(Moved from StackOverflow: http://stackoverflow.com/questions/4589725/sftp-via-shell-how-it-is-possible)

How is it possible for tools like http://mysecureshell.sourceforge.net/ to provide SFTP access by merely specifying them as shell by typing:

usermod -s /bin/MySecureShell myuser

?

I'm on Debian Lenny, with default sshd/OpenSSH.

Is this e.g. a feature of SSH protocol that allows user shell to handle sftp commands? I can't wrap my head around this because usually OpenSSH needs sftp-server module (or the internal one in newer versions) - and this makes me think that sftp commands don't even hit the shell and are handled earlier or by different code path..