[rdiff-backup-users] Can one set up a login for rdiff-backup only (not via sshd_config)?

[Chris G]

Is it possible to set up a dedicated login for rdiff-backup to use
without using sshd_config?

I have a NAS backup system which runs linux but has an ancient version
of ssh on it which I don't want to play about with.  So I'd prefer to
create a dedicated login (a user called 'bak') which *only* allows
rdiff-backup to run.

Is there any way I can do this? I guess rdiff-backup needs a shell to
run in so I can't just set the shell field in /etc/passwd to
"rdiff-backup --server".  Would I get away with a .profile for the
user that runs "rdiff-backup --server" or something like that with a
--remote-schema set to nothing?

-- 
Chris Green