= Introduction =

This is schsh, a schroot-based shell. 
The purpose is simple: I want to provide users with scp, sftp and rsync access
to my server, such that they can only operate in a certain subdirectory.
There are plenty of solutions out there, and all have one drawback in common:
You need to manually set up a bunch of chroots, and copy the files needed for
scp, sftp and rsync into them.
I didn't like that, so here is my alternative solution: Use schroot for the
chroots. This gets OpenSSH out of the loop when it comes to chroots, instead
the relevant users get a special shell (schsh, the schroot shell). That shell
essentially calls schroot and runs the desired command inside the chroot. It
also provides some very basic command restriction (so that you can allow scp,
sftp and rsync and nothing else), but for a more sophisticated command
filtering you can nest this with something like rush[0].
Unfortunately, this still needs a (s)chroot to be set up for each user, but at
least no files have to be copied: Instead, schroot is configured to bind-mount
/bin, /lib, /usr/bin and /usr/lib into the user-chroot. Hence no files are
duplicated, and system updates to the relevant tools are applied inside the
chroots automatically.


[0] http://www.gnu.org.ua/software/rush/

= Setup =

Dependencies:
schsh needs Python 2 (I tested it with version 2.7) and schroot.

Installation is simple: Just run "make install". That will copy two files
to /usr/local/bin, and some configuration to /etc/schroot/.
Before you create any users, make sure the directory /var/lib/schsh and a
group called "schsh" exist.

Before you can set up schsh for a user, you need to create it first:
$ adduser sandboxed --disabled-password

Any existing user can be "sandboxed" by running
$ /usr/local/bin/makeschsh sandboxed
This does the following:
* Change the user's shell to /usr/local/bin/schsh
* Create a chroot base in /var/lib/schsh/sandboxed with some empty subfolders
  as well as /etc/passwd and /etc/group containing only root, this user and
  the "schsh" group
* Add the user to the "schsh" group
* Add a schroot called schsh-sandboxed for the given folder, and an fstab file
  in /etc/schroot/schsh used by this schroot

Now if the user logs in via SSH, /usr/local/bin/schsh will be executed, and
it will lock the user into the schroot schsh-sandboxed. It will only see
/bin, /lib, /usr/bin and /usr/lib and a folder called /data mapped to
/home/sandboxed/data. If you want to give the user access to more folders,
or another folder, simply edit /etc/schroot/schsh/sandboxed.fstab.

= Configuration =

There is not much to configure at the moment. However, there are some
global variables at the top of both Python scripts to change the base
paths, and to tell which commands are allowed.