Is it safe to open
Management interfaces give full control of the system. Make them reachable only from an admin network or VPN, change default credentials and keep the software patched.
How to open port 9100
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 9100 -j ACCEPT # Ubuntu ufw ufw allow 9100/tcp # firewalld firewall-cmd --permanent --add-port=9100/tcp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=9100 action=accept place-before=0 comment="port 9100" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 9100 TCP" -Direction Inbound -Protocol TCP -LocalPort 9100 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 9100 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 9100 -j DROP ufw allow from 203.0.113.0/24 to any port 9100 proto tcp /ip firewall filter add chain=input protocol=tcp dst-port=9100 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=9100 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 ':9100 ' netstat -ano | findstr :9100 # Windows lsof -i :9100 # macOS # test from another machine nc -vz example.com 9100