How 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?
It is important you verify which ports are listing 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.
### How 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:
```
sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo nmap -sTU -O IP-address-Here
```
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:
[
][2]
Fig.01: Check the listening ports and applications with lsof command
