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 520
Open to everyone
# Linux iptables iptables -A INPUT -p udp --dport 520 -j ACCEPT # Ubuntu ufw ufw allow 520/udp # firewalld firewall-cmd --permanent --add-port=520/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=udp dst-port=520 action=accept place-before=0 comment="port 520" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 520 UDP" -Direction Inbound -Protocol UDP -LocalPort 520 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p udp --dport 520 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 520 -j DROP ufw allow from 203.0.113.0/24 to any port 520 proto udp /ip firewall filter add chain=input protocol=udp dst-port=520 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=520 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 ':520 ' netstat -ano | findstr :520 # Windows lsof -i :520 # macOS