Port 21 — FTP control

TCP well-known

What runs on port 21

The command channel for FTP: login, directory listings and transfer commands.

Security considerations

Credentials travel in clear text. Anything exposed to the internet on port 21 will be found by scanners within hours.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :21
sudo ss -lntp | grep :21

# Windows
netstat -ano | findstr :21
Get-NetTCPConnection -LocalPort 21

# is it reachable from outside?
nc -zv example.com 21
curl -v telnet://example.com:21

Freeing the port

# find the process, then stop it
sudo lsof -ti :21 | xargs kill        # Linux / macOS
netstat -ano | findstr :21            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Other common ports

• Port 20 — FTP data
• Port 22 — SSH / SFTP
• Port 23 — Telnet
• Port 25 — SMTP
• Port 53 — DNS
• Port 67 — DHCP server
• Port 68 — DHCP client
• Port 69 — TFTP
• Port 80 — HTTP
• Port 110 — POP3
• Port 123 — NTP
• Port 143 — IMAP
• Port 161 — SNMP
• Port 389 — LDAP
• Port 443 — HTTPS
• Port 445 — SMB
• Port 465 — SMTPS
• Port 514 — Syslog

All port numbers