FTP active mode

How it works

The client sends a PORT command with its address. The server opens the data socket back. NAT and inbound firewall rules block that. Active mode survives only when the client is reachable.

A lab server on a public address uses active mode and a 2 GB instrument file lands. The same client moves behind office NAT and active mode fails. Passive succeeds.

How it differs

Active is not more secure. It is the other direction of the data socket.

A forged PORT can make a careless server connect somewhere you did not intend. Prefer passive on the Internet.

Operators who skip the written rule end up debugging the file when the door was the problem.

On the ticket

• The practical close is a log line: time, actor, byte count, result.
• Without that line the transfer is a story.
• With it, the next person can see whether this door did what the ticket claimed.
• If the path is shared, say so in the partner profile so a later change does not silently pick a different limit, key, or region.
• On a real ticket, write down the door, the byte count, and the clock.
• For ftp active mode, that means naming the host or bucket, the expected size, and the time the other side must have a complete file.
• A progress bar is not that record.
• A 200 response that arrives before the complete call is not that record.
• If a retry is allowed, say how many and whether it resumes.
• If a person must approve the send, name the person.
• Partners who receive ftp active mode files should match on hash or size before they import.
• A same-length corrupt file passes a size check and fails a hash.
• Keep the published hash off the only channel an attacker can edit, or treat it as a corruption check rather than a substitution check.
• When the path changes, new key, new region, new cap, update the profile the same day so the next run does not use a stale limit.