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 636
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 636 -j ACCEPT # Ubuntu ufw ufw allow 636/tcp # firewalld firewall-cmd --permanent --add-port=636/tcp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=636 action=accept place-before=0 comment="port 636" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 636 TCP" -Direction Inbound -Protocol TCP -LocalPort 636 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 636 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 636 -j DROP ufw allow from 203.0.113.0/24 to any port 636 proto tcp /ip firewall filter add chain=input protocol=tcp dst-port=636 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=636 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 ':636 ' netstat -ano | findstr :636 # Windows lsof -i :636 # macOS # test from another machine nc -vz example.com 636