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 4789
Open to everyone
# Linux iptables iptables -A INPUT -p udp --dport 4789 -j ACCEPT # Ubuntu ufw ufw allow 4789/udp # firewalld firewall-cmd --permanent --add-port=4789/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=udp dst-port=4789 action=accept place-before=0 comment="port 4789" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 4789 UDP" -Direction Inbound -Protocol UDP -LocalPort 4789 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p udp --dport 4789 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 4789 -j DROP ufw allow from 203.0.113.0/24 to any port 4789 proto udp /ip firewall filter add chain=input protocol=udp dst-port=4789 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=4789 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 ':4789 ' netstat -ano | findstr :4789 # Windows lsof -i :4789 # macOS