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 43
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 43 -j ACCEPT # Ubuntu ufw ufw allow 43/tcp # firewalld firewall-cmd --permanent --add-port=43/tcp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=43 action=accept place-before=0 comment="port 43" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 43 TCP" -Direction Inbound -Protocol TCP -LocalPort 43 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 43 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 43 -j DROP ufw allow from 203.0.113.0/24 to any port 43 proto tcp /ip firewall filter add chain=input protocol=tcp dst-port=43 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=43 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 ':43 ' netstat -ano | findstr :43 # Windows lsof -i :43 # macOS # test from another machine nc -vz example.com 43