FTP uses separate control (21) and data channels — active mode opens inbound data connections; passive mode has the server pick a high port. SFTP is not FTP over SSH — it is SSH subsystem for file transfer. SCP is simpler one-way copy over SSH.
FTP modes
| Mode | Who connects data | Firewall note |
|---|---|---|
| Active | Server connects to client data port | Client must allow inbound — painful |
| Passive | Client connects to server high port | Open outbound + server port range |
| FTPS | TLS on FTP | Explicit vs implicit TLS — different ports |
| SFTP | Single SSH session | Only 22/tcp — easier to policy |
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