When developing or using services it’s often handy to list all listening sockets or connections on the machine. We have a handy tool at our disposal on most distributions:

On older distributions there was the netstat tool, but on newer distribution it’s been replaced by ss.

$ ss

Netid    State    Recv-Q   Send-Q                     Local Address:Port       Peer Address:Port                 
u_seq    ESTAB    0        0                                 @0001f 101956                * 101957               
u_seq    ESTAB    0        0                                 @0001c 36327                 * 36328                
u_str    ESTAB    0        0                                      * 605506                * 605507               
u_str    ESTAB    0        0        /var/run/dbus/system_bus_socket 40972                 * 39126                
u_str    ESTAB    0        0                                      * 39939                 * 39938                
u_str    ESTAB    0        0                                      * 29377                 * 27430                
u_str    ESTAB    0        0                                      * 620557                * 620556               
u_str    ESTAB    0        0                                      * 602348                * 602349               
u_str    ESTAB    0        0                                      * 34852                 * 32327                
u_str    ESTAB    0        0                     @/tmp/.X11-unix/X0 27347                 * 33301                
u_str    ESTAB    0        0                     /run/user/1000/bus 34057                 * 34056                
u_str    ESTAB    0        0                    @/tmp/dbus-RiJ4OUll 1096107               * 1100088              
u_str    ESTAB    0        0                                      * 681622                * 681621               
u_str    ESTAB    0        0                                      * 107612                * 106793               
u_str    ESTAB    0        0                                      * 36029                 * 34234                
u_str    ESTAB    0        0            /run/systemd/journal/stdout 30269                 * 35283                
u_str    ESTAB    0        0                                      * 35859                 * 32413                
....

a few of my most used options:

       -n, --numeric
              Do not try to resolve service names.

       -a, --all
              Display both listening and non-listening (for TCP this means established connections) sockets.

       -l, --listening
              Display only listening sockets (these are omitted by default).

       -p, --processes
              Show process using socket.

       -t, --tcp
              Display TCP sockets.

       -u, --udp
              Display UDP sockets.

       -x, --unix
              Display Unix domain sockets (alias for -f unix).

Examples:

  • Show all listening tcp sockets (use numeric instead of service names for ports):

    ss -altn

  • Show all processes with listening tcp sockets (use numeric instead of service names for ports):

    ss -altnp

Be aware that you’ll have to execute with sudo for some operations, like showing processes.