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When connecting to a server via SFTP, I can use -o to specify a LogLevel, like so:

sftp -oLogLevel=DEBUG3 username@host

This spits out a ton of debug information about the login process. However, it gets to this point and all debugs cease:

debug1: Sending subsystem: sftp
debug2: channel 0: request subsystem confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel 0: rcvd adjust 131072
debug2: channel_input_status_confirm: type 99 id 0
debug2: subsystem request accepted on channel 0
Connected to host.
sftp> ls
Some    Directories
sftp> cd Some
sftp> ls
sftp> quit
debug2: channel 0: read<=0 rfd 4 len 0
debug2: channel 0: read failed
debug2: channel 0: close_read
debug2: channel 0: input open -> drain
debug2: channel 0: ibuf empty
debug2: channel 0: send eof
debug2: channel 0: input drain -> closed
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
Received disconnect from <IP>: 11: User Disconnected

What about that ls command? Where can I get debugs for that? cd? quit?

Also, I don't have any control over the remote server, so I'm not asking about configuring the SFTP subsystem on the server via sshd_config. I've tried that and it doesn't work.

The sftp-client.c file has calls to debug3, so where are they going?

3

Use the -vvv switch to the sftp. The options -o are passed directly to the ssh process but not effective to the sftp itself.

It is well described in the manual page for sftp:

-v      Raise logging level.  This option is also passed to ssh.

Also the -o explains that it is passed to the ssh and not effective in sftp process:

-o ssh_option

Can be used to pass options to ssh in the format used in ssh_config(5). This is useful for specifying options for which there is no separate sftp command-line flag. For example, to specify an alternate port use: sftp -oPort=24. For full details of the options listed below, and their possible values, see ssh_config(5).

  • Yep, that works. The documentation on that option is rather poor. "This option is also passed to ssh" says nothing about the SFTP subsystem. In fact, I didn't believe you at first when I saw it. Also, I don't see in the docs where it says that -o is not effective in the sftp process. – Brendan Byrd Nov 4 '16 at 13:52
  • It is not a problem of a subsystem. The crucial is understanding that running sftp is running two processes. The sftp client which has its options and ssh which has different options. You were getting the logs from the ssh process, but not from sftp client. With the subsystem, you might be confused by the server side (there it is really a subsystem), but on the client, sftp is running "over" ssh so from this point of view it is not a subsystem, but ssh is a subprocess of sftp. – Jakuje Nov 4 '16 at 13:57

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