Is it safe to open
A database should only be reachable from the application servers. Bind it to localhost or a private network, require authentication and TLS, and connect from outside through a VPN or an SSH tunnel.
How to open port 1521
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 1521 -j ACCEPT # Ubuntu ufw ufw allow 1521/tcp # firewalld firewall-cmd --permanent --add-port=1521/tcp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=1521 action=accept place-before=0 comment="port 1521" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 1521 TCP" -Direction Inbound -Protocol TCP -LocalPort 1521 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 1521 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 1521 -j DROP ufw allow from 203.0.113.0/24 to any port 1521 proto tcp /ip firewall filter add chain=input protocol=tcp dst-port=1521 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=1521 action=drop
If the server is behind a router, you also need a port forward (dst-nat). Behind ISP CGNAT (100.64.0.0/10 on the WAN) forwarding will not help: you need a public IP or a VPN.
What is listening
Check on the server
# what listens on the port ss -lntup | grep ':1521 ' netstat -ano | findstr :1521 # Windows lsof -i :1521 # macOS # test from another machine nc -vz example.com 1521