Registry assignments, software defaults and local listeners are different things. A port number alone does not identify or secure a service.
Configuration and troubleshooting
능동 모드와 수동 모드, 설정된 데이터 포트 범위, NAT 매핑을 확인하세요. 포트 21만 열어서는 수동 모드 전송에 충분하지 않습니다.
Important limitations
FTP 자체는 암호화되지 않습니다. FTPS와 SSH 기반 SFTP는 서로 다른 프로토콜이며 포트 설정만 바꿔 대체할 수 없습니다.
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:LISTENWindows PowerShell
Get-NetTCPConnection -LocalPort 21 -State ListenSources and review
Sources checked on:
This review covers the explanations and sources on this page, not your device or every community software entry below.
이 포트를 사용하는 소프트웨어 (2 개 소프트웨어)
Community software entries: links are references, not individual verification.
사용 시 참고 사항
- 포트 21 사용 전에 다른 서비스가 점유하고 있지 않은지 확인해 주세요
- 포트 충돌이 발생하면 다른 포트를 사용하거나 해당 포트를 점유한 서비스를 중지하는 것을 고려해 보세요
자주 묻는 질문
What is port 21 used for?
Port 21 sits in the File Sharing category. This page lists the registered purpose and the software commonly associated with it; to see what is actually listening on a specific machine, check the process with ss/netstat.
Is port 21 TCP or UDP?
The registered protocol on this page is TCP. The same number behaves independently on each protocol: TCP may be listening while UDP is idle, so check them separately.
How do I check if port 21 is in use?
On Linux: sudo ss -tlnp 'sport = :21' for an exact port match. On macOS: sudo lsof -nP -iTCP:21 -sTCP:LISTEN. On Windows PowerShell: Get-NetTCPConnection -LocalPort 21 -State Listen. A listener counts as confirmed only when you can see the bind address and process name; without root/admin you will not see other users' processes.
Is port 21 secure? Should I open it in my firewall?
A port number has no inherent security — it depends on whether the listening service is correctly configured and whether it needs exposure at all. Keep loopback/internal-only services firewalled; where public access is required, pair it with source restrictions, VPN or authentication. The caution section on this page lists port-specific risks.