Network Port: 21
Network Port Reference Guide
Registered Service Name
FTP Control Connection
Transport Protocol
TCP
Functional Overview
Port 21 is used by FTP to establish control links, authenticate credentials, and issue navigation commands.
Security Risks & Vulnerabilities
User credentials (usernames/passwords) are sent unencrypted, exposing them to MITM attacks. Vulnerable to credential brute-forcing.
How to audit network port status
1. Scan Open Ports: Use our Port Scanner utility to verify if this port is actively listening on your host.
2. Verify Encryption: Audit the service layer to confirm SSL/TLS is protecting transmission payload.
3. Hardening Rules: Apply the firewall instructions below to shut down unauthorized inbound ports.
UFW Firewall Rules
Allow Command
sudo ufw allow 21/tcp
Block Command
sudo ufw deny 21/tcp
iptables Firewall Rules
Allow Command
sudo iptables -A INPUT -p tcp --dport 21 -j ACCEPT
Block Command
sudo iptables -A INPUT -p tcp --dport 21 -j DROP