Is it safe to open
Infrastructure services (DNS, NTP, DHCP, routing) should answer only the networks they serve. Open resolvers and time servers get abused for DDoS amplification.
How to open port 5355
Open to everyone
# Linux iptables iptables -A INPUT -p udp --dport 5355 -j ACCEPT # Ubuntu ufw ufw allow 5355/udp # firewalld firewall-cmd --permanent --add-port=5355/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=udp dst-port=5355 action=accept place-before=0 comment="port 5355" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 5355 UDP" -Direction Inbound -Protocol UDP -LocalPort 5355 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p udp --dport 5355 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 5355 -j DROP ufw allow from 203.0.113.0/24 to any port 5355 proto udp /ip firewall filter add chain=input protocol=udp dst-port=5355 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=5355 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 ':5355 ' netstat -ano | findstr :5355 # Windows lsof -i :5355 # macOS