How to Check Open Ports in Linux?

All the incoming requests need a port to listen to the requests simultaneously. When it comes to checking open ports, it becomes quite unmanageable because there are 65535 ports. In this tutorial, we will see how to check open ports in Linux.

What are listening ports?

Listening ports are the ones, that listen to the incoming network requests. Every port is bonded to an IP and they use TCP and UDP ports for communications.

• Open Ports: Open ports accept all the outside connections that are using the appropriate protocol
• Closed Ports: Closed ports do not accept all the connections. They take only the predetermined connections using other outside services or applications.

One listening port can host only one service at a time. For Example, you have a website hosted on your server, and it uses port 80. Now, if you install another web server, it won’t have to use another port but 80.

How to Check Open Ports in Linux?

There are many networking tools available in Linux that will show you the results. Each tool shows the same info the only difference is the output format of the information.

How to check Open Ports using lsof Command

lsof command shows the listening ports and daemons that maintain active network connections

    # sudo lsof -nP -iTCP -sTCP:LISTEN

Output:

    [sourabh@localhost ~]$ sudo lsof -nP -iTCP -sTCP:LISTEN
    COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
    cupsd   1048 root    6u  IPv6  25040      0t0  TCP [::1]:631 (LISTEN)
    cupsd   1048 root    7u  IPv4  25041      0t0  TCP 127.0.0.1:631 (LISTEN)

If you want to search for a specific port then you can try the given format

    # sudo lsof -nP -i:[port-number]

For Example:

    # sudo lsof -nP -i:443

Output:

    [sourabh@localhost ~]$ sudo lsof -nP -i:443
    COMMAND  PID    USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
    chrome  2389 sourabh   16u  IPv4 156398      0t0  TCP 10.131.47.19:40716->142.250.206.138:443 (ESTABLISHED)
    chrome  2389 sourabh   27u  IPv4 146377      0t0  TCP 10.131.47.19:49718->164.100.15.5:443 (ESTABLISHED)
    chrome  2389 sourabh   29u  IPv4 152185      0t0  TCP 10.131.47.19:38362->142.250.193.227:443 (ESTABLISHED)
    chrome  2389 sourabh   31u  IPv4 156399      0t0  TCP 10.131.47.19:40718->142.250.206.138:443 (ESTABLISHED)
    chrome  2389 sourabh   32u  IPv4 152247      0t0  TCP 10.131.47.19:47020->54.192.142.18:443 (ESTABLISHED)
    firefox 2716 sourabh   40u  IPv4 149169      0t0  TCP 10.131.47.19:51646->34.120.208.123:443 (ESTABLISHED)
    firefox 2716 sourabh   61u  IPv4 151199      0t0  TCP 10.131.47.19:38962->34.107.243.93:443 (ESTABLISHED)

How to check Open Ports using netstat Command

Netstat command shows the network activity overview.

    # sudo netstat -tunpl

This command uses 5 arguments

-t - Checks for TCP ports
-u - check UDP ports
-n - Ignore DNS lookups and only shows IP addresses
-p - Shows process ID and program name that is using the port
-l - Shows listening ports

Output:

    [sourabh@localhost ~]$ sudo netstat -tunpl
    Active Internet connections (only servers)
    Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
    tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      1048/cupsd          
    tcp6       0      0 ::1:631                 :::*                    LISTEN      1048/cupsd          
    udp        0      0 0.0.0.0:55407           0.0.0.0:*                           819/avahi-daemon: r 
    udp        0      0 127.0.0.1:323           0.0.0.0:*                           865/chronyd         
    udp        0      0 224.0.0.251:5353        0.0.0.0:*                           2341/chrome         
    udp        0      0 0.0.0.0:5353            0.0.0.0:*                           819/avahi-daemon: r 
    udp6       0      0 ::1:323                 :::*                                865/chronyd         
    udp6       0      0 :::50165                :::*                                819/avahi-daemon: r 
    udp6       0      0 :::5353                 :::*                                819/avahi-daemon: r 

How to check Open Ports using ss Command

ss command is faster and easier to use. It has also obsoleted the netstat command. ss command offers more statistics along with same options as netstat.

    # sudo ss -tunl

Output:

    [sourabh@localhost ~]$ sudo ss -tunl
    Netid        State         Recv-Q         Send-Q                 Local Address:Port                  Peer Address:Port        Process        
    udp          UNCONN        0              0                            0.0.0.0:55407                      0.0.0.0:*                          
    udp          UNCONN        0              0                          127.0.0.1:323                        0.0.0.0:*                          
    udp          UNCONN        0              0                        224.0.0.251:5353                       0.0.0.0:*                          
    udp          UNCONN        0              0                            0.0.0.0:5353                       0.0.0.0:*                          
    udp          UNCONN        0              0                              [::1]:323                           [::]:*                          
    udp          UNCONN        0              0                               [::]:50165                         [::]:*                          
    udp          UNCONN        0              0                               [::]:5353                          [::]:*                          
    tcp          LISTEN        0              4096                       127.0.0.1:631                        0.0.0.0:*                          
    tcp          LISTEN        0              4096                           [::1]:631                           [::]:*    

So these the most used commands that will show you the list of open ports in Linux.