How to Check Open Ports in Linux

During the troubleshooting of services running on a Linux system, checking open ports is one of the tasks any user or administrator should consider performing. If a service is expected to be running but for some reason it’s not, then most likely the port associated with that service is closed and should be opened.

In this tutorial, we will demonstrate how to check open ports in a Linux from the command line.

1) Check open ports using ss command

The Linux ss command gives you detailed insights on open ports and listening sockets. It draws information from the Linux kernel and is more preferred to the netstat command which has been deprecated.

To display listening  TCP connections, run the command

$ ss -tl

Sample output

ss – socket statistics

l – Shows listening sockets

t – Stands for TCP port

To display listening  UDP connections, issue the command

$ ss -lu

Sample output

u – Stands for UDP port

or

To display both tcp and udp, process name

$ ss -lntup

p – List process name that opened sockets

To print out all socket connections, simply use the ss command in its default format

$ ss

Sample output

2) Check open ports using netstat command

The netstat command is a powerful command tool that is used for checking open TCP and UDP ports alongside other attributes. To check open ports, issue the command:

$ netstat -pnltu

Sample output

Let’s take a closer look at the command options:

p – Displays the Procees ID associated with a service or Program name

n – Displays the numerical number of the port running e.g 3306 for mysqld,  and 22 for sshd.

l –  Shows listening sockets

t – Displays TCP connections

u – Displays UDP connections

3) Check open ports using the lsof command

The lsof command is a network command tool that can also be used to check open ports in a Linux system. To display open ports, issue the command

$ lsof -i

Sample output

If you wish to display open sockets, use the lsof command and pipe the output to grep as shown:

$ lsof -n -P | grep LISTEN

Sample output

To view all TCP connections execute :

$ lsof -i tcp

Sample output

To display all UDP  connections run the command:

$ lsof -i udp

Sample output

4) Check open ports using the Nmap utility

Nmap is a free and opensource network scanning tool usually used for reconnaissance in ethical hacking for discovering open ports of remote systems. By default, Nmap does not come installed on your system. To install Nmap, issue the command

$ sudo apt install nmap  (For Debian/ Ubuntu)

$ sudo yum install nmap (For RedHat/ CentOS)

$ sudo dnf install nmap (For Fedora)

$ pacman -S nmap (ArchLinux)

To scan for open TCP ports, run the command

$ nmap -sT -O localhost

Sample output

To scan for open UDP ports, run the command:

$ nmap -sU localhost

Sample output