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 68
Open to everyone
# Linux iptables iptables -A INPUT -p udp --dport 68 -j ACCEPT # Ubuntu ufw ufw allow 68/udp # firewalld firewall-cmd --permanent --add-port=68/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=udp dst-port=68 action=accept place-before=0 comment="port 68" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 68 UDP" -Direction Inbound -Protocol UDP -LocalPort 68 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p udp --dport 68 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 68 -j DROP ufw allow from 203.0.113.0/24 to any port 68 proto udp /ip firewall filter add chain=input protocol=udp dst-port=68 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=68 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 ':68 ' netstat -ano | findstr :68 # Windows lsof -i :68 # macOS