Test connection to port linux

How to check if port is in use on Linux or Unix

H ow do I determine if a port is in use under Linux or Unix-like system? How can I verify which ports are listening on Linux server? How do I check if port is in use on Linux operating system using the CLI?

It is important you verify which ports are listening on the server’s network interfaces. You need to pay attention to open ports to detect an intrusion. Apart from an intrusion, for troubleshooting purposes, it may be necessary to check if a port is already in use by a different application on your servers. For example, you may install Apache and Nginx server on the same system. So it is necessary to know if Apache or Nginx is using TCP port # 80/443. This quick tutorial provides steps to use the netstat, nmap and lsof command to check the ports in use and view the application that is utilizing the port.

Tutorial details
Difficulty level	Easy
Root privileges	Yes
Requirements	lsof, ss, and netstat on Linux
Est. reading time	3 minutes

How to check if port is in use in

To check the listening ports and applications on Linux:

1. Open a terminal application i.e. shell prompt.
2. Run any one of the following command on Linux to see open ports:
   sudo lsof -i -P -n | grep LISTEN
   sudo netstat -tulpn | grep LISTEN
   sudo ss -tulpn | grep LISTEN
   sudo lsof -i:22 ## see a specific port such as 22 ##
   sudo nmap -sTU -O IP-address-Here
3. For the latest version of Linux use the ss command. For example, ss -tulw

Let us see commands and its output in details.

Option #1: lsof command

The syntax is:
$ sudo lsof -i -P -n
$ sudo lsof -i -P -n | grep LISTEN
$ doas lsof -i -P -n | grep LISTEN ### [OpenBSD] ###
Sample outputs:

Fig.01: Check the listening ports and applications with lsof command

Option #2: netstat command

You can check the listening ports and applications with netstat as follows.

Linux netstat syntax

Run netstat command along with grep command to filter out port in LISTEN state:
$ netstat -tulpn | grep LISTEN
The netstat command deprecated for some time on Linux. Therefore, you need to use the ss command as follows:
sudo ss -tulw
sudo ss -tulwn
sudo ss -tulwn | grep LISTEN

Where, ss command options are as follows:

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• -t : Show only TCP sockets on Linux
• -u : Display only UDP sockets on Linux
• -l : Show listening sockets. For example, TCP port 22 is opened by SSHD server.
• -p : List process name that opened sockets
• -n : Don’t resolve service names i.e. don’t use DNS

FreeBSD/MacOS X netstat syntax

$ netstat -anp tcp | grep LISTEN
$ netstat -anp udp | grep LISTEN

OpenBSD netstat syntax

$ netstat -na -f inet | grep LISTEN
$ netstat -nat | grep LISTEN

Option #3: nmap command

The syntax is:
$ sudo nmap -sT -O localhost
$ sudo nmap -sU -O 192.168.2.13 ##[ list open UDP ports ]##
$ sudo nmap -sT -O 192.168.2.13 ##[ list open TCP ports ]##
Sample outputs:

Fig.02: Determines which ports are listening for TCP connections using nmap

A note about Windows users

You can check port usage from Windows operating system using following command:
netstat -bano | more
netstat -bano | grep LISTENING
netstat -bano | findstr /R /C:»[LISTEING]»

Conclusion

This page explained command to determining if a port is in use on Linux or Unix-like server. For more information see the nmap command and lsof command page online here

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How to ping and test for a specific port from Linux or Unix command line

Linux ping port using telnet command

The syntax is:
telnet telnet www.cyberciti.biz 80
telnet 192.168.2.254 80
Sample outputs:

To close your session, press Ctrl + ] + q .

Use nc command

The syntax is:
nc -vz nc -vz 192.168.2.254 80
nc -vz www.cyberciti.biz 443
Sample outputs:

Unix ping port using nmap command

The syntax is:
nmap -PNp
nmap -p
nmap -p 22 www.cyberciti.biz
nmap -p 443 192.168.2.254
Sample outputs:

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Use bash shell

Use nping command

Nping is an open-source tool for network packet generation, response analysis and response time measurement. Nping allows users to generate network packets of a wide range of protocols, letting them tunevirtually any field of the protocol headers. While Nping can be used as a simple ping utility to detect active hosts, it can also be used as a raw packet generator. The syntax is:
sudo nping —tcp -p
sudo nping —tcp -p 443 www.cyberciti.biz
Sample outputs:

Fig.01: nping just pinged www.cyberciti.biz host at port 443

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Comments on this entry are closed.

The nping utility that comes with NMap allows you to ‘ping’ a port by making a TCP connection to it. This does more than just check if the port is open. It verifies that it can be connected to.

I added a note about nping. Thanks!

Hi,
In line
nmap -p 443 192.168.2.254 443
remove last 443
thanks

Thanks for the heads up!

The shell approach is brilliant! Especially, when you’re working with some random docker containers. Thanks a lot!

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How to check open ports in Linux using the CLI

I need to list all open ports in Linux cloud server. How do I check open ports in Linux using the CLI? Can you give me the command to check open ports in Linux operating system?

To troubleshoot server problems and to avoid security issue, one needs to find out open TCP and UDP ports. In this tutorial, you will learn the different Linux commands to check open ports in Linux for auditing and securing the server.

Tutorial details
Difficulty level	Easy
Root privileges	Yes
Requirements	Linux command line
Est. reading time	5 minutes

What the hell are a TCP and UDP ports?

A port is nothing but a 16-bit number between 0 to 65535. For example, TCP port number 22 may be forwarded to the OpenSSH server. Therefore, 22 port number is a way to identify the sshd (OpenSSH server) process.

Port numbers

• The Well Known Ports are those from 0 through 1023.
• The Registered Ports are those from 1024 through 49151.
• The Dynamic and Private Ports are those from 49152 through 65535.

A registered port is a network port assigned by the Internet Assigned Numbers Authority (IANA) and stored in /etc/services file. Use the cat command or grep command/egrep command to view port numbers and service mappings:

Display a list of applications and their ports assigned by IANA

Check open ports in Linux

The procedure to monitor and display open ports in Linux is as follows:

1. Open a Linux terminal application
2. Use ss command to display all open TCP and UDP ports in Linux.
3. Another option is to use the netstat command to list all ports in Linux.
4. Apart from ss / netstat one can use the lsof command to list open files and ports on Linux based system.
5. Finally, one can use nmap command to check TCP and UDP ports too.

Let us see all commands and examples in details.

Using netstat to list open ports

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• -t : All TCP ports
• -u : All UDP ports
• -l : Display listening server sockets
• -p : Show the PID and name of the program to which each socket belongs
• -n : Don’t resolve names
• | grep LISTEN : Only display open ports by applying grep command filter.

Use ss to list open ports

The ss command is used to dump socket statistics. It allows showing information similar to netstat. It can display more TCP and state information than other tools. The syntax is:
sudo ss -tulpn
Sample outputs:

Listening ports and applications using lsof command

Let us run the following to check open TCP and UDP ports using the lsof command:
sudo lsof -i -P -n | grep LISTEN

Where,

• -i : Look for listing ports
• -P : Inhibits the conversion of port numbers to port names for network files. Inhibiting the conversion may make lsof run a little faster. It is also useful when port name lookup is not working properly.
• -n : Do not use DNS name
• | grep LISTEN : Again only show ports in LISTEN state using the grep command as filter.

nmap command

In addition, to above commands one can use the nmap command which is an open source tool for network exploration and security auditing. We are going to use nmap to find and list open ports in Linux:
$ sudo nmap -sT -O localhost
$ sudo nmap -sU -O 192.168.2.254 ##[ list open UDP ports ]##
$ sudo nmap -sT -O 127.0.0.1 ##[ list open TCP ports ]##
$ sudo nmap -sTU -O 192.168.2.24
Sample outputs:

The open port doesn’t mean anyone from outside can access those ports

So far, you know how to find and list open TCP and UDP ports on Linux. However, those ports can still be blocked by software, cloud, or hardware firewall. Hence, you need to verify that your corporate firewall is not blocking incoming or outgoing access. For instance on Linux server we list or dump firewall rules using the following syntax:
sudo iptables -S
# IPv6
sudo ip6tables -S

Conclusion

In conclusion, finding out open ports is one of the most fundamental duties of a Linux system administrator for security reasons. Therefore, close down all unwanted ports and configure firewall such as UFW and FirewallD to open or block ports as per your requirements. After reading this tutorial, you should have a good understanding of how to check for open ports in Linux. See IANA’s offical list of TCP, UDP and other ports here for more information.

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