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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:

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:

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:

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.

How to check if a certain port is open and unused?

I want to install webserver-apache on a Linux platform which uses port no.80 but I am not sure whether is port is open or not, and is being used by some other application or not.

It gives some IP’s one of which is

Could you please help and tell how can i find out if port 80 is open and unused so that I can start installation.

9 Answers 9

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That should give you pid & name of the process that holds port 80

This can be achieved using the nc command as follows:

It will return TRUE if the port is already in use, or FALSE is it (i.e, available not listening currently).

I don’t recommend lsof or netstat method as it first try to scan all running PIDs to get all bounded ports:

Here 8888 is an unused port. The nc command is

85 times faster in the above example.

If you are trying with a remote IP, it is better to add a timeout to auto-exit if it is not accepting connection for the specified time.

Its Google’s IP which is not used, so it will timeout after trying for 2 seconds.

This one displays bind addresses of TCP listening endpoints. All other endpoints are free; Also if on Unix and you are not root, then you can’t bind to a ‘privileged’ port number (port number lower than 1024).

Explained in more details:

For the general case you still need to care to cut out all irrelevant interfaces; a listening address 0.0.0.0 is listening on all network cards, if there is an IP address then that’s the specific IP of the network card/network interface.

How to check opened/closed ports on my computer?

How to check the opened/closed ports on my computer?

8 Answers 8

There’s a few parameters to netstat that are useful for this :

You use a mix of these to get what you want. To know which port numbers are currently in use, use one of these:

In the output all port mentioned are in use either listening for incoming connection or connected to a peer** all others are closed. TCP and UDP ports are 16 bits wide (they go from 1-65535)

** They can also be connecting/disconnecting from the peer.

You can use this command:

If it shows some process its used. Its closed(not used) if there is no output.

Another alternative command line easy to use to find out which process is using a port:

and now run «pslisten 5060» to see who is grabing my SIP port.

It’s work with Apple Mac OS X too.

Is the port status «LISTENING» indicated that the port is opened?

Yes. It means that some service is listening to that port on your computer for incoming connection i.e. this port is open for establishing new connections.

Any port that are not shown in the output indicated that it’s closed?

5 ways to check if a Port is open on a remote Linux PC

Knowing open ports from a Linux machine allows system administrators to connect to a remote computer, which can fix problems with the system and the cloud server. First, we will show you the TCP and UDP ports and how they are different before we start to find the open ports.

T here is an ample number of ways to check for any open ports on a remote Linux PC. Knowing open ports on a Linux machine helps system administrators to connect to the remote PC for troubleshooting system and cloud server issues.

TCP and UDP ports

TCP stands for Transmission Control Protocol. In this method, the computers get connected directly until the data transfer is taking place. Therefore, with this method, the data transfer is guaranteed and is reliable but puts a higher load on the server as it has to monitor the connection and the data transfer too.

UDP stands for User Datagram Protocol. Using this method, the data is sent in the form of little packages into the network with the hope that it reaches the final destination. It means the two computers are not connected directly to each other. This method does not provide any guarantee that the data you send will ever reach its destination. Load on the server is less, and so this method is used commonly by the system administrators first to try something that’s not so important.

Now that you know the types are ports on a Linux system, let’s get started with ways of finding the ones that are open.

Best ways to check if a Port is open on a Linux PC

There are multiple ways you can do it. However, the most reliable way to do this is by using the following commands:

Let’s go through each method one by one.

1. netcat command

netcat is a simple Unix utility that can be used to write and read data using UDP and TCP protocol across network connections.

The primary reason for its design is to provide a back-end tool that works with the scripts and programs. It is also an exploration and network debugging tool that offers tons of features.

To use it, you need to install it in your distro using the respective installation commands.

By doing so, you can do the following operations with it.

Moreover, it also has three modes

The syntax of the command is as follows.

Let’s try to use it on a remote computer.

Showing nc command succeeding in connecting to an open port

As you can see, the connection succeeded. This means that port 22 is open. If the connection fails, then you will get an error message of “failed: Connection refused”

In the above command, we also used different options. Let’s list them below.

2. nmap command

Nmap command is popular network security, auditing, and exploration command. Nmap stands for Network Mapper.

It also has a way to check for open ports. To do so, it utilizes a novel approach to using IP packets. It can also be used to learn about the services the host is providing. Other vital aspects that it can detect include operating system version, packet firewalls/filters, and so on! It is a useful tool.

Let’s see the nmap syntax below.

As you can see, its syntax matches that of the nc command. Let’s run it to get a better understanding.

Running nmap command to check open port

If the port is closed, then it will show status is closed.

nmap command shows closed status for 103 port on 192.168.0.2

3. telnet command

The next command that we will go through is the telnet command. It is an old interactive communication command.

It is specially created for the remote computer interaction, and that’s why we are going to use it to check for open ports on a remote computer. The command is available on both Windows and Linux systems, but on a Windows system, it needs to be enabled before use. It runs over a TCP/IP network. Also, it connects over a remote computer or network equipment over port 23.

One more thing that you need to know is that it is not a secure protocol and must be used with SSH if you want to be encrypted and secure.

To install telnet in RHEL 7 or CentOS 7, you need to use the following command.

For Ubuntu, use the following command

The syntax of the command is as below.

Shows telnet command in action with a successful connection

If the connection fails, then the port is not open, and you will get the following output.

Shows connection refused using the telnet command

4. echo > /dev/tcp/…

There is another way to check for open ports. In Linux, everything is a file, including the host status and its port availability. This can come handy in cases where no commands are working on the remote host.

The syntax of the command is as below

Port is “open” output

The last command that we are going to discuss is the netstat command. It is a network utility TCP/IP command. It is used to print connections, interface statistics, multicast membership, and other network-related tasks.

The syntax of the command is as below.

It will output the whole list of the IP addresses. The entries that have “Listen” in the “State” column are the open ports.

Shows the output of netstat-tuplen

Conclusion

This leads us to the end of our five ways to check if a Port is open on a remote Linux PC. So, which way are you going to use to connect to your remote Linux PC? Comment below and let us know.

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:

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

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.

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