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 646
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 646 -j ACCEPT iptables -A INPUT -p udp --dport 646 -j ACCEPT # Ubuntu ufw ufw allow 646/tcp ufw allow 646/udp # firewalld firewall-cmd --permanent --add-port=646/tcp && firewall-cmd --reload firewall-cmd --permanent --add-port=646/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=646 action=accept place-before=0 comment="port 646" /ip firewall filter add chain=input protocol=udp dst-port=646 action=accept place-before=0 comment="port 646" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 646 TCP" -Direction Inbound -Protocol TCP -LocalPort 646 -Action Allow New-NetFirewallRule -DisplayName "Port 646 UDP" -Direction Inbound -Protocol UDP -LocalPort 646 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 646 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 646 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 646 -j DROP iptables -A INPUT -p udp --dport 646 -j DROP ufw allow from 203.0.113.0/24 to any port 646 proto tcp ufw allow from 203.0.113.0/24 to any port 646 proto udp /ip firewall filter add chain=input protocol=tcp dst-port=646 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=646 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=646 action=drop /ip firewall filter add chain=input protocol=udp dst-port=646 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 ':646 ' netstat -ano | findstr :646 # Windows lsof -i :646 # macOS # test from another machine nc -vz example.com 646