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Ver puertos abiertos en Linux

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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 commandConsider the last line from above outputs:

sshd    85379     root    3u  IPv4 0xffff80000039e000      0t0  TCP 10.86.128.138:22 (LISTEN)
  • sshd is the name of the application.
  • 10.86.128.138 is the IP address to which sshd application bind to (LISTEN)
  • 22 is the TCP port that is being used (LISTEN)
  • 85379 is the process ID of the sshd process

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

Linux check if port is in use using ss command
Where, ss command options are as follows:

  • -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

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 nmapYou can combine TCP/UDP scan in a single command:
$ sudo nmap -sTU -O 192.168.2.13

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]"

Fuente:
https://www.cyberciti.biz/faq/unix-linux-check-if-port-is-in-use-command/