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Why is ExtremeXOS ssh exit status 255 when no error occurs?

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TitleWhy is ExtremeXOS ssh exit status 255 when no error occurs?
Symptoms
EXOS SSH server return code is always 255, irrespective of the status of command
Environment
  • EXOS 15.6.3
Cause
It seems like client sends disconnect message when it is done with remote command execution, ssh-toolkit server is not able to decode this message properly, and kills the session with SIGTERM.
That’s why client doesn’t receive any exit status and it shows status as 255 or -1(error case).
Resolution
This is described in CR# xos0063330;
Our
development team has further investigated the problem and have found that this issue appears only in our older SSH Server, which is a  SSH-toolkit implementation. From EXOS 16.2 and above releases, we are using OpenSSH for SSH server and this issue is not observed. 

SSH-Toolkit Server supports only the below listed SSH-request types. Here "exec" is used for remote command execution, which works fine. But after the command is executed the client will request for a "exit-status" from the server. In SSH-Toolkit server, this message type is not supported. So, for "exit-status" request, Server replies with an error message. This is because it cannot process the request.


The valid channel requests in SSH-Toolkit and their actions are:    

    "pty-req"       -- setup pty
    "shell"         -- start user shell
    "exec"          -- execute a command
    "subsystem"     -- handle subsystem calls (not implemented yet)
    "env"           -- determine if ENV variable is OK to be set, then set
    "signal"        -- if we have signals, send this signal to the process
    "exit-status"   -- send exit status to client, close connection (not supported - ignored)
    "window-change" -- change terminal window size values
    "xon-xoff"      -- handle flow control (not supported - ignored)
    "x11-req"       -- channel request failure (not supported - ignored)


In EXOS 16.2 and 21.1 releases this issue would not exist as we have upgraded the SSH server to OPENSSH instead of SSH-Toolkit. However for EXOS releases lower than 16.2, this limitation would continue to exist.
Additional notes

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