What is the difference between port 20 and port 21?

Port 21 is the command and control channel (where you log in and type commands like LIST, RETR, and STOR), whereas Port 20 is the dedicated data channel where file bytes flow in Active FTP mode.

Should port 21 be open to the public internet?

Generally no. Legacy FTP on port 21 sends plaintext passwords across the network. If remote file transfers are required, administrators should deploy SFTP on port 22 or restrict port 21 to an encrypted VPN tunnel.

How can I check if an FTP daemon is listening on port 21?

Use our online Port Checker to verify public reachability, or run 'curl -v ftp:// :21' or 'nc -zv 21' locally.

Home/Open Port Checker/Port 21

Port 20 Port 22

Well-Known Port Reference

Port 21: FTP Control (TCP)

File Transfer Protocol control channel used for authentication, directory navigation, and issuing transfer commands.

Port Number

Transport

TCP

Security Risk

High

Standard / RFC

RFC 959

Test Port 21 Reachability Live

Verify whether Port 21 is open, closed, or filtered on your server or public IP address.

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Service Architecture & Function

Port 21 is the default listening port for FTP servers (such as vsftpd, ProFTPD, and Pure-FTPd). When a client connects, port 21 establishes a persistent bidirectional command channel where usernames, passwords, file paths, and transfer commands are exchanged.

Security Profile & Hardening

FTP transmits credentials and commands in cleartext over port 21. It is susceptible to credential interception, brute-force dictionary attacks, and bounce scanning. Prefer SSH File Transfer Protocol (SFTP) or TLS-wrapped FTPS.

Example Firewall Commands:

# UFW (Ubuntu/Debian)

sudo ufw allow 21/tcp

# iptables (Direct Kernel)

iptables -A INPUT -p tcp --dport 21 -j ACCEPT