How to Check Open Ports on Linux

It is important to know the open ports on your system, this is useful to detect an intrusion, or during troubleshooting or to detect a problem, for example if you have a problem with apache, it may be necessary to check s it listens well on ports 80/443.

This guide shows you how to check ports and see applications using those ports using netstat, nmap, and lsof on Linux system.

lsof command :

To check open ports with lsof, you can use the command :

#lsof -i -P -n

Take the output below as an example:

smtpd   25362   root   8u   IPv4   0xffff80000045a000   0t0   TCP   12.58.125.39:25   (LISTEN)

– smtp : application name

– 25362 : ID du process smtpd

– 12.58.125.39 : is the interface’s IP address that smtpd is listening on.

– 25 : snmpd port

netstat command

You can check the open ports with the below netstat commands :

# netstat -tulpn

You can combine the command with grep to filter:

# netstat -tulpn | grep LISTEN

To see other useful netstat commands, check this article.

nmap command

Nmap (Network Mapper) is a free and open-source utility for network discovery and security auditing.

# nmap -sU -O 192.168.15.4 #Pour voir les ports UDP ouverts
# nmap -sT -O 192.168.15.4 #Pour voir les ports TCP ouverts

It is possible to combine the two commands into one to display open UDP/TCP ports:

# nmap -sTU -O 192.168.2.13

nmap is a powerful command that has several useful options, you can discover them by checking this article.