How can I know which program is responsible for an open port?

Question

After running the ss -ltun command on Ubuntu 18.04 I get this:

Netid        State          Recv-Q         Send-Q                  Local Address:Port                   Peer Address:Port         
udp          UNCONN         0              0                             0.0.0.0:55656                       0.0.0.0:*            
udp          UNCONN         0              0                             0.0.0.0:631                         0.0.0.0:*            
udp          UNCONN         0              0                             0.0.0.0:857                         0.0.0.0:*            
udp          UNCONN         0              0                             0.0.0.0:5353                        0.0.0.0:*            
udp          UNCONN         0              0                       127.0.0.53%lo:53                          0.0.0.0:*            
udp          UNCONN         0              0                             0.0.0.0:68                          0.0.0.0:*            
udp          UNCONN         0              0                             0.0.0.0:111                         0.0.0.0:*            
udp          UNCONN         0              0                                [::]:857                            [::]:*            
udp          UNCONN         0              0                                [::]:33678                          [::]:*            
udp          UNCONN         0              0                                [::]:5353                           [::]:*            
udp          UNCONN         0              0                                [::]:111                            [::]:*            
tcp          LISTEN         0              128                           0.0.0.0:111                         0.0.0.0:*            
tcp          LISTEN         0              128                     127.0.0.53%lo:53                          0.0.0.0:*            
tcp          LISTEN         0              5                           127.0.0.1:631                         0.0.0.0:*            
tcp          LISTEN         0              128                         127.0.0.1:9050                        0.0.0.0:*            
tcp          LISTEN         0              128                              [::]:111                            [::]:*            
tcp          LISTEN         0              5                               [::1]:631                            [::]:*  

As an example I do not know what is port 111 used for and why is it open, so I run this:

$ grep 111 /etc/services
sunrpc      111/tcp     portmapper  # RPC 4.0 portmapper
sunrpc      111/udp     portmapper
dicom       11112/tcp
kx      2111/tcp            # X over Kerberos

Now I know it's for sunrpc, but I still don't know which program is responsible for it and how can I close it (and should I close it?). Running systemctl list-sockets shows some units but it does not say which port(s) they are managing.

Answer 1

Add the -p option to ss, it will list the processes using the ports:

ss -ultnp

To see all processes, you’ll need to run it as root:

sudo ss -ultnp