-18 21:51 UTC
Nmap scan report for localhost (127.0.0.1)
Host is up (0.00010s latency).
Other addresses for localhost (not scanned): ::1

PORT     STATE SERVICE
4000/tcp open  remoteanything

Nmap done: 1 IP address (1 host up) scanned in 0.25 seconds

The port has been opened. You have successfully opened a new port on your Linux system.

But this is only temporary, as the changes will be reset every time you reboot the system.

Persisting Rules

The approach presented in this article will only temporarily update the firewall rules until the system shuts down or reboots. So similar steps must be repeated to open the same port again after a restart.

For ufw Firewall

ufw rules do not reset on reboot. This is because it is integrated into the boot process, and the kernel saves the firewall rules using ufw by applying appropriate config files.

For firewalld

If you want to add the port to the firewall’s persistent configuration and apply the changes immediately, you can use the --permanent and --reload flags:

sudo firewall-cmd --permanent --add-port=4000/tcp 
sudo firewall-cmd --reload

Refer to How to Set Up firewalld for more details.

For iptables

You will need to save the configuration rules and use the iptables-persistent command.

Refer to How To Set Up a Firewall Using iptables for more details.

Opening Ports Using Different Tools

Using iptables to open specific ports

To open a specific port using iptables, you can use the following command:

sudo iptables -A INPUT -p tcp --dport <port_number> -j ACCEPT

Replace <port_number> with the actual port number you want to open. For example, to open port 4000, you would use:

sudo iptables -A INPUT -p tcp --dport 4000 -j ACCEPT

This command adds a new rule to the INPUT chain to allow incoming TCP traffic on the specified port.

Managing ports with ufw (Uncomplicated Firewall) on Debian/Ubuntu systems

To open a port using ufw, you can use the following command:

sudo ufw allow <port_number>

Replace <port_number> with the actual port number you want to open. For example, to open port 4000, you would use:

sudo ufw allow 4000

This command adds a new rule to allow incoming traffic on the specified port.

Utilizing firewalld for port management on RHEL/CentOS systems

To open a port using firewalld, you can use the following command:

sudo firewall-cmd --add-port=<port_number>/tcp --permanent
sudo firewall-cmd --reload

Replace <port_number> with the actual port number you want to open. For example, to open port 4000, you would use:

sudo firewall-cmd --add-port=4000/tcp --permanent
sudo firewall-cmd --reload

The first command adds a new rule to allow incoming TCP traffic on the specified port, and the second command reloads the firewall configuration to apply the changes.

Checking Open Ports

Commands to list currently open ports

To list currently open ports, you can use the following commands:

• netstat:

netstat -lntu

• ss:

ss -lntu

• netstat:

netstat -tlnp

This command will display all listening sockets, including the protocol, local address, foreign address, state, and the PID/Program name of the process that is listening on the socket.

• nc (Netcat):

nc -z -v -n localhost 1-1024

This command will scan a range of ports on the localhost to check if they are open or not.

Verifying firewall rules and active services

To verify firewall rules and active services, you can use the following commands:

• iptables:

sudo iptables -L

• ufw:

sudo ufw status

• firewalld:

sudo firewall-cmd --list-all

These commands will display the current firewall configuration, including the rules and services that are allowed or blocked.

Common Errors and Debugging

Addressing issues when ports remain closed after configuration

If a port remains closed after configuration, it may be due to a variety of reasons, including:

• The firewall service is not running or is not configured correctly.
• The port is already in use by another service or process.
• The configuration changes have not been applied or have been overridden by another configuration.

To troubleshoot this issue, you can try the following steps:

• Check the firewall service status and ensure it is running.
• Use tools like netstat or ss to verify if the port is already in use.
• Review the firewall configuration files to ensure the changes have been applied correctly.

If the port is still not open, you can try the following troubleshooting steps:

• Check the system logs for any firewall-related errors.
• Temporarily disable the firewall to see if the port opens. If it does, there may be a rule blocking the port.
• Check for any other firewall management tools that may be conflicting with the current configuration.

Ensuring firewall services are active and correctly configured

To ensure firewall services are active and correctly configured, you can follow these steps:

• Check the firewall service status using commands like systemctl status ufw or systemctl status firewalld.
• Verify that the firewall service is enabled to start automatically on boot using commands like systemctl enable ufw or systemctl enable firewalld.
• Review the firewall configuration files to ensure they are correctly configured and up-to-date.

If the firewall service is not running or not configured correctly, you can try the following troubleshooting steps:

• Restart the firewall service.
• Check for any errors in the firewall configuration files.
• Verify that the correct ports are open in the firewall rules.

Checking for conflicts with other firewall management tools

To check for conflicts with other firewall management tools, you can follow these steps:

• Identify all firewall management tools installed on the system, including iptables, ufw, and firewalld.
• Verify that only one firewall management tool is active and configured to manage the firewall rules.
• Disable or remove any conflicting firewall management tools to ensure a single tool is managing the firewall rules.

If you are still experiencing issues, you can try the following troubleshooting steps:

• Check for any other services or processes that may be using the port.
• Review the system logs for any errors related to the port.
• If the port is being used by a different service, consider reconfiguring that service to use a different port.

Comparison of Different Firewall Tools: Pros and Cons of iptables, ufw, and firewalld for Opening Ports

The example commands used above are for opening port 4000 for TCP connections.

FAQs

1. How do I open a port in Linux?

To open a port in Linux, you need to configure your firewall to allow incoming traffic on that port. The steps to do this vary depending on the firewall software you’re using. Here are examples for ufw, firewalld, and iptables:

• For ufw (Uncomplicated Firewall): sudo ufw allow <port_number>
• For firewalld: sudo firewall-cmd --add-port=<port_number>/tcp --permanent && sudo firewall-cmd --reload
• For iptables: sudo iptables -A INPUT -p tcp --dport <port_number> -j ACCEPT

Replace <port_number> with the actual port number you want to open.

2. How to check if port 443 is open in Linux?

To check if port 443 is open in Linux, you can use the netstat or ss command to list all open ports, including TCP and UDP. Here’s how:

• Using netstat: netstat -lntu | grep 443
• Using ss: ss -lntu | grep 443

If port 443 is open, you should see it listed in the output.

3. How to check if port 22 is open in Linux?

To check if port 22 is open in Linux, you can use the netstat, ss, or nc command to list all open ports, including TCP and UDP. Here’s how:

• Using netstat: netstat -lntu | grep 22
• Using ss: ss -lntu | grep 22
• Using nc: nc -z -v -n localhost 22 If port 22 is open, you should see it listed in the output.

4. How to check if a port is open?

To check if a port is open in Linux, you can use the netstat or ss command to list all open ports, including TCP and UDP. Here’s how:

• Using netstat: netstat -lntu | grep <port_number>
• Using ss: ss -lntu | grep <port_number>

Replace <port_number> with the actual port number you want to check. If the port is open, you should see it listed in the output.

5. How to open 22 port in Linux?

Port 22 is typically open by default for SSH connections. If it’s not open, you can follow the steps to open a port in Linux (FAQ 1) and replace <port_number> with 22. Here are the specific commands to open port 22:

• For ufw: sudo ufw allow 22
• For firewalld: sudo firewall-cmd --add-port=22/tcp --permanent && sudo firewall-cmd --reload
• For iptables: sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT

6. Can I open a range of ports instead of a single port?

Yes, you can open a range of ports instead of a single port. The steps to do this vary depending on the firewall software you’re using. Here are examples for ufw, firewalld, and iptables:

• For ufw (Uncomplicated Firewall): sudo ufw allow <start_port>:<end_port>
• For firewalld: sudo firewall-cmd --add-port=<start_port>-<end_port>/tcp --permanent && sudo firewall-cmd --reload
• For iptables: sudo iptables -A INPUT -p tcp --dport <start_port>:<end_port> -j ACCEPT

Replace <start_port> and <end_port> with the actual port range you want to open.

7. Is it safe to open ports on my Linux system?

Opening ports on your Linux system can be safe if done correctly and with caution. However, it also increases the attack surface of your system, making it more vulnerable to security threats. Here are some best practices to follow:

• Only open the ports that are necessary for your applications or services.
• Use strong passwords and authentication mechanisms for services listening on open ports.
• Regularly update your system and firewall software to ensure you have the latest security patches.
• Monitor your system logs for suspicious activity on open ports.
• Consider using a firewall with a default-deny policy to block all incoming traffic except for the ports you explicitly open.

Conclusion

In this tutorial, you learned how to open a new port on Linux and set it up for incoming connections. You also used netstat, ss, telnet, nc, and nmap to check if a port is open, open a range of ports, and test the newly opened port for TCP connections. Additionally, you explored how to persistently open ports using different firewall tools such as ufw, firewalld, and iptables.

To further expand your knowledge on network connections and security, follow these tutorials:

• How to Use Netcat to Establish and Test TCP and UDP Connections.
• How to Use Nmap to Scan for Open Ports.
• How to Set Up a Firewall Using Firewalld on Rocky Linux 8.

These tutorials will help you master network connections, port scanning, and firewall setup on Linux systems.