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ProFTPD with mod_sftp fails with files > 2GB

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MarisB
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ProFTPD with mod_sftp fails with files > 2GB

Hi!

The latest Ubuntu LTS release (22.04.4) with its default ProFTPD Version 1.3.7c. WinSCP fails to upload files that are greater than 2GB. I am using SFTP (mod_sftp).

Clean Windows 11 installation with FileZilla and WinSCP 6.3.1 (no settings changed):
  1. . Uploading via user interface using both FileZilla and WinSCP works, but both show disconnect/reconnect at 2GB.
  2. Uploading from a script with -resumesupport=on also works.
  3. Uploading from a script with -resumesupport=off hangs indefinitely as it disconnects/reconnects at 2GB and starts uploading from the start again and again.
I believe that this issue is caused by the ProFTPD rekey feature:
http://www.proftpd.org/docs/contrib/mod_sftp.html#SFTPRekey

I have not tried to disable the rekeying feature, but it would be nice if WinSCP worked with the default ProFTPD configuration.

Attaching relevant log files. Currently I don't have a public facing test server, but I can make one if that helps.
  • logs.zip (9.31 KB, Private file)

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martin◆
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Re: ProFTPD with mod_sftp fails with files > 2GB

Well, so it looks like ProFTPD issue, doesn't it? Did you report it to them?

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MarisB

I am not sure if it is a server side or client side bug. As they mention the rekeying in their documentation, so I thought this is a bug in WinSCP. Are the logs say that the bug is on the server side?

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martin◆
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Well, you have the problem even with FileZilla. Does the rekey with your server work with any other SFTP client? How does mention of rekey in ProFTPD documentation indicate a bug in WinSCP?

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MarisB
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I am sorry that my description was not clear enough. FileZilla shows a disconnection in the log, but it automatically handles this and the file is successfully uploaded. WinSCP also uploads successfully with -resumesupport=on. What is failing, is the option -resumesupport=off.

If you still think that this is a server's problem, I will try to report it to their team.

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martin◆
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Sorry. Ok, so indeed, without resume support, WinSCP won't resume the transfer.
Is there a reason why you want to have the resume support turned off?

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MarisB
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I was just testing around without a reason and found a bug. At first I thought that WinSCP is ignoring rekeying (I didn't see its support in the WinSCP docs), but now I have found that there is the rekeying support (https://winscp.net/eng/docs/ui_login_kex).

So there is still a bug, but on which side – WinSCP or ProFTPD? I know that WinSCP is a very reliable app, but not sure about ProFTPD, as I just needed one for testing a different server from OpenSSH, and I took one that is already on Ubuntu by default.

Not sure if this helps, but anyway – I just tested today using an OpenSSH client (scp) to ProFTPD, and it worked without rekeying (as far as -v param shows), however, when I try OpenSSH to OpenSSH the rekeying occurs.

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MarisB

I have just tested and OpenSSH can easily upload a 10GB file to ProFTPD. The verbose mode doesn't show any rekeying, however, it slightly pauses (uploading speed) at about 4096MB and 8192MB, so it seems like rekeying is taking place.

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martin◆
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@MarisB: Sorry for my late answer. It took me while to setup a test environment. And I've actually wasn't able to upload a big file to (local=fast) ProFTPD at all:
https://github.com/winscp/winscp/commit/7ae34b89576244e729e849a103c4624b5b2098a9
But once I've fixed that, I did not have any problem with rekey: 
. 2024-03-19 15:56:47.208 Opening remote file.
> 2024-03-19 15:56:47.208 Type: SSH_FXP_OPEN, Size: 43, Number: 3331
< 2024-03-19 15:56:47.219 Type: SSH_FXP_HANDLE, Size: 25, Number: 3331
> 2024-03-19 15:56:47.219 Type: SSH_FXP_WRITE, Size: 32757, Number: 3846
. 2024-03-19 15:56:47.231 Skipped 17 SSH_FXP_WRITE packets
< 2024-03-19 15:56:47.231 Type: SSH_FXP_STATUS, Size: 24, Number: 3846
< 2024-03-19 15:56:47.231 Status code: 0
. 2024-03-19 15:57:41.181 Initiating key re-exchange (too much data sent)
. 2024-03-19 15:57:41.220 Doing ECDH key exchange with curve Curve448, using hash SHA-512 (unaccelerated)
. 2024-03-19 15:57:41.512 Initialised AES-256 SDCTR (AES-NI accelerated) [aes256-ctr] outbound encryption
. 2024-03-19 15:57:41.512 Initialised HMAC-SHA-256 outbound MAC algorithm
. 2024-03-19 15:57:41.512 Initialised AES-256 SDCTR (AES-NI accelerated) [aes256-ctr] inbound encryption
. 2024-03-19 15:57:41.513 Initialised HMAC-SHA-256 inbound MAC algorithm
. 2024-03-19 15:58:34.913 Initiating key re-exchange (too much data sent)
. 2024-03-19 15:58:35.006 Doing ECDH key exchange with curve Curve448, using hash SHA-512 (unaccelerated)
. 2024-03-19 15:58:35.214 Initialised AES-256 SDCTR (AES-NI accelerated) [aes256-ctr] outbound encryption
. 2024-03-19 15:58:35.214 Initialised HMAC-SHA-256 outbound MAC algorithm
. 2024-03-19 15:58:35.214 Initialised AES-256 SDCTR (AES-NI accelerated) [aes256-ctr] inbound encryption
. 2024-03-19 15:58:35.214 Initialised HMAC-SHA-256 inbound MAC algorithm
. 2024-03-19 15:59:14.229 Skipped 85487 SSH_FXP_WRITE and 85473 SSH_FXP_STATUS packets
> 2024-03-19 15:59:14.229 Type: SSH_FXP_CLOSE, Size: 25, Number: 21893124
. 2024-03-19 15:59:14.269 Skipped 31 SSH_FXP_STATUS packets
< 2024-03-19 15:59:14.269 Type: SSH_FXP_STATUS, Size: 24, Number: 21893124
< 2024-03-19 15:59:14.269 Status code: 0
> 2024-03-19 15:59:14.270 Type: SSH_FXP_RENAME, Size: 50, Number: 21893394
< 2024-03-19 15:59:14.291 Type: SSH_FXP_STATUS, Size: 24, Number: 21893394
< 2024-03-19 15:59:14.291 Status code: 0
. 2024-03-19 15:59:14.291 Preserving timestamp [2024-03-13T09:20:47.000Z]
> 2024-03-19 15:59:14.292 Type: SSH_FXP_SETSTAT, Size: 34, Number: 3593
< 2024-03-19 15:59:14.306 Type: SSH_FXP_STATUS, Size: 24, Number: 3593
< 2024-03-19 15:59:14.307 Status code: 0
. 2024-03-19 15:59:14.307 Transfer done: 'C:\test\big2gb' => '/work/big2gb' [2797710400]
. 2024-03-19 15:59:14.311 Copying finished: Transferred: 2 797 710 400, Elapsed: 0:02:27, CPS: 19 009 929/s
Tested against the very latest (master) ProFTPD.

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MarisB

Thank you for your support! The latest ProFTPD with mod_sftp 1.3.9rc3, which I compiled from the most recent source, is working perfectly. It turns out Ubuntu LTS hasn't been keeping up with the newest features and fixes. I mistakenly thought they were more up-to-date.

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