Intermittent Active mode attempts by server

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Intermittent Active mode attempts by server

#1 Post by » 2019-05-12 14:40

We have the server setup for Passive mode and all client connection specify passive mode. For connections requiring explicit TLS encryption we randomly receive "425 Can't open data connection for transfer of '...'". On retrying the upload for the same file it usually works.

These are our observations:
The ones that fail call the PASV command but instead of the usual '227 Entering Passive Mode'-message it reports '421 Could not create socket.', then followed by the PORT command. This ultimately fails as we are not setup for Active Mode at all and then ultimately we get '425 Can't open data connection for transfer of '...' ' on the client.

We are running the latest release of the server and can produce this with only a 3 separate active uploads - therefore load is not the issue either (on another server with more load we do see it more often, but cannot determine % fails vs successes).

What would be the reason for this scenario and how would we be able to go about finding or resolving the root cause of the 421?

Is this a bug or something to expect?

Thanks in advance

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Re: Intermittent Active mode attempts by server

#2 Post by botg » 2019-05-12 19:34

Check your configured port range, it seems to be too small.

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Re: Intermittent Active mode attempts by server

#3 Post by boco » 2019-05-12 21:01

Please note that an FTP server cannot initiate any command. It's the FTP client that, after the server sends the error reply (because it has run out of ports, or is blocked), falls back to Active mode. Usually it is possible to disable the Active mode fallback in the client, to speed up operation. If you are certain that Active mode will not work for sure, it's a redundant operation and thus not required to even be attempted.
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