Networking
Risk: Critical
Port 21 — FTP (File Transfer Protocol Control)
Port 21 is the command control port for standard File Transfer Protocol. Data transfer occurs over port 20 or dynamic passive ports.
PORT NUMBER
21
PROTOCOL
TCP
DEFAULT PROCESS
vsftpd, proftpd, pure-ftpd
FIREWALL TARGET
21/tcp
Instant Terminal Fixes for Port 21
macOS & Linux Terminal
1. Check who is listening on port 21:
sudo ss -tulpn | grep :212. Kill process occupying port 21 (EADDRINUSE fix):
sudo systemctl stop vsftpd3. UFW Firewall Allow rule:
sudo ufw deny 21/tcpWindows PowerShell (Admin)
1. Check listening port:
Get-NetTCPConnection -LocalPort 212. Force kill process on port 21:
Stop-Service ftpsvc -Force3. Docker Port Forwarding Mapping:
docker run -p 21:21 -d fauria/vsftpdCommon Error Encountered
ftp: connect: Connection refused on port 21
Security Advisory & Hardening
FTP transmits usernames and passwords in clear text over the wire. Deprecate FTP in favor of SFTP (SSH Port 22) or FTPS with mandatory TLS.
Frequently Asked Questions
Port 21 Questions & Answers
Common questions regarding default services, kill commands, and firewall configurations.
Port 21 is commonly used for FTP (File Transfer Protocol Control). Port 21 is the command control port for standard File Transfer Protocol. Data transfer occurs over port 20 or dynamic passive ports.