Linux start listen port

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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How to Create Port Listener in Windows or Linux– Handy for Connectivity Test

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One of the challenging tasks while working in the project team is to perform the necessary connectivity test though services don’t exist.

This is often you have to do when you are working in a DMZ environment for migration or new projects. Let’s take a real-time example – you are working on migration, and you have to ensure connectivity exists between application “A” to “B” on a particular port.

Well. It’s straightforward you can perform telnet but how about when “B” doesn’t have any service running? That’s where you need the port listener to help in this situation.

If you have a similar situation or feel this would be beneficial for you at work, then here are few ways to achieve this in Windows or UNIX platform.

To create post listener in Windows OS

To have a port listener on a specific port in Windows, you can use the “Port Listener” utility.

This utility is available for free for Windows 95 to Windows 10.

• Download Post Listener as zip or exe format from here
• In this guide, I will download the exe format
• Double click on downloaded postlistener.exe file
• It will prompt to select the location to extract the files, click on unzip

• Go to path where you have extracted the files, in this example; I have at c:listener

• Double click on listener to start the utility
• Enter the port number which you want to test and click on start

• In the above example, I have started listening port on 5500, and it’s time to validate if it’s running.

Open a command prompt and run netstat to validate if port 5500 is listening

So yes, now I have created a port listener successfully in Windows.

To create post listener in Linux OS

The procedure is slightly different in Linux; here we will use netcat (nc) command to start the listener.

• To install nc, you can use yum command

• Once installed, use the following command to start the port listener as 5500 in the background.

• To validate, let’s use netstat command

So here I have port 5500 listened successfully. Doing this in Linux is slightly more comfortable, isn’t it?

To create a port listener using Python

The above two examples are limited to OS. How about having a python script that can work on Windows or UNIX?

Well, I found the below python code which works on Windows and Linux both. Create a file – let’s say portlistener.py with below code

Save the file and run it with python command as shown below

Interested in learning Python? Check out this online course.

I hope the above procedure helps you to create a port listener for the connectivity tests.

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Как прослушать новый порт сервера Ubuntu из командной строки?

Я хочу слушать новый порт на сервере Ubuntu.

Я в root-доступе.

Я хочу запустить свой файл.js на порту 3000. Итак, я запустил команду:

Но тогда нет выхода.

Я новичок в Ubuntu, Кто-нибудь может мне помочь с этим?

Я хочу слушать новый порт 3000 так же, как следующий порт:

1 ответ

Сам Ubuntu Server не будет слушать ни один порт. Установленные и запущенные на сервере службы приложений прослушивают порты. Например, по умолчанию:

• sshd слушайте на 22,
• apache2/nginx слушает 80 и 443 и т. д.

Если вы хотите получить имена служб, которые прослушивают порты вашей системы, используйте netstat от root через sudo таким образом:

• 127.0.0.1 означает петлевой интерфейс (localhost),
• 0.0.0.0 означает все доступные интерфейсы (localhost, сетевые адаптеры и т. д.),
• некоторые службы могут прослушивать определенные интерфейсы, т.е. 192.168.1.100 или это может быть какой-то IPv6-адрес.

Если есть брандмауэр, и вы хотите получить доступ к своим службам извне, вам необходимо разрешить входной / выходной трафик на порты, которые они прослушивают.

Используя nc -l 3000 команда nc сам начнет прослушивать порт 3000. ИМО, эта опция предназначена для тестовых целей, то есть вы настраиваете брандмауэр или около того.

Скажем, в терминале, который вы запустили:

Затем в другом терминале вы можете проверить, слушает ли что-то порт 3000:

Вы должны разработать свою службу (программу, сценарий, команду) для прослушивания на желаемом порту 3000, и когда этот порт не используется какой-либо другой службой, когда вы запустите свою службу, она начнет прослушивать порт.

С другой стороны, если вам нужно что-то постоянно слушать . 3000 Самый простой способ, на мой взгляд, для проведения ваших тестов — создать модуль systemd. Для этого:

В качестве содержания /etc/systemd/system/listen-3000.service место:

Включите и запустите listen-3000.service :

Отключить и остановить listen-3000.service :

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Easy steps to open a port in Linux RHEL/CentOS 7/8

Table of Contents

In this article I will share examples to check port status and open a port in Linux. This article was written while using CentOS 8, so it is safe to say that it also fully covers CentOS/RHEL 7/8, Fedora, Oracle Enterprise Linux and generally the whole Red Hat family of operating systems and possibly Novell’s SLES and OpenSUSE.

Before we jump into the examples to open a port in Linux, we must understand the requirement clearly. The very basic question which comes to my mind

1. Do you need to open a port for a service? Such as a custom port 5555 for apache service?
2. Do you mean the port is already listening but blocked by firewall so you want to open a port in firewall?
3. Open a port for custom temporary task such as transfer and receive files using this port and then close the port.

We will cover all these scenarios in this article

Check port status

To check the list of existing ports which are open we will use nmap to check port status:

Currently we see only two ports are open on my CentOS 8 node.

Check list of listening ports

We will use netstat to list the TCP ports which are in listening state. The total number of ports are higher compared to the nmap output.

Open a port for some service

If this is your requirement then you are looking for the wrong question. Basically it is other way round i.e. a service will open a port. For example when you start SSHD service, by default it will start port 22 and not the other way round i.e. if you open port 22, it will not automatically start SSHD service.

Let us observe this in example, we know that port 22 is open on my CentOS 8 node. If I stop the sshd service

You can see that port 22 is not open anymore.

You must use respective service’s configuration file to change the default port. Once done you can restart the service and that should automatically open the respective port on your Linux node.

This covers the first scenario.

firewalld open port

It is also possible that your ports are disabled in firewall. If your port is not listed in nmap then it is most likely blocked by firewall.

We will use firewalld to open a port as this is the most used interface today in RHEL/CentOS 7 and 8. Determine which zone the system’s network interfaces are in. In the following example, the eth0 and eth1 interface is in the ‘public’ zone:

To permanently firewalld open port in a zone use the —add-port option. The example below permanently opens TCP port 1234 in the ‘public‘ zone. Note that permanent changes do not take effect until the firewalld service is reloaded.

Once firewalld open port, next use netstat to check port status:

We still don’t see port 1234 here. This is because currently port 1234 is not bind to any service . So our port is OPEN but NOT LISTENING. As soon as a request or service tries to use port 1234, we will get this in LISTEN state.

Use nc or ncat to open a port in Linux

Let us verify this theory Use nc or ncat to open a port in Linux nc or ncat is delivered as part of nmap-ncat rpm in RHEL/CentOS which you can install using yum or dnf. Use —listen with —port to open a port using nc command. In the below example we open port 1234

Open another terminal of this server and check port status

As you see port 1234 is listening for both IPv4 and IPv6. To only use IPv4 use -4 with the above command

Next on another terminal you can check port status for port 1234

Use nc or ncat to open a port and transfer files

We can also use nc to transfer file from one host to another host. Here I will transfer my » inputfile » from centos-8 to rhel-8 On the client we will open a random port, here we will use 9899. I have enabled verbose so you can see more details on the screen

Next to start the transfer, use the below command

If you face any issues you can check the firewall between your server and client. It is possible that the respective port is blocked and you must use firewalld open port

Lastly I hope the steps from the article to open a port and check port status on Linux was helpful. So, let me know your suggestions and feedback using the comment section.

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3 thoughts on “Easy steps to open a port in Linux RHEL/CentOS 7/8”

A very thorough and helpful post. I was trying to allow ssh on a secondary port and could not get it to work using the usual advice (w/CentOS8.)
The recommendation you provided to add the port using the firewall-cmd was the missing ingredient:

Thanks for this!

Hi
I did below steps and reloaded firewall but still when I do netstat -ntlp port 1234 not showing open

I did explained this part in the article

We still don’t see port 1234 here. This is because currently port 1234 is not bind to any service. So our port is OPEN but NOT LISTENING. As soon as a request or service tries to use port 1234, we will get this in LISTEN state.

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