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 21
Open to everyone
# Linux iptables iptables -A INPUT -p tcp --dport 21 -j ACCEPT # Ubuntu ufw ufw allow 21/tcp # firewalld firewall-cmd --permanent --add-port=21/tcp && firewall-cmd --reload # MikroTik /ip firewall filter add chain=input protocol=tcp dst-port=21 action=accept place-before=0 comment="port 21" # Windows (PowerShell) New-NetFirewallRule -DisplayName "Port 21 TCP" -Direction Inbound -Protocol TCP -LocalPort 21 -Action Allow
Only for your addresses
# allow only one address or network (replace 203.0.113.0/24) iptables -A INPUT -p tcp --dport 21 -s 203.0.113.0/24 -j ACCEPT iptables -A INPUT -p tcp --dport 21 -j DROP ufw allow from 203.0.113.0/24 to any port 21 proto tcp /ip firewall filter add chain=input protocol=tcp dst-port=21 src-address=203.0.113.0/24 action=accept /ip firewall filter add chain=input protocol=tcp dst-port=21 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 ':21 ' netstat -ano | findstr :21 # Windows lsof -i :21 # macOS # test from another machine nc -vz example.com 21