A neat command is `sudo lsof -i -P` which will show something like this: java 1114 elasticsearch IPv6 TCP localhost:9200 nginx 1169 w

A neat command is `sudo lsof -i -P` which will show something like this: java 1114 elasticsearch IPv6 TCP localhost:9200 nginx 1169 www-data IPv6 TCP *:80 nginx 1169 www-data IPv6 TCP *:443 dhclient 1620 root IPv4 UDP *:68 sshd 1697 root IPv6 TCP *:22 I.e. services listening on various ports. Knowing which services are running plus a firewall as an extra precaution goes a long way to avoid issues like this.