How to Check all the Open Ports in Your Linux System

As a Linux system administrator, to transmit data from server to server having a good command over the TCP (Transmission Control Protocol) protocols is very much crucial. In the term of TCP, we get the idea of two types of ports: Open port and Close port. In-network transmission control protocol, all the ports which have accepted transmission of data are called open port, and on the other hand, the ports where data packets are filtered or couldn’t reach are called close port. Those who have been working with Ubuntu server management must know the havoc of not maintaining networks open ports properly. The knowledge between the open port and close port is pretty vice versa. In the Linux networking system, understanding the concept of open ports and checking the available number of open ports is vital.

Checking Open Ports in Linux

Network ports are usually allocated just after the IP address. Let your network address followed by a 16-bit socket, then the total number of available ports will be 2^16= 65536. In computer networking, we all are familiar with the concept and types of networking addresses like physical address and local address.

Every networking address has an endpoint that defines the work-type of that network address. Let, we want to send an email from our Gmail account; in this case, Gmail uses the SMTP (Simple Mail Transfer Protocol) system. We know that SSL (Secure Sockets Layer) is a socket number used for identification and security purpose.

In a word, this socket is called port. For Gmail, the default SSL or port is 465. The internet service provider(ISP) often allows its users to download movies of games from their File Transfer Protocol (FTP) server. Most of the time, FTP servers are made with Apache Ubuntu server tools, where ports are kept open and associated with IP addresses. In the entire post, we cover up the method of how to check open ports in Linux as well as in Ubuntu.

1. Checking Open Ports Using nmap Command in Linux

In Linux, Network Mapper or nmap command is used for checking the status of a system, used devices, checking current network services, and the availability of socket or ports. If your Linux system doesn’t have nmap installed, for Ubuntu and other Linux versions, you can install nmap by the following terminal commands. You can also check the version of your network mapper.

$ sudo apt-get install nmap
$ nmap --version

After the nmap is installed, at the very first step, we can check the ports of our localhost address. Most of the time, the localhost IP address is assigned by 127.0.0.1

$ sudo nmap -sT -O localhost

We can also find the Transmission Control Protocol (TCP) ports using the nmap command. The namp command is also used for finding the available ports for a particular IP Address. Let we want to check the ports for the IP 192.168.0.1

$ sudo nmap 192.168.0.1
$ nmap -open 192.168.0.1
$ nmap google.com

2. Finding Open Ports Using netcat Command in Ubuntu

Previously we have seen how to check available ports using Transmission Control Protocol. Now we will see how to check the available number of ports using the User Datagram Protocol (UDP).

In the Ubuntu server, this kind of networking operations is done by using the nc or netcat command. And I must say that in Linux, the netcat is the most powerful weapon to check network sockets. Let you want to create a connection under a TCP protocol where the port is assigned as 2389.

Now you can run the following terminal command to create a TCP connection. Or, if you are the client, you have access to the localhost; you can run the second terminal command too.

$ nc -l 2389
$ nc localhost 2389

3. Finding Open Ports Using netstat Command in Linux

If you want to check only the UDP User Datagram Protocol ports, you can also use the network statistics or netstat command. The netstat command can display both receiving and sending end transmission data. Finally, we have one command that can find open ports in Linux by netstat.

$ netstat --listen
$ netstat -lntu
$ netstat -vaun

The NGINX command is also used to monitor the networking system of your Linux. Here I’m introducing you with a nignx command that can check the open ports of your Linux system. 

$ sudo netstat -lntup | grep "nginx"

If you find the nignx is not working correctly, try reloading the nignx.

$ sudo nginx -t
$ sudo nginx -s reload

To find all the open ports in your Linux system, you can use this terminal command. 

$ netstat -antplF

For finding the specific port status in Linux, there is a netstat command which can display all listening ports. Let, our specific port is 80. 

$ sudo netstat -lntup | grep ":80"

 4. Monitor Listening Ports Using lsof Command in Ubuntu

In Linux, a list of open files or in short lsof command is used to see the list of files or directories which are open. But we can also use the lsof commands for some simple networking tasks. We can find the list of all open ports by lsof commands. I must mention that in Linux, open port is also aliased by listening port.

Let, we want to check the connected foreign addresses and the listening ports using the lsof command. Here, listening or listen is referred to as open ports. In the terminal, we can use exact data text grabber or Global regular expression print or, in short, the grep command.

$ lsof -i
$ sudo lsof -i -P -n | grep LISTEN
$ lsof -i TCP| fgrep LISTEN

If you want to check the status of a specific port against an IP address of the network, you can use this terminal command in your Linux system. Let, we want to check the status for the port 80.

$ sudo lsof -i :80 

5. Finding Established Ports Using ss Command in Linux

In Linux, the ss command is referred to the term socket statistics. Here the sockets are assigned as ports. So by using the ss command, we can determine the open ports in the Linux system. If you don’t have ss installed in your Linux, you can quickly install ss in your machine from the apt command given below. Then run the sockstat command in your terminal. In the output, you will find the socket/port details.

$ sudo apt install sockstat 
$ sockstat
$ netstat -an |grep LISTEN

For more informative open port checking purposes, there is a lntu command in Linux. The lntu command mainly looks for the details of TCP open ports, UDP open ports, name of the software, and used port number.

$ ss -lntu

Final Thoughts

For networking, open ports can be checked by using C++ or PHP, but the best way to check all open or listening ports is to use the Linux command lines. These will make your time efficient. In this post, we have tried to show some dynamic methods of checking open ports of the Linux system. In some Linux distributions like Ubuntu and Red Hat, checking open ports and close ports are unavoidable for firewall security purposes. You can find SMTP ports, Network Time Protocol (NTP) ports, HTTP ports, and UDP ports using the commands described above.

The entire post was all about how ports are assigned with IP addresses and why you can check the open or closed ports in your Linux system. If you find this post useful, please let us know what you’ve loved in this post. And you can also write a comment about this post. Feel free to share this post among your Linux geeks.