Port 21 — FTP

TCPFile Sharing

FTP uses TCP 21 for its control connection. File data travels over a separate connection, so a successful login does not prove transfers will work.

Registry assignments, software defaults and local listeners are different things. A port number alone does not identify or secure a service.

Configuration and troubleshooting

Check active versus passive mode, the configured data-port range and NAT mappings. Opening only port 21 is insufficient for passive transfers.

Important limitations

FTP itself is not encrypted. FTPS and SSH-based SFTP are different protocols, not interchangeable port settings.

Read-only checks: run on the server host

Choose the commands for your OS. Check TCP and UDP separately. No result is not a lasting availability guarantee; inspect host and container separately.

Linux

sudo ss -ltnp 'sport = :21'

macOS

sudo lsof -nP -iTCP:21 -sTCP:LISTEN

Windows PowerShell

Get-NetTCPConnection -LocalPort 21 -State Listen

Sources and review

Sources checked on: 2026-09-05

This review covers the explanations and sources on this page, not your device or every community software entry below.

RFC 959 — FTP connections

Software Using This Port (2 software)

Community software entries: links are references, not individual verification.

FTP (File Transfer Protocol)

FileTransferProtocolNetworkTransferFile

Reference link ↗

FileZilla Server

FTPServerSoftware

Reference link ↗

Usage Suggestions

Before using port 21, please confirm it is not occupied by other services
If you encounter port conflicts, consider using other ports or stopping the occupying service