Port 21 - Port Encyclopedia リファレンス 日本語
日本語 page. Port 21 reference for Port Encyclopedia. Check meaning, syntax, common usage, related standards, and practical implementation notes. 日本語のページ文脈と関連する WebRecast...
What Port 21 is used for
Port 21 is commonly associated with FTP control traffic. The number alone is not enough for diagnosis. Check the protocol, direction, listening process, and network boundary together.
The main risk for this port is clear-text credentials and legacy firewall exposure. In production, first decide whether the service should exist, then match firewall rules, ingress routing, security groups, and service configuration against the same evidence.
Practical check: confirm whether passive FTP data ports are also required. Port 27017 and Port 22 may belong to different service families, so do not close every port scan finding with the same generic fix.
Port 21 findings should be treated differently from a normal web port because FTP often opens a second data channel. A scanner can show only the control port while the real transfer problem is hidden in passive port ranges, NAT rules, or firewall helpers.
If the service is still required, document who owns the FTP account, whether uploads are restricted to a jailed directory, and why SFTP or a managed object storage flow cannot replace it.
Port checklist
- Port 21 has a confirmed TCP, UDP, or dual-protocol requirement.
- The listening process and container are mapped to a service owner.
- Firewall, ingress, and cloud security rules point to the same intended target.
- External exposure includes TLS, authentication, and logging where needed.
Example searches
- ftp control port open
- is Port 21 secure
- Port 21 firewall
- what service uses Port 21
主な用途
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