Ever wondered what's really going on under the hood of your Ubuntu 20.04 system? One crucial aspect of system administration involves understanding which ports are actively listening for incoming connections. Think of ports as doorways through which applications communicate. Knowing which ports are open and listening can be a game-changer for troubleshooting network issues, bolstering security, and generally keeping tabs on your server's activity. This guide will walk you through several methods to uncover those listening ports, making you a veritable port-detecting pro! So, buckle up, and let's dive into the fascinating world of network ports on Ubuntu 20.04.

Why Should You Care About Listening Ports?

Before we get into the "how," let's quickly cover the "why." Understanding listening ports is essential for a few key reasons:

• Security: Knowing which ports are open allows you to identify potential vulnerabilities. Unnecessary open ports can be exploited by malicious actors. Regularly checking your listening ports helps you maintain a secure system by closing any ports that shouldn't be open.
• Troubleshooting: If you're having trouble connecting to a service on your Ubuntu server, checking the listening ports can help you determine if the service is actually listening on the correct port. This is a fundamental step in diagnosing connectivity issues.
• Resource Management: By identifying the processes associated with each listening port, you can gain insights into resource usage. This can help you optimize your system's performance and identify any resource hogs.

Keeping an eye on your listening ports is like doing a regular health check for your Ubuntu system. It empowers you to proactively identify and address potential issues before they escalate.

Method 1: Using the netstat Command

The netstat command is a classic tool for displaying network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. While it's technically deprecated in favor of ss, it's still widely used and available on most systems, including Ubuntu 20.04. netstat is your old reliable friend when you need a quick overview of network activity. Guys, you may be wondering that it is deprecated, but dont worry. The command is still usable in most machine, and you can still count on this command.

To display listening ports using netstat, open your terminal and run the following command:

sudo netstat -tulnp

Let's break down the options used in this command:

• -t: This option tells netstat to display TCP ports.
• -u: This option tells netstat to display UDP ports.
• -l: This option tells netstat to display only listening ports.
• -n: This option tells netstat to display numerical addresses instead of resolving hostnames. This makes the output faster and easier to read.
• -p: This option tells netstat to display the process ID (PID) and name of the program that is listening on the port. This requires root privileges, hence the sudo.

The output of this command will be a table with several columns, including:

• Proto: The protocol used (TCP or UDP).
• Local Address: The IP address and port number the service is listening on. An address of 0.0.0.0 means the service is listening on all interfaces.
• State: The state of the connection (should be LISTEN for listening ports).
• PID/Program name: The process ID and name of the program listening on the port.

For example, you might see a line like this:

tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      1234/sshd

This indicates that the sshd process (SSH daemon) is listening on TCP port 22 on all interfaces.

netstat is a solid choice for a quick and dirty look at your listening ports. It's widely available, easy to use, and provides a good amount of information in a single command.

Method 2: Embracing the ss Command

The ss command, short for "socket statistics," is the modern replacement for netstat. It's designed to be faster and provide more information than its predecessor. If you're looking for a more efficient and feature-rich tool, ss is your go-to option. While netstat is good, ss is great. It provides a ton more options and is generally faster, especially on busy systems.

To display listening ports using ss, open your terminal and run the following command:

sudo ss -tulnp

The options used in this command are very similar to those used with netstat:

• -t: Show TCP sockets.
• -u: Show UDP sockets.
• -l: Display only listening sockets.
• -n: Show numerical port numbers.
• -p: Show the process using the socket.

The output of ss is also similar to netstat, but it may include additional information, such as the socket's queue length and memory usage. Also, ss command usually provide the same information as netstat, but it may be presented in a slightly different format. Don't be alarmed if the columns are arranged differently or if you see slightly different labels. The key information – protocol, local address, state, and process ID/name – will still be there.

For example, you might see a line like this:

listern  0      128    0.0.0.0:22           0.0.0.0:*                users:(("sshd",pid=1234,fd=3))

This tells you that sshd (SSH daemon) is listening on port 22. The output from ss is generally more concise and easier to parse than netstat.

One of the advantages of ss is its ability to filter results based on various criteria. For example, you can filter by port number:

ss -tulnp | grep :80

This will only show listening ports that are using port 80. This filtering capability makes ss a powerful tool for quickly finding the information you need.

If you're serious about network administration on Ubuntu 20.04, mastering the ss command is well worth the effort. It's a modern, efficient, and feature-rich tool that will become an indispensable part of your toolkit.

Method 3: Leveraging lsof (List Open Files)

The lsof command is a powerful utility that lists all open files on your system. In Linux, everything is a file, including network sockets. This means you can use lsof to find listening ports as well. lsof is like the Sherlock Holmes of your system, meticulously tracking every open file and socket. While it might seem a bit more complex than netstat or ss at first glance, lsof offers unparalleled flexibility and the ability to dig deep into your system's inner workings.

To display listening ports using lsof, open your terminal and run the following command:

sudo lsof -i -P -n | grep LISTEN

Let's break down the options used in this command:

• -i: This option tells lsof to list network files.
• -P: This option tells lsof to disable port name lookup, which can speed up the output.
• -n: This option tells lsof to disable hostname lookup, which also speeds up the output.
• grep LISTEN: This filters the output to only show lines that contain the word "LISTEN," indicating listening ports.

The output of this command will be a list of processes that have open network sockets in the listening state. The output format can be a bit verbose, but it provides a lot of information.

For example, you might see a line like this:

sshd      1234  root    4u  IPv4 0x1234567890abcdef      0t0  TCP *:22 (LISTEN)

This tells you that the sshd process is listening on TCP port 22 on all interfaces.

lsof can be particularly useful when you need to find out which process is using a specific port. For example, to find out which process is listening on port 80, you can use the following command:

sudo lsof -i :80

This will show you all processes that have port 80 open, regardless of whether they are listening or connected.

While lsof might have a steeper learning curve than netstat or ss, its versatility and ability to provide detailed information make it a valuable tool for any system administrator.

Method 4: GUI way using netstat or ss on terminal.

For those who prefer a graphical interface, you can use tools like tcpdump, Wireshark, or Nmap to visualize network traffic and identify listening ports. These tools typically provide a more intuitive way to explore network activity. This method is not for everyone. Some people find it easier to use command line tools, while others prefer a graphical interface. If you are new to Linux, you may find it easier to use a graphical interface.

Nmap

Nmap, short for Network Mapper, is a free and open-source utility for network discovery and security auditing. While primarily known as a port scanner, Nmap can also be used to identify listening ports on your local machine.

1. Install Nmap: If you don't have Nmap installed, you can install it using the following command:

sudo apt update sudo apt install nmap ``` 2. Scan Local Ports: To scan for listening ports on your local machine, use the following command:

```bash

nmap -sT -p 1-65535 localhost ```

This command will scan all TCP ports (1-65535) on your local machine. The `-sT` option specifies a TCP connect scan, which is a reliable way to identify open ports. Nmap is a great tool to see if the port is listening or not.

Wireshark

Wireshark is a powerful network protocol analyzer that allows you to capture and analyze network traffic in real-time. While it doesn't directly list listening ports, you can use it to observe network connections and infer which ports are actively listening.

1. Install Wireshark: If you don't have Wireshark installed, you can install it using the following command:

sudo apt update sudo apt install wireshark ```

During the installation, you'll be prompted to allow non-superusers to capture packets. Choose "Yes" to allow regular users to use Wireshark.

2. Capture Network Traffic: Launch Wireshark and select the network interface you want to monitor (e.g., eth0 or wlan0). Start capturing network traffic by clicking the blue shark fin icon or pressing Ctrl+E.
3. Filter for Listening Ports: In the filter bar, type tcp.port || udp.port and press Enter. This will display all TCP and UDP packets. You can then analyze the captured traffic to identify ports that are actively receiving connections, which indicates they are listening ports.

While Wireshark doesn't directly list listening ports like netstat or ss, it provides a visual representation of network traffic that can be helpful in identifying actively used ports.

Conclusion

So there you have it, guys! Four different methods to uncover the secrets of listening ports on your Ubuntu 20.04 system. Whether you prefer the classic netstat, the modern ss, the versatile lsof, or the visual approach with tools like Nmap and Wireshark, you now have the knowledge and tools to monitor your system's network activity. Remember, keeping an eye on your listening ports is a crucial aspect of system administration, security, and troubleshooting. So, go forth and explore the fascinating world of network ports! You can use any of the tools mentioned above to show listening ports in ubuntu 2004, depending on your preference.