[SSHD-1127] SFTP/SCP SftpErrorStatusDataHandler is not propagating back proper custom messages to the client

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Assignee: Lyor Goldstein
• Reporter: Susmit Sarkar
• Created: 17/Feb/21 13:53
• Updated: 19/May/21 05:19
• Resolved: 19/May/21 05:19

Description

Hi Team,

SftpErrorStatusDataHandler is not propagating back proper custom messages to the client. Although it works fine with clients like FileZilla and WinSCP, we don't get a proper message for Unix-based SFTP command-line client.

We can say that there is no issue with the implementation as it's working for clients like FileZilla and WinScp, so the code is fine. I agree with you on that.

But commercial sftp server libraries are able to successfully propagate messages to all clients FileZilla, WinScp, Putty, Terminal.

As for example, commercial enterprise based SFTP server uses this API:

// code placeholder
import com.xxxx.sshd.SessionChannel;

SessionChannel.sendStderrData("Too many concurrent logins. Please try again later.\n".getBytes("US-ASCII"));

I think a similar implementation should be introduced in the MINA library for future release.

While reading the doc I came across this: (https://github.com/apache/mina-sshd/blob/ac45b617272303058193db193650e7f2db3d1872/docs/sftp.md)

If the registered handler implements ChannelSessionAware then it will also be informed of the registered ChannelSession when it is provided to the SftpSubsystem itself. This can be used to register an extended data writer that can handle data sent via the STDERR channel. Note: this feature is allowed according to SFTP version 4 - section 3.1:

Packets are sent and received on stdout and stdin. Data sent on stderr by the server SHOULD be considered debug or supplemental error information, and MAY be displayed to the user.

however, the current code provides no built-in support for this feature.

sftp> PUT docker-compose.yml
debug3: Looking up docker-compose.yml
Uploading docker-compose.yml to /dmicons/docker-compose.yml
debug3: Sent message SSH2_FXP_OPEN I:8 P:/dmicons/docker-compose.yml
remote open("/dmicons/docker-compose.yml"): Failure

---------------------------------------------------------------

Same operation in WinScp loggers are coming as:

2021-02-17 18:42:44.322 - Uploading a file to a full filesystem (HDD).

• 2021-02-17 18:42:44.322 - Exceeding a user disk quota.
  . 2021-02-17 18:42:44.322 Asking user:
  . 2021-02-17 18:42:44.322 Cannot create remote file '/dmicons/IBM_UserTest.log'. ("General failure (server should provide error description).
  . 2021-02-17 18:42:44.322 Error code: 4
  . 2021-02-17 18:42:44.322 Error message from server: User admin is not allowed to execute this SFTP command because of policy SFTP20

Thanks,

Susmit