When determining what's running on a server, you usually want to know if anything is listening on for connections and on what ports.
Out of the box, socket statistics (ss) is usually available. The older program, netstat, might be installed sometimes too, though it won't be covered here.
A good first step is to run ss -tua, which will list all TCP and UDP sockets:
$ ss -tuaNetid State Recv-Q Send-Q Local Address:Port Peer Address:Port udp UNCONN 0 0 127.0.0.1:323 *:* udp UNCONN 0 0 *:bootpc *:* udp UNCONN 0 0 *:bootpc *:* udp UNCONN 0 0 *:sunrpc *:* udp UNCONN 0 0 *:ntp *:* udp UNCONN 0 0 *:728 *:* udp UNCONN 0 0 ::1:323 :::* udp UNCONN 0 0 :::sunrpc :::* udp UNCONN 0 0 :::728 :::* tcp LISTEN 0 5 *:irdmi *:* tcp LISTEN 0 128 ...
