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 88
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 88 -j ACCEPT iptables -A INPUT -p udp --dport 88 -j ACCEPT # Ubuntu ufw ufw allow 88/tcp ufw allow 88/udp # firewalld firewall-cmd --permanent --add-port=88/tcp && firewall-cmd --reload firewall-cmd --permanent --add-port=88/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=88 action=accept place-before=0 comment="port 88" /ip firewall filter add chain=input protocol=udp dst-port=88 action=accept place-before=0 comment="port 88" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 88 TCP" -Direction Inbound -Protocol TCP -LocalPort 88 -Action Allow New-NetFirewallRule -DisplayName "Port 88 UDP" -Direction Inbound -Protocol UDP -LocalPort 88 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 88 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 88 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 88 -j DROP iptables -A INPUT -p udp --dport 88 -j DROP ufw allow from 203.0.113.0/24 to any port 88 proto tcp ufw allow from 203.0.113.0/24 to any port 88 proto udp /ip firewall filter add chain=input protocol=tcp dst-port=88 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=88 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=88 action=drop /ip firewall filter add chain=input protocol=udp dst-port=88 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 ':88 ' netstat -ano | findstr :88 # Windows lsof -i :88 # macOS # test from another machine nc -vz example.com 88