FTP connection failing: "425 Security: Bad IP connectin

[^rooker]

[PROBLEM]
Recently, someone told me that he was unable to connect to a certain FTP anymore, although the same user could log on from somewhere else.

The errormessage in Internet Explorer (sorry, only got the german version) said the following:

Beim Öffnen des Ordners auf dem FTP-Server ist ein Fehler aufgetreten. Stellen Sie sicher, dass Sie die erforderlichen Zugriffsrechte für den Ordner haben

Here are the more interesting details about that error:

Details:
200 Switching to ASCII mode.
227 Entering Passive Mode (xxx,xxx,xxx,xxx,yyyy)
425 Security: Bad IP connecting.

[SOLUTION]
A view into the logs revealed that this client was connecting with different IP addresses in some scheduling fashion, so the control connection and the data connection didn't come from the same IP.

Interesting was that this was automatically done by their router (some consumer standalone device) by default.

Since they haven't figured out yet how to change their router *not* to do so, they've simply tried deactivating passive mode and it worked.