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SFTP server periodically stalls
I'm trying to SFTP upload files from one computer to another on a periodic
basis. Most of the time this works great but sometimes (on the hour) the
connection is unsuccessful.

In reviewing the logs... the SFTP client is able to login via SSH. The
client sends a SSH_MSG_CHANNEL_REQUEST packet for the sftp subsystem and
the server responds with a SSH_MSG_CHANNEL_SUCCESS. The client then sends
an SSH_MSG_CHANNEL_DATA packet containing a SSH_FXP_INIT packet and then...
nothing. The server *should* be responding with a SSH_FXP_VERSION packet
(wrapped in a SSH_MSG_CHANNEL_DATA packet) but it's not.

On the server end of things the last entry I see in the log files for these
sessions is this:

May 20 09:00:12 sftpacct sshd[22358]: debug3: send packet: type 99


That corresponds to the SSH_MSG_CHANNEL_SUCCESS packet.

Overall it kinda feels to me like that the sftp subsystem just isn't setup
correctly. From my sshd_config file:

Subsystem sftp internal-sftp -f LOCAL7 -l INFO -u 0007


Could that be the source of some of the problems? Where else should I be
looking?

The server is OpenSSH_7.6p1 Ubuntu-4ubuntu0.6.

The full log files are available at https://pastebin.com/ag9q6pVS and
https://pastebin.com/sKYDrJAK. https://pastebin.com/ag9q6pVS has "May 20
09:00:12 sftpuser sshd[22287]: User child is on pid 22358" close to the end
and the activity for the new pid is in https://pastebin.com/sKYDrJAK.

Thanks!
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Re: SFTP server periodically stalls [ In reply to ]
On Fri, 27 May 2022 at 06:40, Terra Frost <terrafrost@gmail.com> wrote:
[...]
> Overall it kinda feels to me like that the sftp subsystem just isn't setup
> correctly.

If it wasn't set up correctly I'd expect it to not work at all.

[...]
> Could that be the source of some of the problems? Where else should I be
> looking?

If you can catch it when it's in the stalled state, I'd check the
netstat entry corresponding to the sshd to see if there's anything in
the "SendQ" (which would indicate a problem with the network) and
strace the sshd and sftp-server processes to see what if anything
they're waiting on.

The "on the hour" nature of it makes me suspicious, though. Do you do
anything like roll the syslogs on the hour?

--
Darren Tucker (dtucker at dtucker.net)
GPG key 11EAA6FA / A86E 3E07 5B19 5880 E860 37F4 9357 ECEF 11EA A6FA (new)
Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.
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Re: SFTP server periodically stalls [ In reply to ]
Darren Tucker wrote:
> Terra Frost wrote:
> > Could that be the source of some of the problems? Where else should I be
> > looking?
>
> The "on the hour" nature of it makes me suspicious, though. Do you do
> anything like roll the syslogs on the hour?

The problem could be quite far away. Such as for some reason the
firewall rules being reloaded at the top of every hour and for just a
very short time hourly it could be blocking packets.

Try disabling cron and/or systemd-timerd (I think on Ubuntu?) and see
if things magically work at the top of the hour? If so then it is
something happening via those other periodic tasks.

Bob
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