Open Ports
Ports are opened on a system by various background services like http server, database server, smtp server etc.
If you are running a webserver and launch any such service then it will open a port so that other systems on the internet or local network can connect to it.
After you start a service on your system, then you need to check if the desired port is opened by the service or not.
If the port is not open, then the service probably failed to start or has stopped working due to some error.
Check open ports with Netstat
A very simple way to check for open ports on a linux server or system is to use the netstat commands. Though the netstat command is meant for checking network connections on a system, it can check and report open ports easily.
The syntax is very simple. You need to use the following options:
"-l" (for listening connections) "-t" (for tcp connections) "-p" (process name and id that opened the port) "-n" (show port numbers instead of names)
Here is a quick example:
$ sudo netstat -ltpn Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 0.0.0.0:33060 0.0.0.0:* LISTEN 1294/mysqld tcp 0 0 0.0.0.0:6600 0.0.0.0:* LISTEN 1/init tcp 0 0 127.0.0.1:3306 0.0.0.0:* LISTEN 1294/mysqld tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN 1283/apache2 tcp 0 0 0.0.0.0:1716 0.0.0.0:* LISTEN 1748/kdeconnectd tcp 0 0 127.0.0.53:53 0.0.0.0:* LISTEN 735/systemd-resolve tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 1471/cupsd
The above example clearly shows that apache has opened the http (80) port and mysql has opened 3306 port.
If you omit the "-n" option then the port names will be displayed instead.
$ sudo netstat -ltp Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 0.0.0.0:33060 0.0.0.0:* LISTEN 1294/mysqld tcp 0 0 0.0.0.0:6600 0.0.0.0:* LISTEN 1/init tcp 0 0 localhost:mysql 0.0.0.0:* LISTEN 1294/mysqld tcp 0 0 0.0.0.0:http 0.0.0.0:* LISTEN 1283/apache2 tcp 0 0 0.0.0.0:1716 0.0.0.0:* LISTEN 1748/kdeconnectd tcp 0 0 localhost:domain 0.0.0.0:* LISTEN 735/systemd-resolve tcp 0 0 localhost:ipp 0.0.0.0:* LISTEN 1471/cupsd
Check specific open port
If you want to list out a specific open port then filter the output with grep. The following command filters out the port number containing 3306
$ sudo netstat -ltpn | grep "3306" tcp 0 0 0.0.0.0:33060 0.0.0.0:* LISTEN 1294/mysqld tcp 0 0 127.0.0.1:3306 0.0.0.0:* LISTEN 1294/mysqld
