Is it safe to open
VPN ports are meant to be open. Use modern protocols (WireGuard, IKEv2), strong keys, and keep the VPN software up to date.
How to open port 51820
Open to everyone
# Linux iptables iptables -A INPUT -p udp --dport 51820 -j ACCEPT # Ubuntu ufw ufw allow 51820/udp # firewalld firewall-cmd --permanent --add-port=51820/udp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=udp dst-port=51820 action=accept place-before=0 comment="port 51820" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 51820 UDP" -Direction Inbound -Protocol UDP -LocalPort 51820 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p udp --dport 51820 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p udp --dport 51820 -j DROP ufw allow from 203.0.113.0/24 to any port 51820 proto udp /ip firewall filter add chain=input protocol=udp dst-port=51820 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=udp dst-port=51820 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 ':51820 ' netstat -ano | findstr :51820 # Windows lsof -i :51820 # macOS