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 853
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 853 -j ACCEPT iptables -A INPUT -p udp --dport 853 -j ACCEPT # Ubuntu ufw ufw allow 853/tcp ufw allow 853/udp # firewalld firewall-cmd --permanent --add-port=853/tcp && firewall-cmd --reload firewall-cmd --permanent --add-port=853/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=853 action=accept place-before=0 comment="port 853" /ip firewall filter add chain=input protocol=udp dst-port=853 action=accept place-before=0 comment="port 853" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 853 TCP" -Direction Inbound -Protocol TCP -LocalPort 853 -Action Allow New-NetFirewallRule -DisplayName "Port 853 UDP" -Direction Inbound -Protocol UDP -LocalPort 853 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 853 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 853 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 853 -j DROP iptables -A INPUT -p udp --dport 853 -j DROP ufw allow from 203.0.113.0/24 to any port 853 proto tcp ufw allow from 203.0.113.0/24 to any port 853 proto udp /ip firewall filter add chain=input protocol=tcp dst-port=853 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=853 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=853 action=drop /ip firewall filter add chain=input protocol=udp dst-port=853 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 ':853 ' netstat -ano | findstr :853 # Windows lsof -i :853 # macOS # test from another machine nc -vz example.com 853