Is it safe to open
File-sharing protocols are not designed for the internet: they leak data and spread ransomware. Keep them inside the LAN and give remote users a VPN or an HTTPS-based service instead.
How to open port 20
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 20 -j ACCEPT # Ubuntu ufw ufw allow 20/tcp # firewalld firewall-cmd --permanent --add-port=20/tcp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=20 action=accept place-before=0 comment="port 20" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 20 TCP" -Direction Inbound -Protocol TCP -LocalPort 20 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 20 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 20 -j DROP ufw allow from 203.0.113.0/24 to any port 20 proto tcp /ip firewall filter add chain=input protocol=tcp dst-port=20 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=20 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 ':20 ' netstat -ano | findstr :20 # Windows lsof -i :20 # macOS # test from another machine nc -vz example.com 20