What is port 21 used for?

Port 21 is the standard TCP port used for FTP (File Transfer Protocol). It is commonly used by vsftpd, ProFTPD, Pure-FTPd, FileZilla Server. Legacy protocol for transferring computer files between client and server across a network.

How do I check if port 21 is open?

Enter your public IP or hostname into the scanner tool and click Check. A sub-second raw TCP socket handshake will be initiated from our edge server to port 21 on your target. If the 3-way handshake finishes with SYN-ACK, the port is OPEN. If an RST packet is returned, the host is reachable but the port is CLOSED. If packets time out without response, the port is FILTERED by an ISP or firewall.

Why does port 21 show as closed or filtered?

If port 21 reports CLOSED or FILTERED, confirm that: 1) Your service is actively running and bound to 0.0.0.0 (not just localhost/127.0.0.1); 2) Your router has a port forwarding rule directing external port 21 to your machine's local IP; 3) Your operating system firewall (Windows Defender Firewall, ufw, or iptables) allows incoming connections on port 21.

PORT 21 TCP File Transfer High Risk

Port 21 (FTP (File Transfer Protocol)) Open Check

Q: Is port 21 safe to expose to the public internet?

Exposing port 21 carries elevated security risks (High risk). Plaintext credentials and complex active/passive secondary data ports make FTP problematic through firewalls. Use SFTP (port 22) or FTPS instead. For remote services, consider placing them behind a secure VPN (such as WireGuard or Tailscale) or restricting inbound access with strict IP firewall rules.

Legacy protocol for transferring computer files between client and server across a network.

Live Socket Reachability Probe

Pre-configured for Port 21

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Port Specifications & Network Behavior

Standard Port Number 21

Primary Service FTP (File Transfer Protocol)

Transport Protocol TCP

Traffic Classification File Transfer

Residential ISP Status Typically unrestricted by consumer ISPs

Security Exposure High Exposure discouraged without VPN or firewall protection

Security Advisory: FTP (File Transfer Protocol) (Port 21)

Plaintext credentials and complex active/passive secondary data ports make FTP problematic through firewalls. Use SFTP (port 22) or FTPS instead.

Recommendation: Keep port 21 closed to WAN traffic. Connect remotely using WireGuard, Tailscale, or an SSH tunnel.

Port Forwarding & Troubleshooting Checklist

If port 21 returns CLOSED or FILTERED, work through this verified diagnostic checklist:

1 Verify FTP daemon is active via 'systemctl status vsftpd'.
2 For passive FTP through NAT, ensure passive port range (e.g. 40000-40100) and external WAN IP are configured in vsftpd.conf.
3 Forward both command port 21 and passive port range on router.

Verify Host Is Listening Locally

Windows (PowerShell) netstat -ano | findstr :21

Linux / macOS (Bash) ss -tulpn | grep :21