TTY PTY support in Windows OpenSSH

PTY

Windows OS does not have inbuilt support for a pseudoterminal.

Windows version of OpenSSH server implements a VT100 PTY by intercepting Windows Console events. This is implemented in ssh-shellhost.exe, connected to sshd via std i/o. At a high level, ssh-shellhost.exe acts as an intermediary between sshd and a Windows console applications performing the following:

• interprets incoming VT100, processes and calls Windows Console IO
• processes Windows Console (output) event queue, translates them to VT100 and spits out on stdout.

TTY

Windows console previously did not have the capability to process any incoming terminal control codes. Support for xterm is added in Windows 10.

To support downlevel platforms (down till Windows 7), a VT100 interpreter is implemented in Windows OpenSSH client (ssh.exe) to support typical TTY scenarios. Although this works well when talking to Windows targets, there are limitations and known issues when dealing with interactive applications on Unix like top, vi, etc.

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If you are using the openssh client,
a) Windows 10+ OS then windows console (cmd.exe/powershell) has the support to interpret the VT sequences received from the SSH server. We simply pass the raw VT sequences to the console (cmd.exe/powershell). If you are facing any issues then it should be fixed by the windows console team.

 If you want to use the inbuilt VT100 ANSI parser (that's part of the openssh client) then set the environment variable "SSH_TERM_CONHOST_PARSER" to 0.

b) Prior versions of windows 10 OS, the raw VT sequences will be parsed by the inbuilt VT100 ANSI parser that's part of the ssh.exe. We have fixed most of the issues. We have no plans to extend to VT100+ sequences.

 If you want support for the VT100+ sequences then please upgrade to windows 10+ OS or use the third party tools like putty, Cygwin, etc.

For UNIX users

If you want to use vi / top / man, etc commands while connecting from windows client to Linux server then please follow these steps

1. Upgrade to windows 10+ OS.

2. Set the non-legacy console mode in the console properties,
   Make sure you uncheck the "use legacy console"
   

3. Set the Screenbuffer width, window size width to >= 90.
   

4. Set the environment variable (TERM) to xterm. Before/After logging into the SSH server.
   c:\test> set TERM=xterm
   c:\test> set TERM (This should show the term set to xterm)

Testing

We have tested different scenarios,

1. Windows client (cmd.exe / powershell) to windows server
2. Windows client (cmd.exe / powershell) to Linux server
3. Linux client to windows server
4. Third party ssh tools like putty/Cygwin to windows server

While making any terminal code changes, please make sure below manual test cases are passed

1. All the arrow key functions should work properly (up, down, left and right arrows).
2. The backspace, delete, home, end functions keys should work properly.
3. "cls" should clear only the visible window (windows 10+ OS) / should clear entire buffer (prior versions of windows 10 OS).
4. When connected to unix ssh server, the basic commands like vi, top, man, clear, etc should work properly.

Known windows 10 console issues (These will be fixed by the windows console team)

When connected to unix ssh server

a) vi command, the up/down/side arrow keys are not working.
b) vim command, intermittently the file is opened in the Replace mode.
c) top command, clears the previous screen contents.