Linux which ports are active

4 Ways to Find Out What Ports Are Listening in Linux

The state of a port is either open, filtered, closed, or unfiltered. A port is said to be open if an application on the target machine is listening for connections/packets on that port.

In this article, we will explain four ways to check open ports and also will show you how to find which application is listening on what port in Linux.

1. Using Netstat Command

Netstat is a widely used tool for querying information about the Linux networking subsystem. You can use it to print all open ports like this:

The flag -l tells netstat to print all listening sockets, -t shows all TCP connections, -u displays all UDP connections and -p enables printing of application/program name listening on the port.

Check Open Ports Using Netstat Command

To print numeric values rather than service names, add the -n flag.

Show Numeric Values

You can also use grep command to find out which application is listening on a particular port, for example.

Find Port of Running Application

Alternatively, you can specify the port and find the application bound to, as shown.

Find Application Using a Port Number

2. Using ss Command

ss command is another useful tool for displaying information about sockets. It’s output looks similar to that of netstat. The following command will show all listening ports for TCP and UDP connections in numeric value.

Find Open Ports Using ss Command

3. Using Nmap Command

Nmap is a powerful and popular network exploration tool and port scanner. To install nmap on your system, use your default package manager as shown.

To scan all open/listening ports in your Linux system, run the following command (which should take a long time to complete).

4. Using lsof Command

The final tool we will cover for querying open ports is lsof command, which is used to list open files in Linux. Since everything is a file in Unix/Linux, an open file may be a stream or a network file.

To list all Internet and network files, use the -i option. Note that this command shows a mix of service names and numeric ports.

List Open Network Files Using lsof Command

To find which application is listening on a particular port, run lsof in this form.

Find Application Using Port

That’s all! In this article, we have explained four ways to check open ports in Linux. We also showed how to check which processes are bound upon particular ports. You can share your thoughts or ask any questions via the feedback form below.

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Checking Open and Listening Ports on Linux Using netstat and ss

One step in securing a Linux computer system is identifying which ports are active. Your system’s active ports give you information about which outside applications may be connected to your system. You can also discover if you are unintentionally exposing an application or service to the internet, like a MySQL database. There are several Linux tools that help you discover which ports are in use and identify both ends of active communications. This guide introduces three common tools you can use with links to guides that dive deeper into each tool.

What is a Port in Computer Networking?

Service names and port numbers are used to distinguish between different services that run over transport protocols. Common transport protocols are TCP, UDP, DCCP, and SCTP. These protocols enable communication between applications by establishing a connection and ensuring data is transmitted successfully. Well-known port assignments, such as HTTP at port 80 over TCP and UDP, are listed at the IANA Service Name and Transport Protocol Port Number Registry. These port assignments help distinguish different types of network traffic across the same connection.

How to Check Which Linux Ports Are in Use?

Three tools to help you check ports in use on a Linux system are:

• netstat: This tool shows your server’s network status.
• ss: You can view socket statistics with the ss tool. For example, ss allows you to monitor TCP, UDP, and UNIX sockets.
• lsof: This Linux utility lists open files. Since everything on a Linux system can be considered a file, lsof provides a lot of information on your entire system.

While all three tools help you learn how to check if a port is open in Linux, each program has its own advantages and disadvantages. See the following examples to identify which tool is the best fit for your purpose.

Using netstat

The netstat tool is great for inspecting the following areas of your Linux system:

• Unix sockets and network connections
• Routing tables
• Network interfaces
• Network protocols
• Multicast group membership

Running netstat without any options displays all open sockets and network connections. While this checks if a port is open in Linux, it can generate a lot of output. You can control the output using netstat’s command-line options. For example, to view the PID and program name for a system’s listening TCP connections, run netstat with the following command-line options:

The output resembles the following:

To learn how to install netstat, interpret its output, and view common command line options, see our Inspecting Network Information with netstat guide.

Using ss

Another way to have Linux check ports is via the ss tool. ss was created to improve upon netstat and provides more functionality. It allows you to monitor TCP, UDP, and UNIX sockets. A socket enables programs to communicate with each other across a network and is comprised of an IP address and a port number.

Running the ss with no options displays TCP, UDP, and UNIX sockets. Similar to netstat, this unrestricted list can get quite big on busy machines, so it is useful to restrict the ss command’s output by using command-line options. For example, to view all listening and non-listening TCP sockets issue the following command:

The output resembles the following:

Using just the -l parameter tells ss to list all Linux’s listening ports, which are omitted by default, making it easier to check for listening ports in Linux.

To take a deeper dive into the ss tool, read our Learning to Use the ss Tool to its Full Potential guide. This guide provides commands specific to each protocol, commands to view general statistics about a system’s current connections, and ways to filter your output.

Using lsof

Since everything on a Linux system can be considered a file, the lsof tool can report on many aspects of a system, including open network interfaces and network connections. By default, it will list open ports in Linux. The lsof tool is preinstalled on many Linux distributions, so you may consider using it before a tool you need to install.

While one of the most frequent uses of lsof is determining which program listens to a given TCP port, one unique feature of the lsof tool is repeat mode*. This mode allows you to run the lsof command continuously on a timed interval. When inspecting your system to find information about which ports are in use, lsof can return information about which user and processes are using a specific port. For example, when working with a local development environment you may want to find which localhost ports are currently in use. Use the following command to retrieve this information:

The output returns a similar response:

lsof is a powerful diagnostic tool capable of a significant number of ways that you can combine its command line options to troubleshoot various issues. To learn more about the lsof command read our How to List Open Files with lsof guide. This guide provides information about command-line options, the anatomy of the lsof output, and filtering your output with regular expressions.

This page was originally published on Thursday, February 25, 2021.

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How to Find Out List of All Open Ports in Linux

In this article, we will briefly talk about ports in computer networking and move to how you can list all open ports in Linux.

In computer networking, and more definitely in software terms, a port is a logical entity which acts as a endpoint of communication to identify a given application or process on an Linux operating system. It is a 16-bit number (0 to 65535) which differentiates one application from another on end systems.

The two most popular Internet transport protocols, Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) and other less known protocols use port numbers for communication sessions (source and destination port numbers in conjunction with the source and destination IP addresses).

In addition, a combination of an IP address, port and protocol such as TCP/UDP is known as a socket, and every service must have a unique socket.

Below are the different categories of ports:

1. 0-1023 – the Well Known Ports, also referred to as System Ports.
2. 1024-49151 – the Registered Ports, also known as User Ports.
3. 49152-65535 – the Dynamic Ports, also referred to as the Private Ports.

You can view a list of different applications and port/protocol combination in /etc/services file in Linux using cat command:

To list all open ports or currently running ports including TCP and UDP in Linux, we will use netstat, is a powerful tool for monitoring network connections and statistics.

1. -l – prints only listening sockets
2. -n – shows port number
3. -t – enables listing of tcp ports
4. -u – enables listing of udp ports

You can also use ss command, a well known useful utility for examining sockets in a Linux system. Run the command below to list all your open TCP and UCP ports:

Make it a point to read through the man pages of the commands above for more usage information.

In summary, understanding the concept of ports in computer networking is very vital for system and network administrators. You can as well go through this netstat guide with simple, precise and well explained examples.

Last but not least, get in touch with us by sharing other methods for listing open ports in Linux or asking a question via the response form below.

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How to Check for Open Ports in Linux

When you are troubleshooting networking issues in Linux or are looking for ways to improve the security of your Linux machine, you will need to know if and which ports are open. In this article we will look at different ways to list or display open ports in Linux.

What is a Port?

A port is a 16-bit number (0 to 65535) to help identify a given application or process on a Linux (Unix) operating system. Port differentiates one application from another on a Linux system.

Below are the different categories of ports:

• 0 – 1023 – Referred to as Well Known Ports
• 1024 – 49151 –Referred to as Registered Ports
• 49152 – 65535 – Referred to as Dynamic Ports

Using the following command, a list of applications and ports is displayed on your terminal:

TCP: TCP stands for Transmission Control Protocol. It is the most commonly-used protocol on the Internet. TCP is not just one-way communication, rather it sends packets back to acknowledge it’s received your packets.

UDP: Also known as User Datagram Protocol. It is an alternative communications protocol to TCP. The UDP protocol works similar to TCP. However, it ignores all error-checking stuff. UDP is necessary when speed is desirable and error correction is not needed.

SOCKETS: Socket allows communication from two different processes on the same or different machines.

Let’s look at different ways to list an open port in Linux.

1. Netstat

In this method we will use the command netstat -atu to check for open ports in Linux.

We used the -a , -t and -u flags for netstat.

• -a: shows all sockets
• -t: shows TCP connections
• -u: shows UDP connections

You can also add the -p flag to show related PID of the process or program name.

To display only UDP ports, you can use the following command:

Also, you can use the following command to search for TCP ports:

2. lsof

Instead of using netstat, we can use the lsof command to display open ports in Linux:

The following command can also help to display open sockets:

Also, you can use the command below to list all TCP connections:

Moreover, you can use the following command for UDP connections:

3. Network Mapped Command

In this method we will use nmap to detect the open port on your system. We can use the following command to show tcp port connections:

Finally, to show udp port connections, we can use the following command:

Conclusion

When it comes to the security of your Linux PC, the first thing to do is to close all unnecessary ports to prevent external access. With the methods listed above, you will be able to easily check for open ports on a Linux system and determine which ports should be closed or remain open.

Michael wears many hat in the opensource industry. He is based in Accra, Ghana. He revels in anything Linux and Devops.

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