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 33434
Open to everyone
# Linux iptables iptables -A INPUT -p udp --dport 33434 -j ACCEPT # Ubuntu ufw ufw allow 33434/udp # firewalld firewall-cmd --permanent --add-port=33434/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=udp dst-port=33434 action=accept place-before=0 comment="port 33434" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 33434 UDP" -Direction Inbound -Protocol UDP -LocalPort 33434 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p udp --dport 33434 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 33434 -j DROP ufw allow from 203.0.113.0/24 to any port 33434 proto udp /ip firewall filter add chain=input protocol=udp dst-port=33434 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=33434 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 ':33434 ' netstat -ano | findstr :33434 # Windows lsof -i :33434 # macOS