[Complete Guide] How to Check Open Ports on Ubuntu and Troubleshoot Issues

By 佐川 直弘 | Naohiro Sagawa (運営者)

※記事内に広告を含みます。Amazonアソシエイトとして適格販売により収入を得ています。　

1. Introduction

In network management and server administration, accurately understanding the status of ports is crucial. Especially when using Ubuntu, checking open ports and active processes helps enhance security measures and enables quick troubleshooting.

This article provides a detailed explanation of the fundamental commands and tools used to check ports on Ubuntu. Designed for beginners to intermediate users, it offers practical and easy-to-follow steps, so be sure to read until the end.

2. What is a Port?

2.1 Basic Concept of Ports

A port is a virtual communication gateway used by computers and network devices to send and receive data. Specifically, it allows multiple applications to communicate simultaneously on the same IP address by identifying and directing data to the appropriate application.

For example, a web server uses port 80 for HTTP communication. If the same server also allows SSH connections, it uses port 22. Since different services are distinguished by their port numbers, checking port status is essential for network management.

2.2 Types and Roles of Ports

Ports are categorized into three major types:

1. Well-Known Ports (0–1023)

• Standardized globally and assigned to widely used services.
• Examples:
• HTTP: 80
• HTTPS: 443
• SSH: 22

1. Registered Ports (1024–49151)

• Used by specific applications or organizations.
• Examples:
• MySQL: 3306
• PostgreSQL: 5432

1. Dynamic Ports (49152–65535)

• Temporary ports often used for client-side communication.

Understanding these classifications makes it easier to determine how a port is used.

3. How to Check Ports on Ubuntu

Ubuntu offers various tools to check port status. Here, we will explain four practical commands.

3.1 Using the ss Command

The ss command is a powerful network management tool in Linux. It is fast and provides detailed connection information.

Basic Command:

sudo ss -ltn

Options Explanation:

• -l: Displays only listening ports.
• -t: Shows only TCP protocol connections.
• -n: Displays addresses and port numbers in numeric format.

Example Output:

State       Recv-Q Send-Q      Local Address:Port        Peer Address:Port  
LISTEN      0      128              0.0.0.0:22               0.0.0.0:*

3.2 Using the netstat Command

The netstat command is a traditional network management tool. Although it is gradually being replaced by ss, it is still available on many systems.

Basic Command:

sudo netstat -ltn

Example Output:

Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN

3.3 Using the lsof Command

The lsof command is useful for identifying processes using a specific port.

Check a Specific Port:

sudo lsof -i :80

Example Output:

COMMAND   PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
apache2   1234  www    4u   IPv4  12345 0t0      TCP *:http (LISTEN)

3.4 Using the nmap Command

The nmap tool specializes in network scanning and security diagnostics.

Scan Localhost:

sudo nmap localhost

Example Output:

Starting Nmap 7.80 ( https://nmap.org ) at 2024-12-21 18:00 JST
Nmap scan report for localhost (127.0.0.1)
Host is up (0.00013s latency).
PORT    STATE SERVICE
22/tcp  open  ssh
80/tcp  open  http

Key Points:

• Displays open ports and their associated services.
• Scanning external servers requires prior authorization.

4. Checking Firewall Settings

In Ubuntu, firewalls are commonly used to enhance security. ufw (Uncomplicated Firewall) is a widely used, simple yet powerful tool. This section explains how to check and modify port permissions using ufw.

4.1 Checking the Firewall Status

Check Firewall Status:

sudo ufw status verbose

Example Output:

Status: active
Logging: on (low)
Default: deny (incoming), allow (outgoing)
New profiles: skip
To                         Action      From
--                         ------      ----
22/tcp                     ALLOW       Anywhere
80/tcp                     ALLOW       Anywhere

Explanation:

• Status: active: Firewall is enabled.
• Logging: on: Logs firewall activity.
• Default: deny (incoming), allow (outgoing): Denies incoming connections by default while allowing outgoing ones.
• ALLOW: Specifies which ports/services are permitted (e.g., SSH and HTTP).

Note: If the firewall is disabled (Status: inactive), activate it with:

sudo ufw enable

4.2 Allowing or Blocking Ports

Allow a specific port:

sudo ufw allow 22/tcp

Explanation:

• Allows TCP connections on port 22 (SSH).

Block a specific port:

sudo ufw deny 80/tcp

Explanation:

• Blocks access to port 80 (HTTP).

Allow only a specific IP address:

sudo ufw allow from 192.168.1.100 to any port 22 proto tcp

Explanation:

• Allows SSH connections only from IP address 192.168.1.100.

4.3 Resetting and Reviewing Firewall Settings

To reset the firewall settings and start over, use the following command:

sudo ufw reset

This command clears all rules and restores the firewall to its default state. Always review the rules after making changes.

5. Practical Example: Checking a Specific Port

Here, we will demonstrate how to check the status of SSH (port 22) as an example.

5.1 Checking Port Status

Example command:

sudo ss -ltn | grep ':22'

Example Output:

LISTEN      0      128        0.0.0.0:22            0.0.0.0:*

Key Points:

• If the output shows LISTEN, the port is open and waiting for connections.
• 0.0.0.0 means the server accepts connections from any IP address.

5.2 Identifying the Process Using the Port

Example command:

sudo lsof -i :22

Example Output:

COMMAND   PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
sshd      1234  root   3u   IPv4  56789 0t0      TCP *:ssh (LISTEN)

Key Points:

• sshd is the daemon process managing SSH connections.
• The process ID (PID) can be used to stop or restart the service.

To stop the process:

sudo kill 1234

5.3 Troubleshooting Examples

Problem: The port is not open or cannot be accessed.

Steps to fix:

1. Check firewall settings:

sudo ufw status verbose

2. If the port is blocked, allow it:

sudo ufw allow 22/tcp

3. Check and restart the service if necessary:

sudo systemctl restart ssh

6. Port Security Best Practices

Managing ports is directly linked to network security. Below are key points to enhance security.

6.1 Closing Unnecessary Ports

Unused ports should be closed to prevent potential attacks.

Example: Closing port 80

sudo ufw deny 80/tcp

6.2 Preventing Port Scanning

Port scanning is a technique used by attackers to find vulnerabilities in a system. The following measures can help prevent it:

1. Strengthening firewall rules:

sudo ufw default deny incoming

2. Monitoring logs:

sudo tail -f /var/log/ufw.log

3. Using intrusion detection tools:

• Tools like fail2ban can automatically block suspicious access attempts.

7. Conclusion

This article provided detailed steps for checking ports on Ubuntu, managing firewall settings using ufw, and implementing security measures.

7.1 Key Takeaways

• Understanding Ports: Ports act as communication gateways and are categorized into well-known, registered, and dynamic ports.
• Using Commands to Check Ports:
• Commands like ss, netstat, lsof, and nmap help check port status and running processes.
• Firewall Management:
• ufw can be used to allow or block specific ports, enhancing security.
• Security Measures:
• Closing unnecessary ports, monitoring logs, and using security tools help maintain a safe network environment.

7.2 Future Applications

Port management is a fundamental aspect of network security. Apply the knowledge from this guide to maintain a secure server environment.

FAQ: Frequently Asked Questions About Checking Ports on Ubuntu

Q1. What should I do if a port is not open on Ubuntu?

A:
If a port is not open, follow these steps:

1. Check firewall settings:

sudo ufw status verbose

If the firewall is blocking the port, allow it with the following command:

sudo ufw allow [port number]/tcp

2. Verify that the corresponding service is running:

sudo systemctl status [service name]

Example: For SSH, use:

sudo systemctl status ssh

If the service is not running, restart it:

sudo systemctl restart [service name]

3. Check if the service is using the correct port:

Verify the configuration file (e.g., for SSH, check /etc/ssh/sshd_config) to ensure the correct port is set.

Q2. What is the difference between ss and netstat?

A:
ss and netstat are both tools for checking network connections, but they have key differences:

• ss:
• Recommended for modern Linux systems.
• Faster and provides more detailed information.
• Example command: sudo ss -ltn
• netstat:
• An older tool that is gradually being deprecated.
• Still available on many systems for compatibility.
• Example command: sudo netstat -ltn

For newer systems, ss is the preferred choice.

Q3. How can I detect if someone is scanning my ports?

A:
To detect port scanning, try the following methods:

1. Check firewall logs:

sudo tail -f /var/log/ufw.log

Look for unusual IP addresses or repeated connection attempts.

2. Use an intrusion detection system (IDS):

• Install and configure tools like fail2ban or Snort to automatically block malicious access.

3. Scan your own server using nmap:

sudo nmap localhost

Check for open ports and close any that are unnecessary.

Q4. How can I check which process is using a specific port?

A:
Use the lsof command to identify processes using a specific port:

sudo lsof -i :[port number]

Example: To check port 80:

sudo lsof -i :80

Example Output:

COMMAND   PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
apache2   1234  www    4u   IPv4  12345 0t0      TCP *:http (LISTEN)

Q5. How do I allow only a specific IP address using ufw?

A:
To allow access from only a specific IP address, use the following command:

sudo ufw allow from [IP address] to any port [port number] proto tcp

Example: Allow SSH access (port 22) only from IP address 192.168.1.100:

sudo ufw allow from 192.168.1.100 to any port 22 proto tcp

Q6. How can I change the port number for a service?

A:
To change a service’s port number, edit its configuration file. Here’s an example for SSH:

1. Open the configuration file:

sudo nano /etc/ssh/sshd_config

2. Find the Port setting and change the port number:

Port 2222

3. Restart the SSH service:

sudo systemctl restart ssh

4. Allow the new port in the firewall:

sudo ufw allow 2222/tcp

Q7. Can I allow multiple ports at once?

A:
Yes, you can allow multiple ports at once using the following methods:

1. Allow a range of ports:

sudo ufw allow 1000:2000/tcp

Explanation: Allows TCP connections on ports 1000 to 2000.

2. Allow multiple specific ports:

sudo ufw allow 22/tcp
sudo ufw allow 80/tcp